Evolution

What to remember

• Life arose by chemical evolution from non-living organic molecules in a reducing atmosphere (Oparin and Haldane); Miller's experiment made amino acids from CH₄, H₂, NH₃ and water vapour with an electric discharge.
• Evidence for evolution: fossils, homologous organs (divergent evolution), analogous organs (convergent evolution), molecular similarities, industrial melanism, and antibiotic and pesticide resistance.
• Natural selection (Darwin) works on small, heritable variations and gives adaptive radiation; de Vries proposed that large, single-step mutations (saltation) drive evolution.
• Hardy-Weinberg: allele frequencies stay constant (p² + 2pq + q² = 1) unless gene flow, genetic drift, mutation, recombination or natural selection acts.

Origin of life

The universe and early Earth. The universe began with the big bang, a single huge explosion. The universe is very old. Earth formed about 4.5 billion years ago. At first the earth had no atmosphere. Gases such as water vapour, methane (CH₄), carbon dioxide (CO₂) and ammonia (NH₃) came out of the molten mass. UV rays split water into hydrogen and oxygen. The light hydrogen escaped. The oxygen combined with NH₃ and CH₄ to form water, CO₂ and other compounds. As the earth cooled, the water vapour fell as rain and filled the low areas to form oceans. Life appeared about 500 million years after the earth formed, nearly 4 billion years ago.

Spontaneous generation and biogenesis. An old idea said that life arises from decaying matter, such as straw or mud. Louis Pasteur showed with careful experiments that life comes only from pre-existing life (biogenesis).

Chemical evolution. Oparin (Russia) and Haldane (England) proposed that the first life came from pre-existing non-living organic molecules such as RNA and proteins. Life was preceded by the formation of diverse organic molecules from inorganic constituents. The conditions were high temperature, volcanic storms and a reducing atmosphere with CH₄ and NH₃.

Miller's experiment (1953). S. L. Miller created an electric discharge in a closed flask containing CH₄, H₂, NH₃ and water vapour at about 800 °C. Amino acids were formed. Similar experiments later gave sugars, nitrogen bases, pigments and fats. Analysis of meteorite content showed similar compounds. So chemical evolution is supported.

First life. The first non-cellular forms of life were probably giant molecules (RNA, protein, polysaccharides) about 3 billion years ago. The first cellular life, probably single-celled, did not arise until about 2000 million years ago. Then unicellular forms gave rise to multicellular life.

Special creation. This theory has three ideas: all living organisms were created as they are now, the diversity was always the same and will remain the same, and the earth is about 4000 years old. The evidence from fossils, from Darwin's study of the Galapagos Islands and from the age of rocks does not support it.

Evidences for evolution

Homologous organs have the same basic structure but different functions. Examples are the forelimbs of whales, bats, cheetah and humans (same bones), and the thorn of Bougainvillea and the tendril of Cucurbita. They show divergent evolution and common ancestry.

Analogous organs have different structure but the same function. Examples are the wings of a butterfly and a bird, the flippers of penguins and dolphins, the eyes of an octopus and a mammal, and sweet potato (root modification) and potato (stem modification). They show convergent evolution.

Embryological evidence. Ernst Haeckel proposed that an embryo passes through the adult stages of its ancestors, based on features common to all vertebrate embryos, such as gill slits, which adults do not have. Karl Ernst von Baer rejected this idea. He pointed out that embryos never pass through the adult stages of other animals.

Industrial melanism. In England before the industrial revolution, white-winged moths (Biston betularia) on light-coloured tree trunks covered with lichens were more common than dark-winged (melanised) moths. After industrialisation, soot darkened the tree trunks. Birds could now easily see the white moths, and the dark moths survived. The dark form increased in number. Natural selection changed the population.

Resistance. Bacteria become resistant to antibiotics. Herbicides and pesticides cause resistant weeds and insects to be selected. These are examples of evolution in a short time, by human action.

Adaptive radiation

Adaptive radiation is the evolution of different species from one ancestor in a geographical area, as they adapt to different habitats.

• Darwin's finches in the Galapagos Islands. From one seed-eating ancestor, several types arose with differently shaped beaks. Some feed on seeds, some are insect eaters and some eat vegetarian food.
• Australian marsupials. Many types of marsupials radiated from one ancestral stock in isolated Australia.
• Convergent evolution of placentals. In Australia, placental mammals that are similar to the marsupials also evolved. This is convergent evolution.

