evolutionary fitness

Meaning: Evolutionary (Darwinian) fitness is an individual's or genotype's relative contribution to the next generation's gene pool, measured by offspring that survive and reproduce, not by strength or lifespan. Natural selection acts on heritable variation in fitness, so alleles that raise fitness tend to rise in frequency. Inclusive fitness also counts the reproduction of relatives, weighted by relatedness, which explains kin-directed altruism.

Equation: w = R/Rmax, where w is relative fitness (unitless), R is the mean number of offspring reaching reproductive age for a genotype (offspring per individual), and Rmax is the same value for the most successful genotype (offspring per individual).

On the MCAT: Passages give offspring or survival data for different phenotypes and ask which is favored or how allele frequency will shift over generations. Common settings are heterozygote advantage (the sickle cell allele where malaria is common) and kin selection, where altruism is favored when relatedness times benefit to the recipient exceeds cost to the actor.

Common confusion: Fitness is not "survival of the strongest." A long-lived animal that leaves no offspring has a fitness of zero. Fitness also depends on the environment, so a trait that raises fitness in one setting can be neutral or harmful in another.