Evolution

Class 11 Biology, unit 14, book pages 376–388

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Evolution means change over time. This unit traces how life changed from the first prokaryotic heterotrophs to the many forms alive today, how the eukaryotic cell may have arisen by endosymbiosis, and how Lamarck, Darwin and the Neo-Darwinists explained the mechanism of change. It ends with the evidence for evolution from biogeography, molecular biology and comparative anatomy, including homologous and analogous structures.

Learning outcomes (7)

Explain that evolution happens due to variations in organisms and the selection pressure that organisms face.
Discuss the evidence that is provided by biogeography.
Analyze the evidence of evolution that comes from molecular biology.
Differentiate between convergent and divergent evolution on the basis of inheritance of homologous and analogous structures.
Describe endosymbiotic theory about the mechanism of evolution of eukaryotes from prokaryotes.
Describe the theory of inheritance of acquired characters, as proposed by Lamarck with example of giraffe neck.
State the drawbacks in Lamarckism.

Origin and evolution on Earth

The Earth is about 4.5 billion years old. The oldest known fossils are 3.7 billion years old. Life has been in the process of transformation since its origin. The word evolution means change over a period of time.

Evolution: Change over a period of time. In biology it is the change of living things since the origin of life.

Evidence suggests that the first organisms were prokaryotic heterotrophs. They depended for their energy on the complex organic molecules present in the ocean "soup". A complex "biochemical evolution" may then have enabled some organisms to harvest solar radiation for making new organic matter inside their cells. This change from the heterotrophic to the autotrophic form may have saved life from becoming extinct, because there would not have been enough organic soup to support the heterotrophs for long.

From the organic soup to life on land

The first organisms were prokaryotic heterotrophs. They obtained energy from the complex organic molecules in the ocean soup.
Some organisms became autotrophs. The earliest photosynthetic autotrophs used hydrogen sulfide as the source of hydrogen to reduce carbon dioxide to sugars. This saved life, because the soup was running short.
Organisms similar to cyanobacteria began oxygen-evolving photosynthesis. They used water instead of hydrogen sulfide as the source of hydrogen, so oxygen was released.
The level of oxygen in the air kept increasing. Its ionization formed the ozone layer.
The thickening ozone layer reduced the ultraviolet radiation reaching the Earth's surface. This made it possible for organisms to become terrestrial and gave stability to the successful forms of life.

Figure 14.1: Origin and evolution on Earth. The time of each event is shown as in the book's figure; the book prints the date for plants as 600 billion years, which should be 600 million.

14.1Evolution from prokaryotes to eukaryotesOften examined

Prokaryotes ruled the world for more than a billion years. According to the fossil record, the eukaryotes appeared 1.9 to 2.1 billion years ago. Several theories have been put forward about how eukaryotes evolved from prokaryotes. A striking one is the endosymbiosis theory.

Endosymbiosis theory

The endosymbiosis theory was proposed by Lynn Margulis. The first step in the evolution of the eukaryotic cell is thought to have occurred when a large anaerobic amoeboid prokaryote ingested a small aerobic bacterium and stabilized its prey as an endosymbiont (a cell living inside another cell) rather than digesting it. This aerobic bacterium developed into a mitochondrion, the site of aerobic respiration in eukaryotic cells. Having such an endosymbiont gave the host the advantage of aerobic respiration. Flagella may have been derived through the ingestion of prokaryotes similar to spirochetes.

Endosymbiosis theory: The theory, proposed by Lynn Margulis, that the eukaryotic cell evolved when a large prokaryote ingested smaller prokaryotes and, instead of digesting them, kept them alive inside itself, where they became organelles.
Endosymbiont: A small cell that lives permanently inside a larger host cell. The aerobic bacterium that became the mitochondrion is an example.

Chloroplasts are thought to be derived from the ingestion of photosynthetic bacteria, which then formed a symbiotic relationship with the host. Both mitochondria and chloroplasts are similar in size to bacteria, have their own DNA, have ribosomes similar to those of bacteria, and produce a limited portion of their own enzymes and proteins. To explain these observations it is suggested that the endosymbionts must have transferred, over time, some of their genes to the host nucleus, and so lost their independence for the sake of the symbiotic relationship.

Figure 14.2: Endosymbiotic origin of eukaryotes. (1) Infoldings in the plasma membrane of an ancestral cell gave rise to endomembrane components, including a nucleus and endoplasmic reticulum. (2) In a first endosymbiotic event the ancestral eukaryote consumed aerobic bacteria that evolved into mitochondria. (3) In a second event the early eukaryote consumed photosynthetic bacteria that evolved into chloroplasts. The bacteria and the organelles they became are drawn in the accent.

The endosymbiotic origin of the eukaryotic cell

Infoldings of the plasma membrane of an ancestral cell formed the endomembrane components, including a nucleus and endoplasmic reticulum. The result is an early eukaryote.
First endosymbiotic event: the ancestral eukaryote ingested an aerobic bacterium and kept it alive. The bacterium became a mitochondrion and gave its host aerobic respiration. This gives the modern heterotrophic eukaryote.
Second endosymbiotic event: a eukaryote that already had mitochondria ingested a photosynthetic bacterium. It became a chloroplast. This gives the modern photosynthetic eukaryote.

Observations that point to an endosymbiotic origin of mitochondria and chloroplasts
Feature	Mitochondria and chloroplasts	Bacteria (prokaryotes)
Size	Similar to a bacterium	Small cells
DNA	Have their own DNA	Have their own DNA
Ribosomes	Similar to bacterial ribosomes	Own ribosomes
Enzymes and proteins	Make a limited portion of their own	Make all of their own

Regardless of the exact mechanism involved, the emergence of the eukaryotic cell led to a dramatic increase in the complexity and diversity of living organisms on Earth. At first, organisms could exist only as independent single cells. Later, some evolved into multicellular organisms in which various cells became specialized for many different functions. These multicellular forms became adapted to life in a great variety of environments.

Worked example: reading the endosymbiotic figure

A student looks at Figure 14.2. (a) Which organelle first appears after event 2 and which after event 3? (b) A cell contains mitochondria but no chloroplasts. At which stage of the figure does it fit? (c) Why do the book's authors say the endosymbionts "lost their independence"?

Event 2 is the ingestion of an aerobic bacterium. Aerobic bacteria carry out aerobic respiration, so the organelle that appears is the mitochondrion.
Event 3 is the ingestion of a photosynthetic bacterium. Photosynthetic bacteria trap light for making food, so the organelle that appears is the chloroplast.
A cell with mitochondria but no chloroplasts is the modern heterotrophic eukaryote. It went through events 1 and 2 but not event 3. A photosynthetic eukaryote has both, because event 3 took place in a cell that already had mitochondria.
Over time the endosymbionts transferred some of their genes to the host nucleus. They could then no longer live and make all their own proteins alone, so they became dependent on the host. That is the loss of independence.

14.2LamarckismOften examined

The idea of evolution may have existed in the minds of early biologists, but JB Lamarck was the first to put forward a theory of evolution. In 1809 he presented his ideas of evolution in his book Philosophie Zoologique.

14.2.1 Main points of Lamarckism

Lamarck's theory helped to prepare the way for the acceptance of the concept of evolution. His proposed mechanism of evolution was based on two conditions: the use and disuse of body parts, and the inheritance of acquired characteristics.

Use and disuse of organs: Organs that an organism uses more frequently are developed and become strong and more efficient. Organs that are not properly used deteriorate, diminish and ultimately disappear in successive generations.
Acquired characters: Characteristics that an individual acquires during its lifetime. They often arise from the use or disuse of organs. Lamarck believed they were passed on to the offspring.

