Heredity - Evolution

1. What This Chapter Covers

Looking at the living world we are struck by two seemingly opposite observations: the fantastic variety of life, and the similarity between its forms. Understanding both is what it takes to understand how life evolves.

The book is precise about the word: when we say something evolves, we mean not only that it changes but that there is some component of direction in that change.

Reproduction usually gives rise to individuals that have some new characters despite their similarity to their parents, and those new characters often produce observable changes in life forms. The chapter's three questions are how new characters are produced, whether they are inherited, and what role they play in evolution.

The index allots this chapter 15 periods in December — the joint largest in the book, shared with Chapter 6 — and runs it from page 176 to page 204.

2. Variation (Textbook 8.1)

Activity 1 asks you to tabulate your own traits against those of your mother, father, brother, sister and grandparents: eye colour, hair colour and type, nose shape, face shape, ear lobes attached or free, and the markings on the inner thumb.

The useful questions are the awkward ones the book asks: is there a character in you like your grandmother's but not your mother's, and where did your mother get a character that her mother did not have?

Activity 2 does the same for friends, comparing skin colour, ear lobes, thumb markings, forehead length and iris colour. Comparing the two tables answers whether you share more characters with your parents or with your friends.

Differences in characters within very closely related groups of organisms are called variations. A new character in a group may lead to variations, and those also get inherited.

Activity 3. Look at the seeds in a pea or bean pod and try to find two identical seeds. You cannot, and the hint the book gives is that seeds form from ovules — so variation is not only about obvious differences but about subtle ones we usually overlook.

3. Mendel and His Experiments (Textbook 8.2)

In 1857 Gregor Johann Mendel started work on how variations pass from one generation to the next. He was an Austrian monk, and he did his experiments not in a university or laboratory but in a monastery garden, working for over seven years before presenting his conclusions as a detailed research paper. He is known as the Father of genetics.

Why the pea plant (8.2.1)

Mendel chose the pea plant for five reasons: it has well defined characters, bisexual flowers, is predominantly self-pollinating, is suitable for cross pollination, and is an annual plant. He worked on nearly 10,000 pea plants of 34 different varieties.

The actual counts

Two things are worth noticing. The ratios are never exactly 3:1 — they run from 2.82 to 3.15 — which is what real counted data looks like, and the largest sample, seed colour with 8023 seeds, gives the closest ratio. And in the printed table the recessive seed colour is shown only as a green seed picture with no word beside it; the text of the chapter supplies the word green.

The procedure (8.2.2)

Mendel selected varieties, obtained their seeds and grew more plants from them. He first allowed self pollination within a variety, and by repeated self-pollination produced seeds of varieties he called pure.

He would then start from such a pure variety, let it flower, and cross-pollinate it: removing the stamens from a purple flower, transferring pollen from the stamens of another plant to the egg-bearing carpel of the purple flower, letting the pollinated carpel mature into a pod, planting the seeds and examining the offspring.

Mendel hypothesized that characters are carried as traits, that an organism always carries a pair of factors for a character, and that distinguishing traits of the same character are present in a population.

4. The Monohybrid Cross (Textbook 8.2.3 to 8.2.7)

Cross-pollinating a pure yellow-seeded plant with a pure green-seeded plant gave, in the F1 generation — filial meaning progeny — all yellow seeds. Self-pollinating those gave an F2 generation of about 75 per cent yellow and 25 per cent green, a 3 : 1 ratio.

Mendel's three assumptions

Assumption 1. Every pea plant has two factors responsible for a particular trait. A pure breed has both factors of the same type: pure yellow is YY, pure green is yy. Same factors means homozygous; a mixed pair, Yy, is heterozygous.

Assumption 2. During reproduction one factor from each parent is taken to form a new pair in the progeny.

Assumption 3. One factor will always dominate the other if they are mixed. The trait expressed in F1 is dominant; the one that does not appear is recessive.

