1. Chap. 23 – Population Evolution

5. How do Populations evolve? Before Drought After Drought 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

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 Galapagos Penguin!‘Happy Feet’ 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? Mutations + Natural Selection + Sexual recombination = Evolution Yes! By mutation, you can change a fixed allele (bb)

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

9. A = pink a = white 500 plants AAAaaa 320 plants 160 plants 20 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 AAAa aa What is the frequency of alleles in a population? p q Aa

10. 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 AA 320 plants Aa 160 plants aa 20 plants Frequency of allele A = 0.8Frequency of allele a = 0.2 p2p2 2pq q2q2 p 2 + 2pq + q 2 = 1

11. 0.25AA0.25Aa 0.25aa 0.5A 0.5 a 0.5A 0.5a 0.8A 0.2a 0.5 A0.5 a 0.64AA0.16Aa 0.04aa Aa X Aa A = 50% a = 50% Aa X Aa A = 80% a = 20% Will this genotype become extinct very soon? 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 p 2 + 2pq + q 2 = 1 p 2 = frequency of AA 2pq = frequency of Aa q 2 = frequency of aa Hardy-Weinberg Equation

14. Hardy- Weinberg Equilibrium p 2 + 2pq + q 2 = 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) q 2 = _____p 2 = _____ q = ___2pq = _____ (carriers) p = ___ 0.0001 0.01 0.99 0.9801 0.0198

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 = bbGREY HAMSTERS = BB or Bb

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

18. Microevolution is caused by departures from the 5 conditions needed for equilibrium: 1.small populations (genetic drift -important) 2.gene flow (migration) 3.mutation 4.nonrandom mating 5.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. Nonrandom Mating 0.8350.1560.0090.1500.7000.150 0.2780.4890.233 0.3320.4860.182.0240.3040.672 Eskimo Egyptian Chinese Australian M/M M/N N/N OBSERVEDEXPECTED 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 1.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 = albino AA/Aa = normal) Aa

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

33. Another ex. for how natural selection maintains variation or undesirable alleles 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.. Host Parasite Interaction= Red Queen

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

35. Types of SELECTION: 1)Directional 2)Diversifying 3)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. Darwinian Fitness: the relative contribution that an individual makes to the gene pool of the next generation ARE YOU A ‘FIT’ INDIVIDUAL?

38. So why aren’t we perfect? 1.organisms are locked into historical constraints 2.adaptations often are compromises 3.not all evolution is adaptive 4.selection only can edit variations that exist