A species is a group of individuals that interbreed and produce viable offspring. Species are made up of populations , which are localized groups of individuals. Fitness of an individual is defined as the relative number of offspring that survive to reproductive age compared to other individuals in a species. Neo-Darwinism includes the idea that an organism can also increase its fitness by increasing the survival of the offspring of related individuals. Evolution occurs by natural selection, gene flow, and genetic drift.
Gene flow is the movement of alleles in or out of a population.
Result of immigration or emigration or, in plants, the movement of sperm via pollen from one population to another. Gene flow often counteracts natural selection. Genetic drift refers to changes in gene frequency that occur by chance alone. Genetic drift is most important in small populations, where alleles are more likely to be lost by chance or to have not been represented in the first place.