Overall, the main sources of genetic variation are the formation of new alleles, the altering of gene number or position, rapid reproduction, and sexual reproduction. Apply the law of segregation to determine the chances of a particular genotype arising from a genetic cross. Observing that true-breeding pea plants with contrasting traits gave rise to F 1 generations that all expressed the dominant trait and F 2 generations that expressed the dominant and recessive traits in a ratio, Mendel proposed the law of segregation.
The law of segregation states that each individual that is a diploid has a pair of alleles copy for a particular trait. Each parent passes an allele at random to their offspring resulting in a diploid organism. The allele that contains the dominant trait determines the phenotype of the offspring. In essence, the law states that copies of genes separate or segregate so that each gamete receives only one allele.
For the F 2 generation of a monohybrid cross, the following three possible combinations of genotypes could result: homozygous dominant, heterozygous, or homozygous recessive. The equal segregation of alleles is the reason we can apply the Punnett square to accurately predict the offspring of parents with known genotypes.
The behavior of homologous chromosomes during meiosis can account for the segregation of the alleles at each genetic locus to different gametes. As chromosomes separate into different gametes during meiosis, the two different alleles for a particular gene also segregate so that each gamete acquires one of the two alleles.
Independent assortment allows the calculation of genotypic and phenotypic ratios based on the probability of individual gene combinations. Use the probability or forked line method to calculate the chance of any particular genotype arising from a genetic cross.
The independent assortment of genes can be illustrated by the dihybrid cross: a cross between two true-breeding parents that express different traits for two characteristics. Consider the characteristics of seed color and seed texture for two pea plants: one that has green, wrinkled seeds yyrr and another that has yellow, round seeds YYRR.
Therefore, the F 1 generation of offspring all are YyRr. For the F2 generation, the law of segregation requires that each gamete receive either an R allele or an r allele along with either a Y allele or a y allele. The law of independent assortment states that a gamete into which an r allele sorted would be equally likely to contain either a Y allele or a y allele.
Thus, there are four equally likely gametes that can be formed when the YyRr heterozygote is self-crossed as follows: YR, Yr, yR, and yr. These are the offspring ratios we would expect, assuming we performed the crosses with a large enough sample size. Independent assortment of 2 genes : This dihybrid cross of pea plants involves the genes for seed color and texture.
Because of independent assortment and dominance, the dihybrid phenotypic ratio can be collapsed into two ratios, characteristic of any monohybrid cross that follows a dominant and recessive pattern. Ignoring seed color and considering only seed texture in the above dihybrid cross, we would expect that three-quarters of the F 2 generation offspring would be round and one-quarter would be wrinkled.
Similarly, isolating only seed color, we would assume that three-quarters of the F 2 offspring would be yellow and one-quarter would be green.
The sorting of alleles for texture and color are independent events, so we can apply the product rule.
These proportions are identical to those obtained using a Punnett square. When more than two genes are being considered, the Punnett-square method becomes unwieldy. It would be extremely cumbersome to manually enter each genotype.
Whereas inbreeding can lead to a reduction in genetic variation, outbreeding can lead to an increase. Sometimes, there can be random fluctuations in the numbers of alleles in a population. These changes in relative allele frequency, called genetic drift , can either increase or decrease by chance over time. Typically, genetic drift occurs in small populations, where infrequently-occurring alleles face a greater chance of being lost. Once it begins, genetic drift will continue until the involved allele is either lost by a population or is the only allele present at a particular gene locus within a population.
Both possibilities decrease the genetic diversity of a population. Genetic drift is common after a population experiences a population bottleneck. A population bottleneck arises when a significant number of individuals in a population die or are otherwise prevented from breeding, resulting in a drastic decrease in the size of the population. Genetic drift can result in the loss of rare alleles, and can decrease the size of the gene pool.
Genetic drift can also cause a new population to be genetically distinct from its original population, which has led to the hypothesis that genetic drift plays a role in the evolution of new species. How does the physical distribution of individuals affect a population?
A species with a broad distribution rarely has the same genetic makeup over its entire range. For example, individuals in a population living at one end of the range may live at a higher altitude and encounter different climatic conditions than others living at the opposite end at a lower altitude.
What effect does this have? At this more extreme boundary, the relative allele frequency may differ dramatically from those at the opposite boundary. Distribution is one way that genetic variation can be preserved in large populations over wide physical ranges, as different forces will shift relative allele frequencies in different ways at either end.
Migration is the movement of organisms from one location to another. Although it can occur in cyclical patterns as it does in birds , migration when used in a population genetics context often refers to the movement of individuals into or out of a defined population.
