Key Concepts of Evolutionary Fitness Measures

Why This Matters

Fitness is the currency of evolution—it's how we quantify who "wins" the evolutionary game and why. When you're asked about natural selection, adaptation, or population genetics on an exam, you're really being asked to think about fitness in its various forms. These measures help explain everything from why altruism evolves to how populations shift toward certain traits over generations to what happens when environments change.

Don't just memorize definitions here. Each fitness measure captures a different angle on the same fundamental question: how do genes make it into the next generation? Understanding when to apply each measure—whether you're analyzing kin selection, comparing phenotypes, or modeling population change—is what separates surface-level recall from the kind of conceptual thinking that earns full credit on FRQs.

---

Measuring Individual Output

These concepts quantify what a single organism contributes to the next generation—the raw numbers that natural selection ultimately acts upon. The key distinction is whether you're counting total offspring or comparing performance against others.

Absolute Fitness

• Total lifetime reproductive output—the actual number of surviving offspring an individual produces, not a comparison to others
• Direct measure of gene pool contribution—higher absolute fitness means more copies of your alleles in the next generation
• Environmentally sensitive—resource availability, predation pressure, and climate all shift absolute fitness values across populations

Relative Fitness

• Fitness as a proportion—compares an individual's reproductive success to the most successful genotype in the population (assigned a value of 1)
• Reveals selection pressure—when relative fitness differs between genotypes, natural selection is actively favoring certain traits
• Essential for population genetics calculations—used to predict allele frequency changes using equations like $\Delta p = pq[p(w_{AA} - w_{Aa}) + q(w_{Aa} - w_{aa})]/\bar{w}$

Darwinian Fitness

• Synonymous with reproductive success—emphasizes that evolution "cares" only about passing genes to the next generation, not longevity or strength per se
• Core of natural selection theory—traits that increase Darwinian fitness spread; those that decrease it disappear
• Context-dependent—a trait boosting fitness in one environment may reduce it in another (think peppered moths before and after industrial pollution)

---

Components of Reproductive Success

Fitness isn't one thing—it's built from survival and reproduction working together. These measures break down the pathway from birth to successful reproduction into its component parts.

Reproductive Success

• Offspring surviving to reproductive age—not just births, but viable descendants who can themselves reproduce
• The ultimate fitness metric—combines survival, mating success, and offspring viability into one outcome measure
• Shaped by multiple factors—mate choice, parental investment, and environmental conditions all feed into this number

Survival Rate

• Proportion reaching reproductive age—you can't reproduce if you don't survive, making this a prerequisite for fitness
• Stage-specific selection—different life stages face different mortality pressures (high juvenile mortality vs. stable adult survival in many species)
• Density-dependent effects—predation, disease, and competition intensify as populations grow, lowering survival rates

Fecundity

• Potential reproductive capacity—the maximum number of offspring an organism could produce (eggs, seeds, sperm counts)
• Trade-off with offspring quality—high fecundity often means less parental investment per offspring (r-selected vs. K-selected strategies)
• Not the same as reproductive success—producing 1,000 eggs means nothing if none survive to adulthood

Viability

• Probability of surviving to reproduce—reflects genetic health, developmental stability, and environmental fit
• Reveals genetic load—populations carrying deleterious alleles show reduced viability, especially in homozygotes
• Selection target—viability selection removes individuals before they can reproduce, shaping allele frequencies

---

Expanding the Fitness Concept

Classical fitness focuses on individual reproduction, but evolution is more nuanced. These concepts capture how genes spread through indirect pathways and how we quantify selection's strength.

Inclusive Fitness

• Your genes in others' offspring—adds indirect fitness (helping relatives reproduce) to direct fitness (your own offspring)
• Explains altruism mathematically—Hamilton's rule states altruism evolves when $rB > C$, where r is relatedness, B is benefit to recipient, C is cost to actor
• Kin selection foundation—why worker bees sacrifice reproduction, why ground squirrels give alarm calls, why siblings cooperate

Selection Coefficient

• Quantifies selection intensity—represented as s, measuring how much a genotype's fitness deviates from the optimum
• Ranges from 0 to 1—$s = 0$ means no selection (neutral); $s = 1$ means complete selection against (lethal or sterile)
• Predicts evolutionary rate—larger selection coefficients drive faster allele frequency change; used in equations like $\Delta q = -spq^2/(1-sq^2)$ for recessive alleles

---

Visualizing Fitness Across Genotypes

Evolution doesn't happen to individuals in isolation—it shapes entire populations across genetic space. This model helps you think about how populations navigate toward higher fitness over time.

Adaptive Landscape

• Fitness mapped across genotype space—peaks represent high-fitness genotype combinations; valleys represent low-fitness combinations
• Explains evolutionary constraints—populations can get "stuck" on local peaks, unable to cross fitness valleys to reach higher peaks
• Dynamic surfaces—environmental change reshapes the landscape, turning peaks into valleys and vice versa (climate change moving fitness optima)

---

Quick Reference Table

---

Self-Check Questions

1. An organism produces 50 offspring while the most successful individual in the population produces 100. What is this organism's relative fitness, and how would you calculate it?

2. Which two fitness concepts would you use to explain why a sterile worker bee's behavior can still be considered evolutionarily "successful"? What equation connects them?

3. Compare and contrast fecundity and reproductive success. Why might an organism with high fecundity still have low fitness?

4. A population sits on a local fitness peak in an adaptive landscape. Explain why it might not evolve toward a nearby higher peak, even though that genotype combination would have greater fitness.

5. If a recessive allele has a selection coefficient of $s = 0.4$, what does this tell you about the fitness of homozygotes for that allele compared to other genotypes? How would this affect the allele's frequency over time?