Biological evolution and natural selection

Charles Darwin made a sea voyage on HMS Beagle and saw the variety of life, especially in the Galapagos Islands. He proposed branching descent and natural selection. Alfred Russel Wallace, who worked on the Malay Archipelago, came to similar conclusions at about the same time. Darwin was influenced by the essay of Thomas Malthus on populations: resources increase slowly and populations increase fast.

The steps in Darwin's idea are as follows. Populations have variations. Resources are limited, so there is a struggle for existence. Individuals that have a variation that suits the environment survive and leave more offspring. This is fitness. Fitness here means reproductive fitness, which is the ability to survive and reproduce. Over many generations, favourable variations spread. New species form.

Lamarck (in brief) proposed that organs used more become better developed and those not used are lost (use and disuse). The giraffe's long neck was his example. He said that these acquired characters are passed to the offspring. This idea is not accepted today.

Mechanism of evolution

Darwin said that variations are small and directional, and that evolution is gradual. Hugo de Vries worked on evening primrose (Oenothera lamarckiana) and proposed the theory of mutation. Mutations are large, random and directionless. They cause speciation in a single step. Single-step large mutation is called saltation.

Hardy-Weinberg principle

Allele frequencies in a population are stable. They are constant from one generation to the next. The gene pool is constant. This is genetic equilibrium or Hardy-Weinberg equilibrium. For a gene with two alleles, A and a, let the frequency of A be p and that of a be q. Then p + q = 1. The frequency of AA is p², of Aa is 2pq and of aa is q².

p² + 2pq + q² = 1

Example: if the frequency of a recessive disease (aa) is 1 in 10,000 (q² = 0.0001), then q = 0.01, p = 0.99, and carriers (2pq) are about 0.0198, nearly 2%.

Five factors that change allele frequency (disturb the equilibrium):

• 1. Gene migration (gene flow): movement of individuals or gametes into or out of a population.
• 2. Genetic drift: chance change in allele frequency, strongest in small populations. The founder effect is a type of genetic drift. A few individuals start a new population, and their alleles are not representative of the original population.
• 3. Mutation: new alleles are produced in the gene pool.
• 4. Genetic recombination: new combinations of alleles.
• 5. Natural selection: better adapted individuals leave more offspring.

Three types of natural selection:

A brief account of evolution

Animals. Invertebrates were formed and active about 500 million years ago. Jawless fish came about 350 million years ago. Lobe-finned fish with strong fins could move on land and return to water. A lobe-fin fish, the coelacanth, was thought to be extinct but was found alive in South Africa in 1938. These lobefins evolved into the first amphibians (such as Ichthyostega), which lived on land and in water. Amphibians evolved into reptiles. Dinosaurs were reptiles, and the largest was Tyrannosaurus rex. Some reptiles went back to water (the ichthyosaurs). The first mammals were small, shrew-like animals.

Plants. The first organisms to invade land were plants. The sequence is: algae → bryophytes → pteridophytes → gymnosperms → angiosperms.

Origin and evolution of man

The increase in brain size over time is a clear trend: Homo habilis (about 650 to 800 cc) → Homo erectus (about 900 cc) → Neanderthal (about 1400 cc).

NEET traps

• Homologous organs: same structure, different function, divergent evolution. Analogous organs: different structure, same function, convergent evolution.
• Wings of a bird and a bat are homologous as forelimbs, but wings of a bird and an insect are analogous.
• Sweet potato (root) and potato (stem) are analogous. Bougainvillea thorn and Cucurbita tendril are homologous.
• Haeckel proposed recapitulation, and von Baer rejected it.
• Darwin's variations are small and directional. De Vries's mutations are large and random.
• Genetic drift is a chance change in a small population. Natural selection is not random.
• Founder effect is a kind of genetic drift. Gene flow is movement of alleles between populations.
• Stabilising selection reduces both extremes. Disruptive selection favours both extremes.
• Industrial melanism: dark moths increased after soot, and birds picked off the white moths.
• Dryopithecus is more ape-like, and Ramapithecus is more man-like. Homo habilis ate no meat (about 650 to 800 cc), and Homo erectus probably ate meat (about 900 cc).

