Biology 210 - Chapter 26: Population Genetics

Biology 210

GENETICS

1 December, 1997

Chapter 26 Population Genetics

1. Darwin's Revolution

A Summary of Darwin's theory of evolution:

1. Principle of Variation.

2. Principle of Heredity.

3. Principle of Selection.

Evolution is a sorting process

2. Variation and Its Modulation

Observations of Variation

Morphological variation

Chromosomal Polymorphism

Immunological Polymorphism

Protein Polymorphism

For structural proteins, about one-third of the loci are polymorphic, and the average heterozygosity in a population over all loci sampled is consistantly around 10%

DNA Sequence Polymorphism

Restriction-Site Variation
Tandem Repeats (including VNTRs)
Complete Sequence Variation

Variation within and between Populations

Quantitative Variation

3. The Effect of Sexual Reproduction on Variation

figure 26-7 Hardy-Weinberg equilibrium frequencies that result from Random Mating.

4.The Sources of Variation

Variation from Mutations

table 26-11 Different Point-mutation rates

figure 26-9. The change over generations in the frequency of a gene A due to mutation from A to a at a constant mutation rate (m = 10^-5)

figure 26-9 Change over generations in frequency of gene A

Variation from Recombination

This is by far the quickest source of variation. Two chromosomes with "normal" survival from Drosophila can account for as much as 75% of the genetic variation seen in the entire population in the wild.

Variation from Migration

M = Dptotal (P - p_o)

The Origin of New Functions

To add new functions requires more genes - this is commonly achieved by divergence of duplicated genes. In plants, frequently polyploidy allows for many copies of the same gene....

figure 26-10. Frequency distribution of haploid chromosome numbers in dicotyledonous plants.

figure 26-10. Frequency distribution of haploid chromosome numbers in plants

Inbreeding and Assortive Mating

Two deviations of random mating:

1. Inbreeding - when mating between relatives occurs more commonly than would occur by pure chance.

2. Assortive mating - when individuals tend to choose each other as mates because of their degree of resemblence to each other (note that this does not necessarily mean they are related).

The Balance between Inbreeding

and New Variation

figure 26-13. Distribution of gene frequencies among island populations after various numbers of generations of isolation, where the number of generations that have passed (t) is given in multiples of the population size (N).

5. Selection

Fitness and the Struggle for Existence

Two Forms of the Struggle for Existence

1. Frequency independent - the organism struggles with the environment directly.

2. Frequency dependent fitness - the organism struggles with other organisms within the same population, as well as with the environment; this is a type of "selection to blend in with the crowd".

figure 26-14. (a). A blue jay eating a monarch butterfly, which (b) induces vomiting in the jay. Because of this experience, the jay later will refuse to eat a viceroy butterflythat is similar in appearance to the monarch, although jays that have never tried monachs will eat the viceroys.

Measuring Fitness Differences

How Selection Works

The allele with the higher than average fitness increases in the population.

6. Balanced Polymorphism

figure 26-18. Changes in the frequency of the inverstion Standard (ST) in competition with chiricahua (CH) in a laboratory population Drosophila pseudoobsura. The points show the actual frequencies in successive generations. The solid line shows the theoretical course of change, if the fitnesses of the genotypes were as given in the text.

7. Multiple Adaptive Peaks

figure 26-19. An adaptive landscape with two adaptive peaks (red), two adaptive valleys (blue), and a topographic saddle in the center of the landscape. The topographic lines are lines of equal mean fitness. If the genetic composition of a population always changes in such a way as tomove the population "uphill" in the landsape, then the final composition will depend on where the population began with respect to the fall (dashed) line. (a.) Topographic map of the adaptive landscape. (b.) A perspective sketch of the surface shown in the map.

figure 26-20. Differences in horn morphology in two geographically separated species of rhinoceros: (a) the African rhinoceros; (b) the Indian rhinoceros.

8. Artificial Selection

figure 26-22. Changes in average egg production in a chicken population selected for its increase in egg-laying rate over a period of 30 years.

9. Random Events

figure 26-24. The appearance, loss, and eventual incorporation of new mutations during the life of a population. If random genetic drift does not cause the loss of a new mutation, then it must eventually cause the entire population to become homozygous for the mutation (in the absence of selection). If the figure, 10 mutations have arisen, of which 9 increased slightly in frequency and then died out. Only the fourth mutation eventually spread into the population.
figure 26-24

10. A Synthesis of Forces

Variation and Divergence of Populations

figure 26-25. The effects of gene frequency of various forces of evolution. The black arrows show a tendency toward increased variations within the population; the red arrows, decreased variation.

The Exploration of Adaptive Peaks

figure 26-26. Selection and random drift can interact to produce different changes in gene frequency in an adaptive landscape. Without random drift, both populations would have moved toward aaBB as a result of selection alone.

11. The Origin of Species
Speciation - the origin of a new species - is the origin of a group of individuals capable ofmaking a living in a new way and at the same time acquiring some barrier to genetic exchange with the species from which it arose.

Reference:

Richard Dawkins, "Climbing Mount Improbable",
(W.W. Norton & Co., New York, 1997).

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