Use and disuse of organs

Lamarck believed that some organs which are used more frequently by an organism are developed and become strong and more efficient, while organs that are not properly used are deteriorated, diminished and ultimately disappear in successive generations.

Development by use: a blacksmith develops a bigger biceps in the arm that works the hammer. The webbed feet of aquatic birds result from constant spreading of the toe bones and the skin between them, in order to swim to find food and escape predators.
Degeneration by disuse: disuse of organs due to environmental changes weakens them and results in degeneracy and atrophy. Snakes creep into small holes and crevices to avoid enemies and obtain food, and have lost their legs. Humans have lost the ability to position the head to collect sound waves, that is, the contractile feature of the ear muscles.

Figure 14.4: Webbed foot of an aquatic bird. Lamarck explained the web as the result of the toes and the skin between them being spread again and again during swimming.

Figure 14.5: Limb vestiges of python. The small leg remnant is what is left of the limbs that snakes have lost. Drawn as a diagram of the book's photograph.

Inheritance of acquired characteristics

Lamarck believed that characteristics which an individual acquired during its lifetime were passed on to the offspring of that individual. Such characteristics are called acquired characters, and they often emerge from the use or disuse of organs. According to Lamarck, through several generations these acquired characters are continuously inherited and accumulated. Gradually a group of organisms would be produced that is better able to "cope" with the environment because of the inherited acquired characters. Evolution, in other words, would occur.

Lamarck's mechanism of evolution in order. The acquired character (in the accent) is the key step: Lamarck assumed it could be inherited.

Evolution of giraffe neck

The long neck of the modern giraffe was, according to Lamarck, the result of generations of short-necked ancestors feeding on leaves at progressively higher levels of trees. Each giraffe, during its lifetime, would constantly stretch its neck to reach the leaves at the top of the trees. As these individuals reproduced, the result of neck stretching (an acquired characteristic) would be passed on to the next generation. Each offspring would be born with a slightly longer neck than its parents. The slightly longer necks and legs produced in each generation were passed on to the subsequent generation, until the size of the present-day giraffe was reached.

Lamarck's explanation of the long neck of the giraffe

Short-necked ancestors fed on leaves, and they fed at progressively higher levels of the trees.
Each giraffe stretched its neck during its lifetime to reach the leaves at the top. This is use of the organ.
The stretched neck (an acquired characteristic) was passed on to its offspring.
Each offspring was born with a slightly longer neck (and legs) than its parents, and stretched it again.
Repeated over many generations, this continued until the size of the present-day giraffe was reached.

Figure 14.6: Neck elongation by stretching in giraffe. Each generation stretches for leaves a little higher up, so the neck becomes longer from the ancestor on the left to the present-day giraffe on the right.

Lamarck's reading of the book's four examples
Case	Use or disuse?	What Lamarck expects to be inherited
Blacksmith's biceps	Use: the arm that works the hammer is used often	A bigger biceps in the children
Webbed feet of aquatic birds	Use: toes and skin spread constantly in swimming	Webbed feet, a little more webbing each generation
Missing legs of a snake	Disuse: creeping into small holes and crevices	Smaller legs, until they are lost
Ear muscles of humans	Disuse: the ability to position the head to collect sound waves is no longer used	Weaker ear muscles, until the contractile feature is lost

14.2.2Drawbacks of LamarckismOften examined

Lamarck's views on the mechanism of evolution were never widely accepted. The role of the environment in producing phenotypic changes in the individual was correct, but these acquired traits are non-genetic, have no influence on the genotype and cannot be inherited.

There is no experimental proof of Lamarckism.
The use and disuse of organs cannot alter the genotype of the organism.
Somatic changes are not heritable.
Organs are not modified by the wish or requirements of an organism.

Why the inheritance of acquired characters fails
Point	Lamarck's claim	What the book says is true
Where the change occurs	In the body of the individual during its lifetime (somatic change), then passed on	Somatic changes are not heritable
Effect on genes	Assumed to pass to the offspring	No influence on the genotype, so they cannot be inherited
Role of the environment	Organs change by need or wish	The environment does produce phenotypic changes in the individual, but organs are not modified by the wish or requirements of the organism
Evidence	Offered as a theory	There is no experimental proof

Worked example: testing a Lamarckian claim

A student claims: "A body-builder's children will be born with big muscles, because the father built them by use." Use the drawbacks of Lamarckism to answer.

Identify the trait: big muscles built by exercise are an acquired character. It appeared in the body (somatic change) during the father's lifetime.
Apply drawback 3 and the book's statement: somatic changes are not heritable. Acquired traits are non-genetic and have no influence on the genotype.
Apply drawback 2: use of an organ cannot alter the genotype, and the genotype is what is passed to the children.
Conclusion: the children are not born with big muscles because of their father's exercise. The environment (exercise) changed the father's phenotype only, so the claim is not supported.

14.3DarwinismOften examined

Charles Darwin was born in England in a wealthy family. His father was a prominent physician. He joined Cambridge University to study theology, even so he attended many lectures in biology and geology. He was only 22 in 1831 when he accepted the position of naturalist abroad on HMS Beagle, a British naval ship about to sail around the world. His major mission was to expand the navy's knowledge of natural resources, for example water and food in foreign lands.

Darwin's observations during his voyage

The Beagle left Plymouth, England and cruised slowly along the east and west coasts of South America. Darwin collected and catalogued thousands of plant and animal specimens and kept notes of his observations. The Beagle spent almost two months at the Galapagos Islands. The islands are 965 kilometres west of Ecuador. Here Darwin made the observations that were most important in the development of his ideas about evolution.

Figure 14.8: Voyage of HMS Beagle (1831 to 1836), shown as a sequence of the stages named in the book. The Galapagos Islands (accent) gave Darwin the observations most important for his ideas. Schematic, not a map.

14.3.1 Observations about South American mainland

Darwin noticed that the flora and fauna of the different regions of the continent had a definite South American stamp, very distinct from the life forms of Europe. Further, the South American fossils that he found, though clearly different from modern species, were distinctly South American in their resemblance to the living plants and animals of the continent.

Observations about Galapagos Islands

Darwin compared the animals and plants of the Galapagos with those of the South American mainland. He was particularly impressed by their similarities and wondered why the organisms of the Galapagos should resemble those from South America more than those from other islands in different parts of the world. Although there were similarities between Galapagos and South American species, there were also distinct differences. The common birds were a group of finches. Closely related species had beaks of very different sizes and shapes, adapted for feeding on completely different kinds of food.

Darwin collected 14 types of finches on the Galapagos, which were quite similar but seemed to be different species. Some were unique to individual islands, while other species were distributed on two or more islands that were close together.

Figure 14.9: Galapagos Islands, 965 km west of Ecuador. Schematic sketch of the book's map: islands on the left, the coast of Colombia, Ecuador and Peru on the right.

Figure 14.10: Darwin finches. All 14 types arose from an ancestral finch (accent). Each food type (plants, insects, seeds, cactus) is linked to finches whose beaks are suited to it. Beak shapes are schematic. The woodpecker finch and the mangrove finch also feed on insects.

Darwin pondered these observations and tried to develop a satisfactory explanation for the distribution of species among the islands. He perceived the origin of new species and adaptations as closely related processes. A new species would arise from an ancestral form by the gradual accumulation of adaptations to different environments, separated from the original habitat by geographical barriers. Over many generations, the two populations could become dissimilar enough to be designated as separate species.

How Darwin explained new species on separated islands: populations that reach different habitats, kept apart by a geographical barrier, accumulate different adaptations (in the accent) until they are separate species.

14.3.2Development of the theory of evolutionOften examined

Darwin began formulating his theory of natural selection in the late 1830s but went on working quietly on it for many years. He wanted to amass a wealth of evidence before publicly presenting his idea. In 1842, Darwin wrote for himself a brief 35-page sketch of his theory. Two years later he enlarged this into an essay of 230 pages, which he showed to his friends but did not publish. For the next fifteen years, Darwin continued to collect facts to support his ideas.

