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Chapter 17 Evolution of Populations

Chapter 17 Evolution of Populations. 17.1 Genes and Variation. Genetics joins evolutionary theory. Recall that a population is a group of individuals of the same species that interbreed They share a common group of genes called a gene pool.

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Chapter 17 Evolution of Populations

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  1. Chapter 17Evolution of Populations

  2. 17.1 Genes and Variation

  3. Genetics joins evolutionary theory • Recall that a population is a group of individuals of the same species that interbreed • They share a common group of genes called a gene pool

  4. Gene pool- all the genes, including all the different alleles for each gene, that are present in a population at any one time

  5. Allele frequency - the number of times an allele occurs in a gene pool compared with the number of times other alleles occur • often expressed as a percentage • Has nothing to do with an allele being dominant or recessive • Example: In the mouse population the allele frequency of fur: B (dominant black fur)- 20% b (recessive brown fur)- 80%

  6. Sources of Genetic variation • Three main sources of genetic variation are: 1. Mutations 2. Genetic recombination from sexual reproduction • Genes are shuffled in crossing-over during meiosis 3. Lateral gene transfer • Some organisms like bacteria swap genes among one another

  7. Single Gene and Polygenetic Traits • The number of phenotypes produced for a given trait depend on how many genes control the trait • Single gene trait- controlled by a single gene with 2 alleles • Leads to only 2 distinct phenotypes • Example: widow’s peak • Polygenetic traits- controlled by two or more genes, each with 2 or more alleles • Leads to many possible genotypes and phenotypes • Example: height

  8. Data from a single gene trait can be displayed on a bar graph

  9. Data from polygenetic trait can be displayed on a curve graph • The graph of the relative frequency of polygenetic traits is often a symmetrical bell-shaped curve • A bell shaped curve is called normal distribution

  10. 17.2Evolution as a Genetic Change in Populations

  11. Evolution is any change over time in the allele frequency in a population • Natural selection on single gene traits can lead to changes in allele frequencies = changes in phenotype frequencies • Example: lizard color (pg (488)

  12. Natural selection on polygenetic can lead to three types of selection by affecting the fitness of phenotypes • Example: finches’ beak size (pg 489) • Three types of selection: • Directional • Stabilizing • Disruptive

  13. Remember that in an average population, phenotypes usually fit a bell curve called normal distrubution

  14. Directional Selection • Occurs when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end • Causes the range of phenotypes to shift as some individuals fail to survive and reproduce while others succeed

  15. Directional Selection Example • Lets say there is a food shortage on the Galapagos Islands • The amount of small and medium sized seeds runs low • Which finch would be the most fit in this habitat based on beak size? • What would happen to the beak size phenotypes over time if the food supply stays the same? http://wps.prenhall.com/wps/media/objects/487/499541/CDA22_2/CDA22_2b/CDA22_2b.htm

  16. Stabilizing Selection • Occurs when individuals near the center of the curve have a higher fitness than individuals at either end of the curve • This situation keeps the center of the curve the same but makes the graph more narrow

  17. Stabilizing Selection Example • Lets say there is a population of lizards whose size determines their fitness • Large lizards are easily seen by predators like birds flying above • Small lizards are too slow to escape from land predators like snakes • What lizards are the most fit? • As natural selection takes place, how will the species’ size change? http://wps.prenhall.com/wps/media/objects/487/499541/CDA22_2/CDA22_2f/CDA22_2f.htm

  18. Disruptive Selection • Occurs when individuals at the upper and lower end of the curve have higher fitness than individuals near the middle • This can lead to two distinct phenotypes

  19. Disruptive Selection Example • Let’s say there is a species of ducks whose habitat is a pond • The food source for these ducks is either floating on top of the water or has sunk down to the bottom of the pond • What length bill would be best for getting food quickly on top? • What length bill would be best for digging into sand for food on the bottom? • As natural selection occurs, what will future generations look like? http://wps.prenhall.com/wps/media/objects/487/499541/CDA22_2/CDA22_2d/CDA22_2d.htm

  20. Genetic Drift • Genetic drift is a random change in allele frequencies that occurs in small populations • new and different populations can form from a large diverse population • Happens by chance, not natural selection

  21. Genetic drift may happen when there is a severe reduction in the population due to natural disaster like disease(not based on genotypes…CHANCE survival not survival of the fittest) • Much of population will die, but by chance the small amount that lives may have alleles different in frequency from the original population • Called the bottleneck effect

  22. Genetic drift can happen when a small group of individuals migrates and colonizes a new habitat to form a subgroup different from the main population • Called the founder effect

  23. Evolution vs. Genetic Equilibrium • Genetic equilibrium- situation in which allele frequencies in a population remain the same • Evolution does not take place • Hardy-Weinberg Principle- states that allele frequencies in a population remain constant unless one or more factors causes those frequencies to change

  24. There are 5 conditions that can disturb genetic equilibrium and cause evolution: • Nonrandom mating • Population must be small • Immigration or emigration • Mutations • Natural selection

  25. 17.3 The Process of Speciation

  26. Species- population or group of populations whose members can interbreed and produce fertile offspring • Speciation -the formation of a new species • It happens either through natural selection or chance events • Gene pool must become separated from the original species

  27. When species splits into two groups changes in gene pools cause them to no longer interbreed • Reproductive isolation - separation of species or populations so that they cannot interbreed and evolve into two separate species • 3 kinds of isolation: behavioral, geographical, and temporal

  28. Behavioral Isolation • Occurs when two populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior • Example: different mating songs of the eastern and western meadowlarks Western meadowlark Eastern meadowlark

  29. Geographical Isolation • Occurs when two populations are separated by geographical barriers such as rivers, mountains, or bodies of water • Example: 10,000 years ago the Colorado River split the Abert squirrel into two separate populations; natural selection led to the formation of the newer Kaibab squirrel

  30. Abert squirrel and Kaibab squirrel

  31. Temporal Isolation • Occurs when two or more species reproduce at different times • Example: 3 similar species of orchids in the same rainforest only release pollen one day; if they all release on different days, they cannot pollinate one another

  32. Speciation in Darwin’s Finches • How might the founder effect and natural selection have produced reproductive isolation that could have led to speciation among Galapagos finches? • Occurred in 5 steps

  33. 1. Founders Arrive • A few finches from South America (species M) by chance landed on Galapagos Islands • Either lost or blown away by storm • Because of founder effect, may have different allele frequencies than mainland population

  34. 2. Geographic Isolation • Because the island’s environment is different from mainland, finch population evolves into new species (species A) • Few birds cross to other islands, but not common so most finches are geographically isolated • No longer share common gene pool

  35. 3. Changes in Gene Pool • Different populations adapted to different island environments through natural selection • Directional selection occurred • New phenotypes were made (long skinny beaks vs short wide beaks)

  36. 4. Behavioral Isolation • If species B crossed back to the island with species A, they still would not breed • Behavioral isolation: only want to mate with finches that have the same beaks

  37. 5. Competition and Continued Evolution • Different species (A and B) on the same island compete for food • Those most different will be most fit because less competition for the same food • Evolution occurs in way that increases differences among species • Eventually 13 species of finches formed among Galapagos Islands

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