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Evolution of Populations

Explore the evolution of populations through genetic variation, mutations, recombination, and natural selection. Learn about single-gene traits, polygenic traits, and mechanisms like gene flow, genetic drift, and sexual selection. Understand the Hardy-Weinberg Equilibrium and speciation through isolation, and discover patterns in evolution.

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Evolution of Populations

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  1. Evolution of Populations Chapter 11

  2. Genetic Variation Within Populations • Variation and gene pools • Gene pool – all genes present in a population • Allele Frequency – the number of times an allele occurs in the gene pool, compared to other alleles

  3. Genetic Variation Within Populations • In genetic terms, evolution is any change in the relative frequency of alleles in a population.

  4. Genetic Variation Within Populations • Scientists now recognize two main sources of genetic variation • Mutations • Recombination

  5. Genetic Variation Within Populations • Mutations • Any change in a sequence of DNA • Occurs Because: • Problems with replication • Exposure to radiation or chemicals

  6. Genetic Variation Within Populations • Recombination • Parents alleles rearrange when forming gametes • The 23 pairs of chromosomes can produce 8.4 million different combinations!

  7. Genetic Variation Within Populations • Crossing Over • Occurs during meiosis • Further increases genetic variation

  8. Natural Selection In Populations • Normal distribution • Highest frequency in middle and lowest at the extremes • Also representative for allele freqencies

  9. Natural Selection In Populations • Microevolution • Observable changes in allelic frequencies • Occurs in a single population • 3 ways • Directional • Stabilizing • Disruptive

  10. Natural Selection In Populations • Directional Selection • Shift in phenotype to one extreme • Mean also shifts

  11. Natural Selection In Populations • Stabilizing Selection • Norm is selected for and becomes more common • Ex. Gall Fly and Goldenrod

  12. Natural Selection In Populations • Disruptive Selection • Selection for the extremes • Ex. Peppered moths

  13. Natural Selection In Populations • Single-gene and polygenic traits • Widows peak is a single gene trait – a single gene with two alleles • Polygenic Traits are controlled by two or more genes • This means that a polygenic trait can have many possible genotypes ant thus phenotypes.

  14. 16-2 Evolution as Genetic Change • Natural Selection on Single-Gene Traits

  15. Other Mechanisms of Evolution • Gene Flow • Some animals move once they are able • When the move into new population, their alleles become part of gene pool • Occurs with movement between populations • Increases variation • If less movement, the more different two species become

  16. Other Mechanisms of Evolution • Genetic Drift – Random change in allele frequency • Bottleneck Effect • Founder effect – when allele frequencies change because of migration

  17. Other Mechanisms of Evolution • Sexual Selection • Females the important factor in reproduction • This makes females “picky” • Intrasexual selection • Intersexual selection

  18. Hardy-Weinberg Equilibrium • Evolution vs. Genetic Equilibrium • Hardy-Weinberg principle – genotype frequencies will stay constant unless some factor changes the frequency • Genetic Equilibrium – when allele frequencies remain constant

  19. Hardy-Weinberg Equilibrium • Five Conditions to Maintain Equilibrium • Random Mating • Large Population • No Movement into or out of the population • No Mutations • No Natural Selection

  20. Hardy-Weinberg Equilibrium • Biologists can use an equation for comparison purposes p2+ 2pq + q2 = 1 p = dominant alleles q = recessive alleles

  21. Hardy-Weinberg Equilibrium • Five factors leading to evolution • Genetic drift • Gene flow • Sexual selection • Natural selection • Mutations

  22. Speciation Through Isolation • Reproductive Isolation – members of populations can no longer mate • Mules or Hinnies • Speciation - when two species come from one existing species

  23. Speciation Through Isolation • Behavioral Isolation • Two populations will not breed because of differences in courtship • Fireflies

  24. Speciation Through Isolation • Geographical Isolation • Rivers, mountains, or bodies of water separate two populations • Snapping Shrimp and placental vs. marsupial mammals

  25. Speciation Through Isolation • Temporal Isolation • Different species mate at different times • Times of day • Times of year • Trees and plants

  26. Patterns in Evolution • Convergent Evolution – evolution towards similar characteristics in unrelated species • Analogous structures • Bird and bat wings

  27. Patterns in Evolution • Divergent Evolution – closely related species in different directions • Results from adapting to different environments • Kit and Red Fox

  28. Patterns in Evolution • Coevolution – two or more species evolve in response to changes in each other • Benefit to both species • Plants and bees

  29. Patterns in Evolution • Coevolution can also drive predator-prey relationships • Cheetahs and gazelles

  30. Patterns in Evolution • Extinctions – species are eliminated • Background extinctions – occur continously, but at very low rates • Mass extinctions – rare, more intense, involve many species

  31. Patterns in Evolution • Punctuated equilibrium – bursts of evolutionary activity followed by periods of inactivity • Adaptive radiation – diversification of multiple species from one ancestral species • Mammals

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