Population Genetics - AP Biology

During prophase I homologous chromosomes will line up with one another, forming tetrads. During this lining up, DNA sequences can be exchanged between the homologous chromosomes. This type of genetic recombination is called crossing over, and allows the daughter cells of meiosis to be genetically unique from one another.

Crossing over can only occur between homologous chromosomes. Cells become haploid after meiosis I, and can no longer perform crossing over.

When chromatids "cross over," homologous chromosomes trade pieces of genetic material, resulting in novel combinations of alleles, though the same genes are still present. Crossing over occurs during prophase I of meiosis before tetrads are aligned along the equator in metaphase I.

By meiosis II, only sister chromatids remain and homologous chromosomes have been moved to separate cells. Recall that the point of crossing over is to increase genetic diversity. If crossing over did not occur until sometime during meiosis II, sister chromatids, which are identical, would be exchanging alleles. Since these chromatids are identical, this swap of material would not actually change the alleles of the chromatids.

Crossing over is a process that happens between homologous chromosomes in order to increase genetic diversity. During crossing over, part of one chromosome is exchanged with another. The result is a hybrid chromosome with a unique pattern of genetic material. Gametes gain the ability to be genetically different from their neighboring gametes after crossing over occurs. This allows for genetic diversity, which will help cells participate in survival of the fittest and evolution.

During crossing over, homologous chromosomes come together in order to form a tetrad. This close contact allows the nonsister chromatids from homolgous chromosomes to attach to one another and exchange nucleotide sequences. The word "nonsister" implies that the chromatids have the same genes, but are not exact copies of one another, as they come from separate chromosomes.

The crossing over of homologous chromosomes occurs in prophase I of meiosis. Prophase I of meiosis is characterized by the lining up of homologous chromosomes close together to form a structure known as a tetrad. A tetrad is composed of four chromatids.

Anaphase I is marked by the separation of homologous chromosomes, whereas in anaphase II there is the separation of sister chromatids. In anaphase I sister chromatids are still intact and connected at the centromere. Prophase II is similar to prophase in mitosis in that there is the break down of the nuclear membrane and the formation of spindle fibers in preparation for the separation of sister chromatids.

The tetrad, which divides into non-sister chromatids, exchanges genetic information in order to make the genetic pool more variant, and result in combinations of phenotypic traits that can occur outside of linked genotypic coding.

During prophase I, homologous chromosomes pair with each other and exchange genetic material in a process called chromosomal crossover. The exchange occurs in segments over a small region of homology (similarity in sequence, ie., the same alleles). The new combinations of DNA created during crossover provide a significant source of genetic variation.

Crossing over occurs when chromosomal homologs exchange information during metaphase of Meiosis I. During this stage, homologous chromosomes line up on the metaphase plate and exchange genetic information.

Crossing over occurs during prophase I when parts of the homologous chromosomes overlap and switch their genes.

The recombination frequency depends upon the distance between the genes; a larger distance between the genes increases the probability of crossing over occurring.

Genetic drift occurs as a result of chance events causing changes in the allele frequency of a population. It doesn't favor the most fit individuals, but occurs at random.

Mutations can contribute to genetic drift, however, genetic drift is a more specific answer and more relevant to the question at hand.

Small populations tend to have less genetic variation to begin with. Introducing a bottleneck effect further reduces variation and population size, amplifying the effect of genetic drift. This leaves them susceptible to changes in the environment that they may not be capable of adapting to due to limited differences among individuals.

A bottleneck effect is the term used to describe the loss of genetic variation that occurs after outside forces destroy most of a population. The few individuals left to reproduce pass their traits on to all of their offspring, which then may thrive without the competition of a large population. Eventually, there may be a large, very genetically similar population based on the traits of the few original survivors.

The founder effect describes the low genetic variation of a population derived from a small group of individuals in a new geographic location. Genetic drift is the random change of allele frequency in a population.

