Chapter 23 How Populations Evolve In 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 changing For Hardy Weinberg equilibrium to be maintained five conditions have to be met 1 very large populations size 2 isolation from other populations no migrations in or out 3 no mutations p2 2pq q2 1 The gene in a population can be presented by the above equation p dominant allele q recessive allele 2 p homozygous dominant q2 homozygous recessive 2pq heterozygous you can have Aa or aA p q 1 A 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 1 Problem 1 out of 10 000 babies has PKU recessive population q2 1 10 000 0 0001 q 0 0001 0 01 p q 1 so to find the dominant allele p 1 0 01 0 99 Heterozygous 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 Selection Natural Selection requires genetic variation Causes of Microevolution A Genetic Drift Changes in the gene pool of a small population due to change 1 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 populations a some Amish communities have high rate of six finger trait b 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 population 1 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 pool How Natural Selection Can Occur 1 Heterozygous preserve recessive genes 2 Crossbreeding produces hybrid vigor loss of harmful recessive genes 3 Stabilizing Selection This selection favors intermediate variants by selecting against extreme phenotypes a 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 change a 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 Distinct 1 Habitat Isolations Geography can keep species apart and distinct 2 Temporal Isolations Some species can live in the same area but reproduce at different time of the year 3 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 mate 5 Gametic Isolation Gametes of different species that meet rarely fuse to form an embryo 6 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 sterile 8
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