Passage
The Namibian cheetah (Acinonyx jubatus jubatus) , the world's fastest land-dwelling animal, hunts across the African grasslands using high-speed sprints. Cheetahs possess unique anatomical features specialized for speed, including an aerodynamic skull, elongated legs, and an enlarged heart. These physiological adaptations have occurred over time and allow cheetahs to achieve speeds greater than 100 km/h. However, due to their small body and slender limbs, cheetahs (50-64 kg) are limited with regard to the size of the animals they can successfully hunt and therefore typically prey on slightly smaller species that inhabit open grasslands.It is believed that around 12,000 years ago, Namibian cheetahs experienced an environmental catastrophe that drastically reduced their population. The small number of surviving cheetahs began mating with one another, which led to severe inbreeding depression. Over time, the offspring of subsequent generations exhibited decreased fitness, reduced fecundity (number of offspring) , and sustained a loss of major histocompatibility complex (MHC) allele diversity. To study if low MHC diversity correlates with low fitness in cheetahs, scientists analyzed the genetic diversity and disease prevalence in a group of free-ranging and captive male cheetahs.Experiment 1MHC allele genotyping and analysis was performed in wild (free-ranging) and captive male cheetahs (Figure 1) . Because MHC diversity is strongly correlated with genetic diversity, MHC diversity is used as a marker for an organism's overall genetic variability.
Figure 1 Evaluation of heterozygosity in (A) wild and (B) captive male cheetahs born between 1976 and 2007 (Note: Each data point represents a single male cheetah.) Experiment 2Disease prevalence was evaluated in wild and captive male cheetahs by quantitative analysis of symptoms relating gastrointestinal, liver, and kidney disease. The percentages of cheetahs affected by each disease are shown in Figure 2.
Figure 2 Evaluation of disease prevalence in wild and captive male cheetahsExperiment 3Fecal cortisol levels were noninvasively evaluated in wild and captive cheetahs over a period of 6 months (Figure 3) .
Figure 3 Average fecal cortisol concentrations of wild and captive male cheetahs
Adapted from Castro-prieto A, Wachter B, Sommer S. Cheetah paradigm revisited: MHC diversity in the world's largest free-ranging population. Mol Biol Evol. 2011;28(4) :1455-68.
-The cheetahs that survived the environmental catastrophe that occurred 12,000 years ago are at greater risk of extinction because of:

Question

Passage
The Namibian cheetah (Acinonyx jubatus jubatus) , the world's fastest land-dwelling animal, hunts across the African grasslands using high-speed sprints. Cheetahs possess unique anatomical features specialized for speed, including an aerodynamic skull, elongated legs, and an enlarged heart. These physiological adaptations have occurred over time and allow cheetahs to achieve speeds greater than 100 km/h. However, due to their small body and slender limbs, cheetahs (50-64 kg) are limited with regard to the size of the animals they can successfully hunt and therefore typically prey on slightly smaller species that inhabit open grasslands.It is believed that around 12,000 years ago, Namibian cheetahs experienced an environmental catastrophe that drastically reduced their population. The small number of surviving cheetahs began mating with one another, which led to severe inbreeding depression. Over time, the offspring of subsequent generations exhibited decreased fitness, reduced fecundity (number of offspring) , and sustained a loss of major histocompatibility complex (MHC) allele diversity. To study if low MHC diversity correlates with low fitness in cheetahs, scientists analyzed the genetic diversity and disease prevalence in a group of free-ranging and captive male cheetahs.Experiment 1MHC allele genotyping and analysis was performed in wild (free-ranging) and captive male cheetahs (Figure 1) . Because MHC diversity is strongly correlated with genetic diversity, MHC diversity is used as a marker for an organism's overall genetic variability.

Figure 1 Evaluation of heterozygosity in (A) wild and (B) captive male cheetahs born between 1976 and 2007 (Note: Each data point represents a single male cheetah.) Experiment 2Disease prevalence was evaluated in wild and captive male cheetahs by quantitative analysis of symptoms relating gastrointestinal, liver, and kidney disease. The percentages of cheetahs affected by each disease are shown in Figure 2.

Figure 2 Evaluation of disease prevalence in wild and captive male cheetahsExperiment 3Fecal cortisol levels were noninvasively evaluated in wild and captive cheetahs over a period of 6 months (Figure 3) .

Figure 3 Average fecal cortisol concentrations of wild and captive male cheetahs
Adapted from Castro-prieto A, Wachter B, Sommer S. Cheetah paradigm revisited: MHC diversity in the world's largest free-ranging population. Mol Biol Evol. 2011;28(4) :1455-68.
-The cheetahs that survived the environmental catastrophe that occurred 12,000 years ago are at greater risk of extinction because of:

Quizplus · Accepted Answer

11ecba2d_0fdd_0433_a3bf_87762df72710_MD0008_00 Genetic drift describes the natural variations in allele frequencies of a population due to random genetic changes that are not related to natural selection (eg, sampling error, chance event). Although all populations are affected by genetic drift, its effects are more significant in smaller populations because they have a reduced gene pool. The smaller gene pool cannot buffer random (good or bad) variations in allele frequencies that occur due to chance events. In other words, the probability of a beneficial allele being removed by genetic drift increases for populations whose genetic diversity is low. Similarly, drift increases the probability of a deleterious allele becoming fixed within the population. According to the passage, cheetahs experienced a catastrophic event that drastically reduced their population size; this is known as a bottleneck event. Bottlenecks (due to environmental events or human action) greatly reduce the genetic diversity of a population. Consequently, the smaller population of cheetahs has a reduced ability to buffer the negative impacts of random changes in allele frequencies (genetic drift) that may result in extinction. (Choice A) Natural selection is an evolutionary mechanism by which only beneficial alleles are selected for while unfavorable traits are selected against. These selected changes alter the population's allele frequencies, which improves the species' fitness. Consequently, these populations have an improved probability for survival rather than an increased probability for extinction. (Choice C) Gene flow, or the changes in allele frequency due to migration, increases genetic diversity by introducing new alleles to the gene pool. Therefore, small populations with low genetic diversity are more likely to benefit from gene flow and therefore are less likely to go extinct. (Choice D) Random mating increases a population's genetic diversity because of the inherent variations that arise during meiosis (recombination) and sexual reproduction (fusion of gametes). Therefore, the variations or changes to the allele frequencies due to random mating will primarily be beneficial to a population as a whole. Educational objective: Genetic drift is a mechanism of evolution; however, unlike natural selection, the variations in allele frequencies occur randomly by chance. Because smaller populations have a smaller gene pool, the random (good or bad) variations due to genetic drift cannot be buffered.

Passage The Namibian Cheetah (Acinonyx Jubatus Jubatus), the World's Fastest Land-Dwelling