Evolution

Biology A2 · Selection and evolution. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Evolution is the formation of new species from earlier species over time. It happens because the gene pool of a population, meaning all the alleles of all its genes, changes from one generation to the next.

A new species forms when two populations stop exchanging alleles. Mutation, selection and drift then change each gene pool separately. In the end the two groups can no longer breed together to give fertile offspring. This is speciation.

DNA shows how closely species are related. Mutations build up with time, so two species with few base differences in the same gene shared a common ancestor recently. Many differences mean they separated long ago.

Rules to remember

Gene pool: all the alleles of all the genes in a population.
Evolution: a change in the allele frequencies of a gene pool over many generations, leading to new species.
Mutation is the original source of new alleles.
Allopatric speciation: populations are separated by a geographical barrier such as a sea, river or mountain range.
Sympatric speciation: populations live in the same area but are separated by ecology (food, habitat, breeding time) or by behaviour (courtship, song).
Fewer base differences in the same gene = a more recent common ancestor.
Speciation is complete when the two groups cannot interbreed to produce fertile offspring.

Common mistake

Students write that a geographical barrier itself makes a new species. The barrier only stops gene flow; different mutations and different selection pressures must then change the two gene pools.

Worked example

A large lake dries into two small lakes. After many thousands of years the lakes join again, but the fish from the two lakes do not breed with each other. Name the type of speciation and explain it.

Dry land between the two lakes was a geographical barrier, so no alleles passed between the two fish populations.
Different mutations arose and conditions differed, so selection and drift changed each gene pool in its own way.
The populations became so different that they cannot interbreed, so they are separate species.

Answer: Allopatric speciation, caused by geographical separation.

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