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UNC-Chapel Hill BIOL 101 - Hardy- Weinberg Theory

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Chapter 23 How Populations EvolveIn evolutionary theory, what evolves and changes is not an individual, but a species, which is a population of interbreeding individuals.The Hardy-Weinberg equilibrium is important to the study of evolution, because it provides a baseline from which evolutionary departures takes place● It tells us what will happen in a non-evolving population● It provides a reference point to compare the frequencies of genes and genotype of natural populations whose gene pools may be changingFor Hardy-Weinberg equilibrium to be maintained, five conditions have to be met1. very large populations size2. isolation from other populations (no migrations in or out)3. no mutationsp2+2pq+q2=1The gene in a population can be presented by the above equationp= dominant alleleq= recessive allelep2= homozygous dominantq2= homozygous recessive2pq= heterozygous (you can have Aa or aA)p + q = 1A population of plants has 500 individuals. Red is dominant over white. There are 20 hite plants (aa). So 480 are red. Out of the 480 red plants, 320 are homozygous dominant (AA) and 160 are heterozygous (Aa).● There are 1000 genes in the populations (500 plants x 2 genes each)● The dominant gene (A) accounts for 800 genes (320 x 2 + 160 genes)● Number of recessive genes would be 200 (160+40 genes)● The frequency of the (A) gene is 80% or 0.8 (800/1000)● The frequency of the (a) gene is 20% or 0.2 (200/1000)● This works 0.8 + 0.2 = 1● (0.8)2+2(0.8)(0.2)+(0.2)2=1Problem1 out of 10,000 babies has PKU (recessive) populationq2= 1/10,000 = 0.0001q = √0.0001 = 0.01p + q = 1 so to find the dominant allele (p). 1 - 0.01 = 0.99Heterozygous would be 2pq = 0.0198 or about 2%.The Hardy-Weinberg Theory shows how Mendel’s Theory of inheritance supports Darwin’s Theory of Natural SelectionNatural Selection requires genetic variationCauses of MicroevolutionA. Genetic Drift: Changes in the gene pool of a small population due to change1. Bottleneck Effect: Results from drastic reduction in population size. This can be due to natural disasters. This will reduce genetic variability since some genes may be absent.2. Founder Effect: When a few individuals colonize a new area genetic drift can occur since the gene pool is small. Eventually, the population might become large enough to allow genetic variability. The founder effect can cause the high frequency of certain genes in a populationsa. some Amish communities have high rate of six finger traitb. Retinitis pigmentosa (blindness) in the island nation of Tristan. The island was colonized by fifteen people in 1814, one of which was a carrier of the gene.B. Gene Flow: The migration of fertile individuals between populations. Genes can be gained or lost over time.C. Non-random Mating: This could increase the number of homozygotes in a population1. Inbreeding: When individuals of a populations mate with close neighbors rather than more distant members of a population. Homozygous recessive phenotypes can increase. D. Natural Selection: The differential success in reproduction that can cause change in the gene poolHow Natural Selection Can Occur1. Heterozygous preserve recessive genes2. Crossbreeding produces hybrid vigor ()loss of harmful recessive genes)3. Stabilizing Selection: This selection favors intermediate variants by selecting against extreme phenotypesa. Human birth weight averages 7 to 9 pounds. Much smaller or larger birth weight babies have higher death rates.4. Directional Selection: Favors rare variants of one extreme. This is most common when individuals migrate environmental changea. The fossil record show that the size of European black bears increased after periods of glaciation, only to increase during warmer periods.5. Sexual Dimorphism: Distinction between males and females. Males have the advantage to attraction of mates.Species: a group of organisms that can interbreed in nature and produce viable offspring, but cannot successfully interbreed with members of other populations. The species concept gives biodiversity on earth.Hybrids can be produced in zoos that would otherwise never occur in nature.How Species Remain Distinct1. Habitat Isolations: Geography can keep species apart and distinct2. Temporal Isolations: Some species can live in the same area, but reproduce at different time of the year3. Behavioral Isolation: Signals and behaviors that are used to attract mates can create reproductive barriers among closely related species that have overlapping habitats.4. Mechanical Isolation: Anatomical incompatibility may prevent sperm transfer when closely related species attempt to mate5. Gametic Isolation: Gametes of different species that meet rarely fuse to form an embryo6. Hybrid inviability: Sometimes different species do mate, but the hybrid offspring die.7. Hybrid Sterility: When two species mate and produce viable offspring, but the offspring are often


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UNC-Chapel Hill BIOL 101 - Hardy- Weinberg Theory

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