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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.
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:
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:
Imagine a situation in which a key nutrient the local bird population needs to build egg shells thick enough to withstand the weight of a parent during incubation occurs in very low levels. In this environment, the eggs of birds with aa genotypes crack twice as often during incubation as the eggs of AA and Aa individuals. A cracked shell always results in chick death.
Would this situation, and if so how, affect the likelihood that the alleles appearing in the next generation come from the buckets of aa , AA and AA individuals? Please explain.
When incubation success varies with genotype as described above, alleles from the 'buckets' of AA and Aa would be twice as likely as alleles from the 'buckets' of aa individuals to make it into the next generation.
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