One-liners

• 1. The big bang started the universe.
• 2. Earth formed about 4.5 billion years ago and the early atmosphere was reducing.
• 3. Oparin and Haldane proposed chemical evolution of life.
• 4. Miller's flask had CH₄, H₂, NH₃ and water vapour, and it made amino acids.
• 5. Pasteur showed that life comes only from pre-existing life.
• 6. Homologous organs show divergent evolution.
• 7. Analogous organs show convergent evolution.
• 8. Von Baer rejected Haeckel's idea that embryos recapitulate adult ancestors.
• 9. Darwin's finches show adaptive radiation.
• 10. Australian marsupials and placentals show convergent evolution.
• 11. De Vries proposed mutation theory based on evening primrose.
• 12. The Hardy-Weinberg equation is p² + 2pq + q² = 1.
• 13. Stabilising, directional and disruptive are the three types of natural selection.
• 14. Neanderthal brain capacity was about 1400 cc.
• 15. Homo erectus fossils were found in Java.

Practice questions

1. The idea that the first forms of life arose from pre-existing non-living organic molecules was proposed by

   1. Louis Pasteur
   2. Oparin and Haldane
   3. Stanley Miller
   4. Charles Darwin
   Answer

   B. Oparin and Haldane

   Oparin and Haldane proposed chemical evolution. Miller later tested the idea in the laboratory.

2. The gases present in the flask of Miller's experiment were

   1. CH₄, H₂, NH₃ and water vapour
   2. CH₄, CO₂, O₂ and NH₃
   3. O₂, N₂, CO₂ and water vapour
   4. N₂, O₂, H₂ and CO₂
   Answer

   A. CH₄, H₂, NH₃ and water vapour

   The flask had a reducing mixture without free oxygen, and an electric discharge was passed through it.

3. Louis Pasteur's experiments disproved

   1. chemical evolution
   2. inheritance of acquired characters
   3. spontaneous generation of life
   4. natural selection
   Answer

   C. spontaneous generation of life

   Pasteur showed that life comes only from pre-existing life (biogenesis).

4. Which of the following pairs of structures is homologous?

   1. Flippers of a penguin and a dolphin
   2. Eyes of an octopus and a mammal
   3. Wings of a butterfly and a bird
   4. Forelimbs of a whale and a bat
   Answer

   D. Forelimbs of a whale and a bat

   Whale and bat forelimbs have the same bones but different functions. The other pairs are analogous.

5. Which of the following pairs of organs is analogous?

   1. Potato and sweet potato
   2. Thorn of Bougainvillea and tendril of Cucurbita
   3. Forelimbs of a whale and a bat
   4. Forelimbs of a cheetah and a human
   Answer

   A. Potato and sweet potato

   Potato is a stem modification and sweet potato is a root modification. They have the same function (food storage) but a different origin.

6. The idea that an embryo passes through the adult stages of its ancestors (recapitulation) was rejected by

   1. Wallace
   2. Malthus
   3. Lamarck
   4. Karl Ernst von Baer
   Answer

   D. Karl Ernst von Baer

   Von Baer noted that embryos never pass through the adult stages of other animals.

7. Dark-winged moths became more common than white-winged moths in industrial areas of England because

   1. lichens grew faster on the tree trunks
   2. white moths changed into dark moths in a single generation
   3. soot darkened the tree trunks, so predators saw the white moths more easily
   4. white moths migrated away from the cities
   Answer

   C. soot darkened the tree trunks, so predators saw the white moths more easily

   Predators ate the more visible white moths. Dark moths survived and left more offspring. This is natural selection.

8. The beaks of Darwin's finches are an example of

   1. industrial melanism
   2. adaptive radiation
   3. convergent evolution
   4. stabilising selection
   Answer

   B. adaptive radiation

   Many forms arose from one ancestral finch as they adapted to different foods.

9. Which Australian marsupial corresponds to the placental wolf?

   1. Flying phalanger
   2. Numbat
   3. Tasmanian wolf
   4. Marsupial mole
   Answer

   C. Tasmanian wolf

   Placental and marsupial pairs show convergent evolution. Numbat matches the anteater and the marsupial mole matches the mole.

10. Hugo de Vries developed the theory of mutation by working on

    1. pea (Pisum sativum)
    2. fruit fly (Drosophila melanogaster)
    3. peppered moth (Biston betularia)
    4. evening primrose (Oenothera lamarckiana)
    Answer

    D. evening primrose (Oenothera lamarckiana)

    De Vries worked on evening primrose and proposed large, sudden changes called saltation.

