Evolutionary Mechanisms and Natural Selection (1C) - MCAT Biological and Biochemical Foundations of Living Systems

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

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.

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

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.

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

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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

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

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

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

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

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.