biology - chp 16 - evolution of populations - powerpoint

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  • 1.Chapter16Evolution of Populations

2. 16-1 Genes and Variation As Darwin developed his theory ofevolution, he worked under aserious handicap He didnt know how heredityworked This lack of knowledge left twobig gaps in Darwins thinking 3. 1. He had no idea how heritable traits pass from one generation to the next2. He had no idea how variation appeared, even though variation in heritable traits was central to Darwins theory During the 1930s Evolutionary biologists connected Mendels work to Darwins By then biologists understood that genes control heritable traits 4. How Common Is GeneticVariation? Many genes have at least 2 formsor alleles Animals such as horses, dogs,mice, and humans often haveseveral alleles for traits such asbody size or coat color 5. Variation and Gene Pools Genetic variation is studied inpopulations 6. Population A group of individuals of the samespecies that interbreed Because members of a populationinterbreed, they share a commongroup of genes called a gene pool 7. Gene pool All the genes including all thedifferent alleles, that are presentin a population 8. Relative Frequency The number of times that theallele occurs in a gene pool,compared with the number oftimes other alleles for the samegene occur In genetic terms, evolution is anychange in the relative frequencyof alleles in a population 9. Sources of Genetic Variation The 2 main sources of geneticvariation are mutations and thegenetic shuffling that results fromsexual reproduction 10. Mutations Any change in a sequence of DNA Mutations can occur because of Mistakes in DNA replication Radiation or chemicals in theenvironment Some mutations dont affect thephenotype but some do 11. Gene shuffling during sexual reproduction Mutations are not the only source of variation Most heritable differences are due to geneshuffling that occurs during the production ofgametes The 23 pairs of chromosomes can produce 8.4milliondifferent combinations of genes Crossing over further increases the number ofdifferent genotypes that can also appear inoffspring 12. Single Gene andPolygenic Traits The number of phenotypesproduced for a given trait dependson how many genes control thetrait 13. Single gene trait Controlled by a single gene thathas two alleles Variation in these genes leads toonly 2 distinct phenotypes 14. Polygenic traits Traits controlled by two or moregenes Each gene of a polygenic trait hastwo or more alleles As a result one polygenic trait canhave many possible genotypes andphenotypesEx.) height 15. Polygenic traits 16. 16-2 Evolution as Genetic Change A genetic view of evolution offers a new way tolook at key evolutionary concepts If each time an organism reproduces, it passescopies of its genes to its offspring We can therefore view evolutionary fitness asan organisms success in passing genes to thenext generation We can also view an evolutionary adaptation asany genetically controlled physiological,anatomical, or behavioral trait that increases anindividuals ability to pass along its genes 17. Evolution as Genetic Change Remember that evolution is anychange over time in the relativefrequency of alleles in a population. This reminds us that it ispopulations, not individualorganisms that can evolve overtime 18. Natural Selection onSingle Gene Traits Natural selection on single genetraits can lead to changes in allelefrequencies and thus to evolution 19. Natural Selection on Polygenic Traits Natural selection can affect thedistributions of phenotypes in anyof three ways 20. 1. Directional Selection When individuals at one end ofthe curve have higher fitness thanindividuals in the middle or at theother end 21. 2. Stabilizing Selection When individuals near the centerof the curve have higher fitnessthan individuals at either end ofthe curve 22. 3. Disruptive Selection When individuals at the upper andlower ends of the curve havehigher fitness than individualsnear the middle Can create 2 distinct phenotypes 23. Genetic Drift Natural Selection is not the onlysource of evolutionary change In small populations, an allele canbecome more or less common bychance 24. Genetic Drift A random change in allele frequency 25. Genetic Drift These individuals may carry alleles indifferent relative frequencies than didthe larger population from which theycame If so, the population that they foundwill be genetically different from theparent population This cause is not natural selection,but chance 26. Founder effect A situation in which allelefrequency changes as a result ofthe migration of a small subgroupof a population 27. Evolution vs. Genetic Equilibrium To clarify how evolutionarychange operates, scientists oftenfind it helpful to determine whathappens when no change takesplace 28. Hardy Weinberg principle States that allele frequency in apopulation will remain constantunless one or more factors causethese frequencies to change 29. Genetic equilibrium The situation in which allelefrequencies remain constant 30. 