Evolutionary Fitness
Evolutionary fitness is a measure of an organism's reproductive success relative to other individuals in the population, quantified as the average number of offspring that survive to reproductive age. It is not a measure of physical strength but of how well an organism's genotype is represented in the next generation. Absolute fitness is the actual number of reproducing offspring, while relative fitness normalises this value against the most successful genotype in the population.
Key Formula
w = W / W_max
LaTeX: w = \frac{W}{W_{\max}}
| Symbol | Meaning | Unit |
|---|---|---|
| w | Relative fitness of a genotype | dimensionless (0 to 1) |
| W | Absolute fitness — average number of surviving offspring for a genotype | offspring per individual |
| W_max | Maximum absolute fitness in the population | offspring per individual |
Worked Example
Problem
In a population of beetles, genotype AA produces 12 surviving offspring on average, Aa produces 10, and aa produces 6. Calculate the relative fitness of each genotype.
Solution
Step 1: Identify maximum absolute fitness — W_max = 12 (genotype AA). Step 2: Relative fitness of AA: w_AA = 12/12 = 1.00 Step 3: Relative fitness of Aa: w_Aa = 10/12 = 0.833 Step 4: Relative fitness of aa: w_aa = 6/12 = 0.50
Answer
w_AA = 1.00, w_Aa = 0.833, w_aa = 0.50. Allele A is favoured by selection.
Absolute vs Relative Fitness for Beetle Genotypes
| Genotype | Avg Surviving Offspring (W) | Relative Fitness (w) | Selection Coefficient (s = 1 - w) |
|---|---|---|---|
| AA | 12 | 1.00 | 0.00 (no selection against) |
| Aa | 10 | 0.83 | 0.17 |
| aa | 6 | 0.50 | 0.50 |
Interactive Tools
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Related Terms
Natural Selection
Natural selection is the process by which individuals with heritable traits that improve survival and reproduction in a given environment leave more offspring than those without such traits, causing those traits to become more common in the population over generations. It is the primary mechanism of adaptive evolution, first described by Charles Darwin and Alfred Russel Wallace in 1858. Natural selection acts on phenotypic variation and requires heritable differences in reproductive success to drive evolutionary change.
Adaptation (evolution)
An adaptation is a heritable trait that increases an organism's reproductive fitness in its environment, shaped by natural selection over many generations. Adaptations can be morphological (structural), physiological (functional), or behavioural, and they arise because individuals carrying the trait leave more offspring than those without it. The concept of adaptation is central to evolutionary biology and explains the remarkable fit between organisms and their environments.
Genetic Bottleneck
A genetic bottleneck is a sharp reduction in the size of a population due to an environmental event (such as a famine, disease, or habitat destruction), resulting in a dramatic loss of genetic diversity in the surviving population. The surviving individuals carry only a small, random subset of the original genetic variation, and subsequent generations inherit this reduced genetic repertoire regardless of population size recovery. Bottlenecks increase homozygosity, reduce adaptive potential, and can cause rare alleles to be lost or increase in frequency by chance.
The word "fitness" derives from Old English "fittan" (to be suitable). Herbert Spencer coined the phrase "survival of the fittest" in 1864 after reading Darwin's work, though Darwin himself used "fitness" to mean reproductive success, not physical strength.