So after cross-pollination every pea carries Yy, one factor from each pure parent, and because Y dominates, all F1 peas are yellow. That is the law of dominance.

Self-pollinating the F1 (8.2.4, 8.2.5)

Crossing Yy with Yy can give YY, Yy, yY or yy, all equally likely. Any pea with a Y is yellow; only yy is green. So roughly 25 per cent YY yellow, 25 per cent Yy yellow, 25 per cent yY yellow and 25 per cent yy green.

The book draws the consequence explicitly: some seeds that appear yellow in F1 produce green-seeded plants when sown, so we cannot determine the internal character from the external visible one.

Phenotype and genotype (8.2.6, 8.2.7)

The externally visible characters are the phenotype, and the F2 phenotypic ratio is 3 : 1.

The constitution shown by the letters — which factors are actually present — is the genotype, and the monohybrid genotypic ratio is 1 : 2 : 1, that is 25 per cent pure YY, 50 per cent heterozygous Yy and 25 per cent pure yy.

Self-pollinating the F2 confirms it: the YY peas give only yellow, the Yy peas give 75 per cent yellow and 25 per cent green all over again, and the yy peas give only green.

The 3:1 is what a farmer would notice; the 1:2:1 is what is actually being inherited. Mendel's point is that the second explains the first, and that you cannot read a plant's factors off its appearance — which is exactly what the yellow F1 seeds producing green plants demonstrates.

5. The Dihybrid Cross and Mendel's Laws (Textbook 8.2.8, 8.2.9)

Two pairs of contrasting characters are taken together: seed colour, yellow Y and green y, and seed shape, round R and wrinkled r.

Pure yellow-round plants (YYRR) crossed with pure green-wrinkled plants (yyrr) gave an F1 in which every pea was YyRr, and since Y and R are both dominant, all F1 seeds were yellow and round.

Self-pollinating the F1 gave an F2 containing all four combinations: yellow and round, green and round, yellow and wrinkled, and green and wrinkled — in the ratio 9 : 3 : 3 : 1.

The three laws

Law of dominance. Among a pair of closely related alleles for a character, only one expresses itself in the first generation, because one allele is dominant over the other.

Law of segregation. A pair of alleles is responsible for each character, one received from each parent. When that generation forms gametes the alleles separate, and each enters a separate gamete randomly.

Law of independent assortment. In the inheritance of more than one pair of characters, the factors for each pair assort independently of the other pair. The book's annexure works this out on the dihybrid checker board: nine squares give round yellow, three round green, three wrinkled yellow and one wrinkled green, and the factors remain independent and keep their identity in the gametes.

Traits that may be passed from one generation to the next are called heritable traits.

Activity 4 models this with paper strips and buttons. Put four long and four short strips into two bags, A for male and B for female, draw one from each at random and pair them in a box on a 2 by 2 chart, and repeat until the bags are empty. Counting how many pairs are long-long, long-short and short-short over several rounds reproduces the ratio.

6. From Parent to Progeny (Textbook 8.3)

Heredity is the transmission of characters or traits from parent to offspring. Inheritance is the process by which traits are passed from one generation to another.

Mendel's "factors" are now known as genes. A gene is a segment of DNA present in the nucleus of every cell, and it controls the expression of a trait; in some viruses RNA controls it instead. The contrasting expressions of the same trait are the alleles.

Traits are determined by the chemical nature of DNA, and a slight inheritable change in its chemical structure leads to variation.

The structure of DNA. In the 1950s it was worked out by Rosalind Franklin, Francis Crick, James Watson and Maurice Wilkins. The molecule looks like a spiral staircase, a double helix, whose framework is alternating sugar and phosphate groups and whose steps are pairs of bases — adenine, guanine, thymine and cytosine. Watson, Crick and Wilkins were jointly awarded the Nobel Prize in 1962; Rosalind Franklin had died by then and so was not included.

