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How Populations Evolve

How Populations Evolve. Charles Darwin : Theory of Evolution by Natural Selection. Evolution – Change over time of heritable characteristics. Selection is a major driving force for evolution Members of a population vary in their inherited traits

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How Populations Evolve

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  1. How Populations Evolve

  2. Charles Darwin : Theory of Evolution by Natural Selection

  3. Evolution – Change over time of heritable characteristics • Selection is a major driving force for evolution • Members of a population vary in their inherited traits • All species are capable of producing more offspring than the environment can support • Individuals whose inherited traits give them a higher probability of reproducing in a given environment leave more offspring • Unequal reproduction leads to accumulation of favorable traits in a population over generations (adaptation to the environment)

  4. Artificial Selection

  5. Natural Selection

  6. Fossil Record – Evidence for Evolution

  7. Tiktaalik – “fishapod”

  8. Archaeopteryx dino to bird transition

  9. Whale evolution

  10. Multiple Lines of Scientific evidence support evolution • Biogeography – closely related species live in close proximity. Especially apparent on islands. • Comparative Anatomy – closely related species share common anatomical features (homology). • Molecular Biology – genetic code is shared across all life. Closely related species have more similar genetic sequences.

  11. Homology across mammal forearms

  12. Vestigial Structures

  13. Phylogeny – evolutionary tree Lungfishes Amphibians 1 Tetrapods Mammals 2 Tetrapod limbs Amniotes Lizards and snakes 3 Amnion 4 Crocodiles 5 Ostriches Birds 6 Feathers Hawks and other birds

  14. Evolution of populations • A population is a group of interbreeding individuals of the same species that live in the same region. • Have variation of traits within the population due to mutation and sexual reproduction. • Populations evolve by changes in allele frequencies.

  15. Mutations create new alleles, very rarely are these new mutations beneficial Sexual reproduction results in reshuffling of allele combinations

  16. Hardy-Weinberg Equilibrium • A population whose allele frequencies do not change (evolve) is in HWE

  17. Hardy-Weinberg Equations p + q = 1 p2 + 2pq + q2 = 1

  18. Assumptions of HWE • Very large population size • No gene flow between populations(no migration) • No mutations • Random mating between individuals • No selection

  19. Genetic Drift • Random change in allele frequency due to sampling effect • Stronger in smaller populations

  20. Natural Selection • Selection results in individuals that are adapted to the environment to reproduce at a higher rate than those that are not. • Over generations the frequency of alleles that are adaptive increase, while those that are maladaptive decrease.

  21. Sexual Selection leads to differences between sexes

  22. Balancing Selection maintains variation

  23. Natural Selection does not produce perfect organisms • Selection can only act on existing variation • Evolution is limited by historical constraints (what was adapted in the past) • Adaptations are often compromises • Drift, selection, and environment interact to produce allele frequencies

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