Variation

The Differences between organisms of the same species.

Discontinuous

Continuous

For some characteristics there is only a small number of possibilities.

Controlled by single genes- monogenic.

For some characteristics there are many possible values

controlled by many genes- polygenic

Competition

Inter-specific competition occurs between individuals of different species competing for the same resources.

Intra-specific competition occurs between individuals of the same species.

Selection Pressure

An environmental factor that can alter the frequency of alleles in a population, when it is limiting.

selective agents

Predation

Overcrowding allows disease to spread

Day length affects reproductive behaviour

Availability of nesting site

Temperature affects survival

Human impact

Some animals raise young in very specific situations

When a population grows, predators have more food and their numbers increase.

some are better camouflaged

Mimic species are harmless but have other species that are toxic to the predator

habitat loss has destroyed breeding grounds

Population genetics

Gene pool

All the alleles present in a population at a given time

Allele frequency

The frequency of an allele is its proportion, fraction or percentage of all the alleles of that gene in a gene pool

Genetic drift

chance variations in allele frequencies in a population

If a sexually reproducing population is in a stable environment and there is no mutation and no immigration or emigration, the frequencies of all alleles stays constant.

Hardy-Weinberg principle

States that in ideal conditions, allele and genotype frequencies in a population are constant from generation to generation.

p + q = 1

p² + 2pq + q² = 1

Natural selection

The increased chance of survival and reproduction of organisms with phenotypes suited to their environment enhancing the transfer of favourable alleles from one generation to the next