7. Sex Determination (Textbook 8.4)

Each human cell contains 23 pairs, 46 chromosomes. Of these, 22 pairs are autosomes — chromosomes whose number and morphology do not differ between males and females. The remaining pair are the allosomes or sex chromosomes, of two types, X and Y.

Females have XX; males have XY. Every ovum a woman produces carries only an X. The sperms a man produces are of two types, one carrying X and one carrying Y.

If a Y-carrying sperm fertilizes the ovum the baby is XY, a boy; if an X-carrying sperm does, the baby is XX, a girl. So the father's sperm decides the sex, and the book turns this into a direct social question: is it right to blame a woman when a girl baby is born?

Discovery of the sex chromosomes. Walter Sutton and Thomas Hunt Morgan studied the small fruit fly Drosophila melanogaster at Columbia University. The discovery of sex-linked traits in Drosophila indicated that genes are on chromosomes, and they worked out the details of inheritance in the fly.

8. Variation and Selection — the Beetles (Textbook 8.5)

Variations arise from sexual reproduction and from errors in DNA copying, and are passed on. Activity 5 follows twelve beetles living on green leaves in bushes, whose population grows by sexual reproduction, with crows eating them.

Situation 1 — natural selection. A colour variation produces a green beetle instead of a red one, and it passes the colour to all its progeny. Crows cannot see green beetles on green leaves but can see red ones, so green beetles increase and red ones decline.

The colour gave a survival advantage; in other words it was naturally selected, and the selection was exerted by the crows. The more crows there are, the more red beetles are eaten and the more green ones remain. Natural selection is directing evolution, and the result is adaptation that fits the population better to its environment.

Situation 2 — genetic drift. This time the variation produces a blue beetle. Crows can see blue beetles on green leaves as easily as red ones, so there is no survival advantage. Then an elephant stamps on the bushes and kills most of the beetles, and by chance the few survivors are mostly blue. The population recovers, now mostly blue. Accidents can therefore change the frequency of genes in small populations, which is genetic drift, and it also provides diversity.

Situation 3 — what is not inherited. A plant disease destroys leaf material, the beetles get less food, they are poorly nourished and their weight decreases — but no change takes place in their DNA.

The book draws the conclusion carefully. Germ cells of a sexually reproducing population are formed in specialised reproductive tissue. Starvation does not change the DNA of the germ cells, so low weight is not a heritable trait, and even if several generations lose weight, that is not evolution.

Change in non-reproductive tissues cannot be passed on to the DNA of the germ cells, so the experiences of an individual during its lifetime cannot direct evolution.

9. Lamarck and Darwin (Textbook 8.6, 8.7)

Lamarckism

Jean Baptist Lamarck (1774-1829) was the first person to propose a theory of evolution, at a time when people believed organisms had never changed.

He thought that at some point the giraffe was the size of a deer, and that because of a shortage of food on the ground it started stretching its neck to reach the lower branches of trees; over several generations of stretching the neck became long. Characters developed during an organism's lifetime are acquired characters, and Lamarck proposed that they pass to the next generation — the theory of inheritance of acquired characters.

August Weismann tested it on rats. He removed the tails of parent rats and found their offspring normal, with tails. He repeated it for 22 generations and the offspring still had tails, proving that bodily changes are not inherited.

Darwinism

Charles Darwin (1809-1882) was born in England and, at 22, voyaged for five years on the survey ship HMS Beagle, visiting the Galapagos Islands among other places and observing their flora and fauna closely.

He read Sir Charles Lyell's Principles of Geology, which suggested geological change happened at a uniform rate; Darwin disagreed, holding that large changes came from the accumulation of small ones. He was also influenced by Malthus, whose Essay on the Principles of Population observed that population grows in geometrical progression (1, 2, 4, 8, 16) while food sources increase in arithmetic progression (1, 2, 3, 4, 5).

