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Lecture 2: Evolution of Populations

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1 Lecture 2: Evolution of Populations
Campbell & Reece chapters: Chapter 23 Microevolution – evolution at the population level = change in allele frequencies over generations

2 Genetics = science dealing with inheritance or heredity, the transmission of acquired traits

3 Ultimate source of heritable variation is change in DNA
Change in DNA caused by: 1) Mutation 2) Genetic Recombination

4 Mutations = change in genotype other than by recombination.
Three types: 1) Point Mutations 2) Chromosome Mutations 3) Change in Chromosome Number

5 1) Point Mutation Change in a single DNA Nucleotide.
Point mutation rate per gene = ~1 in 100,000 gametes. In humans: = 1 mutation/gene x (~25,000 genes) 100,000 gametes = ~0.25 point mutations/gamete

6 E.g., human hemoglobin: 1 in 2,000 people have mutant hemoglobin gene.
2 alpha chains (141 amino acids) 2 beta chains (146 amino acids) 1973 sampling of population (thousands): 169 mutation types recorded: 62 substitutions in alpha 99 substitutions in beta 1 deletion in alpha 7 deletions in beta 1 in 2,000 people have mutant hemoglobin gene. hemoglobin

7 2) Chromosome Mutations
Rearrangements (including losses and gains) of large pieces of DNA. E.g., inversion: A B C D E F G A B F E D C G Re-attaches here and here [3% of pop. of Edinburgh, Scotland have inversion in Chromosome #1] [Humans differ from chimps by 6 inversions, from gorillas by 8 (also difference in chromosome number)]

8 3) Change in Chromosome No.
a) Aneuploidy - change in chromosome number of less than an entire genome. Horse (2n = 64) versus donkey (2n = 62) Humans (2n = 46) versus chimp or gorilla (2n = 48) Some Genetic Diseases Trisomy (addition of a chromosome to the original diploid pair) of chromosome 21 in humans = Down's syndrome. Extra or one sex chromosomes ( e. g., XYY, XXY, X).

9 b) Polyploidy Evolution of chromosome number which is a multiple of some ancestral set. Has been a major mechanism of evolution in plants.

10 Two ways polyploidy can occur:

11 Polyploid evolution of wheat

12 Genetic Recombination (in sexual reproduction)
= Natural, shuffling of existing genes, occurring with meiosis and sexual reproduction Two types: Independent Assortment Crossing over

13 Independent assortment
Sorting of homologous chromosomes independently of one another during meiosis E. g., (where A,B,&C genes are unlinked) AaBBcc X AabbCC ---> AaBbCc (one of many possibilities)

14 Independent assortment
Results in great variation of gametes, and therefore progeny. [E. g., one human: 223 = 8,388,608 possible types of gametes (each with different combination of alleles).]

15 Crossing over Exchange of chromatid segments of two adjacent homologous chromosomes during meiosis (prophase). Greatly increases variability of gametes and, therefore, of progeny.

16 Genetic Variation Genetic recombination - source of most variation (in sexual organisms), via new allele combinations. Mutation - ultimate source of variation, source of new alleles and genes.

17 Fitness = measure of the relative contribution of a given genotype to the next generation Can measure for individual or population.

18 Fitness = allele/genotype freq. in future generation allele/genotype freq. in prev. generation E. g., 1st gen. 25%AA : 50%Aa : 25%aa [freq. A = 25% + .5(50%) = 50%] 2nd gen.: 36%AA : 48%Aa : 16%aa [freq. A = 36% + .5(48%) = 60%] Fitness of A allele is 60/50 = 1.2; a is 40/50 = 0.8 Fitness of AA genotype is 36/25 = 1.44 , etc.

