Biological evolution

What is biological evolution?

In plain words: in every population some individuals inherit traits that help them use resources better. They leave more offspring, so over generations those traits become common and the population changes.

Natural selection and fitness

• The rate at which new forms appear is linked to the life cycle or life span. Microbes that divide fast can multiply into millions of individuals within hours (page 118).
• A colony of bacteria (say A) on a given medium has built-in variation in its ability to use a feed component. If the medium changes, only the part of the population (say B) that can survive the new conditions comes through. In due course this variant population outgrows the others and appears as a new species, within days. For the same to happen in a fish or fowl would take millions of years, as their life spans are in years (page 118). Give this contrast in an NCERT-based answer.
• Beyond the NCERT text, a caution: a population that has come to use a new medium has adapted, and that alone does not make it a new species; generation time sets how many generations, and so how many chances for change, fit into a given time, not a fixed time to speciation (reasoned from NCERT page 118, where B is a variant part of the same population; on what makes a new species, see Wikipedia, “Speciation”).
• We say the fitness of B is better than that of A under the new conditions: nature selects for fitness (page 118).
• Fitness is based on characteristics that are inherited, so there must be a genetic basis for getting selected and evolving. Adaptive ability is inherited, and “Fitness is the end result of the ability to adapt and get selected by nature” (page 118). NCERT adds: “The fitness, according to Darwin, refers ultimately and only to reproductive fitness”, so those who are better fit leave more progeny than others (Section 6.2, page 113).

Branching descent

Branching descent and natural selection are the two key concepts of the Darwinian theory of evolution (page 118). NCERT’s chapter summary adds: “Homology is accounted for by the idea of branching descent” (NCERT Chapter 6, Summary, page 126). Branching descent means that one ancestral form gives rise to several descendant lines, as in the adaptive radiations of Darwin’s finches and the Australian marsupials (reasoned from NCERT Section 6.4, pages 117–118).

Lamarck: use and disuse

• Even before Darwin, the French naturalist Lamarck said that evolution of life forms had occurred, but driven by the use and disuse of organs (pages 118–119).
• His example was the giraffe: trying to forage leaves on tall trees, giraffes adapted by elongating their necks and passed this acquired character to their offspring, so over the years giraffes came to have long necks. NCERT: “Nobody believes this conjecture any more” (page 119). Stretching an individual’s neck does not give its offspring a longer neck.

Darwin’s observations and Malthus

• Is evolution a process or the result of a process? Describing the story of the world, we treat evolution as a process; describing the story of life on earth, as a consequence of a process called natural selection; and NCERT says we are still not very clear which way to regard them (page 119). Beyond the NCERT text: evolution is population change across generations, and natural selection is one of its mechanisms, alongside mutation, genetic recombination, genetic drift and gene flow (the factors NCERT lists in Section 6.7, page 121).
• The work of Thomas Malthus on populations may have influenced Darwin (page 119).
• Natural selection rests on factual observations (page 119): natural resources are limited; populations are stable in size except for seasonal fluctuation; members of a population vary (no two individuals are alike), though they look superficially similar; and most variations are inherited. Beyond the NCERT text: real populations may also grow or decline, and some differences come from the environment or development rather than inheritance, since phenotypes are plastic (Wikipedia, “Natural selection”, facts in our words).
• Populations could in theory grow exponentially if every individual reproduced maximally, as a growing bacterial population shows, yet real population sizes are limited: so there has been competition for resources, and only some survived and grew at the cost of others (page 119).

The novelty and brilliant insight of Darwin was this: he asserted that variations, which are heritable and which make resource utilisation better for few (adapted to habitat better) will enable only those to reproduce and leave more progeny.

NCERT Biology (Class 12), Section 6.5, page 119

Over many generations the survivors leave more progeny, the characteristics of the population change, and new forms appear to arise (page 119).

Solved examples

Common mistakes to avoid

Quick revision summary

• Natural selection, the essence of Darwin’s theory, would have started, NCERT says, with cellular life of differing metabolic capability; it needs heritable variation and differences in reproduction.
• The rate of appearance of new forms depends on the life span: bacteria in days, fish or fowl in millions of years (NCERT). Fitness, which has a genetic basis, is the end result of the ability to adapt and get selected by nature.
• Darwin’s two key concepts: branching descent and natural selection. Lamarck: use and disuse of organs and inheritance of acquired characters (giraffe), no longer believed.
• Malthus may have influenced Darwin. Facts: limited resources, populations stable in size, variation, mostly inherited, and potential exponential growth, hence competition; heritable variations that improve resource use let a few leave more progeny, and the population changes.

Questions students ask

What are the similarities and differences between Lamarckism and Darwinism?

Lamarckism and Darwinism agree that life forms have evolved, slowly and over many generations (page 119), but differ on how: NCERT says Lamarck, before Darwin, held that evolution had occurred “driven by use and disuse of organs”, with a character acquired in an animal’s life passed on to its young (pages 118–119). Darwin’s natural selection starts instead from heritable variation already present in a population, and change comes because those better fit leave more progeny (pages 112–113 and 119). So, by our reading, in Lamarck’s view the individual changes and hands the change on, while in Darwin’s the population changes.

Why was Lamarck’s theory rejected?

Beyond the NCERT text, Lamarck’s theory, published in France in 1809, was rejected because characters an animal acquires in its body during life are not passed to its offspring: August Weismann held that offspring inherit only what the germ cells (sperm and eggs) carry, and in 1888 he reported, as evidence against Lamarck, that mice whose tails were cut off for five generations still had young with normal tails (Embryo Project Encyclopedia, Arizona State University). A cut tail is an injury, not the use or disuse Lamarck described, so, by our reading, the experiment illustrates Weismann’s germ-cell argument rather than testing Lamarck’s own idea.

What is the difference between branching descent and natural selection?

Branching descent is the pattern of evolution and natural selection is the process behind it, by our reading of NCERT: branching descent means that all existing life forms share common ancestors, which lived at different periods of the earth’s history (page 113), so lines of descent have split again and again. Natural selection is how each line changes, as those better fit in an environment leave more progeny (page 113). In short, branching descent tells which forms are related, and natural selection tells how each came to suit its habitat.

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Whose work on populations may have influenced Darwin?

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   Thomas Malthus (NCERT page 119).

2. Q2. What are the two key concepts of the Darwinian theory of evolution?

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   Branching descent and natural selection (NCERT page 118).

3. Q3. Why must there be a genetic basis for natural selection?

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   Fitness is based on characteristics which are inherited, so there must be a genetic basis for getting selected and to evolve; adaptive ability is inherited (NCERT page 118). Beyond the NCERT text: selection can sort any differences, but only heritable ones change the next generation; acquired differences alone need not persist in descendants.