1. Evolutionary Change in Populations & Speciation
Chapter 18 & 19
Evolutionary Change in Populations & Speciation

2. Ch. 18: Population Genetics
Evolution occurs in populations, not individuals
Evolutionary change is inherited from one generation to the next.
Darwin recognized that evolution occurs in populations, but did not understand how traits are passed on

3. Populations
Population = all the individuals of the same species that live in a particular place at the same time
Members of a population vary in many traits.
Ex. p.390
Some variation is due to environment and some is due to heredity.

4. Population Genetics
The study of genetic variability within a population and of the forces that act on it.
We will discuss the 5 factors responsible for evolutionary change:
Nonrandom mating
Mutation
Genetic Drift
Gene Flow
Natural Selection
Then, we consider genetic variation as the raw material for evolution.

5. Calculating Frequencies
Each population has a gene pool:
All the alleles for all the genes present in the population
Genotype frequency: the proportion of a particular genotype in the population
Phenotype frequency: the proportion of a particular phenotype in a population
Allele frequency: the proportion of a specific allele in a population

6. Hardy-Weinberg Principle
This law states an equilibrium of allele frequencies in a gene pool (using a formula p2 + 2pq + q2=1) remains in effect in each succeeding generation of a sexually reproducing population if five conditions are met.

7. 5 conditions: No mutation: no allelic changes occur.
No gene flow: migration of alleles into or out of the population does not occur.
Random mating: individuals pair by chance and not according to their genotypes or phenotypes.
No genetic drift: the population is large so changes in allele frequencies due to chance are insignificant.
No selection: no selective force favors one genotype over another.

8. Hardy-Weinberg Principle
These conditions are rarely met, so allele frequencies in the gene pool of a population do change from one generation to the next, resulting in evolution.

9. Hardy-Weinberg Principle
p2 + 2pq + q2 = 1
p2 = Frequency of AA
2pq = Frequency of Aa
q2 = Frequency of aa
1 = all the individuals in a population
Always begin by determining the frequency of the homozygous recessive genotype.

10. Microevolution Evolution within a population
Small changes over a few generations
Result from 5 microevolutionary processes:
nonrandom mating
mutation
genetic drift
gene flow
natural selection

11. Nonrandom Mating
1. Random mating involves individuals pairing by chance, not according to genotype or phenotype.
2. Nonrandom mating involves inbreeding and assortative mating.
3. Inbreeding is mating between relatives to a greater extent than by chance. a. However, inbreeding decreases the proportion of heterozygotes. b. In human populations, inbreeding increases the frequency of recessive abnormalities.

12. Nonrandom Mating
4. Assortative mating occurs when individuals mate with those that have the same phenotype.
5. Sexual selection occurs when males compete for the right to reproduce and the female selects males of a particular phenotype. (guppies, lions)

13. Mutation
Many traits in organisms are polymorphic, i.e., two or more distinct phenotypes are present in the population due to mutated genes.
Analysis of Drosophila enzymes indicates they have multiple alleles at least at 30% of their gene loci.
In humans, the ABO blood types are an example of polymorphism.
Mutations can be beneficial, neutral, or harmful; a seemingly harmful mutation that requires Daphnia to live at higher temperatures becomes advantageous when the environment changes.

14. Genetic Drift
Genetic drift refers to changes in allele frequencies of a gene pool due to chance, more often in small populations
Genetic drift occurs when founders start a new population, or after a genetic bottleneck with interbreeding.

15. The bottleneck effect prevents most genotypes from participating in production of the next generation.
1) The bottleneck effect is caused by a severe reduction in population size due to a natural disaster, predation, or habitat reduction. 2) The bottleneck effect causes a severe reduction in the total genetic diversity of the original gene pool. 3) The cheetah bottleneck causes relative infertility because alleles were lost due to intense inbreeding when populations were reduced in earlier times.

17. Genetic Drift
The founder effect is an example of genetic drift where rare alleles or combinations occur in higher frequency in a population isolated from the general population.
1) This is due to founding individuals containing a fraction of total genetic diversity of the original population. 2) Which particular alleles are carried by the founders is dictated by chance alone. 3) As an example, dwarfism is much higher in a Pennsylvania Amish community due to a few German founders.

18. Gene Flow
Gene flow (gene migration) is the movement of alleles among populations by migration of breeding individuals.
Gene flow can increase variation within a population by introducing novel alleles
Continued gene flow decreases diversity among populations, causing gene pools to become similar.
Gene flow among populations can prevent speciation from occurring.

