Population Biology by Owen Borville 9.23.2024
Modern synthesis is the integration of genetics and evolution theory, the relationship between natural selection and genetics that took place in the 1940's. Modern synthesis describes how evolutionary processes like natural selection, can affect a population's genetic makeup and, in turn, how this can result in the gradual evolution of population and species.
The theory also connects population change over time, microevolution, with the proposed theory of macroevolution that gave rise to new species and higher taxonomic groups with widely divergent characters.
Population genetics is the study of how selective forces change the allele frequencies in a population over time. The allele frequency or gene frequency is the rate at which a specific allele appears within a population, and can change depending on environmental factors. The gene pool is the sum of all the alleles in a population that the individuals carry.
Genetic drift is the effect of chance on a population's gene pool, when allele frequencies within a population change randomly with no advantage to the population over existing allele frequencies. The founder effect is the event that initiates an allele frequency change in an isolated part of the population, which is not typical of the original population.
The Hardy-Weinberg principle of equilibrium states that a population’s allele and genotype frequencies are inherently stable unless some kind of evolutionary force is acting upon the population, neither the allele nor the genotypic frequencies would change. The Hardy-Weinberg principle assumes a hypothetical and infinitely large population and conditions with no mutations, migration, emigration, or selective pressure for or against genotype.
Genetic structure is the distribution of the different possible genotypes in a population, from which scientists can predict phenotype distribution.
Population variation is the distribution of phenotypes in a population, influenced by the population's genetic structure and the environment. Heritability is the fraction of the phenotype variation that we can attribute to genetic differences, or genetic variability, among individuals in a population. Genetic variability is the diversity of alleles and genotypes within a population.
Inbreeding is the mating of closely related individuals, which can have the undesirable effect of bringing together deleterious recessive mutations that can cause abnormalities and susceptibility to disease. Inbreeding depression is the increased risk in abnormalities and disease in inbreeding populations.
Selection pressure is the driving selective force and environmental factor that causes one phenotype to be better than another. Genetic drift is the effect of chance on a population's gene pool, where the population's allele and genotype frequencies can change. Genetic drift affects smaller populations more than larger populations.
The bottleneck effect is when genetic drift is magnified because of a natural catastrophe event (such as an earthquake or flood) where the entire genetic structure of the population changes suddenly. If a portion of a populations suddenly leaves or is physically separated from the rest of the population, genetic drift can occur, resulting in the founder effect.
Gene flow is the flow of alleles in and out of a population due to the migration of individuals or gametes, such as with pollen in plants. Mutations are changes to an organism’s DNA and are an important driver of diversity in populations, causing change within a species. Nonrandom mating is the changes in a population’s gene pool due to mate choice or other forces that cause individuals to mate with certain phenotypes more than others. Assortative mating is when individuals tend to mate with those who are phenotypically similar to themselves. Another cause of nonrandom mating is physical location.
Environmental variation is when environmental factors determine population variation, such as temperature and sun exposure. Geographical variation is differences in the phenotypic variation between populations that are separated geographically. A cline is a gradual geographic variation across an ecological gradient.
Adaptive evolution is the increase in frequency of beneficial alleles and decrease in deleterious alleles due to natural selection. Evolutionary fitness is the individual's ability to survive and reproduce by natural selection. Relative fitness is an individual's fitness (ability to survive and reproduce) compared to other organisms in the population.
There are several ways that selection can affect a population variation: Stabilizing selection is a natural selection that favors average phenotypes, or selecting against extreme variation. Directional selection is selection that favors phenotypes at one end of the spectrum of existing variation. Diversifying selection is selection that favors two or more distinct phenotypes. Frequency-dependent selection is selection that favors phenotypes that are either common (positive frequency-dependent selection) or rare (negative frequency-dependent selection).
A sexual dimorphism is a phenotypic difference between a population's males and females. Sexual dimorphism is the condition where different sexes of the same species have different physical characteristics, including traits that aren't directly related to reproduction, such as size, color, body parts (like horns, antlers, tusks), behavior (aggressive or nurturing), and vocalizations (songbirds).
The handicap principle is the theory of sexual selection that argues that only the fittest individuals can afford costly traits. The good genes hypothesis is the theory of sexual selection that argues individuals develop impressive ornaments to show off their efficient metabolism or ability to fight disease. An honest signal is a trait that gives a truthful impression of the individual's fitness, found both in the handicap principle and the good genes hypothesis.