The bases for the development of Darwin's theory were not only his observations about the unique distribution of organisms in different regions of the world. He was also inspired by the work of many other scientists of that time, so their ideas also contributed to the early development of Darwinism.

Contribution of Charles Lyell and James Hutton

In the early 1830s, Charles Lyell published a book, Principles of Geology. Darwin took this book on the voyage. The book presented arguments to support a theory of geological change proposed by James Hutton, called the theory of uniformitarianism. Lyell pointed out that the mountains, valleys, deserts, rivers, lakes and coastlines could have come through the action of existing forces and natural conditions. A river slowly carves a valley. Mountains are worn down to hills and finally plains. The slow pace of these geological processes, which still occur today, indicated that the Earth had to be much older than generally believed, a possibility that fired Darwin's imagination. If the Earth of today is so old and so changed, what was it like thousands of years ago? Did it have the kinds of life we now have? What other life may have existed?

Contribution of Thomas R. Malthus

Darwin returned to England in 1836. Soon afterwards, he read a work by the English political economist Thomas R. Malthus (1766 to 1834), An Essay on the Principle of Population. Malthus noted that human populations have the capacity to increase exponentially (1, 2, 4, 8, 16) while the food supply has the capacity to increase only arithmetically (1, 2, 3, 4, 5). Such a relation could result only in a struggle for food and hence for existence itself.

Contribution of Alfred Russell Wallace

In 1858 Darwin received a letter from a fellow naturalist, Alfred R. Wallace (1823 to 1913), who was travelling at that time in the Malay region. Wallace enclosed an essay that he had written and asked Darwin to read it and then forward it to Lyell. In the essay, Darwin found the theory of the origin of species by means of natural selection. Darwin almost yielded to Wallace the honour of being the first man to announce the theory. However, his friends (Charles Lyell and Joseph Dalton Hooker) arranged to present the two papers under joint authorship using a single title, On the Tendency of Species to Form Varieties; and on the Perpetuation of Selection. The papers were presented to the Linnaean Society in London on 1 July 1858.

The scientists whose ideas contributed to Darwinism
Scientist	Contribution that influenced Darwin
Charles Lyell and James Hutton	Uniformitarianism: slow geological processes still going on today show that the Earth is much older than believed
Thomas R. Malthus	Human populations increase exponentially (1, 2, 4, 8, 16) but food only arithmetically (1, 2, 3, 4, 5), so there is a struggle for existence
Alfred Russell Wallace	His 1858 essay put forward the origin of species by natural selection; the two papers were presented together

Dates in the development of Darwin's theory, as given in the book. The 1858 and 1859 events are in the accent. The date 1859 for the Origin of Species is the book's "November of 1859"; the Descent of Man date follows from "twelve years later".

Why the theory was attributed to Darwin

Alfred Russell Wallace was the man who motivated Darwin to publish his book The Origin of Species by Means of Natural Selection. It appeared in November 1859. Twelve years later, Darwin's Descent of Man was published. This was about the evolution of man. Darwin was much more willing than Wallace to explore the implications of natural selection, particularly in relation to humans. In addition, Wallace was a champion of rather radical social causes and later openly embraced spiritualism. All these elements resulted in the down-play of his role in the discovery of natural selection.

Worked example: Malthus's two growth rates

Malthus said that population grows as 1, 2, 4, 8, 16 while food grows as 1, 2, 3, 4, 5. (a) Continue both series to the 10th period. (b) Compare them. (c) What follows for the individuals of the population?

(a) Population doubles every period (exponential growth): the value in period $n$ is $2^{n - 1}$ . Food increases by 1 every period (arithmetic growth): the value in period $n$ is $n$ .

Exponential population growth against arithmetic growth of food
Period	1	2	3	4	5	6	7	8	9	10
Population ( $2^{n - 1}$ )	1	2	4	8	16	32	64	128	256	512
Food ( $n$ )	1	2	3	4	5	6	7	8	9	10
Population per unit of food	1	1	1.3	2	3.2	5.3	9.1	16	28	51

Exponential population growth against arithmetic growth of food
Period	Population ( $2^{n - 1}$ )	Food ( $n$ )	Population per unit of food
1	1	1	1
2	2	2	1
3	4	3	1.3
4	8	4	2
5	16	5	3.2
6	32	6	5.3
7	64	7	9.1
8	128	8	16
9	256	9	28
10	512	10	51

Population and food supply

Time period

Amount

32

6

population

food supply

Population doubling each period (1, 2, 4, 8, 16, 32) against food supply increasing by one unit (1, 2, 3, 4, 5, 6)

(b) The two start equal. From period 3 onwards the population grows faster than food, and the gap widens. By period 10 there are 512 units of population for 10 units of food, about 51 individuals for every unit of food.

(c) Food is limited and cannot keep up, so the individuals must compete for it. Malthus concluded that this must result in a struggle for food and hence for existence itself. This is the idea Darwin took from him.

14.3.3Darwin's theory of natural selectionOften examined

In his book The Origin of Species Darwin developed two main points: (i) descent with modification, and (ii) natural selection and adaptation.

(i) Descent with modification

Darwin believed in, and perceived, unity in life, with all organisms related through descent from some common ancestors, and that adaptation to various environments results in diversity. In the Darwinian view, the history of life is like a tree, with multiple branching and re-branching from a common trunk all the way to the tips of the living twigs, symbolic of the current diversity of organisms. At each fork of the evolutionary tree is an ancestor to all lines of evolution branching from that fork.

Darwin's view of life as a tree: a common trunk, repeated branching, and living organisms at the tips of the twigs. Each fork (one is in the accent) is an ancestor of all the lines that branch from it.

(ii) Natural selection and adaptations

Natural selection: The differential reproductive capacity among the individuals of a population: some individuals of a population are capable of reproducing while others are not. It is also referred to as the survival of the fittest.

Darwin's mechanism of evolution by natural selection consists of four observations about the natural world.

Over production: each species has the capacity to produce more offspring than will survive to maturity. Through reproduction, natural populations may increase exponentially in number over time.
Variations: the individuals in a population exhibit variation in their traits. Some of these traits improve the chances of an individual's survival and reproductive success, whereas other traits do not.
Struggle for existence: over production leads to competition among the individuals of a population for the limited resources: food, water, light and growing space. Because there are more individuals than the environment can support, not all will survive to reproductive age. Other limits on population growth include predators and disease-causing organisms. The struggle may be interspecific, intraspecific and environmental.
Survival of the fittest: those individuals that possess the most favourable combination of characteristics are most likely to survive and reproduce, passing their heritable traits on to the next generation. For example, if there is a sudden flood only those organisms that can swim or respire in water have a better chance to survive and the others will die; if there is an earthquake, the flying animals have a better chance of survival. This is called natural selection. The fittest individuals are those that reproduce most successfully in the environment.

The processes of natural selection thus cause an increase of favourable alleles and a decrease of unfavourable alleles within the population. Over succeeding generations, individual members become better adapted to local conditions, thus leading to the evolution of new species.

The four observations of Darwin lead, in order, to natural selection. The last observation (in the accent) is what is also called survival of the fittest.

