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Mechanisms of Evolution and Effects on Populations

Mechanisms of Evolution and Effects on Populations. SBI 3UO Evolution – Section 9.1. 5 Factors that change allele frequency in populations. Mutations Gene flow (migration) Non-random mating Genetic Drift Natural Selection. Mutations. randomly introduces new alleles into the populations;

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Mechanisms of Evolution and Effects on Populations

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  1. Mechanisms of Evolution and Effects on Populations SBI 3UO Evolution – Section 9.1

  2. 5 Factors that change allele frequency in populations • Mutations • Gene flow (migration) • Non-random mating • Genetic Drift • Natural Selection

  3. Mutations • randomly introduces new alleles into the populations; • changes in allele frequency • Mutation in the DNA of an individual • Entire gene pool affected • Greater genetic diversity – selective advantage Ex. Norway rat resistant to poison Warfarin (some of the rats that were exposed to the poison survived and mated to produce resistant offspring)

  4. Gene Flow • Between 2 interbreeding populations that have different allele frequencies • Changes in allele frequency of both populations Ex. Grey wolf populations in North America

  5. Non-random Mating • Individuals in a population select mates, often on the basis of their physical and behavioural traits (phenotypes) or interbreed • Increases the proportion of homozygous individuals in the population, but does not affect the frequencies of alleles Ex. preferred phenotypes – caribou interbreeding - shar-pei dogs

  6. Genetic Drift • The change in frequencies of alleles due to chance events in a breeding population • More severe in small populations due to loss of certain alleles from generation to generation see Fig. 9.6, pg. 354

  7. Explanation: • BB individuals have big beaks, Bb individuals have medium-sized beaks, and bb individuals have small beaks • These birds live in a place where large and small seeds are abundant, but no medium-sized seeds are available. • Populations of all big-beaked individuals have a very high average fitness — they can crack open big seeds. • Populations smaller beaks are abundant — but not quite as much as the big-beaked individuals. • Medium-beaked individuals have the lowest fitness — they are not particularly good with either big or little seeds (and no medium-sized seeds are available

  8. Genetic Drift – Founder Effect • A change in the gene pool when a few individuals (founders) start a new isolated populations • Diversity in the gene pool is limited • Prevalent on islands like the Galapagos • Also found in human populations polydactilism in Amish families

  9. Genetic Drift – Bottleneck Effect • Changes in gene distribution that result from a rapid decrease in population size due to starvation, disease, natural disasters, etc. • Rapid decrease in population size • Gene pool – diversity loss • Seen in species that are close to extiction Ex. – northern elephant seals – 1890s overhunting reduced pop. to 20 individuals – low genetic diversity in the pop.

  10. Natural Selection • Selective pressures on populations – competition and predation • Some individuals have a higher likelyhood of survival and chance to reproduce • An allele gives a competitive advantage, thus that allele frequency will increase from generation • Causes changes in the allele frequencies of a population  evolutionary change

  11. Natural Selection

  12. Non-poisonous king snakes mimic poisonous coral snakes. Orchids fool wasps into “mating” with them. Katydids have camouflage to look like leaves.

  13. 3 types of Natural Selection • Stabilizing selection – favours intermediate phenotypes and acts against extreme variants • Directional selection – favours the phenotypes at one extreme over the others • Disruptive (diversifying) selection – favours the extremes of a range of phenotypes rather than intermediate phenotypes; can result in the elimination of the intermediate phenotypes See Fig. 9.9, pg. 356

  14. Sexual selection • Natural selection for mating, based in general on competition between males and choices made by females • Sexual dimorphism – difference between physical characteristics of males and females in animal species • Video http://www.pbs.org/wnet/nature/episodes/what-females-want/podcast-meet-the-mating-ritual-experts/832/ • Video: What Males Will Do - Spider Courtship Dance http://www.pbs.org/wnet/nature/episodes/what-males-will-do/video-spider-courtship-dance/956/

  15. Homework • Read pgs. 350-358, answer questions 1-11 pg. 359 • Read this article://www.news.wisc.edu/11476 • Go to the site below to read about a study on finches in the Galapagos http://www.pbs.org/wgbh/evolution/library/01/6/l_016_01.html

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