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Transcript
• Natural selection: Differential reproduction by genetically diverse organisms. • The driving force in evolution, it leads to greater adaptation in of organisms to their environment. • If sub-populations are found in substantially different environments then selection can lead to genetic divergence and speciation. • Both genetic drift and natural selection tend to erode variation. • However, natural selection is non-random and creates order by increasing the adaptedness of the population. • There are three primary modes of selection: Directional, stabilizing and disruptive selection. • Directional selection: One extreme has higher fitness than the other and the mean phenotype shifts after selection • Example 1: The evolution of cryptic coloration in Biston betularia (Kettlewell 1959). • Example 2: A shift in bill size in Darwin’s medium ground finch Geospiza fortis Boag and Grant (1981). • Example 3: The evolution of pesticide resistance in insects. • 1) Human introduction of new selective pressures, can provide dramatic illustration of the power of selection: antibiotics, pesticides. • 2) Introduction of DDT brought a rapid evolutionary response: • a) Introduced in WWII by the 1946 there was resistance in houseflies. • b) In the Korean War used against body louse, it evolved resistance within a year. • c) By 1965, 165 species had evolved resistance. • Stabilizing or normalizing selection. • a) The extremes of a distribution are selected against, and the mean is at a selective advantage. • b) The most common form of selection. • c) There are trade-offs involved in most traits. The advantage that you get from some extreme of a characteristic is balanced by some negative effects. Size is a good example • Stabilizing selection Example 1: Human birth weights. Mean birth weights have the highest survival. Selection Against Both Homozygotes • Stabilizing selection Example 2: Selection on gall size in the goldenrod ball gall-inducer Eurosta solidaginis (Weis et al. 1992). • a) Larvae develop inside the galls and are attacked by several natural enemies. • Eurytoma gigantea has a short ovipositor and attacks small galls exerting selection for larger gall size . • Eurytoma obtusiventris attacks medium size galls. • Birds attack large galls, selecting for smaller gall size. • b) Selection differs at different times and location depending on the prevalence of different natural enemies. For example, if there are trees nearby there is more bird predation. When parasitoids are more prevalent they can produce directional selection. • c) The opposing selection forces create normalizing selection. • Disruptive selection: There is selection against the mean, and both extremes have higher fitness. The mean may not change, but the variance increases. • Disruptive selection in African swallowtail butterfly, Papilio dardanus (Shepard 1961). • a) The butterfly mimics several different inedible models. • b) There is a polymorphism in the banding pattern. • c) Intermediate phenotypes do not look like any model, and so they are selected against. • d) Genes for the polymorphism are close together on the chromosome forming a tight linkage group a “supergene”. • Disruptive selection in African swallowtail butterfly, Papilio dardanus (Shepard 1961). • a) The butterfly mimics several different inedible models. • b) There is a polymorphism in the banding pattern. • c) Intermediate phenotypes do not look like any model, and so they are selected against. • d) Genes for the polymorphism are close together on the chromosome forming a tight linkage group a “supergene”. In the case of a trait that is determined by a single gene with two alleles, there are five combinations of genotypes that nature can select: 1. either homozygote (AA or aa but not both) 2. both homozygotes (AA and aa) 3. either homozygote and the heterozygote (AA and Aa or aa and Aa) 4. the heterozygote (Aa) 5. all alleles (AA, Aa, and aa) Selection against one of the homozygotes (aa) Expected offspring genotypes Possible parent mating patterns AA AA X AA 4 AA X Aa 2 2 Aa X AA 2 2 Aa X Aa 1 2 1 Total 9 ( 56% ) 6 ( 38% ) 1 ( 6% ) Aa aa . Selection against both homozygotes (AA and aa) Expected offspring genotypes Possible parent mating patterns AA Aa aa Aa X Aa 1 2 1 Total 0 ( 0% ) 2 ( 100% ) 0 ( 0% ) ." Selection against the heterozygote and one of the homozygotes (Aa and aa) Expected offspring genotypes Possible parent mating patterns AA AA X AA 4 Total 4 ( 100% ) Aa aa 0 ( 0% ) 0 ( 0% ) There are at least 5,000 genetically inherited human abnormalities and diseases. Apparently, many, if not most, of them are caused by recessive alleles. Selection against the heterozygotes (Aa) Expected offspring genotypes Possible parent mating patterns AA AA X AA 4 Aa AA X aa 4 aa X AA 4 aa X aa Total aa 4 4 ( 25% ) 8 ( 50% ) 4 ( 25% ) Selection against the heterozygotes (Aa) Expected offspring genotypes Possible parent mating patterns AA AA X AA 4 Aa AA X aa (4) aa X AA (4) aa X aa Total aa 4 4 ( 50% ) 0 4 ( 50% )