The three kinds of struggle for existence
Kind of struggle	Between whom	Example in the book's terms
Interspecific	Individuals of different species	Competition with other organisms, predators and disease-causing organisms
Intraspecific	Individuals of the same species (the same population)	Competition among individuals for the limited food, water, light and growing space
Environmental	Individuals and the physical conditions	Surviving a sudden flood or an earthquake

Lamarck and Darwin compared (giraffe neck as the example)
Point	Lamarck	Darwin
Starting point	An organ is used or disused during life	Variations exist among the individuals of a population
Role of the environment	Changes the individual's organs by use and disuse	Selects the individuals best suited to it (those that survive and reproduce most)
What is passed on	Acquired characters	Heritable traits of the survivors
Giraffe neck	Each giraffe stretches its neck and passes on the longer neck	Giraffes with longer necks (a variation) reach more leaves, survive and reproduce more, so the long-necked trait increases

Worked example: natural selection in a flood

In a river valley a population of an animal species is flooded again and again. Only 3 of every 10 animals can swim well, and this ability is heritable. Explain, using Darwin's four observations, how the population changes over one generation.

Over production: each pair produces more young than will reach maturity, so there are more animals than the valley can support.
Variations: the animals differ. Three in ten swim well; seven do not. This trait improves the chance of survival in a flood.
Struggle for existence: the flood (an environmental limit) and competition for the remaining food kill many animals. The non-swimmers are the ones most likely to die.
Survival of the fittest: the swimmers survive and reproduce, passing the heritable trait to the next generation.
Result: the favourable allele for swimming is now more common, while the alleles of non-swimmers decrease. In the figure, 3 of 10 become 9 of 10 (the numbers are for illustration). Over succeeding generations the population becomes better adapted to the floods.

The three stages of the worked example: before the flood, after the flood and the next generation. The numbers are invented to show the pattern.

14.4Neo-Darwinism

When Lamarck and Darwin put forward their ideas, practically nothing was known about heredity. The emergence of population genetics has provided a clear understanding of inheritance and variation among the individuals of a population, and has given firm support to Darwinian theory. This reappraisal of the theory of natural selection in terms of modern population genetics is sometimes called Neo-Darwinism.

Neo-Darwinism: The reappraisal of Darwin's theory of natural selection in terms of modern population genetics.

The three theories of evolution compared
Theory	Mechanism of evolution	What was known about heredity
Lamarckism	Use and disuse of organs and inheritance of acquired characteristics	Practically nothing
Darwinism	Natural selection acting on variations in a population	Practically nothing
Neo-Darwinism	Natural selection explained in terms of population genetics: variation and inheritance among the individuals of a population, with favourable alleles increasing	Population genetics gives a clear understanding of inheritance and variation

14.5Evidences in favour of evolution theory

The concept of evolution is supported by a wide range of evidences, as it leaves unambiguous signs in nature. The geographical distribution of species and the fossils of ancient organisms anticipated Darwin to formulate his theory of evolution. The evidences which validate the theory of evolution come from biogeography, molecular biology, comparative anatomy, comparative embryology, palaeontology, animal and plant breeding, taxonomy and so on. Some important fields are discussed below.

Fields that give evidence for evolution, as listed by the book. The three discussed in this unit (biogeography, molecular biology and comparative anatomy) are in the accent.

14.5.1Evidence from biogeographyOften examined

The study of the geographical distribution of plants and animals on Earth is called biogeography. Biogeography gives evidence of prehistoric climates, habitats and animal distribution pattern. Biogeographic studies show that life forms in different parts of the world have a distinctive evolutionary history. Species originate in a particular area and then disperse outward from that area. The absence of some species from an area indicates the prior separation of that area from the region of origin of those species. It was the geographical distribution of species that created the impression of evolution in the mind of Darwin.

Biogeography: The study of the geographical distribution of plants and animals on Earth.

Figure 14.12: Continental drift in the last 225 million years. One land mass breaks up and the pieces move apart, so that species on separated land masses evolve separately. Schematic outlines, not accurate maps.

Specific pattern of distribution: Darwin noticed that South America lacked rabbits, even though the environment was quite suitable for them. He concluded that there are no rabbits in South America because rabbits originated somewhere else and had no means to reach South America. Similarly, rabbits are not a naturally occurring species of Australia, but when introduced by humans they increased rapidly, which suggests that the Australian environment is suitable for rabbits.
Factors inhibiting distribution of organisms: biogeographical studies show that species have a restricted distribution from the centre of origin because of some kind of barrier (physical, environmental or ecological).

Barriers that restrict the distribution of species
Kind of barrier	Meaning	Examples given in the book
Physical	A feature of the land or water that organisms cannot cross	Ocean, desert, mountain
Environmental	Conditions that the organism cannot tolerate	Unfavourable climate
Ecological	Other organisms in the area	Presence of organisms that compete with it for food and shelter

Worked example: naming the barrier

Say whether the barrier in each case is physical, environmental or ecological, and why: (a) a land species cannot spread to a continent across a wide ocean; (b) a species cannot spread into a region because its climate is unfavourable; (c) a species cannot settle in a region because another species there competes with it for food and shelter; (d) a species cannot cross a high mountain range.

Classifying barriers
Case	Barrier	Reason
(a) Wide ocean	Physical	An ocean is a feature of the land and water that the species cannot cross
(b) Unfavourable climate	Environmental	The conditions of the region, not an object in the way, stop the species
(c) Competing species	Ecological	Another organism in the region competes for food and shelter
(d) Mountain range	Physical	A mountain is a physical feature in the way (like an ocean or desert)

Ask whether the obstacle is part of the land or water (ocean, desert, mountain). If so it is physical.
Ask whether it is the climate or conditions of the region. If so it is environmental.
Ask whether it is another organism living there. If so it is ecological.

14.5.2Molecular biology evidenceOften examined

The presence of similar molecules in a complete range of living organisms suggests the existence of biochemical homology, in the same way as the anatomical homology shown by organs and tissues. Even some molecules are shared by taxonomically very remote organisms. Such common biomolecules and their roles in the cells reflect the evolutionary relationship between different species. It explains the origin of living organisms from some common ancestor.

Biochemical homology: The presence of similar molecules in a complete range of living organisms, which points to a common origin, just as anatomical homology does for organs and tissues.

Common molecules that point to a common ancestor
Molecule	Where it is found	What it shows
DNA with the same kind of nucleotides	Hereditary material of all living organisms, with a universal genetic code	All organisms use the same hereditary material and code
ATP nucleotide	Energy currency molecule from prokaryotes to the most advanced plants and animals	All use the same energy currency
20 types of amino acids	Living organisms synthesize their proteins using these 20 amino acids	All build proteins from the same set
Cytochrome c	A respiratory protein found in the respiratory chain of all aerobic organisms	All aerobic organisms share this respiratory protein

The reasoning of molecular evidence: the same molecules in a complete range of organisms show biochemical homology, which points to a common ancestor (in the accent).

14.5.3Comparative anatomyOften examined

Comparative anatomy is the study of the similarities and differences in the structures of different species. Similar body parts may be homologies or analogies. Both provide evidence for evolution.

Homologous structures

Homologous structures: Structures which look externally different, as they may have different functions, but are internally alike due to common origin. They are similar in related organisms because they were inherited from a common ancestor.
Divergent evolution: The pattern of evolution in which different species have evolved from common ancestors at different habitats.

The arm of man, the forelimb of the horse, the flipper of the whale and the wing of the bat have different external appearances because they have different roles to perform. Being mammalian forelimbs, they have the same basic pattern of bones. Sepals, petals, stamens and carpels are homologous structures, as they are all modified forms of leaf.

Vestigial organs like the appendix and tail vertebrae in humans, and the remains of leg bones in some snakes, are also homologous structures.

Figure 14.13: Bone pattern in forelimb of mammals (the book also shows a bird). In every limb the same bones are present in the same order: humerus (accent), radius and ulna, carpals, metacarpals and phalanges. Only their size and shape differ with the function. Simplified drawings.

