Chapter 3: The Modern Synthesis
1. Population Genetics
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.
Even though humans choose their mates, we cannot select for all 30,000 gene loci, so mating is
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 p
+ 2pq + q
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
The organism’s environment provides constraints for the strength and direction of selection.
2. The Modern Synthesis
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.
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
Behavior, not just morphology, can be affected by natural selection.
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.
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
Three Necessities of Evolution
The character must vary.