Modern synthesis is the integration of genetics and evolution theory, the relationship between natural selection and genetics that took place in the 1940's. Modern synthesis describes how evolutionary processes like natural selection, can affect a population's genetic makeup and, in turn, how this can result in the gradual evolution of population and species.
The theory also connects population change over time, microevolution, with the proposed theory of macroevolution that gave rise to new species and higher taxonomic groups with widely divergent characters.
Population genetics is the study of how selective forces change the allele frequencies in a population over time. The allele frequency or gene frequency is the rate at which a specific allele appears within a population, and can change depending on environmental factors. The gene pool is the sum of all the alleles in a population that the individuals carry.
Genetic drift is the effect of chance on a population's gene pool, when allele frequencies within a population change randomly with no advantage to the population over existing allele frequencies. The founder effect is the event that initiates an allele frequency change in an isolated part of the population, which is not typical of the original population.
The Hardy-Weinberg principle of equilibrium states that a population’s allele and genotype frequencies are inherently stable unless some kind of evolutionary force is acting upon the population, neither the allele nor the genotypic frequencies would change. The Hardy-Weinberg principle assumes a hypothetical and infinitely large population and conditions with no mutations, migration, emigration, or selective pressure for or against genotype.
Genetic structure is the distribution of the different possible genotypes in a population, from which scientists can predict phenotype distribution.
Population variation is the distribution of phenotypes in a population, influenced by the population's genetic structure and the environment. Heritability is the fraction of the phenotype variation that we can attribute to genetic differences, or genetic variability, among individuals in a population. Genetic variability is the diversity of alleles and genotypes within a population.
Inbreeding is the mating of closely related individuals, which can have the undesirable effect of bringing together deleterious recessive mutations that can cause abnormalities and susceptibility to disease. Inbreeding depression is the increased risk in abnormalities and disease in inbreeding populations.
Selection pressure is the driving selective force and environmental factor that causes one phenotype to be better than another. Genetic drift is the effect of chance on a population's gene pool, where the population's allele and genotype frequencies can change. Genetic drift affects smaller populations more than larger populations.
The bottleneck effect is when genetic drift is magnified because of a natural catastrophe event (such as an earthquake or flood) where the entire genetic structure of the population changes suddenly. If a portion of a populations suddenly leaves or is physically separated from the rest of the population, genetic drift can occur, resulting in the founder effect.
Gene flow is the flow of alleles in and out of a population due to the migration of individuals or gametes, such as with pollen in plants. Mutations are changes to an organism’s DNA and are an important driver of diversity in populations, causing change within a species. Nonrandom mating is the changes in a population’s gene pool due to mate choice or other forces that cause individuals to mate with certain phenotypes more than others. Assortative mating is when individuals tend to mate with those who are phenotypically similar to themselves. Another cause of nonrandom mating is physical location.
Environmental variation is when environmental factors determine population variation, such as temperature and sun exposure. Geographical variation is differences in the phenotypic variation between populations that are separated geographically. A cline is a gradual geographic variation across an ecological gradient.
Adaptive evolution is the increase in frequency of beneficial alleles and decrease in deleterious alleles due to natural selection. Evolutionary fitness is the individual's ability to survive and reproduce by natural selection. Relative fitness is an individual's fitness (ability to survive and reproduce) compared to other organisms in the population.
There are several ways that selection can affect a population variation: Stabilizing selection is a natural selection that favors average phenotypes, or selecting against extreme variation. Directional selection is selection that favors phenotypes at one end of the spectrum of existing variation. Diversifying selection is selection that favors two or more distinct phenotypes. Frequency-dependent selection is selection that favors phenotypes that are either common (positive frequency-dependent selection) or rare (negative frequency-dependent selection).
A sexual dimorphism is a phenotypic difference between a population's males and females. Sexual dimorphism is the condition where different sexes of the same species have different physical characteristics, including traits that aren't directly related to reproduction, such as size, color, body parts (like horns, antlers, tusks), behavior (aggressive or nurturing), and vocalizations (songbirds).
The handicap principle is the theory of sexual selection that argues that only the fittest individuals can afford costly traits. The good genes hypothesis is the theory of sexual selection that argues individuals develop impressive ornaments to show off their efficient metabolism or ability to fight disease. An honest signal is a trait that gives a truthful impression of the individual's fitness, found both in the handicap principle and the good genes hypothesis.