Evolutionary forces

What’s Darwin got to do with it?

Charles Darwin saw that individuals with attributes that made them more successful at living and reproducing tended to leave more offspring. His insight was that if these attributes were inherited, they would become more common in the population.

Darwin didn’t know that inheritance depends on genes, but otherwise his ideas about natural selection were right. Other processes that can affect gene frequency include migration, or even chance events.

The Hardy-Weinberg principle

The Hardy–Weinberg principle (p2+2pq+q2=1)

This graph shows the Hardy–Weinberg proportions for two different alleles (A and a) at a single gene locus. Take a closer look at the graph. What are the frequencies of the different genotypes if the allele frequency for p is 0.7 and q is 0.3? (Hint: look at the dotted line.)

Using the equation above, we can expect 0.49 of the population to be AA, 0.42 of the population to be Aa and 0.09 of the population to be aa. Don’t forget – this principle only holds if no evolutionary forces are at work. Use it as standard for comparing changes in populations.

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Adapted from Johnuniq/Wikipedia

The HardyWeinberg principle states that the gene pool in a population will remain constant unless evolutionary forces are at work. Evolution is therefore the change in the frequency of genes (or their alleles) over time.

Deer mice are a good example: a gene variant for light-coloured fur has spread in a Nebraskan population of deer mice since a glacier deposited light-coloured sand on their habitat around 10,000 years ago. Lighter fur provided better camouflage, so those mice with it survived to pass on their genes.

Genetic variation is important as a means for future adaptation to changes in the environment. Small, isolated populations have less variation, and inbreeding can lead to what is called a genetic bottleneck effect. For example, scientists are currently concerned about the lack of genetic variability in endangered bearded vulture populations.

In human populations inbreeding has had some unusual results. The Fugate family of Kentucky became famous for its blue skin, caused by the chance marriage of two individuals with the same rare gene variant, which was then passed on to the next generation through marriages within the family.

Downloadable resources

About this resource

This resource was first published in ‘Populations’ in June 2014.

Topic:
Genetics and genomics
Issue:
Populations
Education levels:
16–19, Continuing professional development