Evolution is defined as the change in allele frequencies of a population. Darwin's Theory of Evolution has three postulates:
- (The struggle for existence) The ability of a population to expand is infinite, but the ability of any environment to support populations is always finite.
- (variation in fitness) Organisms within populations vary, and this variation affects the ability of individuals to survive and reproduce.
- (the inheritance of variation) this variation is transmitted from parents to offsprings.
Darwin observed that slight variations among individuals can significantly affect the chance that a given individual will survive and the number of offspring it will produce. Darwin called this differential reproductive success of individuals natural selection. It is likely that Darwin used this term because he was a pigeon breeder and familiar with artificial selection in the breeding of domesticated animals.Darwin clearly understood a fundamental principle of evolution—that populations, not individuals, evolve and become adapted to the environments in which they live. The term "adaptation" has two meanings in evolutionary biology.
- The first meaning refers to the processes by which adaptive traits are acquired.
- The second meaning refers to the traits that enhance the survival and reproductive success of their bearers.
When Darwin proposed his theory, he had no examples of selection operating in nature and knew nothing of the mechanisms of heredity.
The rediscovery of Gregor Mendel’s publications gave rise to the study of population genetics which provides a major underpinning for Darwin’s theories.
Population geneticists apply Mendel’s laws to entire populations.
Population geneticists study variation within and among species in order to understand the processes that result in evolutionary changes in species through time.
An evolutionary agent causes changes in the allele and genotype frequencies in a population. These are observed as a deviations from the Hardy–Weinberg equilibrium. There are four known evolutionary agents:
- Random genetic drift
- Nonrandom mating
- Natural Selection