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Genes in Populations

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A population is a group of organisms that are members of the same species and that live in the same geographical area. While natural selection occurs at the level of the individual, through variations in fitness among individuals, evolution occurs at the population level. The reason for this is demonstrated in the following table:




Life Span

One generation

Many generations

Genetic Characterization


Allele frequencies

Genetic Variability





Can change over time

The genotype of an individual is, for the most part, determined at the moment of fertilization and cannot be changed during that individual's lifetime. However, because populations are comprised of many individuals from different generations, the allele frequencies in a population can change over time; if so, evolution may be occurring. Population geneticists focus their studies in two areas. The first area is the measurement of genetic variation within a population. Morphological characters (e.g., variations in length, weight, coloration) and molecular characters (e.g., variations in nucleotide sequences of DNA or in amino acid sequences of proteins) are examined and quantified. The second area is the examination of the mechanisms by which genetic variation changes over space and time.

The evolutionary forces that can influence allele frequencies will be examined here. In this tutorial we will examine how allele frequencies are used to make predictions about the genotypes of a population.

Table Reference: After Table 20-1 in Strickberger, M.W. 1996. Evolution. Second Edition. Jones and Barlett, Sudbury, Massachusetts.