Analogous structures

Analogous structures: Structures that are similar in unrelated organisms. They look externally alike because they perform similar functions, but are different in their internal organization because of their different origin. They are similar because they evolved to do the same job, not because they were inherited from a common ancestor.
Convergent evolution: The pattern of evolution in which different species have evolved from different ancestors at a common habitat.

For example, the wings of bats, birds and insects look similar on the outside. They also have the same function. However, wings evolved independently in these groups of animals. Analogous structures are of evolutionary interest because they demonstrate that populations with separate ancestries may adapt in similar ways to similar environmental demands. The leaves of tracheophytes, the leaves of mosses and the photosynthetic blades of some algae are analogous structures.

Figure 14.14: Analogous wings. The bat wing, butterfly wing and bird wing have the same function but different internal structure and different origin. Simplified drawings.

Divergent and convergent evolution. Left: one common ancestor (accent) gives homologous structures in different habitats. Right: different ancestors in a common habitat evolve similar structures (accent), which are analogous.

Homologous and analogous structures compared
Point	Homologous structures	Analogous structures
Origin	Common ancestor	Different ancestors (unrelated organisms)
Appearance and function	Look externally different; may have different functions	Look externally alike; perform similar functions
Internal structure	Alike (same basic pattern)	Different (different internal organization)
Pattern of evolution	Divergent evolution (common ancestors, different habitats)	Convergent evolution (different ancestors, common habitat)
Examples	Arm of man, forelimb of horse, flipper of whale, wing of bat; sepals, petals, stamens and carpels; vestigial organs	Wings of bats, birds and insects; leaves of tracheophytes, mosses and the blades of some algae

Worked example: homologous or analogous?

Decide whether each pair is homologous or analogous, and name the pattern of evolution: (a) forelimb of a horse and flipper of a whale; (b) wing of a bird and wing of an insect; (c) petal and stamen of a flower; (d) leaves of a moss and leaves of a tracheophyte.

Classifying the structures
Pair	Type	Reason	Pattern
(a) Horse forelimb and whale flipper	Homologous	Both mammalian forelimbs with the same basic pattern of bones, inherited from a common ancestor; they look different because their functions differ	Divergent
(b) Bird wing and insect wing	Analogous	Same function (flight) and similar outside, but they evolved independently in unrelated groups	Convergent
(c) Petal and stamen	Homologous	Both are modified forms of leaf	Divergent
(d) Moss leaf and tracheophyte leaf	Analogous	Similar in function but of different origin in unrelated plants	Convergent

Ask whether the two structures were inherited from a common ancestor (same internal pattern, as the same bones or a modified leaf). If yes, they are homologous.
Ask whether they merely do the same job in unrelated organisms. If yes, they are analogous.
Name the pattern: homologous structures come from divergent evolution, analogous structures from convergent evolution.

Key terms

Evolution: Change over a period of time; the transformation of life since its origin.
Organic soup: The complex organic molecules present in the ocean on which the first prokaryotic heterotrophs depended for energy.
Heterotroph: An organism that depends on complex organic molecules (such as those in the ocean soup) for its energy. The first organisms were prokaryotic heterotrophs.
Autotroph: An organism that makes its own organic matter; the earliest photosynthetic autotrophs used hydrogen sulfide as a source of hydrogen to reduce carbon dioxide to sugars.
Oxygen-evolving photosynthesis: Photosynthesis, begun by organisms similar to cyanobacteria, that uses water as the source of hydrogen and releases oxygen.
Ozone layer: The layer formed from the ionization of increasing oxygen; it reduced the ultraviolet radiation reaching the Earth and made life on land possible.
Endosymbiosis theory: The theory of Lynn Margulis that the eukaryotic cell arose when a large prokaryote ingested smaller prokaryotes and kept them alive inside itself as organelles.
Endosymbiont: A small cell kept alive inside a larger host cell, such as the aerobic bacterium that became the mitochondrion.
Mitochondrion: The site of aerobic respiration in eukaryotic cells; thought to be derived from an ingested aerobic bacterium.
Chloroplast: An organelle thought to be derived from an ingested photosynthetic bacterium.
Lamarckism: The theory of JB Lamarck (1809) that evolution occurs by the use and disuse of organs and the inheritance of acquired characteristics.
Use and disuse of organs: Organs used more become strong and efficient; organs not properly used deteriorate and finally disappear in successive generations.
Acquired characters: Characteristics gained by an individual during its lifetime, often through use or disuse of organs. Lamarck believed they were inherited; they are non-genetic and cannot be.
Somatic changes: Changes in the body of an individual during its lifetime; they are not heritable.
Darwinism: The theory of evolution by natural selection put forward by Charles Darwin in The Origin of Species.
Galapagos finches: The 14 types of finches Darwin collected on the Galapagos Islands, with beaks of very different sizes and shapes adapted for different foods.
Geographical barrier: A barrier that separates a population from its original habitat, so that adaptations accumulate and a new species may arise.
Uniformitarianism: Hutton's theory of geological change, supported by Lyell: slow processes still acting today shaped the Earth, which must therefore be very old.
Descent with modification: Darwin's idea that all organisms are related through descent from common ancestors and that adaptation to various environments results in diversity.
Natural selection: The differential reproductive capacity among the individuals of a population; some reproduce while others do not. Also called survival of the fittest.
Over production: Each species can produce more offspring than will survive to maturity.
Variations: Differences in the traits of the individuals of a population; some improve survival and reproductive success.
Struggle for existence: Competition among individuals for limited resources (food, water, light, growing space); it may be interspecific, intraspecific and environmental.
Survival of the fittest: The individuals with the most favourable combination of characteristics survive, reproduce most successfully and pass their heritable traits on.
Adaptation: A trait that suits an organism to its environment; it accumulates over generations through natural selection.
Neo-Darwinism: The reappraisal of the theory of natural selection in terms of modern population genetics.
Biogeography: The study of the geographical distribution of plants and animals on Earth.
Continental drift: The movement of the land masses apart over the last 225 million years, which separated the species on them.
Biochemical homology: The presence of similar molecules in a complete range of living organisms, pointing to a common origin.
Cytochrome c: A respiratory protein found in the respiratory chain of all aerobic organisms.
Universal genetic code: The genetic code shared by all living organisms, whose hereditary material is DNA with the same kind of nucleotides.
Comparative anatomy: The study of the similarities and differences in the structures of different species.
Homologous structures: Structures that look externally different, as they may have different functions, but are internally alike because of common origin.
Analogous structures: Structures that are similar in unrelated organisms because they do the same job, but differ in internal organization because of different origin.
Divergent evolution: The pattern of evolution in which different species evolve from common ancestors at different habitats.
Convergent evolution: The pattern of evolution in which different species evolve from different ancestors at a common habitat.
Vestigial organs: Reduced organs, such as the appendix and tail vertebrae in humans and leg bones in some snakes, that are also homologous structures.

Multiple choice questionsp. 387–388

Lamarck theory of organic evolution is usually known as

ANatural selection
BInheritance of acquired charactersAnswer
CDescent with change
DSurvival of the fittest

Why

Lamarck's theory is known as the inheritance of acquired characters: characters gained during life through the use or disuse of organs are passed on to the offspring (p. 379). Natural selection, survival of the fittest and descent with change are Darwin's ideas, so (a), (c) and (d) are wrong.

According to Neo-Darwinism, natural selection operates through

AFighting between organisms
BVariations
CKilling weaker organism
DDifferential reproductionAnswer

Why

Neo-Darwinism places natural selection in population genetics: selection acts through differential reproduction, so individuals with favourable alleles leave more offspring and those alleles become more frequent (p. 384). Variation is the raw material, not the process. Selection does not work by fighting or by killing weaker organisms, options (a) and (c).

