Genetic drift, gene flow, mutation and the making of species

Chance in a small population: genetic drift

Think of each generation as a sample. The genes of the next generation are drawn from the gene copies of the parents. In a large population the sample is large and its allele frequency is very close to the old one. In a small population the sample is small, and chance can change the frequency by a large amount.

This random change in allele frequency is genetic drift. It has no direction and it has nothing to do with fitness. In a small population, an allele can drift to a frequency of 0, and then it is lost. It can also drift to 1. Then it is fixed, which means that every copy of the gene is that allele.

The three graphs come from a computer model of populations that start with the allele frequency $0.5$. The small populations wander and end at 0 or 1. The large ones barely move.

The founder effect

Sometimes a small group of individuals leaves a large population and starts a new one, for example on an island. The founders carry only a sample of the alleles of the source population. The frequencies in the sample may differ greatly from those of the source. As the new population grows, it keeps the frequencies of its founders.

NCERT says that sometimes the change in allele frequency is so different in the new sample that the new population becomes a different species. The original drifted group is called the founders, and the effect is called the founder effect.

A rare allele may be missing from the founders altogether, or it may be much more common than before. Use the slider to see how likely the first case is.

You should see that a rare allele is often missing when the group is small, and that the chance of this falls fast when more founders come.

A bottleneck is a similar thing. A disaster leaves a few survivors of a large population, which is again a small random sample. NCERT does not use this word, but A-Level and IB do.

Gene flow

Individuals may move from one population to another and breed there. This is gene flow, or gene migration. The alleles that they bring change the allele frequencies of the receiving population. If the same alleles are shared by two populations through migration, their frequencies change in both, and the populations become more alike.

Suppose a fraction $m$ of the breeders on an island each generation are migrants from the mainland. The frequency of $A$ on the island is $p$ and on the mainland $p_m$. The new frequency on the island is

If migration goes on, $p$ moves toward $p_m$ and the island and the mainland become alike.

Mutation and recombination

Mutation is a sudden change in the DNA. It is the only source of completely new alleles, so without it there would be nothing to select. In one generation, it changes the allele frequencies very little, since the rate is low. Over a long time it supplies the raw material of evolution.

Genetic recombination shuffles alleles into new combinations, by crossing over and by the independent assortment of the chromosomes. It makes new genotypes in every generation. NCERT counts it with the factors of the last lesson.

Darwin thought that the variations that matter are small and directional, so that evolution is gradual. Hugo de Vries, who worked on the evening primrose, said that mutations are random and directionless, and large enough to make a new species in a single step. This single-step change is saltation.

The modern synthetic theory

The modern synthetic theory, also called neo-Darwinism, joined Darwin's natural selection with Mendel's genetics, mutation and the study of populations. NEET names it in the syllabus, but NCERT does not use the name. Its main parts are these.

• Variation: produced by mutation and recombination.
• Natural selection: acting on the variation.
• Genetic drift and gene flow: which change allele frequencies without regard to fitness.
• Reproductive isolation: which keeps the gene pools of new species apart.

How a new species forms

This part is not in the NEET syllabus, but A-Level and IB ask it.

A species is a group of natural populations that breed with each other and are reproductively isolated from other such groups. New species arise by speciation.

• Allopatric speciation: a barrier, such as a river, a mountain or the sea, splits a population. The two parts evolve separately by selection, drift and mutation. After a long time they are so different that they cannot interbreed, even if they meet again.
• Sympatric speciation: a new species forms inside the range of its parent species, without a geographical barrier. A common case in plants is a change in the number of chromosome sets, called polyploidy.

The founder effect and drift can start the process in a small new population.

Try it in Python

The program makes 40 populations of each of three sizes, starts each at an allele frequency of 0.5, runs them for 50 generations and counts how many lose or fix the allele. It uses a fixed seed, so the numbers are the same each time.

Practise with new numbers

Common mistakes

• Saying that drift favours the fittest. Drift is random and has nothing to do with fitness.
• Thinking that drift matters equally in large and small populations. The effect is large in small populations.
• Mixing up gene flow and the founder effect. Gene flow is a steady movement of alleles between populations. The founder effect is a single event, a small group that starts a new population.
• Saying that mutation changes the allele frequency quickly. It is slow. Its importance is that it makes new alleles.
• Calling Darwin's variations and de Vries' mutations the same. Darwin: small and directional. De Vries: large, random, directionless.

Remember these

• Genetic drift: random change in allele frequency, strongest in small populations. It can lead to loss or fixation of an allele.
• Founder effect: a small group starts a new population and its allele frequencies may differ from the source.
• Gene flow moves alleles between populations: $p' = (1 - m)p + mp_m$.
• Mutation makes new alleles. Recombination makes new combinations. De Vries: mutation theory and saltation.
• Speciation, in A-Level and IB: allopatric (barrier) and sympatric (no barrier).

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why does genetic drift matter more in a population of 20 than in a population of 20000?

2. Eight birds start a new population on an island. What is this effect called, and why can its allele frequencies differ from the mainland?

3. On an island, $p = 0.4$. Every generation, 25 per cent of the breeders are migrants from the mainland, where $p_m = 0.8$. What is $p$ after one generation?

4. Say what de Vries and Darwin each thought about the size of the variations that drive evolution.

5. Which of the five factors is the only source of new alleles?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 6.
• Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapters 18.2 and 19.3.