The founder effect describes the phenomenon when a smaller group that originally came from a part of a larger population forms their own population. This new population will likely have a biased gene pool that will not be identical to the parent population. For example, if a certain species of bird gains a mutation such that some members are capable of flying farther, these birds may eventually separate to a different location and form their own unique population with a higher predominance of the "sustained flight" mutation than the original population.

The founder effect, after a long time, can lead to speciation, but this is not an essential part of the founder effect. Introducing a new species to native populations may influence the balance of the ecosystem and change genetic frequencies, but is not linked to the founder effect. A decrease in genetic variety due to fluctuation of certain traits would more aptly describe the bottleneck effect.

The bottleneck effect describes the phenomenon when a population has a sudden reduction in the gene pool due to natural environmental events, natural disasters, disease, or human involvement. This reduction in the gene pool will likely cause a bias that did not exist in the original population. For example, suppose a population of birds has a small number with a mutation making them unable to fly. If a disease reaches this population that kills all birds when they reach an altitude above 50m, then the gene pool of the population will suddenly shift to favor the flightless birds.

The bottleneck effect, after a long time, could potentially lead to speciation, but this is not a defining factor of the effect. Introducing a new species can increase the pressures of natural selection, but does not directly relate to the bottleneck effect. A decrease in genetic variety due to a small number of individuals from a larger population establishing a new population more aptly describes the founder effect.

The founder effect describes a scenario in which a new population is started by a small group from a larger population. This smaller population is most likely not representative of the larger group and displays certain genetic bias. The high rate of Huntington's disease is most likely a result of the fact that the small group of European colonists had a high rate of the gene that produces the disease.

Natural selection and sympatric speciation do not apply in this situation. The bottleneck effect occurs when a large population is thinned, and a non-representative group of the original population is all that remains; this does not describe the situation presented above.

Genetic drift is a direct result of independent assortment. Since genes are not inherited by any organized mechanism, there are random fluctuations during which certain alleles experience an increase in frequency over others.

Genetic drift results in random changes in allele frequency; these changes are not a cause of genetic drift. In smaller populations and extreme cases, random changes can result in the loss of an allele entirely within the population. The results of genetic drift are more prominent in smaller populations due to their already reduced gene pool. Since genetic drift is random, both beneficial and harmful alleles can be promoted or eliminated.

Genetic drift cannot increase genetic diversity. The only way to increase genetic diversity is by the introduction of new traits and alleles. Genetic drift can reduce genetic diversity by eliminating alleles from a population, but is incapable of creating new traits. This can only be done through mutation.

Genetic drift describes the random selection of alleles that are passed from one generation to the next due to independent assortment in gametogenesis. Genetic drift cannot create new alleles, so it cannot increase genetic diversity (the number of alleles in a population). It can, however, decrease genetic diversity if an allele of a low frequency is not passed down to subsequent generations due to pure chance.

There is no hard and fast rule for whether genetic drift or natural selection have had a greater effect on shaping populations. Both have greatly shaped the populations present on Earth today, but their relative importance varies between species and has also varied over time. The conditions of Hardy-Weinberg equilibrium require that both natural selection and genetic drift be negligible. If genetic drift is occurring, then the population cannot be in Hardy-Weinberg equilibrium.

Genetic drift is the random process of alleles being passed from parents to offspring. Increasing genetic diversity in a population requires introducing a greater number of alleles, which can only occur through mutations or addition of unrelated members to the population. Genetic drift only affects how already-existing alleles are passed down.

If an allele has a high frequency at baseline, the chance of it being passed down to subsequent generations is higher than alleles of a lower frequency. Through random chance, a high-frequency allele can eventually have a frequency of 100%, becoming fixed in the population. Conversely, a low-frequency allele can eventually disappear from the population if none of the few parents who possess that allele happen to pass it onto their offspring.

Gene flow is a mechanism of evolution in which genes are transferred between populations. Two examples of gene flow are migration and horizontal gene transfer. In the case of migration, the movement of individuals into or out of a population also results in a transfer of alleles. Horizontal gene transfer (common in bacteria) is the transfer of genes through means other than reproduction (i.e. plasmid exchange).