Question

Describe the four evolutionary mechanisms that alter the frequencies of existing genetic variation in natural populations. You need to define each mechanism and describe at least one example of its action in natural populations (e.g., genetic drift in overhunted populations resulting in less genetic diversity).

Answer 1

Evolution in a population may occur due to four fundamental forces of evolution: Natural Selection, Genetic Drift, Mutations and Gene Flow.

1. Genetic drift

• Genetic drift is a mechanism of evolution in which allele frequencies of a population change over generations due to chance (sampling error).

• Genetic drift occurs in all populations of non-infinite size, but its effects are strongest in small populations.

• Genetic drift may result in the loss of some alleles

• example: A couple with brown and blue eyes have children with brown or blue eyes. Even if there is a 50% chance of having blue eyes, brown eyes being the dominant allele, all the children might have brown eyes in the future generations as a matter of chance.

2. Natural selection:

Natural selection is the process in nature by which organisms better adapted to their environment tend to survive and reproduce more than those less adapted to their environment. For example, treefrogs are sometimes eaten by snakes and birds.

3. Mutations:

A mutation is a change that occurs in our DNA sequence, either due to mistakes when the DNA is copied or as the result of environmental factors such as UV light and cigarette smoke.

There are three types of DNA Mutations: base substitutions, deletions and insertions.

example: sickle cell anemia is caused by a substitution in the beta-hemoglobin gene, which alters a single amino acid in the protein produced.

4. Gene flow:

gene flow (also known as gene migration or allele flow) is the transfer of genetic variation from one population to another.  Gene flow is expected to be lower in species that have low dispersal or mobility, that occur in fragmented habitats, where there is long distances between populations, and when there are small population sizes.

For example, many island populations have low rates of gene flow due to geographic isolation and small population sizes. The Black Footed Rock Wallaby has several inbred populations that live on various islands off the coast of Australia. The population is so strongly isolated that lack of gene flow has led to high rates of inbreeding.