1. Chapter 18
Process of Evolution

2. Principles of genetics explain how populations vary and change
18.1 Microevolution
Principles of genetics explain how populations vary and change
population genetics
Modern Evolutionary Synthesis applies natural selection to genetics
microevolution refers to changes within a population, all the members of one species (that interbreed) in a particular area

3. population genetics, cont.
18.1 Microevolution
population genetics, cont.
gene pool: all alleles at all gene loci in all individuals of a population
variation in gene pool is key to natural selection
a gene pool is to a population as a genotype is to an individual
gene pools can be described in terms of gene frequencies, the percentage occurrence of particular alleles or genotypes

4. population genetics, cont.
18.1 Microevolution
population genetics, cont.
gene pool, cont.
example:
suppose that in a Drosophila population 30% of flies are homozygous dominant for grey bodies, 45% are heterozygous, and 25% have black bodies (homozygous recessive)
What are the allele frequencies?
What are the frequencies in the following generation?

5. population genetics, cont.
18.1 Microevolution
population genetics, cont.
gene pool, cont.
we find that sexual reproduction alone does not change allele frequencies
Hardy and Weinberg independently realized the possibility of equilibrium of gene pool frequencies in 1908

6. population genetics, cont.
18.1 Microevolution
population genetics, cont.
Hardy-Weinberg principle: equilibrium of allele frequencies in a gene pool will remain in effect in each succeeding generation if
1. no mutations (changes in alleles)
2. no gene flow (allele migration)
3. random mating (chance pairing)
4. no genetic drift (large population, insignificant chance changes)
5. no selection

7. population genetics, cont.
18.1 Microevolution
population genetics, cont.
Hardy-Weinberg principle, cont.
these conditions are rarely met
these are the factors that cause evolution, the change in allele frequencies over time
natural selection can be seen as a change in allele frequencies
more frequent alleles: more fit
less frequent alleles: less fit (but rarely removed completely)

8. population genetics, cont.
18.1 Microevolution
population genetics, cont.
microevolution example
peppered moth in England
pre-Industrial Revolution, white moths more common, rested on white trees to avoid being bird food
during Industrial Revolution, trees covered with soot
black moths survived, white moths became bird food, so black moth frequency (and allele) increased
called industrial melanism

9. Fig. 18.2a Moths on light tree trunk

10. Fig. 18.2b Moths on dark tree trunk

11. population genetics, cont.
18.1 Microevolution
population genetics, cont.
Hardy-Weinberg equation
allows one to measure allele frequencies
by comparing frequencies over several generations, changes can be detected and measured
if frequencies stay constant over time, the population is in equilibrium

12. population genetics, cont.
18.1 Microevolution
population genetics, cont.
Hardy-Weinberg equation, cont.
if there are only 2 alleles,
let p = frequency of dominant allele
let q = frequency of recessive allele
then p + q = 1 (100%)
individuals have 2 alleles/trait, so
p2 = frequency of AA
2pq = frequency of Aa
q2 = frequency of aa, and
p2 + 2pq + q2 = 1

13. population genetics, cont.
18.1 Microevolution
population genetics, cont.
Hardy-Weinberg equation, cont.
using the equation
vestigial wings are recessive in flies
given a population of 500 flies and 80 flies with vestigial wings, what are the frequencies of the wild and vestigial wing alleles?

14. population genetics, cont.
18.1 Microevolution
population genetics, cont.
Hardy-Weinberg equation, cont.
using the equation
the only observable value is….. q2
q2 = 80/500 = 0.16
q = = 0.4
p + q = 1
p = 1 - q = = 0.6
frequency of dominant allele = 0.6
frequency of recessive allele = 0.4

15. population genetics, cont.
18.1 Microevolution
population genetics, cont.
Hardy-Weinberg equation, cont.
using the equation, cont.
if we measured p and q in a few generations and the values were the same, then the population is in equilibrium
if this is true, then all five conditions are being met
However, suppose it is fit for flies to fly?

