Variation and Evolution

Continuous variation is controlled by two or more genes. A characteristic within a population showing a gradiation from one extreme to the other is showing continuous variation. Eg, height. Best shown in a line graph.

Dicontinuous variation is usually controlled by one gene. An example of this is sex--male OR female. Best shown in a bar graph.

Variation can be heritable or environmental.

Environmental variation affects the phenotype without affecting the genotype, e.g. dying your hair--you have changed your phenotype but your genotype still codes for your original hair colour. Another example is height, as your genotype may code for a long body but environmental factors may cause stunted growth.

Heritable variation is caused by mutations in the germ line and the crossing over/random assortment that occurs during meiosis. It is further produced by the fusion of genetically varied gametes during cross-fertilisation. Mutations are the biggest driver of long-term genetic change within a population.

Competition and Selection

All populations have the capacity to expand in number but rarely do so; this is due to environmental factors coming into play to reduce the population.

Organisms compete for limited resources. Interspecific competition is competition between members of different species for territory and food, etc. Intraspecific competition is competition between members of the same species for mates, food, territory etc. Intraspecific competition drives the evolutionary development of a species as only the fittest of the species survives to reproduce.

Selection pressures are environmental pressures that drive selection in a population. For example, in…