0.1 Why are allele frequencies maintained across generations when

In some respects, understanding how agents of evolution like natural selection, sexual selection and genetic drift drive changes in allele frequencies is easier than understanding why in their absence allele frequencies remain unaltered from one generation to the next . Understanding genetic equilibrium , however, is incredibly important as it forms the foundation of population genetics. It is the null hypothesis postulating the absence of evolutionary forces and, thus, against which the possibility of evolution is assessed. Work through the material below to increase your understanding of mechanism responsible for genetic equilibrium.

What is happening genetically when all individuals are equally likely to survive and reproduce?

Clearly for alleles to be perpetuated in a population through time, they must be passed from parent to offspring via reproduction. There is no other way (in the absence of continuous immigration)!

Thus, genetically speaking, when all individuals in a sexually reproducing population have an equal chance of surviving to reproduce and of producing surviving offspring:

• each individual has an equal chance of contributing one of the two required gametes to every fertilization event and thus, to the next generation.

A simple way to visualize this is depicted below. Each individual in this population holds two buckets of gametes. The buckets represent the two types of gametes the individual produces in equal numbers based on its genotype for a single locus. For example, Individual 1 with genotype Aa will produce equal numbers of A and a allele-containing gametes. In contrast, Individual 2’s two buckets contain equal numbers of A allele-containing gametes reflecting its AA genotype. If you are unsure why we expect 50% of an individual’s gametes to contain one of two alleles for a given locus and 50% the other allele for that locus, please review meiosis.

When no agents of evolution are acting on this population, each individual and therefore each bucket, because they contain equal quantities of gametes, has an equal likelihood (probability) of donating one of the two necessary gametes to a successful fertilization event and thus, to the next generation. Taken as a whole, this population of 10 individuals offers 20 buckets of gametes from which the two gametes for fertilization could possibly come. From which buckets the two gametes actually come depends upon which two individuals end up mating by chance and which of their two alleles the successful gamete contains.

To test your understanding, consider the questions below: