Evolution

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Evolution

  1. 1. 5b: Evolution Evolution
  2. 2. Darwin <ul><li>Darwin observed several things on his voyage on the Beagle: </li></ul><ul><ul><li>Geological oddities: marine shells in the mountains, for example </li></ul></ul><ul><ul><li>Fossils: differed from modern examples </li></ul></ul><ul><ul><li>Animals: similar to some in England but different as well- the Patagonian hare, for example </li></ul></ul>
  3. 3. Darwin <ul><li>Darwin’s observations: </li></ul><ul><ul><li>Geographic change in species: Variation due to environmental differences </li></ul></ul><ul><ul><ul><li>Galapagos finches:many species with different beaks, living on different islands, eating different things </li></ul></ul></ul><ul><ul><ul><li>Galapagos tortoises: One type of tortoise per island </li></ul></ul></ul><ul><ul><li>Adaptation : a feature that allows the organism to survive better in its environment </li></ul></ul>
  4. 4. Natural Selection <ul><li>Darwin’s observations led him to propose a method by which adaptations might arise: Natural Selection </li></ul><ul><li>The 4 steps, revisited: </li></ul><ul><ul><li>Heritable variations passed to offspring </li></ul></ul><ul><ul><li>More offspring produced than environment can support </li></ul></ul><ul><ul><li>Favorable traits in some offspring lead to those having a higher survival rate </li></ul></ul><ul><ul><li>Over time, more and more of the population possesses the favorable traits </li></ul></ul>
  5. 5. Natural Selection <ul><li>IMPORTANT: </li></ul><ul><ul><li>Variation arises from RANDOM changes that happen to arise from genetic mutations-- there is no directedness or ability to predict future needs </li></ul></ul><ul><ul><li>Some mutations are detrimental, and some are neutral </li></ul></ul>
  6. 6. Natural Selection <ul><li>IMPORTANT: </li></ul><ul><ul><li>Natural selection is ongoing continuously because the environment is constantly changing </li></ul></ul>
  7. 7. Natural Selection <ul><li>Because resources are always limited, some individuals will fail to survive and reproduce-- thus removing their genes from the population </li></ul><ul><ul><li>Failing to reproduce is functionally the same as failing to survive </li></ul></ul>
  8. 8. Natural Selection <ul><li>Fitness varies among individuals </li></ul><ul><ul><li>It is the reproductive success of one individual compared to others in the population </li></ul></ul><ul><ul><li>More fit animal: </li></ul></ul><ul><ul><ul><li>Uses more resources </li></ul></ul></ul><ul><ul><ul><li>Avoids death </li></ul></ul></ul><ul><ul><ul><li>Leaves more offspring </li></ul></ul></ul>
  9. 9. Natural vs. Artificial Selection <ul><li>Humans practice artificial selection on domestic animals and plants </li></ul><ul><ul><li>Individuals with desired traits are bred, while those with undesired traits are not </li></ul></ul><ul><ul><ul><li>Eventually, offspring change to resemble the predetermined idea the humans had </li></ul></ul></ul><ul><ul><ul><li>Ex. Any domestic animal, vegetables </li></ul></ul></ul>
  10. 10. Natural vs. Artificial Selection <ul><li>In artificial selection, the result is predesired, and the breeding is directed </li></ul><ul><li>In natural selection, the result is determined by the environment with no direction or desire involved </li></ul>
  11. 11. Natural Selection- Terms <ul><li>Selection pressure: What acts on an animal to either increase or decrease its chances to survive and/ or reproduce </li></ul><ul><ul><li>Selected for: the trait conveys advantage </li></ul></ul><ul><ul><li>Selected against: the trait conveys disadvantage </li></ul></ul>
  12. 12. Adaptations <ul><li>Adaptations may take many generations to evolve </li></ul><ul><li>Explain why animals are suited to their environment and ‘lifestyle’ </li></ul><ul><li>The more ‘ adaptive ’ a trait is, the greater the advantage that the individual has </li></ul>
  13. 13. Convergent Evolution <ul><li>Convergent Evolution : when unrelated species share similar characteristics </li></ul><ul><ul><li>Ex: Flippers on manatees, penguins and sea turtles </li></ul></ul><ul><ul><li>Ex: The horny toad of the US, and the thorny devil of Australia </li></ul></ul>
  14. 14. Wallace <ul><li>Important to realize that Darwin was not the only guy to come up with Natural Selection- Alfred Wallace came to the same conclusions studying different organisms, and they first published at the same time </li></ul>
  15. 15. Evidence for Evolution <ul><li>Fossils: </li></ul><ul><ul><li>Fossils frequently show the pattern of a succession of species from simple to more complex, though this is not universal </li></ul></ul><ul><ul><li>Transitional fossils: Archaeopteryx </li></ul></ul><ul><ul><ul><li>Whales: see p. 226 </li></ul></ul></ul>
