BIO 128 - Chapter 19 The Evolution of Populations

nonrandom mating

- Results in a changing population - One reason it occurs is because of mate choice. For example, female peahens may prefer peacocks with bigger, brighter tails. - Another cause is the physical location, especially in larger populations spread over vast geographic distances where not all individuals have equal access to one another.

modern synthesis

- The coherent understanding of the relationship between natural selection and genetics - Describes how evolutionary processes, such as natural selection, can affect a population's genetic makeup, and how this can result in the gradual evolution of population and species.

Importance of the Hardy-Weinberg Principle

- The principle offers a useful model against which to compare real population changes. - It gives scientists a mathematical baseline of a non-evolving population to which they can compare evolving populations and thereby infer what evolutionary forces might be at play.

environmental variances

- the environment also influence phenotypes. A beachgoer is likely to have darker skin than a city dweller, for example, due to the regular exposure to the sun, an environmental factor - For some species the environment determines some major characteristics, such as gender (temperature and American alligator eggs).

Describe how these different forces can lead to different outcomes in terms of the population variation.

-Stabilizing: happens when extreme phenotypes on both ends of the spectrum are unfavorable. Ex: mouse population. -Directional: selects for phenotypes at one end of the spectrum of existing variation. Ex: peppered moth. -Diversifying: when two or more distinct phenotypes can each have their advantages for natural selection, while intermediate phenotypes are, on average, less fit. Ex: multiple male forms. -Frequency-dependent: favors phenotypes that are either common (positive frequency-dependent selection = decreases a population's genetic variance) or rare (negative frequency-dependent selection = serves to increase genetic variance). Ex: Blue, orange, and yellow Pacific Northwest lizards. -Sexual: selection pressures on males and females to obtain matings. Ex: Peacock's tail.

founder effect

An event that initiates an allele frequency change in an isolated part of the population, which is not typical of the original population. Ex:) causes differentiation between populations that span a great distance.

Explain why only natural selection can act upon heritable variation

Because alleles are passed from parents to offspring, those that confer beneficial traits or behaviors may be selected, while deleterious alleles may not.

alleles

Different forms of a gene The variants of a gene for a particular character that code for different traits associated with that character. Ex.) Flower color in pea plants - dominant purple and recessive white.

Explain how each evolutionary force can influence a population's allele frequencies.

Genetic drift stems from the chance occurrence that some individuals have more offspring than others and results in changes in allele frequencies that are random in direction. When individuals leave or join the population, allele frequencies can change as a result of gene flow. Natural selection is the only mechanism that generates adaption of organisms to their environments.

geographical variation

Geographic separation between populations can lead to differences in the phenotypic variation between those populations.

How do scientists use population genetics in studying population evolution?

It helps them to better understand that when frequencies of alleles or genotypes deviate from the expected values the population is evolving (or vice-versa).

Describe genetic drift and the bottleneck effect

It results in suddenly wiping out a large portion of the genome. At once, the survivors' genetic structure becomes the entire population's genetic structure, which may be very different from the pre-disaster population.

assortative mating

It's one common form of mate choice: an individual's preference to mate with partners who are phenotypically similar to themselves.

microevolution

Population change over time

Hardy-Weinberg Principle

States that a population's allele and genotype frequencies are inherently stable- unless some kind of evolutionary force is acting upon the population. Assumes conditions with no mutations, migration, emigration, or selective pressure for or against genotype, plus an infinite population.

polymorphisms

The different phenotypes a population's individuals display, or the different alleles of a particular gene they express

gene flow

The flow of alleles in and out of a population due to the migration of individuals or gametes. This variable flow of individuals in and out of the group not only changes the population's gene structure, but it can also introduce new genetic variation to the populations in different geological locations and habitats.

genetic structure

The frequencies of the resulting genotypes in a population.

macroevolution

The processes that gave rise to new species and higher taxonomic groups with widely divergent characters.

allele/gene frequency

The rate at which a specific allele appears within a population.

population genetics

The study of genetic variation within populations, which involves the examination and modeling of changes in the frequencies of genes and alleles in populations over space and time.

gene pool

The sum of all the alleles in a population.

genetic drift

When allele frequencies within a population change randomly with no advantage to the population over existing allele frequencies. - Natural events, such as an earthquake disaster that kills, at random, a large portion of the population, can magnify it.

mutation

changes to an organism's DNA

Describe the different types of variation in a population

genetic drift, gene flow, mutation, nonrandom mating, and environmental variances

adaptive evolution

selecting for beneficial alleles and thus increasing their frequency in the population, while selecting against deleterious alleles and thereby decreasing their frequency

Explain the different ways natural selection can shape populations.

stabilizing, directional, diversifying, frequency-dependent, and sexual selection

population variation

the distribution of phenotypes among individuals