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Evolutionary Processes

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BIOL 1500
Alexander Mills

TEXTBOOK NOTES 4 BIOLOGY 1001 – Pg. 81-88 & 223-224 Evolutionary Processes Inbreeding: Mating between relatives  Increases homozygosity  Does not cause evolution because allele frequencies do not change in the population  Changes genotype frequencies only Why Does Inbreeding Depression Occur  Inbreed depression: is a decline in average fitness that takes place when homozygosity increases and heterozygosity decreases in a population  Such a decline has been documented in a small population of wolves on an island in Lake Superior - The 20+ wolves that live on that island are highly inbred - 50% of them have congenital bone deformities  Inbreeding depression results from: - Many recessive alleles represent loss-of-function mutation - There are normally very few homozygous recessive individuals in a population - Most loss-of-function alleles exist in heterozygous individuals - The alleles have little to no effect when they occur in heterozygotes because one normal allele usually produces enough functional protein to support a normal phenotype - Loss of function mutations are deleterious or even lethal when they are homozygous  Another Cause: many genes involved in fighting disease are under intense selection for heterozygote advantage - Fitness declines if an individual is homozygous of this gene - Example of heterozygote advantage in humans is sickle cell anemia: autosomal recessive condition - Individuals with 2 dominant alleles for this have normal red blood cells while the red blood cells of individual with 2 recessive alleles become distorted and sickle shaped - ^These cells damage the capillaries and lead to complications in the kidney heat, and the brain - In Africa, where malaria parasite is prevalent, heterozygotes have a significant advantage over both types of homozygous individuals  Offspring of inbred mating are expected to have lower fitness than the offspring of outcrossed mating  Inbreeding does not change allele frequencies but can speed up the rate of evolutionary change - It increases the rate at which purifying selection eliminates recessive deleterious alleles  Deleterious recessives are rate in populations because they lower fitness - Rare alleles are usually found in heterozygotes because it is more likely for an individual to carry one copy of a rare allele than two  When no inbreeding is occurring, the recessive deleterious alleles found in heterozygotes cannot be eliminated by natural selection - When inbreeding occurs, more recessive deleterious alleles are found in homozygous and are quickly eliminated by selection Assortative Mating  Takes place when mating is non-random with respect to specific traits  In positive assortment, individuals tend to choose mates that share a particular phenotypic trait with them  In negative assortment, individuals tend to choose mates that differ in a specific phenotypic trait as them  Example of positive assortment: lesser snow goose - These geese occur in 2 colours: blue and white - Mating between blue and white geese take place much less than expected by chance - Geese choose mates that resemble their parents and their siblings - Only birds from mixed colour families choose mates that differ from them in colour - 43% of blue white geese are predicted if mating were random  Inbreeding and assortative mating do not cause evolutionary change but may change genotype frequencies Sexual Selection  Aspecial case of natural selection  Sexual selection occurs when individuals within a population differ in their ability to attract mates - It favours individuals with heritable traits that enhance their ability to obtain mates Theory: The fundamental asymmetry of sex  The Bateman-Trivers theory contains two elements: a claim about a pattern in the natural world and a process that causes the pattern - The pattern component of their theory is that sexual selection usually acts on males much more strongly than on females - ^As a result traits that attract member of the opposite sex are much more elaborated in males - Summarization of the theory: “Eggs are expensive but sperm are cheap” - ^ The energetic cost of creating a large egg is enormous whereas a sperm contains few energetic resources  In most species, females invest much more in their offspring than do males - ^ This phenomenon is called: fundamental asymmetry of sex which is the characteristics of almost all sexual species and has 2 important consequences: 1. Since females can only reproduce a few eggs in their lifetime, their fitness is limited not by the ability to find a mate but primarily by her ability to gain the resources needed to produce more eggs and healthier young 2. Amale’s fitness is limited not by the ability to acquire the resources needed to produce sperm but by the number of females he can mate with Predictions of the Bateman-Trivers Theory  If females invest a great deal in each offspring, then they should protect that investment by being choosy about their mates and if males incest little in each offspring, then they should be willing to mate with almost any female  If there are equal number of males and females in the population, and if males are trying to mate with any female possible, then mates will compete with each other for mates  ^ Thus, sexual selection should act more strongly on males than on females - ^ Meaning, that traits that evolve due to sexual selection should be found more in males Kermode, the Spirit Bear  Black and white phase Kermode bears differ by a single nucleotide in a gene that regulates pigment production - The population containing the mutant allele likely became isolated from the other black bear populations during periods of Pleistocene glaciation - The white phase mutation probably became established in a small isolated population due to the founder effect and genetic drift - Limited migration allowed gene flow to carry the w
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