A population is a cluster of individuals of the same species who share a common geographical area and find their mates more often in their own cluster than in others. It is in populations that we can observe and measure the changes in allele frequencies over time (evolution).
Allele frequencies change over time because of mutation, gene flow, genetic drift, and natural selection. However, phenotypic changes can arise due to epigenetic and developmental processes in addition to these four processes of evolution.
The genotype does not generally interact directly with the environment. The phenotype interacts with the environment, and the genotype is affected by the phenotype's success.
Mutation is the only way novel genetic combinations are introduced.
Gene flow is the process by which allele frequencies are changed, largely through migration and nonrandom mating.
Genetic drift is the process by which random events affect the frequencies of alleles from generation to generation. Founder effects are frequent cases of genetic drift. Drift is most noticeable in small populations.
Natural selection can be seen as the filtering of phenotypes (and their genotypes) by the environment, resulting in an overrepresentation of better fit phenotypes (and their associated genotypes) within a population in a given environment over time. The better fit phenotypic variants are considered to be best adapted to the environment. The traits they carry that help them do well in the environment are called adaptations.
Speciation is the process by which new species arise. There are several different definitions for species, but most see the species as a viable evolutionary unit.
A subspecies is a population that is geographically circumscribed and genetically differentiated and that represents a distinct evolutionary lineage within a species.
