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The Mechanisms of Evolution

This overview of evolutionary mechanisms highlights how life on Earth has changed over time, informed by fossil records and modern observations. It discusses the contribution of Charles Darwin, focusing on natural selection, adaptation, and genetic variations within populations. Key concepts such as biological evolution, the role of different environments, and the Hardy-Weinberg equilibrium are explored. Understanding these mechanisms is crucial for comprehending how species evolve, adapt, and survive in their changing habitats.

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The Mechanisms of Evolution

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  1. The Mechanisms of Evolution

  2. Life’s History: Seen in Fossils & Relics • Broad patterns • Change through time • communities change • habitats change • Each of Earth’s biotas replaced a similar, but distinct biota

  3. Modern Life: Seen by Direct Observation • Modern life exhibits characteristic patterns • species are variable • in the short term • species are stable • environments are stable

  4. Biological Evolution • encompasses the changes in Earth’s biotas • detected in remnants of the changes found in the modern biota

  5. The Contribution of Charles Darwin • Darwin proposed a mechanism by which evolution may have occurred • based on observations in South America • SA flora & fauna differed from European • temperate SA forms resembled tropical SA forms more than temperate European forms

  6. The Contribution of Charles Darwin • Darwin proposed a mechanism by which evolution may have occurred • based on observations in South America, especially the Galápagos Islands • ~600 miles west of Ecuador • each with dramatically different conditions and communities

  7. Darwin’s map Darwin’s Travels Figure 23.1

  8. The Contribution of Charles Darwin • Darwin observed stable, variable populations • each possessed adaptations (n) to its environment • Darwin imagined the different island populations came from a founding population • populations underwent adaptation (v) and now thrive under different conditions

  9. The Contribution of Charles Darwin • In 1859 Darwin proposed a mechanism by which adaptation may have occurred • species change over time (are not immutable) • some changes enable species to more effectively inhabit their environments • adaptive changes occur by natural selection

  10. The Concept of Natural Selection • artificial selection of domesticated species mimics natural selection • artificial selection • breeders retain desirable individuals and remove undesirable individuals

  11. Artificial Selection - PracticalFigure 23.4

  12. Artificial Selection - HobbyFigure 23.2

  13. Artificial Selection - ExperimentalFigure 23.5

  14. The Concept of Natural Selection • artificial selection of domesticated species mimics natural selection • natural selection occurs • when some individuals produce more offspring than other individuals • because more individuals are produced than the environment can support • those best-suited to the conditions survive & reproduce, others don’t

  15. The Concept of Natural Selection • natural selection is a conservative process • in a stable environment, “average” individuals will survive and reproduce • in a changing environment changes, those best-suited to new conditions will survive and reproduce

  16. The Contribution of Charles Darwin • Darwin did not know the mechanisms of heredity

  17. Evolution: change in the genetic composition of a population over time • evolution is population-based • phenotypic variation in a population is due to genotypic differences in individuals • evolution results from differential success of individuals with different heritable phenotypes

  18. Evolution: change in the genetic composition of a population over time • at one genetic locus • an individual has two alleles • a population may have many alleles • the sum of all alleles for all loci in a population is its gene pool

  19. a population’s gene pool for the X locus:Figure 23.3

  20. a population’s gene pool for the X locus:Figure 23.3allele frequenciesX1 = 0.2X2 = 0.5X3 = 0.3

  21. a population’s gene pool for the X locus:Figure 23.3genotypefrequenciesX1X1 = 0.1 X1X2 = 0.1 X1X3 = 0.1 X2X2 = 0.3 X2X3 = 0.3 X3X3 = 0.1

  22. Evolution: change in the genetic composition of a population over time • a population’s genetic structure • allele frequencies • genotype frequencies

  23. Evolution: change in the genetic composition of a population over time • a genotype’s or phenotype’srelative contribution to the next generation = fitness • depends on thesurvivaland reproductive success of individuals with it

  24. the mathematics of population genetics • for a population with only two alleles, A & a, at a locus • the frequency of allele A is p and • the frequency of a is q = 1- p • allele frequencies can be calculated from genotype frequencies p = (2NAA + NAa)/2N and q = (2Naa + NAa)/2N

  25. the mathematics of population genetics • equal allele frequencies do not imply equal genotype frequencie • Figure 23.6

  26. the mathematics of population genetics • undisrupted, a population’s genetic structure remains the same over time

  27. undisrupted, a population’s genetic structure remains the same over timeFigure 23.7 p = 0.55 p = 0.55 q = 0.45 q = 0.45

  28. Hardy and Weinberg did the math • a population in Hardy-Weinberg equilibrium • has allele frequencies p & q • has genotype frequencies p2, q2 and 2pq and • succeeding generations will have the same genetic structure

  29. Hardy and Weinberg did the math • Hardy-Weinberg equilibrium requires • random mating • a large population size • no migration • negligible mutation • stabilizing natural selection

  30. Hardy-Weinberg agents of evolution • changes in a population’s genetic structure occur because of agents of evolution • mutation • spontaneous, random changes • usually detrimental or neutral • may be pre-adaptive • natural rates are very low • rates of accumulation vary

  31. Hardy-Weinberg agents of evolution • changes in a population’s genetic structure occur because of agents of evolution • gene flow • migration incorporates new alleles or changes allele frequencies • migration is typical among populations of the same species

  32. bottlenecks shrink populations abruptlyFigure 23.8

  33. Prairie Chicken -millions to <hundredFigure 23.9

  34. Hardy-Weinberg agents of evolution • changes in a population’s genetic structure occur because of agents of evolution • random genetic drift: • chance events that alter allele frequencies • most likely in small populations • bottlenecks shrink populations abruptly • the founder effect occurs when a small sub-population is displaced

  35. founder effect occurs when a smallsub-population isdisplacedFigure 23.10

  36. Hardy-Weinberg agents of evolution • changes in a population’s genetic structure occur because of agents of evolution • random genetic drift: • chance events that alter allele frequencies • bottlenecks & the founder effect produce low allelic variation compared to the parent population

  37. Hardy-Weinberg agents of evolution • changes in a population’s genetic structure occur because of agents of evolution • assortative mating • one genotype prefers another genotype • results in changed genotype frequencies

  38. Assortative Mating in PrimulaFigure 23.11

  39. Hardy and Weinberg did the math • a population in Hardy-Weinberg equilibrium • has allele frequencies p & q • has genotype frequencies p2, q2 and 2pq and • succeeding generations with have the same genetic structure IF…

  40. Hardy and Weinberg did the math • Hardy-Weinberg equilibrium requires • random mating • a large population size • no migration • negligible mutation • stabilizing natural selection

  41. Hardy-Weinberg agents of evolution • changes in a population’s genetic structure occur because of agents of evolution • natural selection • enhanced reproductive success by individuals with particular genotypes • may lead to a change in allele frequency • leads to adaptation (v.)

  42. Natural Selection May Have Different Effects Under Different CircumstancesFigure 23.12

  43. Figure 23.13

  44. See page 460

  45. Figure 23.14

  46. two food sources that differ significantly in hardness produce a bimodal distribution in beak sizesFigure 23.15

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