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