11. The long neck of the giraffe was explained by Lamarck through

    1. use and disuse of organs
    2. genetic drift
    3. natural selection of small variations
    4. saltation
    Answer

    A. use and disuse of organs

    Lamarck said that organs used more are developed more and that the acquired changes are passed to offspring.

12. In the Hardy-Weinberg equation, the frequency of heterozygotes is given by

    1. q²
    2. 2pq
    3. p + q
    4. p²
    Answer

    B. 2pq

    p² is AA, 2pq is Aa and q² is aa.

13. A few individuals found a new population, and its allele frequencies differ from those of the original population by chance. This is called

    1. gene flow
    2. genetic recombination
    3. disruptive selection
    4. the founder effect
    Answer

    D. the founder effect

    The founder effect is a form of genetic drift in a small new population.

14. The type of natural selection that favours both extremes of a trait and reduces the mean is

    1. disruptive selection
    2. genetic drift
    3. stabilising selection
    4. directional selection
    Answer

    A. disruptive selection

    Disruptive selection gives two peaks in the distribution curve.

15. The brain capacity of Homo erectus was about

    1. 450 cc
    2. 1400 cc
    3. 900 cc
    4. 650 to 800 cc
    Answer

    C. 900 cc

    Homo habilis had about 650 to 800 cc and Neanderthal man had about 1400 cc.

16. Which of the following was the first group of organisms to invade land?

    1. Amphibians
    2. Plants
    3. Reptiles
    4. Fishes
    Answer

    B. Plants

    Plants came onto land first. Animals followed later.

17. Given below are two statements: Statement I: Homologous organs show divergent evolution. Statement II: Analogous organs show convergent evolution.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    A. Both Statement I and Statement II are correct

    Both are correct. Homologous organs have a common origin, and analogous organs have a similar function with different origin.

18. Given below are two statements: Statement I: In the theory of de Vries, the mutations are small and directional. Statement II: Saltation is a single-step large mutation.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    D. Statement I is incorrect but Statement II is correct

    De Vries said mutations are large, random and directionless, so Statement I is incorrect. Statement II is correct.

19. Given below are two statements: Statement I: In genetic drift the change in allele frequency is directional and is caused by the environment. Statement II: The founder effect is seen in large populations.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    B. Both Statement I and Statement II are incorrect

    Drift is a chance change and is strongest in small populations. The founder effect occurs when a few individuals start a new population. Both statements are incorrect.

20. Which of the following statements about types of natural selection are correct? 1. Stabilising selection reduces both extremes. 2. Directional selection shifts the peak toward one extreme. 3. Disruptive selection favours the mean.

    1. 1 and 2 only
    2. 2 and 3 only
    3. 1 and 3 only
    4. 1, 2 and 3
    Answer

    A. 1 and 2 only

    Disruptive selection favours both extremes and not the mean.

21. Given below are two statements: Statement I: The early atmosphere of the earth had no free oxygen. Statement II: The early atmosphere of the earth was oxidising.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    C. Statement I is correct but Statement II is incorrect

    The early atmosphere was reducing (CH₄, NH₃, H₂ and water vapour). Statement I is correct and Statement II is incorrect.

22. Which of the following statements about human evolution are correct? 1. Homo habilis had a brain capacity of about 650 to 800 cc. 2. Neanderthal man had a brain capacity of about 900 cc. 3. Fossils of Homo erectus were found in Java.

    1. 1 and 2 only
    2. 1 and 3 only
    3. 2 and 3 only
    4. 1, 2 and 3
    Answer

    B. 1 and 3 only

    Neanderthal man had about 1400 cc. About 900 cc is the figure for Homo erectus.

23. Given below are two statements: Statement I: Antibiotic resistance arises because antibiotics cause the mutations that make the bacteria resistant. Statement II: Resistant bacteria survive treatment and leave more progeny.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    D. Statement I is incorrect but Statement II is correct

    Resistant variants already exist and are selected by the antibiotic. So Statement I is incorrect. Statement II is correct.

24. Given below are two statements: Statement I: Potato and sweet potato are analogous organs. Statement II: The thorn of Bougainvillea and the tendril of Cucurbita are homologous organs.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    A. Both Statement I and Statement II are correct

    Both are correct. Potato (stem) and sweet potato (root) have a different origin and the same function, while the thorn and the tendril are both modified stems.