On the Galapagos he observed a small group of related birds, the finches, differing in the structure of their beaks: the large ground finch eating seeds, the cactus ground finch, the vegetarian finch eating buds, and the woodpecker finch eating insects.

From this he proposed natural selection: nature decides which organism survives and which perishes, which is what survival of the fittest means. Organisms with useful traits survive; those whose traits are not useful are eliminated. Alfred Russel Wallace independently reached the same conclusion.

When Darwin was formulating the theory, he received a letter and an article from Alfred Russel Wallace about his studies in the Indonesian islands — on natural selection. Darwin was astonished to find the same theory he had in mind. The two jointly published an article in the Journal of the Linnaean Society, and only after that did Darwin publish The Origin of Species in 1859.

The theory in the book's own eight points

1. Any population develops variations, and all its members are not identical.
2. Variations are passed from parent to offspring through heredity.
3. Over-abundance of offspring leads to a constant struggle for survival.
4. Individuals with variations that help them survive and reproduce live longer and have more offspring.
5. The offspring of survivors inherit the useful variations, and the process repeats each generation until the variation becomes a common feature.
6. As the environment changes, organisms within it adapt to the new conditions.
7. Over a long period each species can accumulate so many changes that it becomes a new species, similar to but distinctly different from the original. All species arose this way.
8. Evolution is a slow and continuous process.

The book notes there are limitations and objections to Darwin's theory, and that newer theories such as the synthetic theory and the mutation theory have been put forward.

10. Speciation (Textbook 8.8)

Small changes within a species — red and green beetles — are micro evolution. The formation of new species is speciation, also called macro evolution.

Red and green beetles can mate and produce offspring. But suppose they are separated for a long time, as when crows carry some of them far away and drop them. Over those years many variations accumulate in each group. When the two groups meet again they can no longer mate and produce offspring; each can breed only within its own population. At that point a new species has been formed.

11. Evidence for Evolution (Textbook 8.9)

Homologous and analogous organs

Compare the forelimb of a whale which swims, the wing of a bat which flies, the leg of a cheetah which runs, the claw of a mole which digs and the hand of a man which grasps. Their external form and function are entirely different, yet their anatomy shows a common pattern in the arrangement of bones.

That indicates all vertebrates evolved from a common ancestor, and such organs are homologous. This kind of evolution is divergent evolution.

But similarity of shape does not always mean common ancestry. The wings of insects are outgrowths of their exoskeleton; the wings of birds have a feathery covering all along the arm. Their design, structure and components are different, and they look alike only because they share a common use — flying. Organs that are structurally different but functionally similar are analogous, and this kind of evolution is convergent evolution.

Embryology (8.9.2)

Embryology is the study of embryos and their development. The tadpole of a frog resembles a fish more than it resembles a frog.

There are remarkable similarities in the embryos of different animals from fish to man — the book's figure compares fish, salamander, tortoise, chick, pig, calf, rabbit and human. The resemblance at an early stage is so close that even an experienced embryologist would find it difficult to tell one embryo from another. This strengthens the view of a common ancestor.

Fossils (8.9.3)

Fossils are evidence of ancient life forms or habitats preserved by natural processes, typically within sediments deposited beneath water and land.

They may be actual remains such as bones or seeds, or traces of past events such as a dinosaur footprint or ripple marks on a prehistoric shore. Usually a dead body decomposes and is lost; but if, say, a dead insect is caught in mud, the mud hardens and retains the impression of the body parts.

The study of fossils is palaeontology. Palaeontologists determine a fossil's age by the carbon dating method: radioactive isotopes of elements such as carbon, uranium and potassium break down at a known rate, so the age of the rock or mineral containing them can be calculated.

A rare and magnificent fossil of the dinosaur Ketosaurs, of the lower Jurassic age going back about 160 million years, was collected from Yamanapalli in Adilabad district of Telangana. It is 14 metres long and 5 metres high and is preserved at the BM Birla Science Centre, Hyderabad.

