Processes Of Evolution: Population Genetics

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the average number of offspring contributed by an individual of a particular genotype to the next generation.

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absolute fitness

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the proportionate reduction in gametic contribution of a particular genotype compared with the standard, most favored genotype due to selection.

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selection coefficient

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a measure of the proportionate contribution of offspring to the next generation by a particular genotype, compared with a standard genotype, which is the most favored genotype whose contribution is defined as 1.0.

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relative fitness

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In this model, the fitness of the three possible genotypes are as follows: AA = 1-s(1) Aa = 1 aa = 1-s(2)

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the hybrid vigor model or heterozygote superiority

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These two mechanisms are the only means by which new genetic factors may be introduced into a population.

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gene flow and mutation

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In the Continent-Island Model, this represents the frequency of q in the mainland population.

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q-beta

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Banding patterns on water snakes in Lake Erie illustrate this effect of gene flow on populations.

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homogenization

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This can be defined as changes in gene frequencies due to sampling error in finite populations.

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genetic drift

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This is the number of parents actually responsible for the genetic composition of the next generation.

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effective breeding size or effective population number

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Given the same mutation rate for a lethal and a deleterious allele, this would have a higher equilibrium frequency because dividing by a number that is between zero and one yields a larger number than the original numerator.

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deleterious allele (deleterious = u/s; lethal = u)

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When considering populations in which both selection and mutation are occurring, the equilibrium frequencies of dominant and recessive alleles in a population are directly related to this.

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mutation rate

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When considering populations in which both mutation and selection are occurring, the equilibrium frequencies of dominant and recessive alleles are indirectly related to this.

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the selection coefficient

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This is the BIOLOGICAL explanation for how lethal and deleterious recessive alleles can be maintained at a higher equilibrium frequency than lethal and deleterious and lethal dominant alleles.

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heterozygotic shielding

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This is the MATHEMATICAL explanation for how calculations of lethal and deleterious recessive alleles compute to higher equilibrium frequencies than those for dominant lethal and recessive alleles.

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the square root of a number between zero and one

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The fitness effects of mutations depend on this, as seen in the example of sickle-cell anemia and malaria prevalence.

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environment

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The best information on rate at which new alleles are created come from this type of mutation, which results in either no protein products or a dysfunctional protein for the gene. These estimates usually underestimate the rate of mutation.

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loss-of-function mutations

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These are enzymes differing in electrophoretic mobility as a result of allelic differences at a single locus.

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allozyme

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This type of locus has two or more alleles that exist at frequencies that are too large to be explained solely by recurrent mutation. In practice, at a locus exhibiting this, the most common allele has a frequency < 0.95.

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polymorphic locus

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This is the most important source of new genes, which results from retrotransposition and unequal crossing over during meiosis.

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gene duplication

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This is the mathematical representation of the Hardy-Weinberg law of equilibrium.

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p^2 + 2pq + q^2

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This is a local population that interbreeds in a panmictic manner.

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deme

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This is shuffled every generation by meiosis and reproduction mechanisms with the individuals in each generation just playing the role of the "temporary holding vessels."

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gene pool

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The fractional representation among all alleles at on locus in a population.

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gene frequency

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The fractional representation of the genetic characteristic of interest in the inheritance patterns of individuals.

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genotypic frequency

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In this case of extreme genetic drift the new population is established by very few individuals.

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founder effect

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In this case of genetic drift, populations may regain original size, but chance variations in allelic frequencies in the population are drastically reduced. The Northern Elephant Seal is an example of this effect.

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bottleneck effect

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This is a type of assortive mating that increases the frequency of homozygotes and decreases the frequency of heterozygotes.

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positive assortment

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Inbreeding affects THIS but not THIS, from generation to generation. (name them in order)

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genotypic frequencies and gene frequencies

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The fixation coefficient measures this.

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degree of inbreeding

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This type of trait can be divided into few, discrete phenotypic categories, and is determined by genes at 1-2 loci.

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discontinuous trait (aka, discrete or qualitative trait)

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This type of selection favors one extreme phenotype and results in a shifting of the mean phenotype.