25. Given below are two statements: Statement I: In a population at Hardy-Weinberg equilibrium, allele frequencies do not change from generation to generation. Statement II: Natural selection is one of the factors that keep the equilibrium unchanged.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    C. Statement I is correct but Statement II is incorrect

    Statement I is correct. Natural selection is one of the factors that disturb the equilibrium, so Statement II is incorrect.

26. Assertion (A): Darwin's finches are an example of adaptive radiation. Reason (R): Different forms arose from one ancestral seed-eating finch as they adapted to different foods and habitats.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    A. Both A and R are true and R is the correct explanation of A

    The varied beaks suit different food, and all of the finches came from one ancestor.

27. Assertion (A): Australian marsupials and placental mammals show convergent evolution. Reason (R): Both groups evolved from one common ancestral stock in Australia.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    C. A is true but R is false

    Convergent evolution means that similar forms arose independently from different ancestors. Only the marsupials radiated from an ancestral stock in Australia. So R is false.

28. Assertion (A): Genetic drift changes allele frequencies by chance. Reason (R): The founder effect is a type of genetic drift.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    B. Both A and R are true but R is not the correct explanation of A

    Both are true. But the founder effect is an example of drift and does not explain why drift is a chance process.

29. Assertion (A): Lamarck proposed that variations arise by natural selection. Reason (R): Lamarck suggested the use and disuse of organs.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    D. A is false but R is true

    Natural selection was proposed by Darwin, so A is false. R is true.

30. Assertion (A): Stabilising selection reduces variation in a population. Reason (R): It removes both extreme phenotypes and favours the mean.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    A. Both A and R are true and R is the correct explanation of A

    More individuals come to have the mean value, so the curve becomes narrower.

31. Assertion (A): Miller's experiment supports chemical evolution. Reason (R): It was done in an oxidising atmosphere that contained free oxygen.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    C. A is true but R is false

    The experiment supports chemical evolution. But the flask had a reducing mixture (CH₄, H₂, NH₃) without free oxygen, so R is false.

32. Match List I with List II. List I: a. Oparin and Haldane b. Miller c. Pasteur d. Darwin. List II: i. Biogenesis ii. Natural selection iii. Chemical evolution of life iv. Electric discharge in a flask of gases.

    1. a-i, b-iv, c-iii, d-ii
    2. a-iii, b-iv, c-i, d-ii
    3. a-iv, b-iii, c-i, d-ii
    4. a-iii, b-iv, c-ii, d-i
    Answer

    B. a-iii, b-iv, c-i, d-ii

    Oparin and Haldane proposed chemical evolution (iii), Miller used a discharge (iv), Pasteur showed biogenesis (i) and Darwin proposed natural selection (ii).

33. Match List I with List II. List I: a. Homologous organs b. Analogous organs c. Adaptive radiation d. Industrial melanism. List II: i. Common origin, different functions ii. Different origin, same function iii. Darwin's finches iv. Biston betularia.

    1. a-ii, b-i, c-iii, d-iv
    2. a-iii, b-ii, c-i, d-iv
    3. a-i, b-ii, c-iv, d-iii
    4. a-i, b-ii, c-iii, d-iv
    Answer

    D. a-i, b-ii, c-iii, d-iv

    Homologous organs share an origin (i), analogous organs share a function (ii), finches show radiation (iii) and the peppered moth shows industrial melanism (iv).

34. Match List I with List II. List I: a. Wolf b. Mole c. Anteater d. Flying squirrel. List II: i. Numbat ii. Flying phalanger iii. Tasmanian wolf iv. Marsupial mole.

    1. a-iii, b-iv, c-ii, d-i
    2. a-iv, b-iii, c-i, d-ii
    3. a-iii, b-iv, c-i, d-ii
    4. a-i, b-iv, c-iii, d-ii
    Answer

    C. a-iii, b-iv, c-i, d-ii

    The marsupial counterparts are Tasmanian wolf, marsupial mole, numbat and flying phalanger.

35. Match List I with List II. List I: a. p² b. 2pq c. q² d. p + q. List II: i. Frequency of heterozygotes ii. Frequency of homozygous recessive individuals iii. Sum of the two allele frequencies iv. Frequency of homozygous dominant individuals.

