Theories of Evolution

Theories of evolution explain how new forms of life arise and spread. This page covers Darwin's natural selection and branching descent, Lamarck's use and disuse of organs, de Vries's mutation theory, the Hardy-Weinberg principle and the five factors that disturb it, the three kinds of natural selection, and a brief account of how plants and vertebrates evolved through geological time, as in the NCERT Class 12 chapter Evolution. NEET often asks Hardy-Weinberg calculations, saltation and the types of natural selection in questions on theories of evolution.

1. Biological Evolution

• Evolution by natural selection, in a true sense, would have started when cellular forms of life with differences in metabolic capability originated on earth.
• ★ Exam imp The essence of Darwinian theory about evolution is natural selection.
• The rate of appearance of new forms is linked to the life cycle or life span.
• Microbes divide fast and can become millions of individuals within hours.

1.1 Selection in a bacterial colony

1. A colony of bacteria (A) growing on a given medium has built-in variation in its ability to use a feed component.
2. A change in the medium composition brings out only that part of the population (B) that can survive the new conditions.
3. In due course, the variant population B outgrows the others and appears as a new species, within days.

• The same change in a fish or fowl would take millions of years, as their life spans are in years.
• The fitness of B is better than that of A under the new conditions: nature selects for fitness.
• So-called fitness is based on characteristics that are inherited, so there must be a genetic basis for getting selected and evolving.
• Put another way, some organisms are better adapted to survive in an otherwise hostile environment.
• ★ Exam imp Adaptive ability is inherited and has a genetic basis; fitness is the end result of the ability to adapt and get selected by nature.

2. Lamarck's Theory: Use and Disuse

• Even before Darwin, the French naturalist Lamarck said that evolution of life forms had occurred.
• ★ Exam imp He held that it was driven by the use and disuse of organs.
• Example: giraffes, trying to forage leaves on tall trees, had to adapt by elongating their necks.
• They passed on this acquired character to succeeding generations, so giraffes slowly came to have long necks.
• Nobody believes this conjecture any more.

3. Is Evolution a Process or a Result?

• The world we see, inanimate and animate, is only the success stories of evolution.
• When we describe the story of this world, we describe evolution as a process.
• When we describe the story of life on earth, we treat evolution as a consequence of a process called natural selection.
• We are still not clear whether to regard evolution and natural selection as processes, or as end results of unknown processes.

4. The Basis of Natural Selection

• The work of Thomas Malthus on populations possibly influenced Darwin.
• Natural selection rests on observations that are factual:

1. Natural resources are limited.
2. Populations are stable in size, except for seasonal fluctuation.
3. Members of a population vary in characteristics; in fact no two individuals are alike, even though they look similar.
4. Most variations are inherited.

• In theory, population size would grow exponentially if everybody reproduced maximally, as seen in a growing bacterial population.
• In reality, population sizes are limited, so there must have been competition for resources.
• Only some survived and grew at the cost of others that could not flourish.

4.1 Darwin's insight

1. Some variations are heritable and make resource use better for a few individuals (they are better adapted to the habitat).
2. Only those individuals reproduce and leave more progeny.
3. Over many generations, the survivors keep leaving more progeny, and the population's characteristics change.
4. Hence, new forms appear to arise.

5. Mechanism of Evolution: Mutation Theory

• What is the origin of variation, and how does speciation occur?
• Mendel had talked of inheritable 'factors' influencing the phenotype, but Darwin either ignored these observations or kept silent.
• ★ Exam imp In the first decade of the twentieth century, Hugo de Vries, working on the evening primrose, put forward the idea of mutations.
• Mutations: large differences arising suddenly in a population.
• He believed that mutation causes evolution, not the minor heritable variations that Darwin talked about.
• Mutations are random and directionless, while Darwinian variations are small and directional.
• Evolution for Darwin was gradual; de Vries believed mutation caused speciation, and called it saltation (single-step large mutation).
• Studies in population genetics later brought out some clarity.

6. Hardy-Weinberg Principle

• In a given population, one can find the frequency of occurrence of the alleles of a gene or a locus.
• This frequency is supposed to remain fixed, and even remain the same through generations.
• The Hardy-Weinberg principle stated this using algebraic equations.
• ★ Exam imp The principle says that allele frequencies in a population are stable and constant from generation to generation.
• Gene pool: the total genes and their alleles in a population; it remains constant.
• Genetic equilibrium: this constancy of the gene pool.
• The sum total of all the allelic frequencies is 1.

6.1 The equation

• In a diploid, let $p$ and $q$ be the frequencies of allele A and allele a, so $p+q=1$.
• The frequency of AA individuals is simply $p^2$: the probability that allele A (frequency $p$) appears on both chromosomes is the product of the probabilities.
• Similarly, the frequency of aa is $q^2$, and of Aa is $2pq$.