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directional selection

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This type of selection has two adaptive norms within the population. Selection favors extreme phenotypes, and increases variance.

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disruptive selection

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The distribution of birth weights among human babies provides an example of this kind of selection.

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stabilizing selection

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This is the measure of the difference between the mean of the selected parents and the mean of all the individuals in the original unselected parent population.

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selection differential

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These are the two statistical variables that are used when samples are drawn from populations to characterize and analyze continuous traits.

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mean and variance

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Measure of the proportion of phenotypic variance in a trait due to genotypic differences.

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heritability

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This is equal to the slope (b) of the parent-offspring regression line for a given trait.

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narrow-sense heritability (h2)

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This type of trait variance can be mathematically represented as follows: V(p)= V(g)+V(e)+ V(ge), where V(g)- genotypic variance V(e)- environmental variance, V(ge) interaction variance

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total phenotypic variance

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This type of heritability is most important for determining the rate and amount of response to differential selection.

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heritability in the narrow sense

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In Candace Galen's studies of Skypilots (a type of flower), tundra skypilots were pollinated almost exclusively by this pollinator whereas timberline skypilots were known to be pollinated by many different insects, including flies, small bees, and some bumblebees. The difference in the pollinator type caused the tundra skypilots to be 12% larger than their timberline counterparts.

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bumblebees

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H. C. Bumpus (1899) measured English sparrows after a severe winter storm in New England. Survivors had lower variance in all traits when compared to non-survivors, providing evidence for this type of selection.

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stabilizing selection

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In the study of sticklebacks by Mike Bell, this type of predator was noted to select for reduction of the pelvic girdle and its components.

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invertebrate predators (insects)

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On an island in Lake Erie, this phenotype of the native water snakes is selected against because it is more vulnerable to predation on the island. However, the allele for this phenotype is maintained in the population due to migration from the mainland population where the phenotype is adaptive.

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the banded phenotype in Nerodia sipedon

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In Drosophila subobscura, this trait is controlled by an inversion. These inversions vary geographically in frequency of occurrence and appear to be adaptive because they resulted from natural selection in response to the same selective pressures on different continents.

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body size

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This term describes the reduction in mean fitness of a population from inbreeding resulting in homozygotes with deleterious or lethal recessive alleles.

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inbreeding depression

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This model of gene flow is similar to the island model but it is one dimensional & immigrants arrive only from neighboring populations (migrant only move to the next deme in line).

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the stepping stone model

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This is a type of point mutation that results in no amino acid change.

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a silent or samesense substitution

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These are mutations that have no effect on fitness.

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neutral mutations

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A type of chromosomal alteration where the chromosome breaks in two places, resulting in an intermediary segment that flips 180 degrees. The resulting alleles inside of this are inherited as a unit called a supergene.

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inversions

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the forward mutation rate

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the rate of dominant alleles (proportion / probability) mutating torecessive alleles (per generation, per locus)

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u = mutation rate of A→a,

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forward mutation rate

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v = mutation rate of a→A,

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backward mutation rate

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the backward mutation rate

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the rate of recessive alleles (proportion / probability) mutating todominant alleles (per generation, per locus)

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q^ = u / u + v [note p^ = v / u + v]

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The equilibrium frequency for q

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when q < q^

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»∆q is POSITIVE(q INCREASES)

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when q > q^

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»∆q is NEGATIVE(q DECREASES)

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»Notwithstanding the forces of natural selection, _____ _____ will reintroduce a disadvantageous or lethal allele into the population.

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recurrent mutation

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–In the case of a disadvantageous recessive allele, for example, the reduction in the frequency due to natural selection slows as the allele becomes more and more rare.
–As selection reduces the frequency of such an allele, mutation pressure increasing its frequency will eventually balance with natural selection.
–This process will lead to an equilibrium in the gene frequency, in which case the increases and decreases in gene frequency are counterbalanced.
–The allele frequency remains stable despite the ongoing processes of mutation and selection.