    1. a-i, b-iv, c-ii, d-iii
    2. a-ii, b-i, c-iv, d-iii
    3. a-iv, b-ii, c-i, d-iii
    4. a-iv, b-i, c-ii, d-iii
    Answer

    D. a-iv, b-i, c-ii, d-iii

    p² is AA (iv), 2pq is Aa (i), q² is aa (ii) and p + q equals 1 (iii).

36. Match List I with List II. List I: a. Homo habilis b. Homo erectus c. Neanderthal man d. Australopithecines. List II: i. Brain about 900 cc ii. Brain about 1400 cc iii. Brain about 650 to 800 cc iv. Essentially fruit eaters of the East African grasslands.

    1. a-iii, b-ii, c-i, d-iv
    2. a-iii, b-i, c-ii, d-iv
    3. a-iii, b-i, c-iv, d-ii
    4. a-i, b-iii, c-ii, d-iv
    Answer

    B. a-iii, b-i, c-ii, d-iv

    Habilis had about 650 to 800 cc (iii), erectus about 900 cc (i), Neanderthal about 1400 cc (ii) and the australopithecines ate mostly fruit (iv).

37. Match List I with List II. List I: a. Stabilising selection b. Directional selection c. Disruptive selection d. Founder effect. List II: i. Peak shifts toward one extreme ii. Two peaks are formed iii. Both extremes are reduced iv. New population started by a few individuals.

    1. a-iii, b-i, c-iv, d-ii
    2. a-i, b-iii, c-ii, d-iv
    3. a-iii, b-i, c-ii, d-iv
    4. a-iii, b-ii, c-i, d-iv
    Answer

    C. a-iii, b-i, c-ii, d-iv

    Stabilising reduces both extremes (iii), directional shifts the peak (i), disruptive forms two peaks (ii) and the founder effect begins with a few individuals (iv).

38. In a population in Hardy-Weinberg equilibrium, the frequency of a recessive homozygous genotype (aa) is 0.04. What is the frequency of heterozygous carriers (Aa)?

    1. 0.32
    2. 0.16
    3. 0.64
    4. 0.04
    Answer

    A. 0.32

    q² = 0.04, so q = 0.2 and p = 0.8. Then 2pq = 2 x 0.8 x 0.2 = 0.32.

39. In a population at equilibrium, the frequency of allele A is 0.7. What is the expected frequency of the heterozygous genotype Aa?

    1. 0.21
    2. 0.42
    3. 0.09
    4. 0.49
    Answer

    B. 0.42

    q = 0.3, so 2pq = 2 x 0.7 x 0.3 = 0.42. The value 0.49 is p² and 0.09 is q².

40. A recessive disorder affects 1 in 10,000 people in a population at Hardy-Weinberg equilibrium. What is the frequency of the recessive allele?

    1. 0.0001
    2. 0.001
    3. 0.1
    4. 0.01
    Answer

    D. 0.01

    q² = 1/10,000 = 0.0001, so q = 0.01.

41. A few bacteria in a culture survive an antibiotic, and the surviving population becomes resistant. This is best explained by

    1. inheritance of acquired characters
    2. the antibiotic causing directed mutations
    3. natural selection acting on existing variation
    4. use and disuse of organs
    Answer

    C. natural selection acting on existing variation

    Resistant variants were already present. The antibiotic removed the sensitive cells, so the resistant ones multiplied.

42. In a bird population, medium-sized individuals survive and leave more offspring, while very small and very large individuals die. This is an example of

    1. stabilising selection
    2. genetic drift
    3. disruptive selection
    4. directional selection
    Answer

    A. stabilising selection

    The mean is favoured and both extremes are reduced.

43. A group of 20 individuals from a large mainland population settles on an island. Their allele frequencies differ from those of the mainland population. This is mainly due to

    1. genetic recombination
    2. the founder effect
    3. gene flow
    4. stabilising selection
    Answer

    B. the founder effect

    A small group starts a new population and carries only some of the original alleles. This is genetic drift of the founder type.

44. A fossil skull has a brain capacity of about 1400 cc, and it was found with signs of burial of the dead and the use of hides. It most probably belonged to

    1. Australopithecus
    2. Homo habilis
    3. Homo erectus
    4. Neanderthal man
    Answer

    D. Neanderthal man

    Neanderthal man had a brain capacity of about 1400 cc, used hides and buried the dead.