Chapter 6 - Concept of Selection

Notes for Understanding Evolution - Chapter 6

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Notes for Chapter 6: Concept of Selection

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Introduction

I. Selection Against Recessive Defects

Key Terms: complete selection, recessive trait

RQUE6.1: Using the formula on UE:62, with the initial value (q0) of 0.4, compute the frequency of a recessive allele after ten generations (q10).

RQUE6.2: Why do harmful recessive traits tend to stay in a population once they get rare through the action of selection?

II. Significance of the Heterozygote

RQUE6.3: If only one out of about 2,500 individuals are afflicted with cystic fibrosis disease (i.e., they are homozygous for recessive trait), why is there still four chances in 100 that a person chosen at random is a carrier for the gene?

III. Interplay of Mutation and Selection

RQUE6.4: If complete selection operates against a trait one would expect, in theory, that such a trait should eventually be lost. So why is it that such traits are often persistent in real populations?

RQUE6.5: Why do harmful high-energy radiations and improvements in healthcare both tend to promote the increase of deleterious recessive alleles?

IV. Partial Selection

Key Terms: partial selection, lethal vs. semi-lethal vs. subvital mutant genes, sterile vs. semi-sterile vs. slightly less fit individuals, selection intensity, s (= selection coefficient)

RQUE6.6: What is the expectation for the persistence of an allele as the selection coefficient decreases?

V. Selection Against Dominant Defects

Key Terms: retinoblastoma (cancer of the eye)

RQUE6.7: What differences must be considered when one is investigating a harmful dominant genetic allele (e.g., for retinoblastoma), in contrast to the recessive alleles considered above?

VI. Concealed Variability in Natural Populations

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Featured Organism: Drosophila pseudoobscura
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Drosophila spp. (other species in same genus)
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Featured Scientist: Theodosius Dobzhansky
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Links: 1 - 2 - 3 - 4 - 5 - 6

RQUE6.8: What was the first experimental evidence that, as predicted, that typical populations of organisms harbor an enormous store of recessive deleterious alleles?

VII. Genetic Load in Human Populations

RQUE6.9: Are humans exempt from such problems?

VIII. Selection Relaxation in Humans

IX. General Effects of Selection

Key Terms: directional vs. stabilizing vs. disruptive selection

RQUE6.10: Contrast, with an example, the following three modes of selection: directional, stabilizing, and disruptive.

X. Natural Selection and Pregnancy Loss

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or back to Chapter 5 or ahead to Chapter 7