  16. 16. Evidence for Evolution <ul><li>Biogeography: the study of the distribution of organisms </li></ul><ul><ul><li>Different mix of organisms when geography separates areas </li></ul></ul><ul><ul><li>Ex: Cacti and Spurges in deserts </li></ul></ul><ul><ul><li>Ex: Marsupials in Australia </li></ul></ul><ul><ul><ul><li>Americas and Australia connected at one time, but marsupials able to evolve separately from placental mammals in Australia </li></ul></ul></ul>
  17. 17. Evidence for Evolution <ul><li>Anatomy: </li></ul><ul><ul><li>Vestigial structures: fully developed in one group, but reduced and nonfunctional in others </li></ul></ul><ul><ul><ul><li>Ex: Appendix, tailbone in humans </li></ul></ul></ul><ul><ul><ul><li>Ex: Hipbones in snakes </li></ul></ul></ul><ul><ul><ul><li>Ex: Wings in flightless birds </li></ul></ul></ul><ul><ul><li>These occur because organisms inherit structures from their ancestors </li></ul></ul>
  18. 18. Evidence for Evolution <ul><li>Anatomy: </li></ul><ul><ul><li>Homologous structures: Anatomically similar structures, frequently used for different purposes, explained by a common ancestor </li></ul></ul><ul><ul><ul><li>Ex: Horse legs vs. bird wings. vs. bat wings vs. whale flippers vs. human arms </li></ul></ul></ul>
  19. 19. Homologous Structures <ul><li>Example from humans: </li></ul><ul><ul><li>Pharyngeal pouches- seen in all vertebrate embryos </li></ul></ul><ul><ul><ul><li>Develop into gills in fish and amphibians </li></ul></ul></ul><ul><ul><ul><li>Develop into tonsils, inner ear canal, thymus and glands in humans </li></ul></ul></ul><ul><ul><li>It is easier (therefore more likely to arise randomly) to modify an existing structure than create an entirely new one </li></ul></ul>
  20. 20. Evidence for Evolution <ul><li>Molecular Evidence: </li></ul><ul><ul><li>All living organisms use the same biological molecules-- DNA, RNA, ATP, etc. </li></ul></ul><ul><ul><li>DNA code is the same for all living organisms </li></ul></ul><ul><ul><li>Many of the same genes have been modified to result in the wide variety we see </li></ul></ul><ul><ul><li>Gene sequences are more different the further apart organisms are evolutionarily </li></ul></ul>
  21. 21. Evolution on a Small Scale <ul><li>Remember: individual organisms can not evolve, populations/ species evolve </li></ul><ul><li>Microevolution : small measurable changes in a population from generation to generation </li></ul>
  22. 22. Hardy-Weinberg Equilibrium <ul><li>Hardy-Weinberg Equilibrium allows us to measure small changes in the frequency of alleles in a population </li></ul><ul><ul><li>This is how we know microevolution is occurring </li></ul></ul><ul><ul><li>Population genetics is the study of the occurrence and flow of genes in populations </li></ul></ul><ul><ul><li>We will look at the example of peppered moths </li></ul></ul>
  23. 23. Peppered Moths <ul><li>Peppered moths can be either light or dark, and this is controlled by a single set of alleles: </li></ul><ul><ul><ul><li>D= dark color </li></ul></ul></ul><ul><ul><ul><li>d= light color </li></ul></ul></ul><ul><ul><li>We know the frequency of genotypes in the population: </li></ul></ul><ul><ul><ul><li>4% DD </li></ul></ul></ul><ul><ul><ul><li>32% Dd </li></ul></ul></ul><ul><ul><ul><li>64% dd </li></ul></ul></ul>
  24. 24. Peppered Moths <ul><li>From genotypes, we can figure out the frequency of each allele in the population: </li></ul><ul><li>.04 + .16 =.2 D </li></ul><ul><li>.16 + .64 =.8 d </li></ul><ul><li>Frequency of each gamete type will be the same as the frequency of occurrence of each allele </li></ul><ul><li>We can use a Punnett square to figure out the gene frequencies in the next generation </li></ul>
  25. 25. Peppered Moths ** This is a punnett square for freqencies, not individuals .2D .8d .2D .04DD .16Dd .8d .16Dd .64dd
  26. 26. Peppered Moths <ul><li>Notice: the allele frequencies in the next generation are EXACTLY THE SAME </li></ul><ul><li>This means that sexual reproduction alone can not change the frequency of gene/ allele frequencies in a population, provided some assumptions are met </li></ul><ul><ul><li>Notice the dominant allele does not increase in frequency </li></ul></ul><ul><ul><li>This is Hardy-Weinberg equilibrium </li></ul></ul>
  27. 27. The Assumptions <ul><li>No mutations: allelic changes do not occur </li></ul><ul><li>No gene flow into or out of the population </li></ul><ul><li>Random mating </li></ul><ul><li>No genetic drift- large population, changes in frequency due to chance are insignificant </li></ul><ul><li>No selection: one genotype is not favored over another </li></ul>
  28. 28. HW Equilibrium <ul><li>However, those assumptions are basically never met </li></ul><ul><li>Changes in allele frequency do occur, and we can see those by comparing RL to the numbers predicted by HW equilibrium </li></ul><ul><li>Also, we know that deviation from the assumptions is what causes evolution </li></ul>
  29. 29. Back to the moths <ul><li>After the industrial revolution, peppered moths that were light did not blend into trees with lots of soot on their bark, and they were eaten by birds </li></ul><ul><li>The frequency of D then increased in the population to around 80% </li></ul>
