Although the phrase “survival of the fittest” appears nowhere in Darwin’s Origin of Species[*], it has become synonymous with the term “natural selection” in the minds of many. The phrase is not only an inaccurate summary of Darwin’s theory, its common usage often implies that “only the strong survive” or a similar misunderstanding of the term “fitness”. Just what is fitness, anyway? And, if not size or strength, then what qualities make an individual “fit”?

Evolution happens because of differential reproductive success. Some individuals possess traits that increase the probability that they will successfully reproduce. Others lack those traits or possess traits that make them less likely to reproduce.

The end result is that the genetic makeup of the population changes over time because some individuals pass on their genes to a greater number of offspring[#](Endler 1986)(Futuyma 1998). Over time, some traits (and their underlying genes) disappear while other traits become more and more common in successive generations, and may eventually be found in every individual.

Genes that increase fitness (as defined above) tend to be preserved and increase in frequency.

Some of the many factors upon which fitness depends:

• PhylogenyThe evolutionary history of a taxonomic group. - Plants, animals, fungi, protists, archaeans and monerans (and each of the phyla, classes, orders, and families within these 6 Kingdoms) all experience the world in fundamentally different ways.
• Trophic level (predator, prey [or both], parasite, photosynthesizer)
• Number, identity and abundance of actual or potential predator, prey or parasite species
• Number, identity and abundance of actual or potential competing species
• Number, identity and abundance of actual or potential pollinators
• Mating system (monogamyMating system in which females and males form exclusivemating pairs, either for one mating season or for life., polygynyMating system in which males mate with multiple females., polyandryMating system in which females mate with multiple males.)
• Life cycle (larval stageStage in the development of many animals (eg, insects or amphibians), occurring after birth or hatching and before the adult form is reached.These immature, active forms arestructurally different from the adults and are usually adapted to a different environment., age at first reproduction, frequency of reproduction)
• Number of offspring
• Asexual or sexual reproduction, self-fertilizationFusion of male and female gametes (sex cells) produced by the same individual. Self-fertilization occurs in most flowering plants, numerous protozoans, and many invertebrates.
• Gender
• Mate choice made by female, male, or none
• Parental care and type of offspring (spores, seeds, eggs, live birth)
• Seed type and mode of dispersal (wind, water, bird, mammal, insect)
• Social system (solitary, herd, family groups, territoriality, dominance hierarchyA social system by which individuals within a population or group controlaccess to resources within the group. In some hierarchies, one individualdominates. In others, each individual has a rank in the hierarchy., harems, or eusocialityA social system of reproductive specialization. The basic group is the colony, headed by a reproductive queen, and includes classesof sterile members which carry out specialized tasks as in many ants, bees and termites)
• Activity cycle (nocturnalActive at night./ diurnalActive during daylight., hibernationA state of inactivity and metabolic depression in animals, characterizedby lower body temperature, slower breathing, and lower metabolic rate. or dormancy periods)
• Frequency, regularity, severity, type of disturbanceA change in average environmental conditions, sometimes resulting in theremoval of large amounts of biomass. Ecological disturbances include fires, flooding, and volcanic activity.
• Prevailing climate, latitude, environmental conditions
• Mode of transportation (fly, glide, drift, swim, walk, hop, burrow, none)

There are certainly more, but the point is that with so many combinations possible for all of these factors, there are many and diverse conditions that impact whether, how often and how successfully individuals reproduce. Size and strength may be advantageous for a few species, but certainly they are unimportant to most others. What matters is that some individuals in a population have traits that improve their reproductive output, which means that relatively more individuals in the next generation carry the genes for those traits. Any trait that results in relatively more offspring than others in the population will tend to persist and become more common.

It is also important to remember that if we want to talk about the “fittest,” we make the comparison among individuals of the same species in a population, not among members of different species. A weasel may be an efficient predator, but it is not “more fit” than the mice it catches. Prey that escape are not “more fit” than the predator. But some mice are faster or more agile or better able to find food or mates, and by leaving more offspring they prove more fit than other mice in the population.

Spencer, RJ. 2002. Experimentally testing nest site selection: Fitness trade-offs and predation risk in turtles. Ecology 83: 2136-2144

Arntz, AM, EH Delucia, & N Jordan. 1998. Contribution of photosynthetic rate to growth and reproduction in Amaranthus hybridus. Oecologia 117: 323-330