The bones of forelimbs of whale, bat, horse and man are similar in structure, because

Aone organism has given rise to another
Bthey share a common ancestorAnswer
Cthey perform the same function
Dthey have biochemical similarities

Why

The forelimbs have the same basic bone pattern (Fig 14.13) because they are homologous structures inherited from one mammalian ancestor. They do different jobs (arm, hoof, flipper, wing), so (c) is wrong: shared function gives analogy, not homology. Option (a) is not the book's explanation, and biochemical similarity (d) is molecular evidence, not the reason for the bone pattern.

Fitness according to Darwin refers to

Anumber of species in a community
Buseful variation in population
Cstrength of an individual
Dreproductive fitness of an organismAnswer

Why

For Darwin, fitness is reproductive fitness: the fittest individuals are those that reproduce most successfully and pass on their heritable traits (p. 384). A strong individual that leaves no offspring is not fit in Darwin's sense, so (c) is wrong. Variation (b) is a cause of differences in fitness, not its definition.

Analogous organs arise due to:

Adivergent evolution
Bartificial selection
Cgenetic drift
Dconvergent evolutionAnswer

Why

Analogous structures look alike because unrelated organisms face similar demands, so they arise through convergent evolution (p. 386). Divergent evolution from a common ancestor produces homologous structures, option (a). Artificial selection (b) and genetic drift (c) are not the book's explanation of analogy.

First life form on earth was a

Acyanobacterium
BheterotrophAnswer
Cautotroph
Dphotoautotroph

Why

The book says the first organisms were prokaryotic heterotrophs that depended on the complex organic molecules of the ocean 'soup' (p. 377). Autotrophs came later: the earliest photosynthetic autotrophs used hydrogen sulphide, and cyanobacteria-like organisms later evolved oxygen-producing photosynthesis. So (a), (c) and (d) are wrong.

Which of the following statements is true?

AWings to birds and insects are homologous organs.
BHuman hands and wing of bat are analogous organs.
CWing of butterfly and wing of bat are homologous organs.
DFlipper of penguin and dolphin are analogous organs.Answer

Why

Penguin flipper and dolphin flipper look alike and do the same job in water, but come from unrelated ancestors, so they are analogous (option d).

Bird and insect wings, and bat and butterfly wings, evolved independently for flight, so they are analogous, not homologous (a and c are wrong). The human hand and the bat wing share the same basic bone pattern from a common ancestor, so they are homologous, not analogous (b is wrong).

Note. The book prints no answer key. Worked from its definitions of homologous and analogous structures (pp. 385 and 386). The penguin and dolphin pair is not in the book; it applies the book's definition of analogy.

Which one of the following phenomena supports Darwin's concept of natural selection in organic evolution?

ADevelopment of transgenic animals
BProduction of "Dolly", the sheep by cloning
CPrevalence of pesticide resistant insectsAnswer
DDevelopment of organs from 'stem cells' for organ transplantation

Why

Insects vary in their tolerance of a pesticide. The pesticide kills the susceptible ones, the resistant survivors reproduce and pass the trait on, so resistance spreads through the population. This is natural selection in action: survival and reproduction of the fittest (p. 384).

By the statement 'survival of the fittest', Darwin meant that

Athe strongest of all species survives
Bthe most intelligent of the species survives
Cthe cleverest of the species survives
Dthe species most adaptable to changes survives.Answer

Why

Darwin's 'fittest' are the individuals with the most favourable combination of characteristics for their environment, which reproduce most successfully (p. 384). Strength (a), intelligence (b) and cleverness (c) are not what Darwin meant.

Note. Option (d) says 'species', but the book applies survival of the fittest to individuals. (d) is still the closest of the four options.

According to Lamarckism, long necked giraffes evolved because

Anature selected only long necked ones
Bhumans preferred only long necked ones
Cshort necks suddenly changed into long necks
Dof stretching of necks over many generations by short necked onesAnswer

Why

Lamarck explained the long neck by use. Each giraffe stretched its neck to reach leaves, the lengthened neck was an acquired character passed to its offspring, and the lengthening accumulated over generations (p. 380). Option (a) is Darwin's explanation, not Lamarck's. Option (c) is a sudden change that Lamarck did not propose, and (b) is human selection, which is not in the book's account.

Short answer questionsp. 388

13 marks

Natural selection is random or systematic, explain your answer.

Solution

Natural selection is systematic (non-random). The variation it acts on arises by chance, but selection itself is not random:

Individuals with favourable, heritable traits survive and reproduce more successfully than others, so those traits become more common.
Individuals with unfavourable traits leave fewer offspring, so those traits decline.
The environment (food, competition, predators, climate) consistently favours the same traits, so the change is directed towards better adaptation, not left to chance.

22026 Annual I, Set B, Q.2 (xiii)3 marks

How is convergent evolution different from divergent evolution?

Solution

Convergent and divergent evolution are both ways in which species change, but they differ in ancestry, habitat and the structures produced:

Convergent and divergent evolution (pp. 385 and 386).
Feature	Convergent evolution	Divergent evolution
Ancestry	Different, unrelated ancestors	Species evolved from a common ancestor
Habitat	Species share a common habitat	Species live in different habitats
Structures produced	Analogous structures (wings of bat, bird and insect)	Homologous structures (forelimbs of man, horse, whale and bat)

33 marks

Explain endosymbiont hypothesis.

Solution

The endosymbiont hypothesis (Lynn Margulis) proposes that eukaryotic cells arose when prokaryotes were engulfed by a larger host cell and kept as permanent partners:

A large anaerobic amoeboid prokaryote ingested a small aerobic bacterium and kept it as an endosymbiont instead of digesting it. The bacterium became the mitochondrion, the site of aerobic respiration.
Later, an early eukaryote consumed photosynthetic bacteria, which evolved into chloroplasts.
Mitochondria and chloroplasts are similar in size to bacteria, have their own DNA and ribosomes similar to those of bacteria, and make a limited portion of their own enzymes and proteins. This suggests they were once free-living bacteria.

42025 Annual I, Set D, Q.2 (iv)3 marks

What are homologous structures, explain with an example.

Solution

Homologous structures look different externally and may do different jobs, but they have the same internal basic plan because they are inherited from a common ancestor:

Example: the forelimbs of man, horse, cat, bat, bird and whale all have the same basic pattern of bones (Fig 14.13), even though they serve as arm, hoof, paw, wing and flipper.
Sepals, petals, stamens and carpels of a flower are homologous, because all are modified leaves.
Vestigial organs, such as the appendix and tail vertebrae in humans, and the remnants of leg bones in some snakes, are also homologous structures.

53 marks

What are vestigial organs? Give few examples from human body.

Solution

Vestigial organs are reduced remnants of organs that were functional in ancestors. They have lost most or all of their original function through evolution and are still present in the body:

The appendix, a small reduced structure of the human digestive system.
The tail vertebrae (coccyx), the small remnant of the tail that the human ancestors had.
The reduced ear muscles, which humans have lost the contractile function of, because we no longer need to move the ears to locate sounds.

Note. The book names only the appendix and tail vertebrae as human examples (p. 385); the third point follows from the book's remark that humans have lost the contractile feature of the ear muscles (p. 379).

63 marks

Enlist few characteristics found only in mammals.

Solution

Mammals have a set of features that no other group of animals has. Three examples are:

Hair or fur covering the body, which helps to keep body temperature steady.
Mammary glands that produce milk to feed the young.
Three bones in the middle ear (malleus, incus and stapes) that carry sound to the inner ear.

Note. The book does not list mammalian characteristics. This answer uses the standard features of mammals and stays at the level of the book.

Extensive answer questionsp. 388

How natural selection operates to create new species? Discuss in details.

