Darwin theorized that it was the process of natural selection that drove the process of evolution


Darwin theorized that it was the process of natural selection that drove the process of evolution. (Darwin had no knowledge of genetic material at the time) An organism that had characteristics that gave it reproductive success in its environment would pass its traits (genes) on over time. Beneficial changes in characteristics as a result of genetic mutation accumulate to create new species. Other reasons may affect evolutionary change >> genetic fluctuations in small populations mating opportunities are nonrandom such as in animal husbandry or sexual selection (what is attractive to the opposite sex, beauty-proportionality-symmetry) migration transfers alleles from one population to another Genetic Drift - changes in environment result in decrease in a large fraction and by chance eliminating an allele this will not occur as easily in a large healthy unthreatened population Founder Effect - when a small number of individuals break away from a population and establish a new closed population the allele frequency of the new pop. may not be the same as the old pop. Japan, Quebec, , many finches on the Galapagos islands show founder effects Mutations Harmful mutations occur often but are selected against and these alleles become rare Beneficial Mutations are rare but they are selected for and accumulate in the gene pool
Types of Selection Pressures Imagine the coevolution that occurs between hummingbirds and flowers they each benefit from nectar feeding behaviors. Imagine now that how the flower structure itself will act to stabilize, direct or disrupt the allelic frequency of beak length.
Stabilizing > Directional > Disruptive >

Sexual selection Sexual selection favors traits that enhance the mating success of an individual. One such general trait is sexual dimorphism. This is readily apparent in the fiddler crab who's giant pincer claw is a sexual enhancement to mating. In many species of birds it is the quality of a males song that song that tells the female something of his sexual vigour and health (all meaning good genes!!). Often male birds posses elaborate feather structures that are an obvious impediment to survival. Many marine pinnipeds (seals) display sexually selected traits such as size and aggressive behavior in males. The male that can commandeer the limited breeding ground areas on remote islands will have the greatest fitness.