Ecological speciation is a form of speciation arising from reproductive isolation that occurs due to an ecological factor that reduces or eliminates gene flow between two populations of a species. Ecological factors can include changes in the environmental conditions in which a species experiences, such as behavioral changes involving predation, predator avoidance, pollinator attraction, and foraging; as well as changes in mate choice due to sexual selection or communication systems. Ecologically-driven reproductive isolation under divergent natural selection leads to the formation of new species. This has been documented in many cases in nature and has been a major focus of research on speciation for the past few decades.
Ecological speciation has been defined in various ways to identify it as distinct from nonecological forms of speciation. The evolutionary biologist Dolph Schluter defines it as "the evolution of reproductive isolation between populations or subsets of a single population by adaptation to different environments or ecological niches", while others believe natural selection is the driving force. The key difference between ecological speciation and other kinds of speciation is that it is triggered by divergent natural selection among different habitats, as opposed to other kinds of speciation processes like random genetic drift, the fixation of incompatible mutations in populations experiencing similar selective pressures, or various forms of sexual selection not involving selection on ecologically relevant traits. Ecological speciation can occur either in allopatry, sympatry, or parapatry—the only requirement being that speciation occurs as a result of adaptation to different ecological or micro-ecological conditions.
Ecological speciation can occur pre-zygotically (barriers to reproduction that occur before the formation of a zygote) or post-zygotically (barriers to reproduction that occur after the formation of a zygote). Examples of pre-zygotic isolation include habitat isolation, isolation via pollinator-pollination systems, and temporal isolation. Examples of post-zygotic isolation involve genetic incompatibilities of hybrids, low fitness hybrids, and sexual selection against hybrids.
Some debate exists over the framework concerning the delineation of whether a speciation event is ecological or nonecological. "The pervasive effect of selection suggests that adaptive evolution and speciation are inseparable, casting doubt on whether speciation is ever nonecological". However, there are numerous examples of closely related, ecologically similar species (e.g., Albinaria land snails on islands in the Mediterranean, Batrachoseps salamanders from California, and certain crickets and damselflies), which is a pattern consistent with the possibility of nonecological speciation.
Ecological causes of divergent selection Divergent selection is key to the occurrence of ecological speciation. Three ecological causes of divergent selection have been identified: differences in environmental conditions, ecological interactions, and sexual selection. The causes are outlined in the following list:
Differences in environmental conditions as a prerequisite to speciation is incontrovertibly the most studied. Predation, resource availability (food abundance), climatic conditions, and the structure of a habitat are some of the examples that can differ and give rise to divergent selection. Despite being one of the most studied factors in ecological speciation, many aspects are still less understood such as how prevalent the process is in nature as well as the origin of barriers for post-zygotic isolation (as opposed to the much easier detectable pre-zygotic barriers). Laboratory experiments involving single-environmental differences are limited and have often not tracked the traits involved in isolation. Studies in nature have focused on a variety of environmental factors such as predation-caused divergent selection; however, little has been studied in regards to pathogens or parasites. Ecological interactions can drive divergent selection between populations in sympatry. Examples of these interactions can be intraspecific (between the same species) and interspecific (between different species) competition or relationships such as those of ecological facilitation. Interspecific competition in particular has support from experiments; however it is unknown if it can give rise to reproductive isolation despite driving divergent selection. Reinforcement (the strengthening of isolation by selection favoring the mating of members of their own populations due to reduced fitness of hybrids) is considered to be a form of, or involved in, ecological speciation. Though, debate exists as to how to determine ultimate causes since reinforcement can complete the speciation process regardless of how it originated. Further, character displacement can have the same effect. Sexual selection can play a role in ecological speciation as the recognition of mates is central to reproductive isolation—that is, if a species cannot recognize its potential mates, the flow of genes is suspended. Despite its role, only two types of sexual selection can be implicated in ecological speciation: the spatial variation in secondary sexual traits (sexual traits that arise specifically at sexual maturity) or communication and mating systems. This restriction is based on the fact that they produce diverging environments in which selection can act. For example, isolation will increase between two populations where there is a mismatch between signals (such as the feather display of a male bird) and the preferences (such as the sexual preferences of a female bird). This pattern has been detected in stickleback fish.