  30. 30. Causes of microevolution <ul><li>Genetic mutations </li></ul><ul><ul><li>The ultimate source for allele differences </li></ul></ul><ul><ul><li>Mutations can be harmful in one environment and helpful in another </li></ul></ul>
  31. 31. Causes of microevolution <ul><li>Gene flow: </li></ul><ul><ul><li>Movement of alleles among populations by migrating animals </li></ul></ul><ul><ul><li>Can increase variation in a population by introducing new mutations </li></ul></ul><ul><ul><li>Can prevent speciation by making the gene pool the same across populations </li></ul></ul>
  32. 32. Causes of microevolution <ul><li>Nonrandom mating: </li></ul><ul><ul><li>Assortive mating: individuals mate with those that are similar </li></ul></ul><ul><ul><ul><li>Ex. tall people mating with each other </li></ul></ul></ul><ul><ul><ul><li>Causes two groups of homozygotes to become more common, and hets to become less common </li></ul></ul></ul><ul><ul><li>Sexual selection: Favors characteristics that increase the chance of mating </li></ul></ul><ul><ul><ul><li>Ex. Male birds with bright colors or crazy feathers </li></ul></ul></ul><ul><ul><ul><li>Animals that compete for mates </li></ul></ul></ul>
  33. 33. Causes of microevolution <ul><li>Genetic drift: changes due to chance </li></ul><ul><ul><li>Allele frequencies ‘drift’ over time </li></ul></ul><ul><ul><li>More common in small populations, like on islands or other isolated areas </li></ul></ul><ul><ul><li>Can result in the loss of rare alleles completely, and fixation of others as they are the only ones left </li></ul></ul>
  34. 34. Causes of microevolution <ul><li>Bottleneck Effect: Catastrophic loss of most of a population, only a few individuals survive by chance </li></ul><ul><ul><li>Those few are all that is left to pass on to later generations </li></ul></ul><ul><ul><ul><li>Ex. cheetas and poor sperm </li></ul></ul></ul><ul><li>Founder Effect: Rare alleles are more common in a population isolated from the main population </li></ul><ul><ul><li>Only a few individuals founded the new population, so their alleles are the ones that are represented </li></ul></ul><ul><ul><ul><li>Ex. Amish have higher rate of two limb mutations </li></ul></ul></ul>
  35. 35. Causes: Natural Selection <ul><li>There are several types of selection, so it gets its own category </li></ul><ul><li>Directional Selection : an extreme phenotype is more successful and population shifts in that direction </li></ul><ul><ul><li>Ex. the peppered moths </li></ul></ul><ul><ul><li>Ex. drug resistance in bacteria </li></ul></ul><ul><ul><li>Ex. Anacondas </li></ul></ul>
  36. 36. Natural Selection <ul><li>Stabilizing Selection : an intermediate phenotype is best, extremes are selected against </li></ul><ul><ul><li>Ex. male anacondas </li></ul></ul><ul><li>Disruptive Selection : Two or more extremes are favored over the intermediate </li></ul><ul><ul><li>Favors polymorphism: the occurrence of different forms in the same species </li></ul></ul><ul><ul><li>Ex. Snail with two different shell colorings, in two different habitats </li></ul></ul>
  37. 37. Perfection? <ul><li>NO!! Animals do NOT tend to be perfectly adapted </li></ul><ul><ul><li>Evolution does not start from scratch, it is modifying what is already there </li></ul></ul><ul><ul><li>Compromises: there may be costs to the adaptive benefit </li></ul></ul><ul><ul><li>Sexual selection may not result in adaptive traits- giant feathers may be energetically expensive and make it difficult to fly, for example </li></ul></ul>
  38. 38. Maintenance of Variation <ul><li>Populations will always show variation </li></ul><ul><li>New mutations always arising </li></ul><ul><li>Gene flow may be occurring </li></ul><ul><li>Diploidy and heterozygotes: </li></ul><ul><ul><li>Only alleles that are expressed in phenotype can feel selective pressures </li></ul></ul><ul><ul><li>Heterozygous animals can protect alleles that might otherwise be selected against </li></ul></ul><ul><ul><ul><li>The homozygous recessive will still occur occasionally </li></ul></ul></ul>
  39. 39. Example of Sickle Cells <ul><li>Individuals with sickle cell are homozygous for S and typically die very young due to change in shape of RBCs </li></ul><ul><li>Individuals that are heterozygous are OK, because cells are normal shape until in low O2 environment </li></ul><ul><li>Individuals that are homozygous A are usually the most fit (most of us fall into this group) </li></ul>
  40. 40. Sickle Cell <ul><li>However, individuals with African descent have higher frequency of S- more sickle cell disease, but also: </li></ul><ul><li>Heterozygote is immune to malaria </li></ul><ul><li>So, in parts of Africa w/ malaria, heterozygote is selected for, but homozygotes continue to exist to do statistical frequency of each genotype in the next generation </li></ul>

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