19. Natural Selection
Natural selection requires a. variation (i.e., the members of a population differ from one another), b. inheritance (i.e., many of the differences between individuals in a population are heritable genetic differences), c. differential adaptedness (i.e., some differences affect how well an organism is adapted to its environment), and d. differential reproduction (i.e., better adapted individuals are more likely to reproduce).
Fitness is the extent to which an individual contributes fertile offspring to the next generation.
Relative fitness compares the fitness of one phenotype to another.

20. Types of Selection
Directional selection occurs when an extreme phenotype is favored; the distribution curve shifts that direction.
a. A shift to dark-colored peppered moths from light-colored correlated with increasing pollution.
b. Drug-resistant strains of bacteria are a serious health threat and represent this type of selection.

21. c. Increases in insecticide-resistant mosquitoes and resistance of the malaria protozoan Plasmodium to medications are also examples of directional selection.
d. The gradual increase in the size of the modern horse, Equus, correlates with a change in the environment from forest-like conditions to grassland conditions.

22. Types of Selection
Stabilizing selection occurs when extreme phenotypes are eliminated and the intermediate phenotype is favored.
a. The average number of eggs laid by Swiss starlings is four or five. b. If the female lays more or less than this number, fewer survive. c. Genes determining the physiology of yolk production and behavior are involved in clutch size.

23. Types of Selection
Disruptive selection occurs when extreme phenotypes are favored and can lead to more than one distinct form. a. British snails (Cepaea nemoralis) vary because a wide range causes natural selection to vary. b. In forest areas, thrushes feed on snails with light bands. c. In low-vegetation areas, thrushes feed on snails with dark shells that lack light bands.

25. Macroevolution and Speciation
Chapter 19
Macroevolution and Speciation

26. Macroevolution
Macroevolution refers to any evolutionary change at or above the species level.
Speciation is the splitting of one species into:
two or more species or
the transformation of one species into a new species over time

27. What is a species?
Linnaeus separated species based on morphology, i.e., their traits differed
Darwin saw that similar species are related by common descent.
Ernst Mayr (1942) developed the biological species concept: a species is a group of interbreeding populations that are reproductively isolated from other such groups.
*The biological definition of a species - members of one species interbreed and have a shared gene pool, and each species is reproductively isolated from every other species.

28. Species are based on interfertility, not physical similarity.
For example, the eastern and western meadowlarks may have similar shapes and coloration, but differences in song prevent interbreeding between the two species.
In contrast, humans have considerable diversity, but we all belong to the same species because of our capacity to interbreed.
Fig. 24.2
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

29. Speciation The key to speciation is reproductive isolation. Two types:
Anagenesis – small, progressive evolutionary changes over long periods
Cladogenesis – 2 or more populations of an ancestral species split and diverge, also called divergent speciation
Anagenesis

30. Anagenesis

31. Cladogenesis

32. Allopatric Speciation
Allopatric speciation occurs when new species result from populations being separated by a geographical barrier that prevents their members from reproducing with each other.
While geographically isolated, variations accumulate until the populations are reproductively isolated.
Examples: rivers shifting their courses, glaciers melting, mountain ranges forming, large lakes diminishing into several smaller, geographically separated pools
Ex. Kaibab squirrel – p.413

33. Sympatric Speciation
Sympatric speciation would occur when members of a single population develop a genetic difference (e.g., chromosome number) that prevents them from reproducing with the parent type.
The main example of sympatric speciation is in plants.
Failure to reduce chromosome number produces polyploid plants that reproduce successfully only with polyploids.

34. Evolutionary Change Can Occur Rapidly or Gradually
Punctuated Equilibrium – history of a species, long periods of stasis (little or no evolutionary change) are punctuated, or interrupted, by short periods of rapid speciation
i.e. evolution proceeds in “spurts”
Gradualism – evolution proceeds continuously over long periods
Difficult to observe because fossil record is not always complete
Maintains that populations evolve slowly by accumulating adaptations due to selective pressures

35. Adaptive Radiation
The evolutionary diversification of many related species from one or a few ancestral species in a short time period.
Ex. Hawaiian honeycreepers p.422
Ex. Finches of Galapagos Islands
13 species from 1 founder mainland species

36. Extinction
End of a lineage – occurs when the last individual of a species dies
Permanent
Only 1 species is alive today for every 2000 that have become extinct

37. Chapter 18 & 19 Review Chapter 18: p. 404-405 Chapter 19: p. 426
Post-Test: 1, 4, 5, 7, 9-15
Review: 1, 2, 6
Chapter 19: p. 426
Post-Test: 1, 7, 9, 11, 15
Review: 4, 6, 9