Test: Evolution and Population Dynamics - 1 - MCAT MCQ

Recessive Lethal Allele: A recessive lethal allele is one that is expressed phenotypically only when an individual inherits two copies of the allele (one from each parent). This means that individuals who carry only one copy of the recessive lethal allele (referred to as carriers) do not exhibit the lethal phenotype and can lead normal lives, reproduce, and pass on the allele to their offspring.

Heterozygous Carriers: Heterozygous carriers of a recessive lethal allele have one copy of the normal allele and one copy of the recessive lethal allele. Since the normal allele is dominant over the recessive lethal allele, carriers do not show any negative effects. As a result, carriers have equal chances of survival and reproductive success as individuals without the allele.

Persistence in the Population: The recessive lethal allele can persist in the population through generations because carriers, who are phenotypically normal, can pass on the allele to their offspring without any negative effects on their own fitness. In each generation, some individuals will inherit the recessive lethal allele from their carrier parents, but they will only exhibit the lethal phenotype if they inherit two copies. This "hidden" nature of the allele in carriers allows it to remain in the gene pool over time.

The bottleneck effect occurs when a population undergoes a dramatic reduction in size, often due to a severe environmental event such as a natural disaster or disease outbreak. In this case, the drought caused a significant decrease in the fly population.

During the drought, the population experienced a severe reduction in numbers, with almost the entire population dying. This resulted in a small surviving population that was likely composed of a random subset of the original population. As a result, the genetic diversity within the population was greatly reduced.

After the drought, as the population begins to recover, the next generation consists of twice as many black-bodied flies as tan-bodied flies. This change in the relative proportions of different traits within the population suggests that the surviving individuals had a higher representation of black-bodied flies compared to tan-bodied flies.

The bottleneck effect can lead to changes in allele frequencies within a population, as certain traits may become more or less common due to chance. In this case, the drought acted as a bottleneck event, reducing the population size and leading to a random change in the frequency of black-bodied and tan-bodied flies.

• The data in the table shows that over three generations the number of purple flowers is increasing, whereas the number of white flowers is decreasing.
• Stabilizing selection describes a process in which extremes of a trait (being very tall or very short for example) are selected against and an intermediary trait (like being average height) is selected for. This does not appear to be happening in this scenario.
• A high fecundity means an organism is able to produce more offspring. In this case, it is possible that purple flowers have a HIGHER fecundity, but probably not the other way around.

II. Gametic Isolation: Gametic isolation occurs when the gametes (sperm and egg) of different species are unable to fuse or undergo successful fertilization. This can be due to differences in gamete structure, biochemistry, or other mechanisms that prevent successful fertilization.

IV. Temporal Isolation: Temporal isolation occurs when two species have different mating times or breeding seasons, preventing them from coming into contact and mating with each other.

To calculate the number of healthy carriers of the recessive allele, we need to use the Hardy-Weinberg equilibrium equation. According to the equation, in a population in equilibrium, the frequency of the homozygous dominant genotype (DD) is given by the square of the frequency of the dominant allele (D), and the frequency of the heterozygous genotype (Dd) is given by 2 times the frequency of the dominant allele (D) multiplied by the frequency of the recessive allele (d).

Given that the frequency of the recessive allele (d) is 10% or 0.10, the frequency of the dominant allele (D) would be 1 - 0.10 = 0.90.

The frequency of the DD genotype would be (0.90)² = 0.81.
The frequency of the Dd genotype would be 2 × 0.90 × 0.10 = 0.18.

To calculate the number of people who are healthy carriers (Dd genotype), we multiply the frequency of the Dd genotype by the total population size:

Number of healthy carriers = Frequency of Dd genotype × Total population size
= 0.18 × 100,000
= 18,000

Therefore, the expected number of healthy carriers of the recessive allele in the population is 18,000.

The correct answer is D. 18,000.

The term that describes alterations in the composition of a gene pool due to migration of individuals between different populations is B. Gene flow.

Gene flow refers to the movement of genes from one population to another through the migration of individuals. It occurs when individuals from one population migrate and interbreed with individuals from another population, exchanging genetic material in the process. This movement of genes can introduce new genetic variants into a population or change the frequency of existing genetic variants.

Directional selection (A) is a process in which natural selection favors individuals with a particular trait, causing a shift in the allele frequencies in a consistent direction over time.

Genetic drift (C) refers to random changes in the allele frequencies of a population due to chance events. It is more pronounced in smaller populations and can lead to the loss or fixation of certain alleles.

Assortive mating (D) is a mating pattern where individuals with similar phenotypes or genotypes preferentially mate with each other. It can lead to changes in the frequencies of certain alleles within a population but is not specifically related to migration between populations.

Therefore, the correct term that describes alterations in the gene pool due to migration between populations is B. Gene flow.

The scenario that represents disruptive selection within a population is A. Heterozygous pink flowers being selected against while homozygous red and homozygous white flowers are selected for.

Disruptive selection, also known as diversifying selection, occurs when individuals with extreme phenotypes at both ends of the spectrum have higher fitness than those with intermediate phenotypes. This can result in the population becoming divided into two distinct groups or the maintenance of genetic variation within the population.

In scenario A, the heterozygous pink flowers are being selected against, while the homozygous red and homozygous white flowers are being selected for. This means that individuals with extreme phenotypes (red or white) have higher fitness compared to those with an intermediate phenotype (pink). This is a classic example of disruptive selection, as it leads to the divergence and maintenance of the red and white flower phenotypes within the population.

The piece of evidence that would be LEAST likely to help the researchers demonstrate a close evolutionary relationship between two extant species is C. The identification of several analogous structures between the species.

Analogous structures are structures that have similar functions or appearances in different species but do not share a common evolutionary origin. They arise due to convergent evolution, where different species independently evolve similar traits in response to similar selective pressures. Analogous structures do not provide strong evidence for a close evolutionary relationship between species.

traits that benefit the group as a whole, even if those traits may have some costs for the individuals within the group. In this case, the warning behavior of the vervet monkeys benefits the entire group by alerting others to the presence of a predator and increasing the chances of survival for the group members.

While the individual making the warning call may be at increased risk of being preyed upon, the group as a whole benefits from the warning behavior. If individuals who make warning calls increase the survival and reproductive success of the group, then the genes associated with this behavior can spread through the population, even if individuals who exhibit the behavior are at a disadvantage in certain situations.

Group selection is a less commonly invoked mechanism in evolutionary biology compared to natural selection acting at the level of individuals. However, in certain cases where cooperative behaviors or traits that benefit the group are involved, group selection can play a role.

Adaptive radiation refers to the diversification of a common ancestral species into multiple different species that occupy different ecological niches or environments. This diversification occurs when populations of a species encounter new and diverse environmental conditions, leading to adaptations that allow them to exploit those environments more effectively.

In the case of prehistoric humans, as they migrated to different regions with varying levels of sunlight and UV radiation, the different populations experienced different selective pressures related to their exposure to sunlight. The concentration of melanin in the skin is an adaptation that provides protection against the harmful effects of UV radiation. Higher concentrations of melanin provide greater protection against UV radiation, while lower concentrations allow for increased synthesis of vitamin D in regions with less sunlight.

Over time, populations in regions with higher UV radiation, such as near the equator, evolved to have higher concentrations of melanin in their skin, providing better protection against UV damage. Populations in regions with lower UV radiation, such as closer to the poles, evolved to have lower concentrations of melanin, allowing for increased synthesis of vitamin D.