Chap. 23 – Population Evolution

1. Chap. 23 – Population Evolution

5. How do Populations evolve? • Some characteristics become: • More/Less common within the population • Microevolution = a change in the genetic makeup of a population from one generation to the next Before Drought After Drought

6. Definitions: Population– group of individuals of the same species living in a certain area Species – a group of populations whose members can interbreed and produce fertile offspring ‘Happy Feet’ Penguin! Galapagos Penguin!

7. Gene pool = ALL alleles of ALL genes of ALL the individuals in a population A gene is FIXED if all members of the population are homozygous for the same allele. ex. all bb  Can this change? Yes! By mutation, you can change a fixed allele (bb) Mutations + Natural Selection + Sexual recombination = Evolution

8. So, why and when does a population evolve? • Backwards approach: • What will be the hallmarks of a NONEVOLVING population?

9. What is the frequency of alleles in a population? A = pink a = white 500 plants AA Aa aa 20 plants 160 plants 320 plants There are 1000 copies of the gene in the population A = 2(320) + 160 = 800 = 80% frequency =0.8 for A a = 2(20) + 160 = 200 = 20% frequency =0.2 a for a p AA Aa q aa Aa

10. AA Aa aa 160 plants 20 plants 320 plants Frequency of allele A = 0.8 Frequency of allele a = 0.2 Genotype frequencies are: AA (320/500) = .64 = .8 x .8 Aa (160/500) = .32 = 2(.8 x .2) aa (20/500) = .04 = .2 x .2 p2 2pq q2 p2 + 2pq + q2 = 1

11. Will this genotype become extinct very soon? 0.5A 0.5 a 0.5 A 0.5 a 0.25AA 0.25Aa 0.64AA 0.16Aa 0.8A 0.5A 0.25Aa 0.25aa 0.16Aa 0.04aa 0.5a 0.2a Aa X Aa A = 50% a = 50% Aa X Aa A = 80% a = 20% Aa

12. Hardy-Weinberg Theorem— Frequencies of alleles and genotypes in a population’s gene pool remain constant over generations (unless, of course, there are outside circumstances)

13. p = frequency of dominant allele (A) q = frequency of recessive allele (a) p + q = 1 p2 + 2pq + q2 = 1 p2 = frequency of AA 2pq = frequency of Aa q2 = frequency of aa Hardy-Weinberg Equation

14. Hardy- Weinberg Equilibrium p2 + 2pq + q2 = 1

15. Given: how many people are homozygous recessive for a certain gene Can you can figure out the allele frequencies in the population? ex: 1/10,000 babies has PKU (recessive) q2 = _____ p2 = _____ q = ___ 2pq = _____ (carriers) p = ___ 0.0001 0.9801 0.01 0.0198 0.99

16. Hardy-Weinberg equilibrium only occurs if a population is NOT evolving. MICROEVOLUTION is a change in allele frequency from one generation to the next in a population. WHITE HAMSTERS = bb GREY HAMSTERS = BB or Bb

17. This will keep populations in Hardy Weinberg Equilibrium (NO EVOLUTION): • 1. Very large population size • 2. No Migrations • 3. No net mutations • 4. Random mating • 5. No natural selection (NO “reproductively fit” allele)

18. Microevolution is caused by departures from the 5 conditions needed for equilibrium: • small populations (genetic drift -important) • gene flow (migration) • mutation • nonrandom mating • natural selection (important)

19. Population Genetics— Darwin – Natural Selection Mendel – genetic basis for variation

20. 1. Genetic Drift = change in allele frequency due to CHANCE. Ex: Billy goat determines which plants survives by randomly chewing off some flowers. So the allele frequency may be not 0.5 R and 0.5r in each generation. 2 types of drift….

