1. Population Genetics
And Speciation

2. Population Genetics – Sec. 1 Genetic Equilibrium
Objectives
Identify traits that vary in populations and that may be studied.
Explain the importance of the bell curve to population genetics.
Compare three causes of genetic variation in a population.
Calculate allele frequency and phenotype frequency.

3. Population Genetics – Sec. 1 Genetic Equilibrium
Evolution of Populations
Evolutionary changes take place over a wide range of time, from days to millennia.
Types
Macroevolution refers to large-scale evolutionary changes that takes place over long time periods. Such as the splitting of one species into two. Not often observable in human time frames.
Microevolution refers to small changes in the characteristics of a population. Such as changes in a single gene, physical trait, or behavior. These occur over very short time periods and is often observable.

4. Population Genetics – Sec. 1 Genetic Equilibrium
Evolution of Populations (cont) The study of evolution at the level of the population is known as population genetics. Population geneticists (or population biologists) examine a population’s gene pool (the total of all the different variations genes that exist in a given population) and observe changes in the frequency of different alleles (single genes) and genotypes (the collection of alleles in an individual) over time.

5. Population Genetics – Sec. 1 Genetic Equilibrium
Variation of Traits Within a Population
Population biologists study many different traits in populations, such as size and color.
Causes of Variation
Traits vary and can be mapped along a bell curve, which shows that most individuals have average traits, whereas a few individuals have extreme traits.
Variations in genotype arise by mutation, recombination, and the random pairing of gametes.

6. Population Genetics – Sec. 1 Genetic Equilibrium
Variation of Traits Within a Population (cont.) Mutation – is a random change in a gene (DNA sequence) that is passed on to offspring. This is the only process that produces new alleles! Recombination – is the reshuffling of genes in a diploid individual Random Pairing of Gametes – occurs because each organism produces large number of gametes.

7. Population Genetics – Sec. 1 Genetic Equilibrium
The Gene Pool
The total genetic information available in a population is called the gene pool.
Allele frequency is determined by dividing the total number of a certain allele by the total number of alleles of all types in the population.
Predicting Phenotype
Phenotype frequency is equal to the number of individuals with a particular phenotype divided by the total number of individuals in the population.

8. Population Genetics – Sec. 1 Genetic Equilibrium
Phenotype Frequency

9. Population Genetics – Sec. 2 Disruption of Genetic Equilibrium
Objectives
List five conditions under which evolution may take place.
Explain how migration can affect the genetics of populations.
Explain how genetic drift can affect populations of different sizes.
Contrast the effects of stabilizing selection, directional selection, and disruptive selection on populations over time.
Identify examples of nonrandom mating.

10. Population Genetics – Sec. 2 Disruption of Genetic Equilibrium
Five Conditions Under Which Evolution May Occur
Evolution may take place when populations are subject to genetic mutations, gene flow, genetic drift, nonrandom mating, or natural selection.
1. Mutation
Mutations are changes in the DNA.

11. Population Genetics – Sec. 2 Disruption of Genetic Equilibrium
2. Gene Flow
Emigration and immigration cause gene flow between populations and can thus affect gene frequencies.
3. Genetic Drift
Genetic drift is a change in allele frequencies due to random events. Can occur slowly or it can be the result of a sudden decrease in population size.
Genetic drift operates most strongly in small populations with a less stable gene frequency.

12. Population Genetics – Sec. 2 Disruption of Genetic Equilibrium
4. Nonrandom Mating
Mating is nonrandom whenever individuals may choose partners.
Sexual Selection
Sexual selection occurs when certain traits increase an individual’s success at mating.
Sexual selection explains the development of traits that improve reproductive success but that may harm the individual.

13. Population Genetics – Sec. 2 Disruption of Genetic Equilibrium
5. Selection
Natural selection can influence evolution in one of three general patterns.
A. Stabilizing Selection
Stabilizing selection favors the formation of average traits. Individuals with average phenotypes are favored, and those with phenotypic extremes are selected against.

14. Population Genetics – Sec. 2 Disruption of Genetic Equilibrium
5. Selection (cont)
B. Directional Selection
Directional selection favors the formation of more-extreme traits. Individuals at one phenotypic extreme are favored, and those at the other extreme are selected against.
C. Disruptive Selection
Disruptive selection favors extreme traits rather than average traits. Individuals at both phenotypic extremes are favored, and those with intermediate phenotypes are selected against.

15. Population Genetics – Sec. 2 Disruption of Genetic Equilibrium
Three Kinds of Selection
Natural selection is evident when the distribution of traits in a population changes over time, shifting from the original bell curve (indicated in green) toward another pattern (shown in blue). Directional selection is a shift in one direction only away from the center of the bell curve. Stabilizing selection is a shift toward the center of the original bell curve. Disruptive selection is a shift in both directions away from the center.

16. Population Genetics – Sec. 3 Formation of Species
Objectives
Relate the biological species concept to the modern definition of species.
Explain how the isolation of populations can lead to speciation.
Compare two kinds of isolation and the pattern of speciation associated with each.
Contrast the model of punctuated equilibrium with the model of gradual change.

17. Population Genetics – Sec. 3 Formation of Species
The Concept of Species
According to the biological species concept, a species is a population of organisms that can successfully interbreed but cannot breed with other groups.
Speciation describes the process by which new species arise through evolution.

18. Population Genetics – Sec. 3 Formation of Species
Isolation and Speciation – Genetic Divergence
Geographic Isolation
Geographic isolation results from the separation of population subgroups by geographic barriers.
Allopatric Speciation
Geographic isolation may lead to allopatric speciation. Occurs when two geographically isolated populations split into different species.

19. Population Genetics – Sec. 3 Formation of Species
Isolation and Speciation - Genetic Divergence (cont.)
Reproductive isolation results from the separation of population subgroups by barriers to successful breeding.
Sympatric Speciation
Reproductive isolation within the same geographic area is known as sympatric speciation. Occurs when one species splits into two through means other than geographic isolation.

20. Population Genetics – Sec. 3 Formation of Species
Isolation and Speciation – Genetic Divergence (cont.)
Reproductive isolation mechanisms are traits that prevent different groups from successfully interbreeding.

21. Population Genetics – Sec. 3 Formation of Species
Isolation and Speciation – Genetic Divergence (cont.)
Reproductive Isolation (cont)
Reproductive isolation mechanisms – There are two main mechanisms:
Prezygotic Isolation Mechanisms traits that prevent mating and fertilization between different species – prevents zygote formation. There are five methods:
a. Habitat Isolation
b. Temporal Isolation
c. Behavioral Isolation
d. Mechanical Isolation
e. Gametic Isolation

22. Population Genetics – Sec. 3 Formation of Species
Isolation and Speciation – Genetic Divergence (cont.)
Reproductive Isolation (cont)
Reproductive isolation mechanisms
Postzygotic Isolation Mechanisms traits that prevent hybrids, the offspring of two different species, from developing into healthy, fertile adults. There are two main methods:
a. Hybrid Inviability
b. Hybrid Sterility

23. Population Genetics – Sec. 3 Formation of Species
Rates of Speciation
Speciation does not proceed at a single, constant tempo. It occurs rapidly in some groups and slowly in others.
In the gradual model of speciation (gradualism), species undergo small changes at a constant rate.
Under punctuated equilibrium, new species arise abruptly, differ greatly from their ancestors, and then change little over long periods.