<p>can someone please explain balanced polymorphism?
i read cliffs, and didn't get that part, any input would be good :)</p>
<p>Balanced polymorphism is when natural selection maintains two or more phenotypic forms in a population. There are two things that cause balanced polymorphism: heterozygote advantage and frequency-dependent selection.</p>
<p>For heterozygote advantage, think of sickle cell disease. Homozygous recessive individuals for sickle cell don't do well because their blood is inefficient at carrying oxygen and homozygous dominant individuals are susceptible to malaria, so heterozygotes have the advantage. For example, some populations in Africa have a sickle-cell allele frequency of .2, which maintains the stable frequencies of .04 for homo. rec., .32 for hetero., and .64 for homo. dom. Natural selection favors this balanced polymorphism because the 32% of heterozygotes have such a resistance to malaria that it balances the harm of the 4% of the people who have sickle cell disease.</p>
<p>Frequency-dependent selection is best exemplified by predator-prey relationships. The more common prey is going to be most recognized by the predator, so natural selection favors greater phenotypic variation, which makes the prey less recognizable by the predator, leading to greater fitness. Consequently, a good deal of phenotypic forms are maintained by natural selection, which is exactly what balanced polymorphism is.</p>