Chapter 3: The Modern Synthesis

Chapter Outline

1. Population Genetics

1. Genotype Frequencies
• Frequencies of genotypes give geneticists a description of the genetic composition of a population.
• Genotype frequencies can be changed by means of sexual reproduction, natural selection, mutation, and genetic drift.
2. Sexual Reproduction
• Even though humans choose their mates, we cannot select for all 30,000 gene loci, so mating is considered random.
• To calculate the effect of sexual reproduction on genotype frequencies, first calculate the allele frequencies of the gene in question, then calculate the genotypes.
• If a population is unaffected by forces of evolution, the distribution of genotypes will operate in Hardy-Weinberg equilibrium, denoted with the equation p² + 2pq + q² = 1.
3. Natural Selection
• If one phenotype results in better fitness, it will increase in frequency in subsequent generations.
• Selection can only produce change when there is variation in the population.
• The frequency of phenotypes is changed by natural selection, not individual genes or gene frequencies directly.
• The organism’s environment provides constraints for the strength and direction of selection.

2.  The Modern Synthesis

1. Continuous Variation
• Darwin didn’t understand how variation was maintained in a population. It wasn’t until the 1930s that biologists figured it out.
• Fisher, Haldane, and Wright showed how Mendelian genetics explain continuous variation.
• Continuous variation, natural selection, and the fieldwork of Dobzhansky, Mayr, and Simpson combined to create what is known today as the modern synthesis.
2. Maintenance of Variation
• The expression of phenotypes depends to some extent on environmental variation.
• Although selection can deplete variation, mutation can introduce new gene variants.
• Hidden variation accounts for cumulative evolutionary change, as in the case of a Pekingese dog whose ancestor was a wolf.

3. Natural Selection and Behavior

1. Behavior, not just morphology, can be affected by natural selection.
2. An example is the soapberry bug, which engages in mate guarding in Oklahoma but not in Florida because of a difference in the sex ratio.
3. Behavioral traits can be canalized, or found in a wide variety of environments, or can be plastic, allowing an organism to adjust its behavior in relation to local conditions.

4. Constraints on Adaptation

1. Three Necessities of Evolution
• The character must vary.
• Variation must affect reproductive success.
• The variation must be heritable.
2. Correlated Characteristics
• Many characters are correlated, either positively or negatively.
• Some genes control more than one character and are called pleiotropic.
• Sometimes evolutionary changes cause maladaptive responses.
• When evolution occurs quickly or the environment changes, populations can be in disequilibrium.
3. Genetic Drift
• Small populations can be greatly affected by changes in genetic variation.
• Genetic drift leads to unpredictable evolution that can isolate a population.
• Inbreeding can result in genetic drift, in terms of a loss of variation and potential increase in frequency of deleterious genes.
4. Local Adaptations
• Sometimes the final result achieved in adaptation is inferior but functional, as in the case of a compound eye versus a camera-type eye.
• Selection cannot favor intermediate types, so the local adaptation remains and does not evolve further.
5. Laws of Physics and Chemistry
• The strength of bones is proportional to their cross-section.
• Animal evolution negotiated a trade-off between body size and movement: an elephant is very large but moves slowly, while a squirrel is small and moves quickly.

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