Ecological selection (or environmental selection or survival selection or individual selection or asexual selection) refers to natural selection minus sexual selection, i.e. strictly ecological processes that operate on a species' inherited traits without reference to mating or secondary sex characteristics. The variant names describe varying circumstances where sexual selection is wholly suppressed as a mating factor.
Ecological selection can be said to be taking place in any circumstance where inheritance of specific traits is determined by ecology alone without direct sexual competition, when e.g. sexual competition is strictly ecological or economic, there is little or no mate choice, females do not resist any male who wishes to mate, all traits will be equally propagated regardless of mating, or the species is hermaphroditic or asexually reproducing, an ecological selection is taking place. In sexually-reproducing species, it is applicable mostly to situations where ecological pressures prevent most competitors from reaching maturity, or where crowding or pair-bonding or an extreme suppression of sexual selection factors prevents the normal sexual competition rituals and selection from taking place, but which also prevent artificial selection from operating, e.g. arranged marriages, where parents rather than the young select the mate based on economic or even astrology factors, and where the sexual desires of the mated pair are often subordinated to these factors, are artificial unless wholly based on an ecological factor such as control of land which is held by their own force.
In cases where ecological and sexual selection factors are strongly at odds, simultaneously encouraging and discouraging the same traits, it may also be important to distinguish them as sub-processes within natural selection.
For instance, one species of Pleistocene Horned Gopher left in the fossil record a series of individuals with successively longer and longer horns, that seemed to be unrelated or maladaptive to its ecological niche. Modern scientists theorize that the horns were useful or impressive in mating rituals among males, and that it was an example of runaway evolution. The species seems to have suddenly died out when horns reached approximately the body length of the animal itself, presumably because it could no longer run or evade predators - thus ecological selection ultimately trumped sexual.
It is also important to distinguish ecological selection in cases of extreme ecological abundance, e.g. the human built environment, cities or zoos, where sexual selection must generally predominate, as there is no threat of the species or individuals losing their ecological niche. Even in these situations, however, where survival is not in question, the variety and the quality of food, e.g. as presented by male to female monkeys in exchange for sex in some species, still has an impact on reproduction, however it becomes a sexual selection factor. Similar phenomena can be said to exist in humans e.g. the "mail order bride" who primarily mates for economic advantage.
Differentiating ecological selection from sexual is useful especially in such extreme cases; Above examples demonstrate exceptions rather than a typical selection in the wild. In general, ecological selection is assumed to be the dominant process in natural selection, except in highly cognitive species that do not, or do not always, pair bond, e.g. walrus, gorilla, human. But even in these species, one would distinguish cases where isolated populations had no real choice of mates, or where the vast majority of individuals died before sexual maturity, leaving only the ecologically selected survivor to mate - regardless of its sexual fitness under normal sexual selection processes for that species.
For example, if only a few closely related males survive a natural disaster, and all are able to mate very widely due to lack of males, sexual selection has been suppressed by an ecological selection (the disaster). Such situations are usually temporary, characteristic of populations under extreme stress, for relatively short terms. However, they can drastically affect populations in that short time, sometimes eliminating all individuals susceptible to a pathogen, and thereby rendering all survivors immune. A few such catastrophic events where ecological selection predominates can lead to a population with specific advantages, e.g. in colonization when invading populations from more crowded disease-prone conditions arrive with antibodies to diseases, and the diseases themselves, which proceed to wipe out natives, clearing the way for the colonists.
In humans, the intervention of artificial devices such as ships or blankets may be enough to make some consider this an example of artificial selection. However it is clearly observed in other species, it seems unreasonable to differentiate colonization by ship from colonization by walking, and even the word "colony" is not specific to humans but refers generically to an intrusion of one species on an ecology to which it has not wholly adapted. So, despite the potential controversy, it may be better to consider all examples of colonist-borne diseases to be ecological selection.
For another example, in a region devastated by nuclear radiation, such as the Bikini Atoll, capacity to survive gamma rays to sexual maturity and (for the female) to term is a key ecological selection factor, although it is neither "natural" nor sexual. Some would call this too artificial selection, not natural or ecological, as the radiation does not enter the ecology as a factor save due to man's effort. Ambiguous artificial-plus-ecological factors may reasonably be called "environmental", and the term environmental selection may be preferable in these cases.