Pre AP Biology -EVO 2.1 Natural Selection Theory- FRQ Exam Style Questions -New Syllabus

Relative fitness considers the number of surviving offspring that an individual produces in comparison to the number of offspring that other individuals produce in the same population. It is a measure of differential reproductive success, where favorable traits lead to higher fitness within a given environment .

In evolutionary biology, fitness is not simply about an organism’s strength or speed; it is specifically a measure of reproductive success. Relative fitness takes this a step further by comparing the reproductive contribution of one individual (or genotype) to the average contribution of the rest of the population.

Key aspects of relative fitness include:

• Comparative Success: It is a “relative” measure because it depends on how well an individual performs compared to its peers. If an organism produces $5$ surviving offspring but the average for the population is $10$, its relative fitness is lower than if the population average were only $2$.
• Survival to Maturity: Fitness requires that offspring not only be born but also survive long enough to reach reproductive age themselves to ensure the passage of heritable traits .
• Selection Pressure: Relative fitness is the engine of natural selection. Individuals with higher relative fitness possess adaptations—heritable traits that increase fitness—which will become more frequent in the gene pool over successive generations.

Mathematically, relative fitness ($w$) is often expressed by scaling the most successful genotype to $1.0$, with all other genotypes represented as a fraction of that maximum success. This allows scientists to predict how allelic frequencies in a population might shift in response to environmental changes.