Medium MCAT Evolution Practice Questions

Concept Explanation

Evolution is the change in the heritable characteristics of biological populations over successive generations, driven by mechanisms such as natural selection, genetic drift, and gene flow. Understanding these processes is critical for the MCAT, as the exam frequently tests how genetic variation leads to differential reproductive success. Central to this topic is the Hardy-Weinberg Principle, which provides a mathematical model for a non-evolving population where allele frequencies remain constant. Evolution can be viewed through different lenses: microevolution involves changes in allele frequencies within a single population, while macroevolution describes patterns of changes in groups of related species over broad periods of geological time. Mastery of evolution requires integrating concepts from MCAT Genetics Practice Questions to explain how mutations and recombinations provide the raw material upon which natural selection acts. Key concepts include fitness—a measure of an individual's reproductive success—and the various modes of selection (stabilizing, directional, and disruptive) that shape the phenotypic distribution of a population over time.

Solved Examples

Example 1: Hardy-Weinberg Equilibrium
In a population of 1000 rabbits, 90 individuals exhibit a recessive white fur phenotype ($aa$). Assuming the population is in Hardy-Weinberg equilibrium, calculate the frequency of the dominant allele ($A$) and the number of heterozygous individuals.

1. Identify the frequency of the homozygous recessive genotype: $q^2 = \frac{90}{1000} = 0.09$.
2. Calculate the frequency of the recessive allele: $q = \sqrt{0.09} = 0.3$.
3. Use the equation $p + q = 1$ to find the dominant allele frequency: $p = 1 - 0.3 = 0.7$.
4. Calculate the frequency of heterozygotes: $2pq = 2(0.7)(0.3) = 0.42$.
5. Determine the number of individuals: $0.42 \times 1000 = 420$.

Example 2: Types of Selection
A population of birds lives on an island where only very small seeds and very large seeds are available. Birds with medium-sized beaks struggle to eat either seed type, while those with small or large beaks thrive. What type of selection is occurring?

1. Analyze the selection pressure: The intermediate phenotype (medium beak) is being selected against.
2. Identify the mode: When both extreme phenotypes are favored over the intermediate, it is called disruptive selection.
3. Predict the outcome: This can lead to speciation if the two extreme groups stop interbreeding.

Example 3: Genetic Drift vs. Gene Flow
A small group of 10 iguanas is washed away from the mainland by a storm and settles on a distant uninhabited island. Over time, the new island population has much lower genetic diversity than the mainland population. Is this gene flow or genetic drift?

1. Define the event: A small subset of a population starting a new colony is a specific type of genetic drift known as the founder effect.
2. Contrast with gene flow: Gene flow involves the movement of alleles between existing populations (migration).
3. Conclusion: This is genetic drift, which significantly impacts small populations by changing allele frequencies due to chance.

Practice Questions

1. A researcher observes that over several generations, the average birth weight of human newborns remains relatively constant around 7.5 pounds, as very light and very heavy babies have lower survival rates. This is an example of:

2. In a population in Hardy-Weinberg equilibrium, the frequency of a certain autosomal recessive disease is 1 in 10,000. What is the frequency of the carriers (heterozygotes) in this population?

3. Two species of frogs live in the same pond but do not interbreed because one species mates in early spring and the other in late summer. This is an example of what type of reproductive isolation?

4. Which of the following conditions is NOT required for a population to remain in Hardy-Weinberg equilibrium?

5. A massive wildfire kills 95% of a squirrel population. The remaining squirrels have a different distribution of fur colors than the original population. This phenomenon is known as:

6. According to the endosymbiotic theory, which organelles in eukaryotic cells originated from an evolutionary relationship with prokaryotes?

7. A scientist compares the amino acid sequences of cytochrome c in humans, chimpanzees, and dogs. This study provides evidence for evolution based on:

8. Which of the following best describes "inclusive fitness" in evolutionary biology?

9. In a specific environment, dark-colored moths have a higher survival rate than light-colored moths due to industrial soot covering the trees. This shift in the population's phenotype is an example of:

10. Divergent evolution is most likely to result in which of the following?

Answers & Explanations

1. Stabilizing Selection: This occurs when the intermediate phenotype is favored and extreme phenotypes are selected against, maintaining the status quo for a trait.
2. 0.0198 (approx. 2%): If $q^2 = 0.0001$, then $q = 0.01$. Since $p + q = 1$, $p = 0.99$. The carrier frequency is $2pq = 2(0.99)(0.01) = 0.0198$.
3. Temporal Isolation: This is a prezygotic barrier where species are prevented from mating because they breed at different times of the day, season, or year.
4. Natural Selection: For Hardy-Weinberg equilibrium to hold, there must be no natural selection, no mutations, no migration (gene flow), a very large population size, and random mating.
5. Bottleneck Effect: This is a form of genetic drift that occurs when a population's size is drastically reduced by a random event, leaving behind a small sample that may not represent the original gene pool.
6. Mitochondria and Chloroplasts: These organelles have their own circular DNA and double membranes, suggesting they were once free-living bacteria. This is a core topic in MCAT Cell Biology Practice Questions.
7. Molecular Homology: Comparing DNA or protein sequences helps determine phylogenetic relationships; the more similar the sequences, the more recently the species shared a common ancestor.
8. The sum of direct and indirect fitness: It accounts for an individual's own offspring as well as the offspring of relatives who share similar genes, explaining altruistic behaviors.
9. Directional Selection: This occurs when natural selection favors one extreme phenotype over others, causing the allele frequency to shift over time in that direction.
10. Homologous Structures: Divergent evolution occurs when two species evolve from a common ancestor but develop different traits; however, they retain underlying structural similarities (e.g., the forelimbs of mammals).

1. Which evolutionary mechanism is most likely to decrease genetic variation within a single small population over time?

• AGene flow
• BGenetic drift
• CMutation
• DRandom mating

Frequently Asked Questions

What is the difference between microevolution and macroevolution?

Microevolution refers to small-scale changes in allele frequencies within a population over a few generations, often due to natural selection or drift. Macroevolution refers to large-scale evolutionary patterns above the species level, such as the origin of new taxonomic groups over geological time.

How does the bottleneck effect impact a population's survival?

The bottleneck effect reduces genetic diversity, which can make a population more vulnerable to environmental changes or diseases. Because the gene pool is limited, harmful recessive alleles are more likely to be expressed through inbreeding.

What are the five conditions for Hardy-Weinberg equilibrium?

The five conditions are: no mutations, no gene flow (isolation from other populations), random mating, a very large population size (to prevent genetic drift), and no natural selection. If these are met, allele frequencies will remain constant.

What is the difference between divergent and convergent evolution?

Divergent evolution occurs when related species develop different traits due to different environments, leading to homologous structures. Convergent evolution occurs when unrelated species develop similar traits (analogous structures) because they occupy similar ecological niches.

Why is mutation considered the ultimate source of genetic variation?

Mutations are the only process that can create entirely new alleles in a population. While processes like crossing over and independent assortment reshuffle existing genes, mutations provide the raw novel material necessary for evolution to occur.