MCAT - Bio/Biochem Foundations:: Evolution

Natural selection, Darwin's central mechanism of evolutionary change, requires which combination of underlying conditions to operate?

- **A)** All individuals in a population must be genetically identical, with no variation
- **B)** Traits must never be heritable, changing randomly each generation
- **C)** Population size must remain exactly constant with no reproduction occurring
- **D)** Heritable variation must exist within a population for a given trait, that variation must affect an individual's relative reproductive success (fitness) in its environment, and there must be differential reproduction based on that variation - over generations, this process tends to increase the frequency of traits associated with higher relative fitness

Fitness, in the evolutionary biology sense, is best defined as:

- **A)** An organism's physical strength or size relative to others in its population
- **B)** An individual's relative reproductive success - specifically, the number of viable, fertile offspring it contributes to the next generation relative to other individuals in the population, NOT simply survival or physical characteristics like strength or speed in isolation
- **C)** An organism's overall health status, unrelated to reproduction
- **D)** A fixed, universal property of a species, identical across all environments

A selective pressure/agent (e.g., predation, food scarcity, climate, or availability of mates) drives natural selection by:

- **A)** Directly and consciously altering an individual organism's DNA sequence during that individual's own lifetime
- **B)** Having no actual effect on which individuals survive or reproduce
- **C)** Differentially affecting the survival and/or reproductive success of individuals with different heritable trait variants, such that individuals with traits better suited to that particular pressure/environment tend to leave more offspring, gradually shifting the population's trait/allele frequencies over successive generations
- **D)** Only ever acting on a single individual, with no effect on population-level allele frequencies

Directional selection, one of several recognized patterns of natural selection acting on a continuously varying (polygenic) trait, favors:

- **A)** Individuals at one extreme of the trait distribution (e.g., unusually large body size), shifting the population's average trait value progressively toward that extreme over successive generations
- **B)** Individuals with intermediate/average trait values, favoring the middle of the distribution
- **C)** Both extremes of the trait distribution simultaneously, disfavoring intermediate values
- **D)** No particular trait value, having no directional effect on the population's phenotype distribution at all

Stabilizing selection, in contrast to directional selection, favors:

- **A)** Individuals at one extreme of the trait distribution only
- **B)** Individuals at both extremes of the distribution simultaneously, disfavoring the average/intermediate value
- **C)** Individuals with intermediate/average trait values, actively selecting AGAINST both extremes of the distribution - tending to reduce overall phenotypic variation in the population while keeping the average trait value relatively unchanged over time
- **D)** A random, unpredictable subset of the population with no relationship to trait value

Disruptive (diversifying) selection, the third classic pattern of selection on a continuous trait, favors:

- **A)** Only the intermediate/average trait value, identical to stabilizing selection
- **B)** A completely random subset of the population regardless of trait value
- **C)** Only a single extreme of the trait distribution, identical to directional selection
- **D)** BOTH extremes of the trait distribution simultaneously, while actively selecting AGAINST the intermediate/average phenotype - potentially increasing overall phenotypic variance and, in some cases, contributing to divergence into two distinct phenotypic subgroups within the same population over time

Sexual selection, a specific form of natural selection related to differential mating success (rather than survival per se), can explain the evolution of certain traits (e.g., elaborate peacock tail feathers) that:

- **A)** Always directly and straightforwardly improve an individual's survival prospects, with no tradeoff
- **B)** May actually be somewhat costly or even disadvantageous for basic survival (e.g., making an individual more visible to predators or metabolically costly to maintain), but nonetheless persist and spread in a population because they significantly increase that individual's mating success/reproductive opportunities, which can outweigh the survival cost in terms of overall net evolutionary fitness
- **C)** Have absolutely no relationship to reproductive success of any kind
- **D)** Only ever occur in asexually reproducing species

A biological species, under the widely used biological species concept, is generally defined as a group of organisms that:

- **A)** Can interbreed with one another under natural conditions and produce viable, fertile offspring, while being reproductively isolated from other such groups (unable to successfully interbreed with them, or producing infertile/inviable hybrid offspring if they do)
- **B)** Simply look physically identical to one another, regardless of any reproductive compatibility
- **C)** Share the exact same geographic habitat, regardless of any reproductive relationship
- **D)** Belong to the same genus, with no further reproductive criteria required