16. Causes of Microevolution
Genetic Mutations
result in multiple alleles
Gene Flow
movement of alleles between populations by migration of breeding individuals
can increase variation in a population, decreases isolation
makes gene pools similar
can prevent speciation

17. Fig Gene flow

18. Causes of Microevolution, cont.
Nonrandom Mating
assortative mating: individuals mate with others of the same phenotype
intrasexual selection: males fight for the right to mate
example: Bighorn sheep
intersexual selection: females exhibit choosiness
example: peahens

19. Causes of Microevolution, cont.
Genetic Drift
changes in allele frequencies due to chance
more likely in small populations
bottleneck effect: prevents most genotypes from participating in production of next generation
example: California condors, population dropped to 20 birds, limits variation

20. Fig Genetic drift

21. Causes of Microevolution, cont.
Genetic Drift, cont.
founder effect: small, “strange” population breaks off of larger population
example: Amish have more polydactyl dwarves then rest of the world
Natural Selection
biggest influence on frequencies
results in adaptation (others don’t)

22. Fig Founder effect

23. Causes of microevolution

24. Natural selection results in adaptation to the environment
natural selection is the process that results in adaptation of a population to the biotic and abiotic environments
requires
variation
inheritance
differential adaptiveness
differential reproduction

25. Effect of selection on finch beak size

26. 18.2 Natural Selection Types of Selection
natural selection usually acts on polygenic traits
polygenic traits display a range of phenotypes
directional selection occurs when an extreme phenotype is favored
example: antibiotic resistance in bacteria

27. Normal distribution

28. Directional selection

29. Fig. 18.6 Directional selection

30. Types of Selection, cont.
18.2 Natural Selection
Types of Selection, cont.
stabilizing selection occurs when an intermediate phenotype is favored
examples: clutch size in Swiss starlings, size of galls made by gall-flies

31. Stabilizing selection

32. Fig. 18.7 Stabilizing selection

33. Types of Selection, cont.
18.2 Natural Selection
Types of Selection, cont.
disruptive selection occurs when extreme phenotypes are favored over the intermediate phenotype
examples: British land snail coloration, male lazuli bunting coloration

34. Disruptive selection

35. Fig. 18.8 Disruptive selection

36. Maintenance of Variations
18.2 Natural Selection
Maintenance of Variations
genotypic variation is maintained by:
mutation
recombination (ex: flowers prevent self-pollination)
gene flow (ex: male wolves out of pack)
disruptive selection

37. Maintenance of Variations, cont.
18.2 Natural Selection
Maintenance of Variations, cont.
diploidy makes heterozygotes possible
heterozygotes maintain recessive alleles
heterozygotes sometimes have an advantage over homozygotes
example: malaria resistance and sickle cell anemia

38. Heterozygote advantage

39. Fig. 18.9 Sickle cell disease

40. Macroevolution requires reproductive isolation
macroevolution: evolutionary change at or above the level of species
speciation: the splitting of one species into two or more species or the transformation of one species into a new one

41. 18.3 Macroevolution What Is a Species?
biological species concept: a group of populations that can breed among themselves to produce fertile offspring
members of one species cannot reproduce with members of another species
members of a species have a shared gene pool

42. Fig Species concept

43. 18.3 Macroevolution What Is a Species?, cont.
reproductive isolating mechanism: any structural, functional, or behavioral charac-teristic that prevents successful reproduction from occurring
prezygotic isolating mechanisms prevent repro-duction attempts or successful fertilization

44. 18.3 Macroevolution What Is a Species?, cont.
reproductive isolation, cont.
prezygotic isolating mechanisms
habitat isolation
temporal isolation
behavior isolation
mechanical isolation
gamete isolation

45. Fig. 18.11 Temporal isolation

46. 18.3 Macroevolution What Is a Species?, cont.
reproductive isolation, cont.
postzygotic isolating mechanisms prevent hybrid offspring from developing or breeding
zygote mortality
hybrid sterility
F2 fitness

47. Isolating mechanisms

48. 18.3 Macroevolution Modes of Speciation
allopatric speciation: origin of new species between populations that are separated geographically
examples: squirrels across Grand Canyon, salamanders in CA
sympatric speciation: origin of new species in populations that overlap geographically
example: polyploid plants

49. Fig. 18.12 Allopatric speciation

50. Fig. 18.13 Sympatric speciation

51. Forms of speciation

52. Modes of Speciation, cont.
18.3 Macroevolution
Modes of Speciation, cont.
adaptive radiation involves many new species arising from a single ancestral species when members become adapted to different environments
particular form of allopatric speciation
examples: Galapagos finches, Hawaiian honeycreepers

53. Steps in adaptive radiation

54. Fig. 18.14 Adaptive radiation

55. Hawaiian honeycreepers