1. Natural Selection
To make a contribution to the gene pool, an organism must not only survive to maturity, it must also reproduce.
Genetic variation can occur by the following
- Independent segregation of chromosomes at meiosis This mixes the genes into new combinations.
- Crossingover New Combinations of genes arise, as a piece of one chromosome exchanges with its homologous pair.
- Mutations Give new variations of genes for evolution to work on.
Agents that change gene frequenceies
Every population has a large range of phenotypes, and these usually fall into a normal bell shaped pattern of distribution. The selective forces such as predators, competition, disease, lack of food water climatic forces, etc., act on the phenotypes in the following ways:
- Stabilising selection- favours the avarage over the extremes
- Directional selection - favours one extreme over the average or the other extreme
- Disruptive selection- favours both extremes over the average
2. Genetic Drift
This is a change in allele frequencies of a population as a result of chance processes. It occurs in small populations and with alleles not greatly affected by selection.
When the beetles reproduced, just by random luck more brown genes than green genes ended up in the offspring. In the diagram at right, brown genes occur slightly more frequently in the offspring (29%) than in the parent generation (25%).
3. Founder Effect
The term "founder effect" refers to the observation that when a small group of individuals breaks off from a larger population and establishes a new population.
4. Mutation
It is the ultimate source of variation that can change equilibrium.
Some “green genes” randomly mutated to “brown genes” (although since any particular mutation is rare, this process alone cannot account for a big change in allele frequency over one generation).
5. Gene Migration
When immigrants arrive from another population possessing a different gene pool, new alleles are introduced.
Some beetles with brown genes immigrated from another population, or some beetles carrying green genes emigrated.