Connecting links

Archeopteryx has some avian characters and some reptilian ones. Organisms bearing the characters of two different groups are connecting links, and Archeopteryx is recognised as the connecting link between aves and reptiles.

12. Human Evolution (Textbook 8.10)

There is great diversity in human forms across the planet, and for a long time people spoke of human races, most often identified by skin colour. The question long debated was whether these apparent groups evolved differently.

The evidence has become very clear: there is no biological basis to the notion of human races. All humans are a single species with a common ancestor.

Regardless of where we have lived for the past few thousand years, we all come from Africa. The earliest members of Homo sapiens can be traced there, and our genetic footprints trace back to African roots. A couple of hundred thousand years ago some ancestors left Africa while others stayed.

The migrants spread from Africa to west Asia, then Central Asia, Eurasia, South Asia and East Asia; down through Indonesia and the Philippines to Australia; and across the Bering land bridge to the Americas. They did not travel in a single line — they went forwards and backwards, groups separating, some even moving back into Africa.

Evolution of man through the ages. Homo habilis lived between 1.6 and 2.5 million years ago; Homo erectus between 1 and 1.8 million years ago; Homo neanderthalensis between 1,00,000 and 40,000 years ago; and Homo sapiens, present man, appeared about 10,000 years ago. The book's line of descent runs Diopithicus, Ramapithicus, Astrolopithicus, Homo habilis, Homo erectus, Homo neanderthalensis, Cromagnan, Homo sapiens.

The dates in the book are not internally consistent — the main text says the first sure fossil of our own species indicates that true man appeared 2 lakh 50 thousand years ago, while the box gives about 10,000 years ago — so quote whichever the question asks about and be aware that the two figures differ in the book itself.

13. Vestigial Organs (Textbook 8.11)

During the course of evolution some organs remain in an organism without a use. The appendix has no role in human digestion, but in herbivores such as the rabbit it plays an important part. Organs that are no longer useful are vestigial organs.

Sometimes a vestigial organ appears abruptly in a human being, and that phenomenon is called atavism — the book's example is a baby born with a tail.

There are nearly 180 vestigial organs in human beings, including the pinna, hair on the skin, and mammary glands in males, which is why the human being is described as a moving museum of vestigial organs.

Key words from the chapter

Variations, offspring, traits, phenotype, genotype, heterozygous, homozygous, independent assortment, alleles, heredity, autosomes, allosomes, natural selection, analogous organs, embryological evidences, human evolution.

14. Summary

Variations are quite apparent among closely related groups of organisms. In about 1857 Gregor Johann Mendel began working on how variations pass from one generation to the next, choosing seven distinguishing traits of the pea plant.

In the monohybrid experiment the F1 generation expresses the dominant trait. In F2, about 75 per cent show the dominant character and about 25 per cent the recessive: that is the phenotype, ratio 3 : 1. Of the 75 per cent, 25 per cent are pure dominant and 50 per cent carry the recessive factor, with the remaining 25 per cent pure recessive: that is the genotype, ratio 1 : 2 : 1.

Every pea plant has two factors, called alleles, responsible for a particular trait, and each parent passes a randomly selected copy of only one of them to its offspring. The factors for each pair of characters assort independently of the other pairs, which is the law of independent assortment.

Acquiring characters from parents is heredity. Each human cell contains 23 pairs of chromosomes, of which 22 pairs are autosomes and one pair are allosomes.

Lamarck proposed that acquired characters pass to the next generation — the theory of inheritance of acquired characters — which Weismann's rats disproved. Each species tends to produce a large number of offspring, but only the fittest survive.

Homologous organs, analogous organs and embryological evidence all explain evolutionary relationships, and traits shared across different organisms are often similar because they are inherited from a common ancestor. Fossils are evidence of ancient life forms or habitats preserved by natural processes.