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7.4 Population Genetics

Keywords

English Term 中文翻译 Definition & Explanation
Microevolution 微进化 A change in allele frequencies in a population over generations.
Genetic Drift 遗传漂变 A process in which chance events cause unpredictable fluctuations in allele frequencies from one generation to the next, most pronounced in small populations.
Bottleneck Effect 瓶颈效应 Genetic drift that occurs when the size of a population is reduced, as by a natural disaster or human actions.
Founder Effect 奠基者效应 Genetic drift that occurs when a few individuals become isolated from a larger population and form a new population.
Gene Flow 基因流 The transfer of alleles into or out of a population due to the movement of fertile individuals or their gametes.

1. Evolution Beyond Natural Selection

In the previous sections, we saw how natural selection drives evolution by favoring advantageous phenotypes. However, natural selection is not the only driver of evolution. Evolution is also driven by random occurrences.

Evolution, at its most fundamental level (microevolution), is simply a change in the allele frequencies of a population over time.

  1. Mutation: As we learned, mutation is a completely random process. It creates genetic variation, which provides the raw phenotypes on which natural selection acts.
  2. Genetic Drift: Evolution can happen purely by chance.
  3. Gene Flow: Evolution can happen through migration.

2. Genetic Drift: The Power of Randomness

Genetic drift is a nonselective process. It is simply statistical "luck." It describes how allele frequencies fluctuate unpredictably from one generation to the next.

Crucial Rule of Genetic Drift

Genetic drift affects all populations, but small populations are vastly more susceptible to random environmental impact than large populations. In a tiny population, a random event (like a tree falling and squashing three bugs) can completely wipe out a rare allele.

There are two classic examples of genetic drift:

A. The Bottleneck Effect

A sudden change in the environment, such as a fire or flood, may drastically reduce the size of a population. The few survivors that pass through this "bottleneck" may have a gene pool that no longer reflects the original population's gene pool—certain alleles may be overrepresented, and others completely lost, purely by chance.

B. The Founder Effect

When a few individuals become isolated from a larger population, this smaller group may establish a new population. The gene pool of this new "founder" population will differ from the source population. (For example, a bird blowing a few seeds to an isolated island).

(Placeholder: A classic diagram showing a bottle full of red, blue, and yellow marbles. When poured, only a few red and blue marbles make it through the narrow neck to form the surviving population. The yellow allele is lost forever by random chance.)

3. Gene Flow (Migration)

Gene flow is the movement of alleles between populations. When individuals migrate into a new population and mate, they introduce new alleles.

  • If two populations constantly exchange members, they will mix their gene pools, reducing the genetic differences between them.
  • Conversely, a reduction of gene flow and genetic variation within a given population can increase the differences between isolated populations of the same species, ultimately setting the stage for them to evolve into completely different species.

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