21. The Bottleneck Effect (skewed representation of alleles after disasters) can lead to genetic drift. ‘Bottle neck’ is the disaster! Alleles left after disaster may not be 0.5R and 0.5r….Ex: cheetahs and hunting

22. The Founder Effect—a small number of individuals colonize a new, isolated area— this can lead to genetic drift. Ex: eye disease alleles have a high frequency in the founders of a colony                                                                      <> 1814 - Tristan da Cunha – colonized by 15 people!

23. 2. Gene Flow - Migration If populations aren’t completely isolated, individuals can migrate and introduce alleles into another population. Ex: wind… pollinators… This may cause a change in allele frequency in the next generation.

24. 3. Mutations Mutations can change the frequency of alleles in a population, but this is a very slow effect in humans. Bacteria - this is fast! Mutation is one of the sources of genetic variation that leads to natural selection.

25. 4. Nonrandom Mating Inbreeding and assortive mating cause an increase in homozygotes. Allele frequencies will not change, but genotype frequencies will.

26. .024 0.835 0.278 0.332 0.304 0.156 0.489 0.486 0.672 0.009 0.233 0.182 0.150 0.700 0.150 Nonrandom Mating M/M M/N N/N M/M M/N N/N Eskimo Egyptian Chinese Australian OBSERVED EXPECTED from Hardy-Weinberg Theorem

27. 5. Natural Selection Differential success in reproduction If ‘aa’ is less ‘reproductively fit’, then the frequency of the ‘a’ allele will decrease. Ex: Cow likes red flower!

28. Allele under case study: The ‘Asian’ getting a B is = failing gene -AA : must have an A+ -Aa - will be okay with an ‘A’ -aa - what a shame even an ‘A-’ is just awlright! (tsk tsk tsk) Initial frequency of these genes in a populations is: p = 0.8 (A) q = 0.2 (a) What 5 ways can you influence this allele frequency to change and cause microevolution? Break Hardy Weinberg’s equilibrium…

29. Variation within populations • quantitative characteristics are • Polygenic (height) • 2. discrete characteristics are • determined by a single gene locus • (freckles) • --2 or more forms represented = • polymorphic

30. Geographical variation—there are differences in gene structure between populations in different areas

31. Genetic variation is created by Mutation and sexual recombination (Crossing over during Prophase 1, Independent assortment during Anahase 1, and Randomn fertilization). This variation leads to natural selection if there is a selective advantage in one allele type. Diploidy prevents natural selection from quickly getting rid of an unfavorable allele. (Aa; ‘a’ hides) Aa (aa = albino AA/Aa = normal)

32. How are undesirable alleles maintained in a population? Sickle Cell Anemia 1) Diploidy -heterozygote carries the hidden recessive gene 2) Heterozygote advantange also maintains undesirable alleles. (aa – anemia Aa = carrier + malaria protection AA = normal)

33. Another ex. for how natural selection maintains variation or undesirable alleles Host Parasite Interaction= Red Queen Frequency-dependent selection— Reproductive success of one ‘morph’ declines when it becomes too common in the population due to coevolution of predators. Ex: Brown/green Fox and mite..

34. Balanced polymorphism maintains genetic diversity in a population via natural selection Frequency- dependent selection Heterozygote advantage Undesirable alleles/variation is preserved by Variation is caused by Sexual Recombination Heterozygote advantage Frequency- dependent selection And Mutations Diploidy Variation is the Raw Material for Natural Selection Natural selection TO SUMMARIZE:

35. Types of SELECTION: • Directional • Diversifying • Stabilizing

36. Types of SELECTION: • 4) Sexual Selection - natural selection for traits that increases mating chance • Intersexual Selection – female selects reproductively fit male • Intrasexual selection – males combat for female

37. ARE YOU A ‘FIT’ INDIVIDUAL? Darwinian Fitness: the relative contribution that an individual makes to the gene pool of the next generation

38. So why aren’t we perfect? • organisms are locked into historical constraints • adaptations often are compromises • not all evolution is adaptive • selection only can edit variations that exist