A summary of the various types of ecological isolation and its drivers.Reproductive isolation typePre-zygotic or post-zygoticEcological cause of selectionDivergent environmentsEcological interactionsSexual selectionReinforcementHabitatPre✓✓✓Sexual/PollinatorPre✓✓✓✓TemporalPre✓✓✓Selection against migrantsPre✓✓✓Post-matingPre✓✓✓✓Selection against hybridsPost✓✓✓✓Ecologically-independentPost✓✓✓✓Ecologically-dependentPost✓✓
Types of reproductive isolation Habitat isolation
Populations of a species can become spatially isolated due to preferences for separate habitats. The separation decreases the chance of mating to occur between the two populations, inhibiting gene flow, and promoting pre-zygotic isolation to lead to complete speciation. Habitat isolation is not equivalent to a geographic barrier like that of allopatric speciation. Instead, it is based on genetic differences, where one species is unable to exploit a different environment, resulting from fitness advantages, fitness disadvantages, or resource competition.
Jerry Coyne and H. Allen Orr posit two different forms of habitat isolation: microspatial habitat isolation (where matings between two species are reduced by preferences or adaptations to ecologically differing areas, despite occupying the same generalized area) and macrospatial habitat isolation (defined by fully allopatric habitats that inhibit gene flow.) Identification of both forms of habitat isolation in nature is difficult due to the effects of geography. Measuring microspatial isolation demands several factors: the spatial separation of different species' members is greater than those of members of the same species during simultaneous breeding periods, the spatial separation reduces gene flow decreased gene flow is directly the result of decreased mating genetic differences correspond to the spatial separation Allopatric distributions pose several problems for detecting true habitat isolation in that different habitats of two allopatrically isolated species does not imply ecologically caused speciation. Alternative explanations could account for the patterns: species differences may be caused by geographic isolation the species may or may not occupy different habitats if they existed in sympatry in cases of similar habitats in allopatry, species may be adapted to unknown ecological factors if the species existed in sympatry, competition may drive habitat segregation that would be undetectable in allopatry These issues (with both micro- and macro-spatial isolation) can be overcome by field or laboratory experiments such as transplantation of individuals into opposite habitats (though this can prove difficult if individuals are not completely unfit for the imposed habitat). Habitat isolation can be measured for a species pair ( and ) during a breeding period by:
Here, is the proportion of encounters between matings that involve partners of a different species that are observed. is the proportion of total individuals of species . is the proportion of total individuals of species . The expected proportion of mating encounters between different species if mating is random is denoted by . A statistic of indicates no mating encounters of different species where indicates random mating of different species.
Geography Ecological speciation caused by habitat isolation can occur in any geographic sense, that is, either allopatrically, parapatrically, or sympatrically. Speciation arising by habitat isolation in allopatry (and parapatry) is straightforward in that reduced gene flow between two populations acquire adaptations that fit the ecological conditions of their habitat. The adaptations are reinforced by selection and, in many cases such as with animals, are reinforced by behavioral preferences (e.g. in birds that prefer specific vocalizations). A classic example of habitat isolation occurring in allopatry is that of host-specific cospeciation such as in the pocket gophers and their host chewing lice or in the fig wasp-fig tree relationship and the yucca-yucca moth relationship—examples of ecological speciation caused by pollinator isolation. In sympatry, the scenario is more complex, as gene flow may not be reduced enough to permit speciation. It is thought that selection for niche divergence can drive the process. In addition, if sympatry results from the secondary contact of two previously separated populations, the process of reinforcement, the selection against unfit hybrids between the two populations, may drive their complete speciation. Competition for resources may also play a role.
Habitat isolation is a significant impediment to gene flow and is exhibited by the common observation that plants and animals are often spatially separated in relation to their adaptations. Numerous field studies, transplantation and removal experiments, and laboratory studies have been conducted to understand the nature of speciation caused by habitat isolation. Horkelia fusca, for example, grows on California slopes and meadows above 4500 feet, where its closet relatives H. californica and H. cuneata grow below 3200 feet in coastal habitats. When species are transplanted to alternate habitats, their viability is reduced, indicating that gene flow between the populations is unlikely. Similar patterns have been found with Artemisia tridentata tridentata and A. tridentata subsp. vaseyana in Utah, where hybrid zones exists between altitudinal populations, and transplant experiments reduce the fitness of the subspecies.
Speciation by habitat isolation has also been studied in serpentine leaf miner flies, ladybird beetles (Epilachna), goldenrod gall flies, Rhagoletis pomonella, leaf beetles, and pea aphids.
Sexual isolation