• This is the binomial expansion of $(p+q{)}^2$.
• When the measured frequency differs from the expected value, the difference (its direction) shows the extent of evolutionary change.
• A disturbance in genetic (Hardy-Weinberg) equilibrium, that is, a change of allele frequency in a population, is interpreted as resulting in evolution.

6.2 Factors that affect Hardy-Weinberg equilibrium

• ★ Exam imp Five factors are known: gene migration or gene flow, genetic drift, mutation, genetic recombination and natural selection.
• Gene migration: when a section of a population migrates to another place and population, gene frequencies change in both the original and the new population.
• New genes or alleles are added to the new population and lost from the old one.
• Gene flow: occurs if this gene migration happens multiple times.
• Genetic drift: the same change in gene frequency occurring by chance.
• Sometimes the change in allele frequency in the new sample of population is so different that they become a different species.
• Founder effect: the original drifted population becomes the founders, and this effect is called the founder effect.

6.3 Natural selection and its three outcomes

• Microbial experiments show that pre-existing advantageous mutations, when selected, result in new phenotypes; over a few generations this results in speciation.
• Natural selection: a process in which heritable variations enabling better survival are enabled to reproduce and leave a greater number of progeny.
• Variation due to mutation, recombination during gametogenesis, gene flow or genetic drift changes the frequency of genes and alleles in future generations.
• Coupled with enhanced reproductive success, natural selection makes it look like a different population.
• Natural selection can lead to three outcomes:

1. Stabilisation: more individuals acquire the mean character value.
2. Directional change: more individuals acquire a value other than the mean character value.
3. Disruption: more individuals acquire peripheral character values at both ends of the distribution curve.

7. A Brief Account of Evolution

7.1 From the first cells to land plants

1. About 2000 million years ago (mya), the first cellular forms of life appeared. How non-cellular aggregates of giant macromolecules evolved into cells with a membranous envelope is not known.
2. Some of these cells could release ${\mathrm{O}}_2$. The reaction could have been similar to the light reaction of photosynthesis, where water is split using solar energy captured and channelled by light-harvesting pigments.
3. Slowly, single-celled organisms became multicellular life forms.
4. By about 500 mya, invertebrates were formed and active.
5. Jawless fish probably evolved around 350 mya.
6. Sea weeds and a few plants probably existed around 320 mya.

• ★ Exam imp The first organisms that invaded land were plants; they were widespread on land when animals invaded it.

7.2 From fish to reptiles

1. Fish with stout and strong fins could move on land and go back to water, about 350 mya.
2. In 1938, a fish caught in South Africa turned out to be a Coelacanth, which was thought to be extinct.
3. These animals, called lobefins, evolved into the first amphibians, which lived on both land and water. No specimens of these are left, but they were the ancestors of modern frogs and salamanders.
4. Amphibians evolved into reptiles. Reptiles lay thick-shelled eggs, which do not dry up in the sun, unlike those of amphibians. Their modern descendants include turtles, tortoises and crocodiles.
5. In the next 200 million years or so, reptiles of different shapes and sizes dominated the earth.

• Giant ferns (pteridophytes) were present, but they all fell to form coal deposits slowly.
• Some land reptiles went back into water and evolved into fish-like reptiles, probably about 200 mya (for example, Ichthyosaurs).
• ★ Exam imp The land reptiles were the dinosaurs. The biggest, Tyrannosaurus rex, was about 20 feet tall, with huge, fearsome, dagger-like teeth.
• About 65 mya, the dinosaurs suddenly disappeared from the earth. The true reason is not known.
• Some say climatic changes killed them; some say most of them evolved into birds. The truth may lie in between.
• Small reptiles of that era still exist today.

7.3 The rise of mammals

• The first mammals were like shrews; their fossils are small-sized.
• Mammals were viviparous: they protected their unborn young inside the mother's body.
• Mammals were more intelligent, at least in sensing and avoiding danger.
• When the reptiles came down, mammals took over the earth.
• South America had mammals resembling the horse, hippopotamus, bear, rabbit, etc.
• Due to continental drift, when South America joined North America, these animals were overridden by North American fauna.
• ★ Exam imp Due to the same continental drift, the pouched mammals of Australia survived, because of the lack of competition from any other mammal.
• Some mammals live wholly in water: whales, dolphins, seals and sea cows.
• The evolution of the horse, elephant, dog, etc., are special stories of evolution.
• The most successful story is the evolution of man, with language skills and self-consciousness.

• Variations in a population result in variable fitness.
• Other phenomena, such as habitat fragmentation and genetic drift, may accentuate these variations, leading to new species and hence evolution.

8. Exam Essentials

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