Two hierarchies. Genes Chromosomes Organisms Demes Populations Species Clades

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1 Evolution cont d

2 Two hierarchies Genes Chromosomes Organisms Demes Populations Species Clades Molecules Cells Organisms Populations Communities Ecosystems Regional Biotas

3 At its simplest level Evolution can be viewed as a change in an allele frequencies. Consider a gene, A with 2 alleles. Begin with a frequency of 70% for A 1 and 30% for A 2 in the population. How do we expect these frequencies to change through sexual reproduction

4 Assume random mating. Consider the alleles as contributing to a common gene pool from which genotypes are formed by combining 2 randomly selected alleles.

5 Use a form of punnet square 0.70 A A A A 1 A A 1 A A A 1 A A 2 A 2

6 Conclusions Sexual reproduction (gamete formation and fusion, meiosis) do not, in and of themselves, change allele frequencies. Given allele frequencies we can calculate frequencies of the corresponding genotypes provided certain assumptions are met.

7 What are the assumptions? Random mating No natural selection No random drift No gene flow No mutation

8 Another example Frequencies of three genotypes involving the MN gene from AINU people of Japan are as follows: MM: 0.179, MN: 0.502, NN: What are the frequencies of the M and N allele of this gene in the same population? What genotype frequencies are expected under strictly Hardy Weinberg conditions? Is the population under HW equilibrium for this gene?

9 p = frequency of the M allele. q = frequency of the N allele MM individuals carry only the M alleles whereas MN individuals carry 1 M and 1 N allele. So p should equal the frequency of MM individuals plus 1/2 of the frequency of MN individuals.

10 Given estimates of p and q We can estimate what the expected genotype frequencies would be: Frequency of MM individuals =? Frequency of MN individuals =? Frequency of NN individuals =?

11 Comparing our estimates with observed data observed expected MM MN NN

12 The Hardy Weinberg Principle Provides a null hypothesis about genotype (or allele) frequencies in a population. Assumes: no mutation, no gene flow, no natural selection, random mating and no genetic drift. If a population is not in Hardy Weinberg balance, this suggest one of the assumptions is wrong.

13 Mutation A change to the nucleotide sequence in the DNA of an organism. Causes: replication error, meiotic error, mutagen (e.g., UV light, or chemical), or the effect of a mutagen. Random with respect to the fitness of the allele

14 For mutation to be passed on It must occur in cells that gives rise to subsequent generations. In multicellular organisms this means somatic mutations will not be passed on.

15 mutation The ultimate source of all variation. Very low rates: 1 in 1,000,000 to 10,000,000 per base pair per generation; but varies somewhat with gene. Higher in bacteria and highest rates seen in certain viruses.

16 Point Mutations

17 duplications Unequal crossing over between chromatids

18 Other Chromosomal mutations Human chromsome pair 2 has telomeric like regions near the centrosome.

19 Mutation s role in evolution By itself very slow. Per generational change will be negligible at any given locus. But can become important (see Fig 25.9). Mutation is the ultimate source of all genetic information

20 Gene flow Movement (dispersal) of individuals among different populations Measure: direct observation, biotelemetry; in some controlled situations genetic markers (example in text).

21 Gene Flow What is its overall effect?

22 Genetic drift Three main ways of acting: Inbreeding effect; although mating may be random, decreased availability of genetic partners makes inbreeding likely in very small populations. Founder effect Bottleneck effect

23 Small population size inbreeding effect

24 Founder effect Humans have experienced several in their history: E.g. out of Africa: highest genetic diversity in Africa, next highest in India. Asian, European, Australasian and American populations successively less diverse.

25 Bottleneck effect Acinonyx jubatus Very low polymorphism even at MHC locus.

26 Non random mating May be positive assortative mating or negative When positive, produces inbreeding: exposes deleterious recessives to selection. N.B. inbreeding occurs naturally in many species (see population size and drift). It is a problem when moving from an outbreeding situation to an inbreeding one: Royal families, isolated communities, some religious communities.

27 When negative, can maintain heterozygosity. Wedekind, C., Seebeck, T., Bettens, F. and Paepke, A.J MHC- Dependent Mate Preferences in Humans. Proc. R. Soc. Lond. B 22, 260,

28 Sexual selection Asymmetry of sex: eggs more expensive than sperm; females fitness increased more by ability to gain resources to produce eggs (and care for progeny) than by ability to find mates. Sperm cheap and plentiful; fitness for males often simplifies into maximizing number of matings.

29 Figure 1 Bill coloration and attractiveness of males in relation to experimental diet (control males, open symbols; carotenoid-supplemented males, closed symbols). J D Blount et al. Science 2003;300:

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31 Mate competition E.g. elephant seals: females choose beaches that are most suitable. But males compete for the best beachfront and must defend it.

32 Marlene Zuk, UC Riverside A modern analyst of sexual selection theory. Pioneered a less male centred view of the field. Postulated and tested the good genes hypothesis (that female choice is often about selecting good genetic males by using specific cues such as color, song, dance, etc).

33 Natural Selection At simplest level, three modes recognized: directional, stabilizing or disruptive.

34 Directional selection If it involves one gene, can lead to fixation or the favoured allele. Reduction in genetic diversity. If multigenic then can give rise to more complex possibilities. E.g. Galapagos finches, cliff swallows in Fig 25.3.

35 Stabilizing Selection E.g. human birth weight.

36 Diversifying or disruptive Female swallowtails are Batesian mimics of different unpalatable species. Males always of yellow form selection

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