Evolution, speciation and scientific validationEdexcel A-Level Biology A: Revision notes

A population is a group of interbreeding individuals of one species in an area. Its gene pool is all of the alleles of all of its genes. Allele frequency is how common an allele is in the gene pool.

Evolution is a change in allele frequency in a population over generations. Individuals do not evolve; populations do. Evolution can come about through gene mutation and natural selection.

A gene mutation is a change in the base sequence of a gene. It is random and can produce a new allele, which may be harmful, neutral or advantageous in the environment.

Natural selection:

1. There is variation in the population because of different alleles.
2. A change in the environment creates a selection pressure, such as a predator, disease or antibiotic.
3. Individuals with an advantageous allele are more likely to survive and reproduce.
4. They pass the allele to their offspring.
5. Over generations the allele frequency increases.

Examples: antibiotic resistance in bacteria, where a resistance allele that arose by mutation becomes common when antibiotics are used, and dark peppered moths on soot-darkened trees, where camouflage reduces predation by birds.

New evidence only becomes part of accepted theory when the scientific community has checked it.

• Scientific journals publish reports of research so that others can read and use it.
• Peer review: before publication, independent experts in the field check that the methods, data, analysis and conclusions are valid. This reduces bias and error.
• Conferences: scientists present findings by talks and posters, answer questions and receive feedback, and can discuss ideas and collaborate. Results are challenged and may be repeated by others.

Conclusions are accepted when results are repeated and supported by other independent research.

Genomics compares the base sequences of DNA (genomes) of different species. Proteomics compares the amino acid sequences of proteins, such as haemoglobin or cytochrome c.

The fewer the differences, the more recently two species shared a common ancestor. This supports the theory of evolution because the pattern of differences matches the relationships from fossils and anatomy. These data are validated by being published, peer reviewed and repeated by other laboratories.

Worked example: if species A and B differ by 1 amino acid in a protein and A and C differ by 14, A and B share a more recent common ancestor.

Gene flow is the transfer of alleles between populations by interbreeding. When populations are isolated, gene flow is reduced or stopped, so each population changes separately:

• different random mutations arise
• different selection pressures favour different alleles
• allele frequencies diverge over many generations.

If the populations become unable to interbreed and produce fertile offspring, they are different species. This is reproductive isolation.

Allopatric speciation: populations are separated by a geographical barrier, such as the sea, a mountain range or a river. There is no gene flow, so different mutations and selection pressures cause divergence until the populations cannot interbreed.

Sympatric speciation: populations live in the same area, but gene flow is reduced by other types of isolation, such as differences in breeding time, courtship behaviour, food or habitat choice, or a change in chromosome number (common in plants). Isolation is often incomplete at first.

In both, speciation requires reproductive isolation followed by genetic divergence.