BI SC 002 Lecture Notes - Lecture 35: Natural Selection, Sickle-Cell Disease, Birth Weight
Document Summary
Directional selection: results from selection against one extreme phenotype, example: Disruptive selection: two extreme phenotypes each confer a selective advantage over an intermediate phenotype, example: Medium pigment: more likely to be found by predators. Light colored shell: easier to camouflage. Stabilizing selection: maintains an intermediate expression of a trait by selecting against extreme variants, example: Small or large baby: less likely to survive than intermediate sized babies. Harmful alleles can be maintained in a population. Sickle cell disease and malaria: sickle cell allele is common where malaria is prevalent. Heterozygotes for the sickle cell allele are most likely to survive to reproductive age. Natural selection selects for adaptations that maximize reproductive success. But: there are other key factors that are involved in shaping the evolution of species. Change in the dna sequence of an organism. Can be: beneficial, harmful, make no difference. Raw material for evolution: genes contribute to phenotypes.
