Chapter 3

Population Genetics: The branch of biology dealing with the processes that change the genetic composition of populations through time.

Genotypic Frequency: The fraction of individuals in a population that have a particular genotype.

Gene Frequency: The fraction of the genes at a genetic locus that are a particular allele (also called allele frequency). For example, a population that contains 250 AA individuals, 200 AS individuals, and 50 SS individuals has 700 copies of the A allele and 300 copies of the S allele; therefore, the frequency of the S allele is 0.3.

Modern Synthesis: An explanation for the evolution of continuously varying traits that combines the theory and empirical evidence of both Mendelian genetics and Darwinism.

Environmental Variation: Phenotypic differences between individuals that exist because those individuals developed in different environments.

Mutation: A spontaneous change in the chemical structure of DNA.

Mate Guarding: A behavior in which the male defends his mate after copulation to prevent other males from mating with her.

Sex Ratio: The number of individuals of one sex in relation to the number of individuals of the opposite sex. By convention, sex ratios are generally expressed as the number of males to the number of females.

Canalized: Of traits: the quality of being very insensitive to environmental conditions during development, resulting in similar phenotypes in a variety of environments

Plastic: Of traits: the quality of being very sensitive to environmental conditions during development, resulting in different phenotypes in different environments

Positively correlated: Of two variables: having a statistical relationship such that larger values of one variable tend to co-occur with larger values of the other variable. For example, in human populations, the height and weight of individuals are positively correlated.

Negatively Correlated: Of two variables: having a statistical relationship such that the larger values of one variable tend to co-occur with smaller values of the other variable. For example, the size and number of seeds produced by an individual plant are negatively correlated in some plant populations.

Pleiotropic effect: A phenotypic effect created by a gene that influences multiple characters.

Correlated response: An evolutionary change in one character caused by selection on a second, correlated character. For example, selection favoring only long legs will also increase arm length if arm length and leg length are positively correlated.

Maladaptive: Detrimental to fitness.

Sampling Variation: The variation in the composition of small samples drawn from a large population.

Genetic Drift: Random change in gene frequencies due to sampling variation that occurs in any finite population. Genetic drift is more rapid in small populations than in large populations.

Fixation: Of a population: a state in which all the individuals in the population are homozygous for the same allele at a particular locus.

Cameratype eye: An eye in which light passes through a transparent opening and is then focused by a lens on photosensitive tissue. Camera-type eyes are found in vertebrates, mollusks, and some arthropods.

Compound Eye: An eye in which the image is formed by many discrete photoreceptors. Compound eyes are found in insects and other arthropods.

Development: All the processes by which the single-celled zygote is transformed into a multicellular adult.

Mendel believed variation to be discontinuous (yellow vs green, wrinkled vs round)

Darwin believed evolution worked through gradual accumulation of small changes in continuously variable traits

How variation is maintained

Mutation in gametes
Variation hidden in polygenic traits
Heterozygote individuals

Correlated Characters:

Example people with albinism also have vision problems

Pleiotropy: when one gene influences several traits

Example: mutation in the gene coding for melanin results in white hair, skin, and light eyes. Alongside changing how the eye develops causing shaky eyes, light sensitivity

Ghosts of evolution

Example: avocado have massive seeds because they used to be consumed by giant sloths

Evolutionary Mismatch:

Example: humans crave salty sweet fatty foods as those used to be rare however they are no abundant and cheap yet we still crave them

Humans have a nearly insatiable appetite for sugar, fat, and salt because:

such appetites were an adaptive response to pre-modern environments.

Path Dependence and Fitness Landscapes:

The relationship between trait values and fitness is not linear – it is curvy like a mountain profile.
Selection causes populations to “go uphill” and increase fitness from their current position to a local optimum (A, B, C).
To travel from A or C to B, a population would need to cross a valley of lower fitness, but selection opposes traits in those zones

Constraints of physics and chemistry:

Animals are limited in how large in size they can become as mass increases as a function of body length and strength of bones and muscles has an upper limit

Goal of evolutionary theory is to determine how genotype frequencies change over time

Evolutionary change can occur through random genetic drift

Mate guardian is a male strategy to prevent other males from mating with an individual female

genotype frequencies must add up to 1.0 and the frequency of the heterozygote is 0.5, that means that the total of the two homozygote frequencies must also be 0.5. Since the frequencies of the two homozygotes are equal, each must have a frequency of 0.25.

For selection to occur, there must be variation within that character or trait. Otherwise, there would be no alternatives for selection to work on.

For a character to be subject to natural selection, it must be able to be inherited from one generation to the next.

Natural selection requires there to be an impact on reproductive success.

Correlated characters can occur because of pleiotropy

Some genes have a pleiotropic effect, meaning one gene affects more than one character.

A correlated response to selection can cause characters to change in maladaptive directions.

In some cases, the shift in one correlated character can lead to a shift in the other character; however, one of the shifts may be maladaptive

Correlated characters are nearly always positively correlated.

Although some correlations are positive (i.e., when one character increases, so too does the other), some correlations are negative (i.e., when one character increases, the other character decreases).

When selection changes the mean value of one correlated character, the other character's mean value will also change.

Although natural selection does increase the adaptedness of a population, it doesn't necessarily lead to the best phenotype.

Sampling variation is a function of genetic drift, wherein random changes in gene frequencies may occur in small populations.

Negative correlations refer to statistical relationships between two traits, in which the increase in one is associated with a decrease in the other.

Behavioral plasticity is most likely to evolve when environments are variable

Genetic drift causes more rapid change in small populations than in large populations.