5 conditions are required to maintain genetic equilibrium1. There must be random mating2. The population must be very large3. There can be no movement into or out of the population4. No mutations5. No natural selections 31. 16-3 The Process of Speciation Factors such as natural selectionand chance events can changethe relative frequencies of allelesin a population But how do these changes lead tospeciation? 32. Speciation The formation of new species 33. Isolating Mechanisms Since members of the same speciesshare a common gene pool, in orderfor a species to evolve into 2 newspecies, the gene pools must beseparated into 2 As new species evolve, populationsbecome reproductively isolated fromeach other 34. Reproductive isolation When the members of 2populations cannot interbreed andproduce fertile offspring 35. Behavioral Isolation When two populations arecapable of interbreeding but havedifferences in courtship rituals orother reproductive strategies 36. Eastern & Western Meadowlark 37. Geographical Isolation When two populations areseparated by geographic barrierssuch as rivers, mountains, orbodies of water 38. Albert & Kaibab Squirrels 39. Temporal Isolation When 2 or more species reproduceat different timesRana aurora - breeds January - Rana boylii - breeds late March -MarchMay 40. Testing Natural Selection in NatureQ: Can evolution be observed in nature?A: YES 41. The Grants 42. Testing Natural Selection in Nature Darwin hypothesized that finches had descended from a common ancestor and overtime, natural selection shaped the beaks of different bird populations as they adapted to eat different foods The Grants, realized that Darwins hypothesis relied on two testable assumptions 43. 1. There must be enough heritable variation in these traits to provide raw materials for natural selection2. Differences in beak size and shape must produce differences in fitness that cause natural selection to occur 44. Variation The Grants identified andmeasured every variablecharacteristic of the birds on theisland Their data indicated that there is agreat variation of heritable traitsamong the Galapagos finches 45. Natural Selection During the Rainy season enough food foreveryone, no competition Dry season some foods becomescarce At that time, differences in beak sizescan mean the difference between lifeand death Birds become feeding specialists 46. Natural Selection The Grants discovered that individualbirds with different size beaks haddifferent chances of survival during adrought 47. Speciation in Darwins Finches Speciation in the Galapagos finchesoccurred by founding of a newpopulation, geographical isolation,changes in the new populations genepool, reproductive isolation andecological competition 48. Founders Arrive Many years ago, afew finches fromSouth Americanmainland Species A, flew orwere blown to oneof the GalapagosIslands 49. Geographic Isolation Later on, some birds fromspecies A crossed toanother island in theGalapagos group The finches then becameunable to fly from islandto island and becomeisolated from each otherand no longer share acommon gene pool 50. Changes in the Gene Pool Overtime, populations on each island became adapted to their local environments 51. Reproductive Isolation Now imagine that afew birds from thesecond island crossback to the first island Q: Will the populationA birds, breed with thepopulation B birds? A: Probably not 52. Ecological Competition As these two newspecies live togetherin the sameenvironment, theycompete with eachother for availableseeds The more differentbirds are, the higherfitness they have, dueto less compitition 53. Continued Evolution This process of isolationon different islands,genetic change, andreproductive isolationprobably repeated itselftime and time againacross the entireGalapagos island chain Over many generations, itproduced the 13 differentfinch species found theretoday 54. Studying Evolution SinceDarwin It is useful to review and critique the strengthand weakness of evolutionary theory Darwin made bold assumptions about heritablevariation, the age of the Earth, and therelationships among organisms New data from genetics, physics, andbiochemistry could have proved him wrong onmany counts, and they did not Scientific evidence supports the theory that livingspecies descended with modification fromcommon ancestors that lived in the past 55. Limitations of Research The Grants data shows howcompetition and climate changeaffects natural selection However, they did not observe theformation of a new species 56. Unanswered Questions Many new discoveries have led tonew hypotheses that refine andexpand Darwins original ideas No scientist suggests that allevolutionary processes are fullyunderstood. Many unansweredquestions remain 57. Why Understanding Evolution isImportant? Evolution continues todayEx.) Drug resistance in bacteria and viruses Pesticide resistance in insects Evolutionary theory helps us understandand respond to these changes in waysthat improve human l