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–This is called a mutation-selection equilibriumor mutation-selection balance.

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What is a point mutation?

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Single-base substitutions.

A single point in the nucleotide sequence is altered. (An allele)

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p^≈u

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–For complete selection against the dominant allele(s = 1)

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• Given the same mutation rate for a deleterious allele and a lethal allele, the equilibrium frequency of a deleterious allele will be ______ than a lethal allele.

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greater

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q^ ≈√u

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For complete selection against the recessive allele(s = 1)

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–What is effect of selection and mutation on the frequency of recessive allele?

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» Given same mutation rate for a deleterious allele and a lethal allele, the equilibrium frequency of a deleterious allele will be greater than a lethal allele.

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p^ ≈u / s

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– In the case of incomplete selection against the dominant allele(s < 1) the equilibrium frequency of the dominant allele is:

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» equilibrium frequencies will be ___ when u is greater or when s is smaller.

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greater

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• Absolute fitness is calculated by...

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multiplying the probability of survival and the expected number of offspring produced an individual surviving to reproduce.

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____ ____ is a ratio of absolute fitness

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relative fitness

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relative fitness symbolized by

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• "W" and values range 0 -1

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Relative intensity (strength) of selection against each genotype is the ______ _______.

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selection coefficient

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Selection coefficient symbolized by

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S

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survival rate symbol

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λ, lambda

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This is the proportional increase or decrease of each genotype after a selective event

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survival rate

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– N genotype after selection /N genotypebefore selection

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survival rate

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• Use the survival rate to get _____ ____ of respective genotypes (compared with the most fit):

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relative fitness(W, relative reproductive ability)

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» The continent-island model represents

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one-way dispersal from a large, continental population to a smaller, island population which is isolated.

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CONTINENT-ISLAND MODEL
q'alpha= qalpha-m(qalpha-qbeta)

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» Frequency of a after one generation represents contributions of non-migrant and migrant portions of the population.

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CONTINENT-ISLAND MODEL
»Change in gene frequency of island population is due to

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• difference in frequencies of a between the two populations
• the proportion of genes contributed to the next generation of the island population by immigrants from the continent.

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CONTINENT-ISLAND MODEL
»The direction of changein gene frequency of a is determined by the ________ in the two populations, i.e. the sign of (qbeta-qalpha).

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relative frequency of a

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CONTINENT-ISLAND MODEL
»______ occurs either when m = 0 (no dispersal)or qalpha= qbeta(equal gene frequencies in both populations).

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Equilibrium(∆q = 0)

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»________ Model --Each of a number of populations inhabits a small "island" of its own.
• An individual moving from one deme (population) to another is equally likely to move to any other deme.
• In considering any one deme, a small percentage (proportion) of mindividuals in the demewill have dispersed from another deme.

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Island Model

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» __________ Model--similar to the island model, but it is one-dimensional and immigrants arrive only from neighboring populations; that is, migrants always move to the next demein line.

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Stepping-Stone

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» ________ Model--populations are continuously distributed, and mates are born close together as dispersal distances are rather short.

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Isolation-By-Distance (Continuous)

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s2= pq/2N

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–In a population of N breeding adults which have 2N genes of proportions p and q,
• the variance(s2) of the allele frequency in the next generation is

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s =√(pq/2N).

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–In a population of N breeding adults which have 2N genes of proportions p and q,
• the standard deviation(s) of the allele frequency in the next generation is

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» Loss of an allele results in the loss of heterozygosity at the locus, which is called _______.

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decay of variabilty

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_______ leads to loss of heterozygosity through the random fixation and loss of alleles

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Genetic drift

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Genetic drift is more powerful in ____populations than it is in_____ ones.

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small, large

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–--random mating where each individual member of a population has an equal chance (probability) of mating with every other individualof the opposite sex.

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Panmixia

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• Types of non-random mating:

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– Sexual selection
– Assortive mating
-- Positive assortment

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--individuals preferentially mate with their own phenotype

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»Positive assortment

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» Individual preferences for a particular phenotype where genotype affects choice of a mate.