19 Hardy-Weinberg Equilibrium (1908)
The frequency of a gene / allele does not change over time (given certain conditions). A,a = alleles of one gene, combine as AA, Aa, or aa Generation 1: p = freq. A q = freq. a p + q = 1 (100%) pA qa pA p2AA pqAa qa pqAa q2aa } =gene frequencies in generation 1 p2AA + 2pqAa + q2aa = 1

20 Hardy-Weinberg Equilibrium (1908)
Example: Generation 1: p = q = p + q = 1 (100%) 0.4A 0.6a 0.4A 0.16AA 0.24Aa 0.6a 0.24Aa 0.36aa } =gene frequencies in generation 1 p2AA + 2pqAa + q2aa = = 1

21 Hardy-Weinberg Equilibrium (1908)
The frequency of a gene / allele does not change over time (given certain conditions). What will be the frequency of alleles in the second generation? p2AA + 2pqAa + q2aa = 1 freq. A (generation 2) = (p2 + pq) / (p2 + 2pq + q2) = p(p + q) / (p + q)2 = p / (p + q) = p Therefore, freq. A = p; freq. a = q, same as in generation 1. } =gene frequencies in generation 1

22 Hardy-Weinberg Equilibrium
Maintained only if: 1) No mutation Mutations rare, but do occur (1 new mutation in 10, ,000,000 genes per individual per generation)

23 Hardy-Weinberg Equilibrium
2) No migration (no gene flow into or out of population) But, can occur . . .

24 Hardy-Weinberg Equilibrium
3) Population size large Two things can disrupt: a) Population bottleneck (large pop. gets very small) b) Founder effect (one or a few individuals dispersed from a large pop.)

25 Hardy-Weinberg Equilibrium
4) Mating is random But, most animals mate selectively, e.g., 1) harem breeding (e. g., elephant seals); 2) assortative mating (like mates with like) 3) sexual selection

26 Hardy-Weinberg Equilibrium
5) All genotypes equally adaptive (i.e., no selection) But, selection does occur . . .

27 If any conditions of Hardy-Weinberg not met:
Genotype frequencies change Evolution occurs! Evolution = change in gene frequency of a population over time.

28 Selective Pressure = agent or causative force that results in selection. E. g., for dark skin, selective pressure = UV radiation (UV increases sunburn and skin cancer in lighter skinned individuals) E. g., for light skin, selective pressure = Vitamin D synthesis

29 Genetic Drift = change in genotype solely by chance effects random!
promoted by: Population Bottleneck -drastic reduction in population size Founder Effect - isolated colonies founded by small no. individuals

30 Population Bottleneck
Fig. 23-9 Fig Range of greater prairie chicken Pre-bottleneck (Illinois, 1820) Post-bottleneck (Illinois, 1993) (a) Original population Bottlenecking event Surviving Figure 23.9 The bottleneck effect

31 Summary: Evolution can occur by two major mechanisms:
Natural Selection (non-random) Genetic Drift (random)

32 Pepper Moth: Biston betularia Selective pressure=predation by birds
Single gene: AA/Aa = dark aa = light Camoflague selected for!

33 Result: Balanced polymorphism
E.g., Sickle Cell Anemia: Mutation = single amino acid subst. in beta chain of hemoglobin --> single a.a. difference. Sickle blood cells Normal blood cells

34 Sickle Cell Anemia Homozygotes for sickle mutation (HsHs): lethal

35 Sickle Cell Anemia Heterozygotes (HsHn): resistant to malaria, selected for in malaria-infested regions, selected against where malaria not present.

36 General Principle: Selection dependent on the environment!
If environmental conditions change, selective pressure can change!!

37 Stabilizing selection - selection against the two extremes in a population (e.g., birth weight in humans, clutch size in birds)

38 Directional selection - selection for one extreme in a population, against the other extreme (e.g., pesticide resistance in insects antibiotic resistance in bacteria)

39 Disruptive selection - selection for the two extremes in a population, against the average forms (e.g., limpets w/ 2 color forms: light & dark in mosaic environment; flies on two hosts: apple & hawthorn)

40 Sexual Selection - selection resulting in greater reproductive fitness in certain individuals of one sex

41 Sexual Selection Intrasexual selection – within one sex; competition between members of one sex (usually males)

42 Sexual Selection Intersexual selection – between two sexes; preference by one sex for features of the other sex. Usu. female choice.

43 Sexual Selection

44 Sexual Selection Balance between survivorship (decreased) reproductive potential (increased)

45 Sexual Selection: decreased survivorship

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