Solution

Starting point: variation within a population

Natural selection does not create variation. Variation already exists among the individuals of a population, and selection sorts it. Darwin's mechanism rests on four observations about the natural world:

Darwin's four observations on which natural selection rests (p. 383).
Observation	What it means	Result
Over production	Each species produces more offspring than can survive to maturity	Competition for limited resources
Variation	Individuals differ in their traits; some traits improve survival and reproduction	Some individuals are better equipped
Struggle for existence	Competition for food, water, light and growing space, plus predators and disease-causing organisms	Not all individuals reach reproductive age
Survival of the fittest	Individuals with the most favourable combination of characters reproduce most successfully	Their heritable traits are passed on

Steps from variation to a new species

Over production: each generation produces more offspring than the environment can support.
Variation: the offspring differ, and some differences are heritable and improve survival and reproduction.
Struggle: competition for limited food, water, light and space, and pressure from predators and disease-causing organisms, means only some individuals reach reproductive age.
Differential reproduction: the individuals with the most favourable heritable traits reproduce most successfully, so their alleles are passed on more often.
Change in frequency: over many generations favourable alleles increase and unfavourable ones decrease, so the population becomes better adapted to its local conditions.
Isolation: a geographical barrier (ocean, desert or mountain) separates part of the population from its original habitat, so the two groups stop mixing and each keeps adapting to its own surroundings.
Divergence: over many generations the two populations become so different that they are designated as separate species.

The stages of natural selection leading to a new species. Each cycle is one generation; the isolation step is what splits a population.

Example: Darwin's finches on the Galapagos Islands

Darwin collected 14 types of finches on the Galapagos Islands. Some were unique to single islands, while others were spread over islands that were close together. Closely related species had beaks of very different sizes and shapes, each adapted to a different food (Fig 14.10). Darwin explained this by an ancestral form that reached the islands and spread to different habitats. Geographical barriers separated the groups, and each group accumulated adaptations to its own food supply until the groups became new species.

Descent with modification

Darwin pictured the history of life as a tree. All organisms descend from a common trunk, and at each fork an ancestor gives rise to all the lines above it. Each branch is shaped by selection for its own habitat.

Descent with modification. The shared trunk is the common ancestor; each branch adapts to its own habitat and can eventually become a new species.

Role of population genetics

Neo-Darwinism explains the same process in genetic terms. Selection acts through differences in reproduction, which change the frequency of alleles in the population, and this change is the basis of new species (p. 384).

22025 Annual I, Set B, Q.2 (vii) OR

Explain evidences from molecular biology in support of evolution.

Solution

All living organisms share the same kinds of biomolecules. Similar molecules across a complete range of living organisms show biochemical homology, which parallels the anatomical homology of organs and tissues. Molecules that are shared even by taxonomically very remote organisms point to a common origin (p. 385).

Evidence from shared molecules

Biomolecules shared by living organisms (p. 385).
Molecule or process	What is shared	Why it supports common ancestry
DNA	Same kind of nucleotides; the hereditary material of all organisms	A universal genetic code: all living things read DNA in the same way
ATP	The energy currency molecule	Used from prokaryotes to the most advanced plants and animals
Amino acids	Proteins are made from the same 20 types of amino acid	Protein synthesis uses the same building blocks everywhere
Cytochrome c	A respiratory protein of the respiratory chain	Found in all aerobic organisms, so it is an ancient shared molecule

Why shared molecules indicate a common ancestor

A universal genetic code is most easily explained if all life inherited the same code from one ancestor. Unrelated origins would be expected to produce different codes.
Remote organisms, such as bacteria, plants and animals, still share molecules such as ATP and cytochrome c. Such matches are unlikely by chance, so the molecules were inherited.
The shared molecules and their roles in the cell reflect the evolutionary relationship between species, which explains the origin of living organisms from a common ancestor.

Shared biomolecules point back to one common ancestor of all living organisms.

Suppose a new mammalian species has originated in planes of sub-continent, what kind of barriers can restrict its dispersal in other regions of the world?

Solution

Species originate in one area and disperse outward from it. The absence of a species from an area shows that it was separated from its region of origin (p. 384). A new species spreads only as far as it can cross. Anything that prevents it from crossing is a barrier. The book groups barriers into physical, environmental and ecological ones (p. 385).

Physical barriers

Oceans, deserts and mountains physically separate populations. A mammal that cannot cross open sea, a dry desert or a high mountain range stays in its region of origin.

Environmental barriers

An unfavourable climate, such as extreme cold, heat or dryness, stops a species from surviving in a region, even when it can physically reach that region.

Ecological barriers

Organisms that compete for the same food and shelter, and predators and disease-causing organisms, can prevent a species from establishing itself in a new region.

Barriers that restrict dispersal (p. 385).
Type of barrier	Examples	Effect on dispersal
Physical	Ocean, desert, mountain	Blocks movement across the barrier
Environmental	Unfavourable climate	The species cannot survive in the new region
Ecological	Competing organisms, predators, disease-causing organisms	The species cannot establish a population

Barriers stop a new species from spreading out of its centre of origin.

Evidence from the book: rabbits

Darwin found no rabbits in South America, although the environment seemed suitable. He concluded that rabbits had originated elsewhere and had no means of reaching South America. When humans introduced rabbits into Australia they increased rapidly, which shows that the environment was suitable and that the absence was caused by a barrier (p. 385).

FBISE 2025 and 2026 questions

Questions from the FBISE HSSC-I 2026 (1st Annual) and FBISE HSSC-I 2025 (1st Annual) papers that belong to this unit, answered the way an examiner expects.

2026 Annual I, Set B, MCQ 161 mark

Molecular evidence of evolution is obtained from:

ADNA analysisAnswer
BEmbryology
CAnatomy
DFossils

Why

Molecular evidence comes from the common biomolecules in living organisms: DNA with a universal genetic code, ATP, the 20 amino acids and cytochrome c. Embryology and anatomy are different fields (comparative anatomy studies structures, not molecules), and fossils are evidence from palaeontology.

2025 Annual I, Set B, MCQ 171 mark

Which of the following is an example of homologous organs?

AWing of an insect & a bird
BForelimb of human & wing of a batAnswer
CWing of an insect & a bat
DGills of fish & lungs of mammals

Why

Homologous organs have the same basic internal pattern because of a common ancestor. The forelimb of a human and the wing of a bat are both mammalian forelimbs with the same bone pattern. The other pairs are analogous (insect wing and bird wing, insect wing and bat wing: same function, different origin) or unrelated (gills and lungs).

2025 Annual I, Set B, Q.2 (vii) OR3 marks

Molecular biology provides evidence of evolution. Justify the statement with reference to any three examples.

Solution

Molecular biology shows biochemical homology: the same kinds of molecules are present in a complete range of living organisms, just as organs and tissues show anatomical homology. Even taxonomically very remote organisms share them, so they reflect an evolutionary relationship and a common ancestor. Three examples:

DNA: the hereditary material of all living organisms is DNA made of the same kind of nucleotides, and the genetic code is universal.
ATP: the energy currency molecule is the same from prokaryotes to the most advanced plants and animals.
Amino acids: all living organisms make their proteins from the same 20 types of amino acid.

Cytochrome c, a respiratory protein of the respiratory chain of all aerobic organisms, is a fourth example. Because such molecules are shared even by remote species, they point to the origin of living organisms from a common ancestor.

2026 Annual I, Set B, Q.2 (xii)1x3 = 3 marks

Briefly elaborate three drawbacks of Larmarckism.

Solution

Lamarck was right that the environment can change an individual, but his mechanism fails for three main reasons:

No experimental proof: there is no experimental evidence that acquired characters are inherited.
Use and disuse cannot alter the genotype: the changes are non-genetic, so the organism's genes stay the same and nothing is passed on.
Somatic changes are not heritable: changes of the body made during life (such as a blacksmith's biceps) are not carried to the offspring. Organs are also not modified by the wish or requirements of an organism.