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Assortive mating

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» Members of one sex show a consistent preferencefor a particular phenotypeof the opposite sex.

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–Sexual selection

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• The _________ of a trait is the variance in the phenotypic measurements for a particular trait, and it provides an indication of the variation in the trait

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phenotypic variance(VP)

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• There are three components of the phenotypic variance, which represent the three sources of variation in the trait under consideration:

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• Genotypic variance(VG)
• Environmental variance(VE)
• Interaction variance(VGE)

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• VP= VG+ VE+ VGE

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The total phenotypic variance

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• VP= VA+ VD+ VI+ VE+ VGE

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equation for partitioning the total phenotypic variance

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–______is a measure of the proportion of the phenotypic variance in a trait that is due to genotypic differences.

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Heritability(H)

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This measure of heritability includes all genetic influences on the phenotype, whether due to additive, dominance, or interactive effects.

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broad-sense heritability

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–If H2= 0,

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none of the phenotypic variance is due to genetic differences among individuals.

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a value of H2= 1 indicates that

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all of the variation in the phenotype is determined by genetic differences.

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H2= VG/ VP, where VG= VA+ VD+ VI

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broad-sense heritability

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–The ratio of the additive genetic variance to the total phenotypic variance.
–As such, it is a measure of only the additive genetic influences on the phenotype.

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–Heritability in the Narrow Sense (h2)

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– h2= VA/ VP

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–Heritability in the Narrow Sense (h2)

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»The amount of change in a character in one generation is the ______.

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selection response(R).

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The two types of point mutations

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Replacement substitution and silent substitution

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»The ________ is the difference between the mean of the phenotype of the individuals selected to be parents of the next generation(selected population) and mean phenotype of the original, unselected population.

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The selection differential

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»Artificial selection involves a special type of directional selection, which is called _________.

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truncation selection

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»______ is the simplest form of directional selection where all individuals above a given threshold value are selected for breeding purposes.

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Truncation selection

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–Selection Response
»When 0 < h2< 1, the response to selection is quantified as ______

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R = h2S.

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R = h2S.

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»It is the change in a quantitative trait resulting from one generation of natural selection.
»Thus, it is the difference between the unselected, parental population mean and the offspring mean for a phenotypic trait.

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»The _________ is the difference between the mean of the selected parents and the mean of all individuals in the original, unselected (parental) population.

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selection differential (S)

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In ________ selection, fitness constantly increases (or decreases) with the value of the trait.

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directional

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Does direction selection alter the amount of variation in a population?

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Yes, it reduces the variation, but not dramatically.

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In _______ selection, individuals with intermediate values of a trait have highest fitness.

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stabilizing

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Does stabilizing selection alter the variance in a population?

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Yes, it reduces the tails of the trait's distribution, thereby reducing the variation.

Front 116

In _______ selection, individuals with extreme values of a trait have the highest fitness.

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disruptive

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Does disruptive selection alter the variance in a population?

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Yes, it trims off the top of the trait's distribution, thereby increasing the variance.

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The mutation rate is ___________.

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the best information on the rate at which new alleles are created.

Front 119

Rates of mutation are specified for ______ loci.

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individual

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Mutation rate is based off of ____.

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loss of function mutations

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Selectionist (balance) theory

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Theory trying to explain why pop'ns are so genetically diverse.

- States that genetic diversity occurs b/c natural selection favors rare individuals, heterozygotes, or different alleles at different times or places thus maintaining genetic diversity

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Neutral theory

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Theory trying to explain why pop'ns are so genetically diverse.

- States that most alleles at most polymorphic loci are selectively equivalent and most genetic diversity occurs b/c it's not eliminated by natural selection

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Most important source of new genes is ____ _____, which results from retrotransposition and unequal crossing over during meiosis

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gene duplication

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What is an inversion?

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affects the linkage of alleles at different loci due to breakage

They don't align properly during synapsis.

If crossing over occurs within an inversion, the resulting chromosome is dysfunctional.

As a result, the alleles inside an inversion are inherited as a supergene