MCAT Biological and Biochemical Foundations of Living Systems Flashcards: 1c Evolution Natural Selection

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This deck focuses on 1c Evolution Natural Selection, giving you a quick way to review the definitions, rules, and examples that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

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Work through these flashcards in short sessions. Try to answer each prompt before flipping the card, then revisit any cards you miss until the explanation feels automatic.

Flashcard 1: What is gene flow (migration) and its typical effect on population differences?

Answer: Allele movement between populations; reduces population divergence. Gene flow homogenizes allele frequencies across populations by introducing alleles via migration, counteracting differentiation caused by drift or local selection.

Flashcard 2: What is fitness in evolutionary biology (as used in natural selection questions)?

Answer: Relative reproductive success of a genotype or phenotype. Fitness measures an organism's ability to pass on genes relative to others, driving natural selection by favoring advantageous genotypes or phenotypes.

Flashcard 3: What is directional selection?

Answer: Selection favoring one extreme phenotype; shifts the mean. Directional selection drives evolutionary change by increasing the frequency of advantageous extreme traits, shifting the population's phenotypic mean accordingly.

Flashcard 4: What is the ultimate source of new alleles in a population?

Answer: Mutation. Mutations introduce novel genetic variation by altering DNA sequences, providing raw material for evolution unlike other mechanisms that redistribute existing alleles.

Flashcard 5: What is the bottleneck effect?

Answer: Drift after a drastic population size reduction. Bottlenecks reduce genetic diversity through drastic population declines, amplifying drift as surviving alleles are randomly sampled from the remnant group.

Flashcard 6: Which five conditions must hold for Hardy–Weinberg equilibrium to apply?

Answer: No selection, no mutation, no migration, random mating, large population. These conditions ensure no changes in allele frequencies, enabling mathematical modeling of genotype proportions under equilibrium.

Flashcard 7: What is disruptive selection?

Answer: Selection favoring both extremes; can increase variance/bimodality. Disruptive selection promotes polymorphism by favoring phenotypic extremes over intermediates, potentially leading to speciation through increased variance.

Flashcard 8: State the Hardy–Weinberg genotype frequency equation in terms of $p$ and $q$.

Answer: $p^2+2pq+q^2=1$. This equation derives from binomial expansion, predicting genotype frequencies from allele frequencies under equilibrium assumptions.

Flashcard 9: What is stabilizing selection?

Answer: Selection favoring intermediate phenotypes; reduces variance. Stabilizing selection maintains phenotypic optima by eliminating extremes, preserving the population mean while narrowing trait distribution.

Flashcard 10: Identify the genotype frequencies under Hardy–Weinberg equilibrium in terms of $p$ and $q$.

Answer: $AA=p^2,\ Aa=2pq,\ aa=q^2$. Under random mating, genotype frequencies follow binomial probabilities, with homozygotes as squares and heterozygotes as twice the product.

Flashcard 11: What is the definition of Hardy–Weinberg equilibrium?

Answer: Allele and genotype frequencies remain constant absent evolutionary forces. Hardy–Weinberg equilibrium assumes no evolutionary forces, allowing prediction of stable frequencies from random mating in idealized populations.

Flashcard 12: What is the definition of evolution in a population genetics context?

Answer: Change in allele frequencies in a population over generations. Evolution is defined as shifts in allele frequencies across generations due to mechanisms like selection, drift, mutation, and gene flow acting on genetic variation.

Flashcard 13: What is the founder effect?

Answer: Drift from a new population started by a small number of individuals. The founder effect arises when a small group colonizes a new area, carrying only a subset of alleles that then drift in the isolated population.

Flashcard 14: What is frequency-dependent selection?

Answer: Fitness of a phenotype depends on its frequency in the population. Frequency-dependent selection stabilizes polymorphisms as rare phenotypes gain advantages, preventing fixation and promoting coexistence of multiple strategies.

Flashcard 15: What is sexual selection?

Answer: Selection on traits that increase mating success. Sexual selection evolves traits that enhance mating opportunities, often through competition or choice, even if they reduce overall survival fitness.

Flashcard 16: What is heterozygote advantage (overdominance)?

Answer: Heterozygote has higher fitness than either homozygote. Heterozygote advantage maintains genetic diversity by conferring superior fitness to carriers of both alleles, as seen in sickle-cell anemia resistance to malaria.

Flashcard 17: Find the heterozygote frequency if $p=0.6$ and $q=0.4$ under Hardy–Weinberg.

Answer: $2pq=0.48$. Heterozygote frequency is twice the product of allele frequencies, derived from Hardy–Weinberg principles for random mating.

Flashcard 18: What is the key evolutionary consequence of inbreeding on genotype frequencies?

Answer: Increases homozygosity (decreases heterozygosity) without changing allele frequencies. Inbreeding increases mating between relatives, elevating homozygote frequencies via non-random mating while allele frequencies remain unchanged.

Flashcard 19: What is natural selection, stated in terms of fitness and heritable variation?

Answer: Differential reproductive success due to heritable trait differences. Natural selection occurs when heritable traits confer varying reproductive success, leading to changes in trait frequencies over generations.

Flashcard 20: Identify the type of selection if heterozygotes have the lowest fitness (both homozygotes favored).

Answer: Disruptive selection (underdominance). When heterozygotes are least fit, selection disrupts intermediates, favoring homozygous extremes and potentially leading to bimodal distributions.

Flashcard 21: What is genetic drift?

Answer: Random change in allele frequencies due to chance sampling. Genetic drift causes unpredictable fluctuations in allele frequencies from random sampling errors, especially pronounced in small populations.

Flashcard 22: State the Hardy–Weinberg allele frequency equation using $p$ and $q$.

Answer: $p+q=1$. For a locus with two alleles, their frequencies sum to unity, forming the basis for Hardy–Weinberg genotype calculations.

Flashcard 23: Which mechanism is more influential in small populations: genetic drift or selection?

Answer: Genetic drift. In small populations, random sampling errors dominate over selective pressures, making genetic drift the primary driver of allele frequency changes.

Flashcard 24: Find $q$ under Hardy–Weinberg if the recessive phenotype frequency is $0.04$.

Answer: $q=0.2$. Recessive phenotype frequency equals $q^2$, so $q$ is the square root, assuming Hardy–Weinberg equilibrium holds.

Flashcard 25: Find $p$ under Hardy–Weinberg if $q=0.3$ for a two-allele locus.

Answer: $p=0.7$. Since allele frequencies sum to 1, $p$ is calculated as $1 - q$ for a biallelic locus in equilibrium.