AP Syllabus focus:
‘Evolutionary fitness is measured by reproductive success, not simply by an individual organism’s survival.’
Evolution often sounds like “survival of the fittest,” but in biology, “fittest” has a precise meaning. Fitness is about passing genes to the next generation, so reproduction—not longevity—determines evolutionary success.
Core idea: fitness is about genetic contribution
Natural populations change over generations because some individuals leave more descendants than others. This difference is captured by evolutionary fitness, which links an organism’s traits to how many copies of its alleles appear in future generations.
Evolutionary fitness
Evolutionary fitness: the ability of an individual (or genotype) to contribute viable, fertile offspring to the next generation, relative to others in the population.
Fitness is therefore a population-level comparison (who leaves more descendants) rather than a label for an individual’s strength, health, or lifespan.
Reproductive success
Reproductive success: the measured reproductive output of an individual, typically quantified as the number of offspring that survive to reproduce (or the number of gene copies passed on).
Because evolution tracks heritable genetic contributions, a trait that improves survival but does not increase successful reproduction may have little evolutionary impact.
Survival is not the same as fitness
An organism can survive well yet have low fitness if it produces few or no offspring. Conversely, short-lived organisms can have high fitness if they reproduce effectively.
Idealized survivorship curves (Types I, II, and III) plot the proportion surviving versus age (often on a log scale). The figure highlights that species can achieve high evolutionary fitness through very different strategies—e.g., long life with few well-cared-for offspring versus short life with many offspring. This reinforces the key idea that reproductive success, not lifespan alone, drives allele representation in the next generation. Source
Key reasons survival alone is insufficient:
Survival without reproduction yields zero genetic contribution.
Timing matters: reproducing earlier can increase total descendants even if lifespan is shorter.
Trade-offs are common: energy invested in growth, immunity, or maintenance may reduce energy available for reproduction.
This plot depicts a life-history trade-off as a negative relationship between investment in survival/maintenance and investment in reproductive output. It illustrates the core allocation principle: with finite energy, increasing one component commonly reduces the other. This makes “fitness” easier to interpret as a balance of multiple components rather than a single trait like longevity. Source
Examples of trade-offs (conceptual, not calculations):
High parental care may reduce total number of offspring but increase the number that reach reproductive age.
Producing many offspring may lower survival of each offspring if resources are limited.
What determines reproductive success?
Reproductive success is multifaceted, so biologists often break it into components that can be observed or inferred.
Common components (ways fitness can differ)
Mating success: ability to obtain mates (number of matings, mate quality).
The polygyny threshold model graph plots female fitness against environmental (territory/resource) quality, comparing expected fitness under monogamy versus joining a polygynous male. The intersection and “threshold” illustrate when choosing an already-mated male can still yield higher reproductive success because resource gains offset the costs of sharing. It provides a clear visual example of how mating system dynamics can alter fitness outcomes. Source
Fecundity: number of offspring produced (e.g., clutch size).
Fertility: success of fertilisation and production of viable embryos.
Offspring viability: probability offspring survive to reproductive age.
Offspring fertility: whether offspring can reproduce successfully themselves.
Reproductive lifespan: how long an individual remains capable of reproduction.
Reproductive timing: age at first reproduction and spacing of reproductive events.
These components can compensate for one another: fewer offspring may still yield higher fitness if each offspring has a higher chance of surviving and reproducing.
Fitness is often expressed relatively
Because fitness is inherently comparative, biologists commonly use relative fitness to compare genotypes or phenotypes within the same population and environment.
= relative fitness (unitless)
= absolute fitness (average number of viable, fertile offspring per individual)
= absolute fitness of the most successful genotype in the comparison
Relative fitness connects directly to evolution: variants with higher tend to become more common over generations because they contribute disproportionately to the next generation’s gene pool.
Important clarifications for AP Biology
Fitness applies to phenotypes and genotypes: you can discuss fitness of an observable trait or the genetic variant producing it.
Fitness is environment- and context-specific: a trait can increase reproductive success in one context and not in another, so fitness is not an absolute “best.”
“Fittest” does not mean strongest: it means most reproductively successful, consistent with the syllabus emphasis that reproduction—not mere survival—drives evolutionary outcomes.
FAQ
They often combine field observations with genetic methods.
Behavioural data: mating pairs, nesting success, number of fledglings
Long-term monitoring: tracking which offspring later reproduce
Genetic parentage analysis: DNA markers to assign offspring to parents, revealing extra-pair paternity and true reproductive output
Inclusive fitness extends reproductive success by including gene copies passed on indirectly through relatives.
It considers:
Direct fitness: your own offspring
Indirect fitness: helping relatives reproduce, weighted by relatedness (e.g., siblings share ~0.5 of alleles)
Yes. Fitness depends on offspring quality and long-term genetic contribution.
Differences can arise from:
Offspring survival to reproductive age
Offspring fertility
Offspring future reproductive output (grand-offspring)
Earlier reproduction can increase the total number of descendants because offspring have more time to reproduce as well.
Also, if mortality risk is high, reproducing sooner increases the chance of leaving any descendants at all.
They use proxies closely tied to successful reproduction, such as:
Lifetime reproductive output (viable offspring produced)
Survival to breeding age plus breeding success
Relative fitness estimates scaled to the best-performing genotype (e.g., $w = W/W_{\max}$)
Practice Questions
Define evolutionary fitness and explain why an organism’s survival alone is not a sufficient measure of fitness. (3 marks)
Correct definition: fitness is measured by contribution to the next generation via viable, fertile offspring (1).
States fitness is relative/comparative within a population (1).
Explains survival alone is insufficient because without successful reproduction (or offspring surviving to reproduce), genes are not passed on (1).
In a population of birds, phenotype A individuals live longer on average than phenotype B individuals. However, phenotype B individuals begin breeding earlier and produce more offspring that survive to adulthood. Using the concept of evolutionary fitness, explain which phenotype is expected to have higher fitness and justify your answer using at least three components of reproductive success. (6 marks)
Identifies phenotype B as expected to have higher fitness based on greater reproductive success, despite shorter lifespan (1).
Links fitness to number of viable, fertile offspring and/or gene copies passed to next generation (1).
Uses earlier age at first reproduction as a factor increasing fitness (1).
Uses higher fecundity (more offspring produced) as a factor increasing fitness (1).
Uses higher offspring viability/survival to adulthood as a factor increasing fitness (1).
Explicitly contrasts survival/longevity with reproduction, stating longevity alone does not determine fitness (1).