What effect does migration have on relative allele frequencies? If the migrating individuals stay and mate with the destination individuals, they can provide a sudden influx of alleles. After mating is established between the migrating and destination individuals, the migrating individuals will contribute gametes carrying alleles that can alter the existing proportion of alleles in the destination population. How do populations respond to all these forces?
As relative allele frequencies change, relative genotype frequencies may also change. Each genotype in the population usually has a different fitness for that particular environment.
In other words, some genotypes will be favored, and individuals with those genotypes will continue to reproduce. Other genotypes will not be favored: individuals with those genotypes will be less likely to reproduce. What type of genotype would be unfavorable? Unfavorable genotypes take many forms, such as increased risk of predation, decreased access to mates, or decreased access to resources that maintain health. One reason is simple mate choice or sexual selection; for example, female peahens may prefer peacocks with bigger, brighter tails.
Traits that lead to more matings for an individual lead to more offspring and through natural selection, eventually lead to a higher frequency of that trait in the population. Assortative mating in the American Robin : The American Robin may practice assortative mating on plumage color, a melanin based trait, and mate with other robins who have the most similar shade of color.
However, there may also be some sexual selection for more vibrant plumage which indicates health and reproductive performance. Another cause of nonrandom mating is physical location. This is especially true in large populations spread over large geographic distances where not all individuals will have equal access to one another.
Some might be miles apart through woods or over rough terrain, while others might live immediately nearby. Genes are not the only players involved in determining population variation. Phenotypes are also influenced by other factors, such as the environment. A beachgoer is likely to have darker skin than a city dweller, for example, due to regular exposure to the sun, an environmental factor. Some major characteristics, such as gender, are determined by the environment for some species.
For example, some turtles and other reptiles have temperature-dependent sex determination TSD. TSD means that individuals develop into males if their eggs are incubated within a certain temperature range, or females at a different temperature range. Temperature-dependent sex determination : The sex of the American alligator Alligator mississippiensis is determined by the temperature at which the eggs are incubated. Eggs incubated at 30 degrees C produce females, and eggs incubated at 33 degrees C produce males.
Geographic separation between populations can lead to differences in the phenotypic variation between those populations. Such geographical variation is seen between most populations and can be significant. One type of geographic variation, called a cline, can be seen as populations of a given species vary gradually across an ecological gradient.
Geographic variation in moose : This graph shows geographical variation in moose; body mass increase positively with latitude.
This is considered a latitudinal cline. Alternatively, flowering plants tend to bloom at different times depending on where they are along the slope of a mountain, known as an altitudinal cline. If there is gene flow between the populations, the individuals will likely show gradual differences in phenotype along the cline. Restricted gene flow, on the other hand, can lead to abrupt differences, even speciation.
Privacy Policy. Skip to main content. The Evolution of Populations. Search for:. Population Genetics. Genetic Variation Genetic variation is a measure of the variation that exists in the genetic makeup of individuals within population.
Learning Objectives Assess the ways in which genetic variance affects the evolution of populations. Key Takeaways Key Points Genetic variation is an important force in evolution as it allows natural selection to increase or decrease frequency of alleles already in the population.
Genetic variation is advantageous to a population because it enables some individuals to adapt to the environment while maintaining the survival of the population. Key Terms genetic diversity : the level of biodiversity, refers to the total number of genetic characteristics in the genetic makeup of a species crossing over : the exchange of genetic material between homologous chromosomes that results in recombinant chromosomes phenotypic variation : variation due to underlying heritable genetic variation ; a fundamental prerequisite for evolution by natural selection genetic variation : variation in alleles of genes that occurs both within and among populations.
Genetic Drift Genetic drift is the change in allele frequencies of a population due to random chance events, such as natural disasters. Learning Objectives Distinguish between selection and genetic drift. Key Takeaways Key Points Genetic drift is the change in the frequency of an allele in a population due to random sampling and the random events that influence the survival and reproduction of those individuals.
The bottleneck effect occurs when a natural disaster or similar event randomly kills a large portion i. The founder effect occurs when a portion of the population i.
Small populations are more susceptible genetic drift than large populations, whose larger numbers can buffer the population against chance events. Key Terms genetic drift : an overall shift of allele distribution in an isolated population, due to random sampling founder effect : a decrease in genetic variation that occurs when an entire population descends from a small number of founders random sampling : a subset of individuals a sample chosen from a larger set a population by chance.
Learning Objectives Explain how gene flow and mutations can influence the allele frequencies of a population. Key Takeaways Key Points Plant populations experience gene flow by spreading their pollen long distances. Animals experience gene flow when individuals leave a family group or herd to join other populations. The flow of individuals in and out of a population introduces new alleles and increases genetic variation within that population.
Some mutations are harmful and are quickly eliminated from the population by natural selection; harmful mutations prevent organisms from reaching sexual maturity and reproducing.
0コメント