Biology Lecture 13: Genetic Variation
Clicker Q: Most genetic disorders are associated with recessive alleles. Why?
A. In general, recessive alleles tend to be rare.
B. By definition, recessive alleles reduce an individual’s fitness.
C. Selection eventually weeds out all copies of a harmful dominant allele and a harmful
recessive allele can hide within a population of heterozygotes.
Dominance status of an allele describes its effect on the phenotype of heterozygotes… NOT
whether the allele is helpful, harmful or neutral
Dominance affects evolution rate and outcome:
o If a harmful allele is dominant, it eventually is weeded out through selection.
o If a harmful allele is recessive, selection is not able to weed out every recessive allele,
and it stays within the population.
o If a beneficial allele is dominant it increases in frequency very quickly.
o If a beneficial allele is recessive it increases in frequency, but much slower.
Other constraints on selection:
o By genetic correlations among traits (Darwin’s finches and their beak depth, which
correlates with body size, beak length, and more)
o A particular trait may be associated with a physical advantage in some instances but
disfavors in another (Example: male peacocks have flashy tail/feathers which are more
impressing to hens, but also more conspicuous to predators)
o Time – in some cases the environment may change more rapidly than evolution can
actually occur. Population will evolve to apply for conditions that may have already
changed. (Example: timing for migration, food sources, etc.)
o Available genetic variation (Example: cheetahs inbreed so they are individually similar to
each other, thus they can’t evolve well in response to environmental change)
Why is genetic variation important:
o Total amount of genetic variation affects the evolutionary potential
o Inbreeding decreases fitness in individuals because they are not genetically diverse
o It affects individual and