2026 Annual I, Set B, Q.2 (xiii)1x3 = 3 marks

Differentiate between convergent and divergent evolution. (Three points)

Solution

Convergent and divergent evolution are both ways in which species change, but they differ in ancestry, habitat and the structures produced:

Convergent and divergent evolution.
Feature	Convergent evolution	Divergent evolution
Ancestry	Different, unrelated ancestors	Species evolved from a common ancestor
Habitat	Species share a common habitat	Species live in different habitats
Structures produced	Analogous structures (wings of bat, bird and insect)	Homologous structures (forelimbs of man, horse, whale and bat)

2025 Annual I, Set B, Q.2 (xiv)3 marks

What are the limitations of Lamarckism?

Solution

The limitations (drawbacks) of Lamarckism are:

No experimental proof: there is no experimental evidence that acquired characters are inherited.
Use and disuse cannot alter the genotype: the changes are non-genetic, so the organism's genes stay the same and nothing is passed on.
Somatic changes are not heritable: changes of the body made during life (such as a blacksmith's biceps) are not carried to the offspring. Organs are also not modified by the wish or requirements of an organism.

In short, the environment can change an individual, but such acquired traits are non-genetic and cannot be inherited.

2026 Annual I, Set D, MCQ 161 mark

Which molecule provides strong evidence for evolution due to similarity among species?

ADNAAnswer
BLipids
CCarbohydrates
DMinerals

Why

DNA is found in all organisms, is built of the same kind of nucleotides and uses a universal genetic code, so its similarity among species shows common ancestry. Lipids, carbohydrates and minerals are not named in the book as molecular evidence of evolution (the book names DNA, ATP, the 20 amino acids and cytochrome c).

2025 Annual I, Set D, Q.2 (i) OR3 marks

Write the major points of theory of natural selection. (Any three)

Solution

In The Origin of Species Darwin explained natural selection through four observations (any three are enough for 3 marks):

Over production: each species produces more offspring than will survive to maturity, so populations can increase exponentially.
Variations: the individuals of a population differ in their traits, and some traits improve the chances of survival and reproduction.
Struggle for existence: there are more individuals than the limited food, water, light and growing space can support, so they compete (interspecific, intraspecific and environmental struggle).
Survival of the fittest: individuals with the most favourable combination of characteristics survive and reproduce, passing their heritable traits on. This is natural selection.

Result: favourable alleles increase, unfavourable alleles decrease, and over generations the population becomes better adapted and new species can arise.

2025 Annual I, Set D, Q.2 (iv)1x3 = 3 marks

What are the homologous organs? Write an example. Which type of evolution is caused by them?

Solution

Homologous organs (structures) look externally different and may do different jobs, but are internally alike because they were inherited from a common ancestor.

Example: the forelimbs of man, horse, whale and bat have the same basic pattern of bones, although they serve as arm, leg, flipper and wing.
Type of evolution: they are caused by divergent evolution, in which different species have evolved from a common ancestor at different habitats.
Other homologous structures: sepals, petals, stamens and carpels (all modified leaves) and vestigial organs such as the human appendix.

2026 Annual I, Set D, Q.2 (xii)1.5+1.5 = 3 marks

Give one example each supporting use and disuse of organs according to Lamarck theory.

Solution

Lamarck's theory rests on use and disuse of organs. One example of each:

Use (development by use): a blacksmith develops a bigger biceps in the arm that works the hammer. Another example is the webbed feet of aquatic birds, which result from constant spreading of the toes and skin in swimming.
Disuse (degeneration by disuse): snakes creep into small holes and crevices and have lost their legs. Another example is the loss of the ability of humans to position the head to collect sound waves (the ear muscles).

Lamarck believed that such changes, gained during life, were inherited by the offspring.

2026 Annual I, Set D, Q.2 (xiii)1x3 = 3 marks

Compare homologous and analogous structures. (three points)

Solution

Homologous and analogous structures are both found by comparing the anatomy of different species, but they differ in origin, appearance and internal structure:

Homologous and analogous structures compared.
Point	Homologous structures	Analogous structures
Origin	Common ancestor	Different ancestors (unrelated organisms)
Appearance and function	Look externally different; may have different functions	Look externally alike; perform similar functions
Internal structure	Alike (same basic pattern)	Different (different internal organization)
Example	Arm of man, forelimb of horse, flipper of whale, wing of bat	Wings of bats, birds and insects

Homologous structures arise by divergent evolution; analogous structures arise by convergent evolution.

2025 Annual I, Set D, Q.2 (xiii) OR3 marks

How evolution of Giraffe neck can be explained on the basis of Lamarckism?

Solution

According to Lamarck the long neck of the modern giraffe was the result of generations of short-necked ancestors feeding on leaves at progressively higher levels of trees. It is explained in three steps:

Use of the neck. Each short-necked giraffe constantly stretched its neck during its lifetime to reach the leaves at the top of the trees. By use, the neck became a little longer (an acquired character).
Inheritance of the acquired character. The longer neck gained by stretching was passed to the offspring, so each offspring was born with a slightly longer neck than its parents.
Accumulation over generations. The same stretching and inheritance went on in every generation, so the neck grew longer step by step until the modern long-necked giraffe appeared.

Exam focus

Differentiate between homologous and analogous structures, or convergent and divergent evolution (three points each), and give examples. (short)
The most repeated topic: Set B 2026 Q.2 (xiii), Set D 2026 Q.2 (xiii), Set B 2025 MCQ 17, Set D 2025 Q.2 (iv); also book short questions 2 and 4.
Go to answer
Drawbacks (limitations) of Lamarckism. (short)
Asked as a 3-mark question in Set B 2026 Q.2 (xii) and Set B 2025 Q.2 (xiv); an SLO of the unit.
Go to answer
Lamarck's use and disuse and the giraffe neck, with one example each. (short)
Set D 2026 Q.2 (xii) (one example each of use and disuse) and Set D 2025 Q.2 (xiii) OR (giraffe neck); also book exercise MCQ 10.
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Molecular evidence of evolution: DNA, ATP, 20 amino acids and cytochrome c. (short)
Set B 2026 MCQ 16, Set D 2026 MCQ 16 and Set B 2025 Q.2 (vii) OR (three examples); also book long question 2.
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The four observations of Darwin's natural selection and how it forms new species. (long)
Set D 2025 Q.2 (i) OR asked the major points; book long question 1 and short question 1 cover the full answer.
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Endosymbiont hypothesis for the origin of eukaryotic cells. (short)
No paper question in this unit in 2025 or 2026, but it is book short question 3 and an SLO of the unit.
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Biogeography and barriers that restrict dispersal of a new species. (long)
Not asked in 2025 or 2026, but it is book long question 3 and an SLO of the unit.
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Period	Population ( $2^{n - 1}$ )	Food ( $n$ )	Population per unit of food
1	1	1	1
2	2	2	1
3	4	3	1.3
4	8	4	2
5	16	5	3.2
6	32	6	5.3
7	64	7	9.1
8	128	8	16
9	256	9	28
10	512	10	51

Period	Population ( $2^{n - 1}$ )	Food ( $n$ )	Population per unit of food
1	1	1	1
2	2	2	1
3	4	3	1.3
4	8	4	2
5	16	5	3.2
6	32	6	5.3
7	64	7	9.1
8	128	8	16
9	256	9	28
10	512	10	51

Period	Population ( $2^{n - 1}$ )	Food ( $n$ )	Population per unit of food
1	1	1	1
2	2	2	1
3	4	3	1.3
4	8	4	2
5	16	5	3.2
6	32	6	5.3
7	64	7	9.1
8	128	8	16
9	256	9	28
10	512	10	51