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In the previous lesson, you explored how natural selection drives evolutionary change and how speciation occurs when populations become isolated. This lesson takes the next step: it examines the raw material that makes evolution possible (mutation and sexual reproduction), explores the different ways populations can become reproductively isolated, and then zooms out to the global scale to understand why biodiversity is concentrated in certain parts of the planet.

Sources of genetic diversity

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3.1.10 Mutation and sexual reproduction increase genetic diversity.

Mutation generates new variants of genes; sexual reproduction generates new combinations of genes.

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Evolution by natural selection requires variation within a population. But where does that variation come from in the first place? There are two sources: mutation and sexual reproduction. They work in different but complementary ways.

Mutation: creating new variants

A mutation is a random change in the base sequence of DNA. When a mutation occurs, it creates a new allele: a different version of a gene. Most mutations are neutral or harmful, but occasionally a mutation produces an allele that gives an individual an advantage in its environment. This new variant can then be acted upon by natural selection.

Mutation is the only source of entirely new genetic information. Without mutation, there would be no new alleles, no new traits, and ultimately no evolution. Every piece of genetic variation in every population on Earth can be traced back to a mutation event.

Case study: Chernobyl tree frogs

![Extremes of the colour gradient of the Eastern San Antonio frog (Hyla orientalis). On the left, a specimen captured in Chernobyl inside the high contamination zone; on the right, a specimen captured outside the Exclusion Zone. [Source

Extremes of the colour gradient of the Eastern San Antonio frog (Hyla orientalis). On the left, a specimen captured in Chernobyl inside the high contamination zone; on the right, a specimen captured outside the Exclusion Zone. [[Source

The eastern tree frog (Hyla orientalis) provides a striking contemporary example of mutation and natural selection working together. Research published in 2022 found that tree frogs living within the Chernobyl Exclusion Zone in Ukraine have significantly darker skin than those in uncontaminated areas nearby.

The likely explanation is that genetic variation in melanin production already existed within the frog population, having arisen through earlier mutations. The nuclear disaster in 1986 created an environment with elevated radiation. Because melanin can help protect cells from radiation damage, frogs carrying alleles for darker pigmentation had a survival advantage. Over the roughly 14 generations since the disaster, darker frogs survived and reproduced more successfully, shifting the population towards darker colouration. This illustrates how mutation first creates new variants, and natural selection then increases the frequency of advantageous variants when the environment changes.

1. Distinguish between a gene and an allele.

2. Explain why mutation is described as the "ultimate source" of genetic variation.

3. In the Chernobyl tree frog example, identify the mutation, the selective pressure, and the resulting change in the population.

Sexual reproduction: creating new combinations

While mutation creates new alleles, sexual reproduction generates new combinations of existing alleles. In sexual reproduction, genetic material from two parents is combined during fertilisation, producing offspring that are genetically different from both parents and from each other. This is why siblings look similar but never identical (unless they are identical twins).

Sexual reproduction does not create new alleles; it shuffles the alleles that already exist into new arrangements. Think of it like a deck of cards: mutation adds new cards to the deck, while sexual reproduction shuffles the deck into a new hand each time.

Together, mutation and sexual reproduction maintain and increase the genetic diversity of a population. High genetic diversity is crucial for resilience: it means a population is more likely to contain individuals with traits suited to surviving environmental change.

1. Complete the table below:

1. Explain why a population that reproduces only asexually would have lower genetic diversity than one that reproduces sexually.

2. Explain why a population with high genetic diversity is more likely to survive a sudden environmental change than a population with low genetic diversity.

Reproductive isolation and speciation