Fecundity selection, also known as fertility selection, is the fitness advantage resulting from selection on traits that increases the number of offspring (i.e. fecundity). Charles Darwin formulated the theory of fecundity selection between 1871 and 1874 to explain the widespread evolution of female-biased sexual size dimorphism (SSD), where females were larger than males.

Along with the theories of natural selection and sexual selection, fecundity selection is a fundamental component of the modern theory of Darwinian selection. Fecundity selection is distinct in that large female size relates to the ability to accommodate more offspring, and a higher capacity for energy storage to be invested in reproduction. Darwin's theory of fecundity selection predicts the following:

Although sexual selection and fecundity selection are distinct, it still may be difficult to interpret whether sexual dimorphism in nature is due to fecundity selection, or to sexual selection. Examples of fecundity selection in nature include self-incompatibility flowering plants, where pollen of some potential mates are not effective in forming seed, as well as bird, lizard, fly, and butterfly and moth species that are spread across an ecological gradient.

In 1944, Reginald Ernest Moreau suggested that in more seasonal environments or higher latitudes, fecundity depends on high mortality. David Lack suggested in 1954 that differential food availability and management across latitudes play a role in offspring and parental fitness. Lack also highlighted that more opportunities for parents to collect food due to an increase in day-length towards the poles is an advantage. This means that moderately higher altitudes provide more successful conditions to produce more offspring. However, extreme day-lengths (i.e. at the poles) may work against parental survival as repetitive food searching would exhaust the parent.

Together, the Moreau–Lack rule hypothesizes that fecundity increases with increasing latitude. Evidence supporting and doubting this claim has led to the consolidation of other predictions, which may better explain Moreau–Lack's rule.

Ashmole (1963) suggested (bird) fecundity depends on seasonality patterns. Food differences in availability between seasons are greater towards higher latitudes, so birds are predicted to experience low survival during the winter due to limited resources. This decline in population may be advantageous for survivors, since there is more food available by the next breeding season. This leads to an enhancement of energy when invested in fitness as a result of higher fecundity. Therefore, Ashmole's hypothesis is dependent upon resource availability as a factor fecundity.

Areas with severe nest predation tend to be those of large clutches/litters, especially in the tropics, as they are more noticeable to predators (frequent parental care, noisier offspring). This predation pressure may lead to the selection for multiple nests of smaller size, with shorter development time.

A criticism of this hypothesis is that it indirectly assumes that these nest-predators are visually-oriented, however, they may be chemically oriented, too, with heightened olfactory senses.