The mechanisms of behavioral isolation work to ensure that a species produces viable offspring and allow that species to remain viable by reducing the amount of gene flow between closely related species. Different species exhibit different behaviors, and these behaviors put constraints on with which organisms they can reproduce. These mechanisms form a core part of evolutionary theory and are necessary for the occurrence of speciation; i.e. the process by which a population diverges into a new species of organism.
It is an obvious fact of biology that species reproduce. It is also an obvious fact that there are constraints on which species can reproduce with which species. One of these constraints on reproduction between species is the behavior of different species.
“For the butterfly, mating and propagation involve the sacrifice of life, for the human being, the sacrifice of beauty.” — Johann Wolfgang von Goethe
Mechanisms of Behavioral Isolation
The different processes that prevent species from mating with each other are known generally as the mechanisms of reproductive isolation. The zoologist Ernst Mayr divided the mechanisms of reproductive isolation into two main categories: pre-zygotic and post-zygotic mechanisms. Pre-zygotic mechanisms act before the fertilization of a zygote while post-zygotic mechanisms act after the fertilization of the zygote. Pre-zygotic isolation mechanisms include the incompatibility of two species’ reproductive organs or differences in the mating cycles of two species. Post-zygotic mechanisms would include the non-viability of zygotes or the sterility of hybrid offspring.
Behavioral isolation, then, can be described as a pre-zygotic mechanism by which species are reproductively isolated from others due to differences in behavior. An extremely simple example of behavioral isolation would be the existence of mating rituals. Several species of organisms have mating rituals that include elaborate dances, displays of affection, the release of pheromones, and in some cases gift giving. These behaviors act as powerful barriers to reproductive by reducing the pool of potential mates to only those organisms that are able to perform those behaviors. Behavioral isolation differs from other forms of reproductive isolation in that the main mechanism that limits reproduction is the behavior of organisms, not genetics or geographical location.
Many species of bird, for example, have elaborate courtship rituals. These mating rituals frequently involve complex sequences of behaviors, such as long dances with many steps and the building of nests. Even the smallest defect in the performance of these rituals can prevent mating. Courtship rituals are a form of behavioral isolation as they reduce the pool of potential mates to only those organisms that can perform the correct rituals. Any potential mate must be able to reproduce these behaviors so the existence of these behaviors and associated practices serves as a mechanism to reproductively isolate that species of bird from any other species. Evolutionarily, these courtship rituals function to ensure the viability of offspring and ensure that most instances of mating between organisms are co-specific; that is, between members of the same species. The existence of these behaviors also prevents organisms from wasting resources and energy pursuing mating opportunities that would be unfruitful or produce non-viable offspring.
“All nature’s creatures join together to express nature’s purpose. Somewhere in their mounting and mating, rutting and butting is the very secret of nature itself.” — Graham Swift
Several experiments have shown that different selection pressures can result in increased levels of behavioral isolation between different species. In a widely cited study by K.F Koopman published in 1950, Koopman demonstrated that by simulating the effects of natural selection on populations of flies, he could significantly reduce the amount of interspecific mating in subsequent generations of flies. Koopman initially took 2 separate but closely related species of Drosophila flies, D. pseudoobscure and D. persimilis and stored equal numbers of males and female specimens together.
Initially, almost half of the offspring created by the flies were hybrid organisms. By subsequently removing the hybrid flies from each generation of specimens, thus mimicking the processes of natural selection, Koopman found that by the 10th generation of specimens, almost all the flies were co-specific. These findings confirm the hypothesis that selection pressure such as the non-viability of hybrid offspring can increase behavioral isolation between two species.
What Role Does Behavioral Isolation Play in Evolution?
Of course, this finding raises the question: what is so evolutionary beneficial about having a species that are behaviorally isolated? At first glance, it may seem that the opposite would be true: organisms that can successfully mate with more species than their own would have a greater chance of passing down their genes than organisms with a limited pool of potential mates. It is well known that for many animals, successive generations of inbreeding between closely related organisms can increase the likelihood of offspring possessing deleterious recessive traits. Therefore, it seems that it would be likely that successful species would show low levels of behavioral isolation, as more potential mates means more opportunities to pass down your genes and a greater variety of genes would allow for more possible beneficial genetic combinations.
The preceding line of thought rests on 2 incorrect assumptions about evolution. First, while it may be beneficial for the individual organism to have potential partners from multiple different species, that does not necessarily mean that it would be beneficial at the species level if each organism in that species could produce offspring with multiple species. Selection pressures work at the species level, not the level of the individual organism, so looking at the potential benefits to an individual organism that biological traits may bring is not necessarily an indicator that the trait will be beneficial at the species level. In fact, sometimes the exact opposite occurs; a trait or behavior that is detrimental to individual organisms can end up being beneficial for the species as a whole. There are many examples of behavior detrimental to individual flourishing that can contribute to species fitness; many social behaviors fit this description.
A second more conceptual issue is that behavioral isolation is not a trait that species have that contributes to their survival. Rather, behavioral isolation, or reproductive isolation in general, is a necessary condition for the occurrence of speciation in the first place.
Consider a hypothetical population of some species. Initially, the population is able to breed with each other to produce fertile offspring and so under the most common definition of a biological species, would count as a single species. Over time though, it is inevitable that subsets of organisms will develop small differences that will end up being barriers to reproduction with other subsets of organisms. For example, let’s say that small genetic mutations manifest into behavioral differences between two groups of the population. Over time, these small differences build up and differences in behavior start to have a significant effect on the selection of mates for reproduction. Perhaps small differences in hunting patterns end up leading to differences in the sexual selection of mates. After a sufficient amount of changes, the two populations will have diverged enough so that they would no longer be able to interbreed to produce viable offspring. At that point, we would say that speciation has occurred and we now have two distinct species of organism.
“One of the basic steps in saving a threatened species is to learn more about it: its diet, its mating and reproductive processes, its range patterns, its social behavior.” — Dian Fossey
Notice that in our hypothetical example, behavioral isolation occurs before speciation. Speciation is a consequence of behavioral isolation, not a cause. So behavioral isolation is not a trait or tendency that a species develops but is a precursor to the formation of different species. In this sense, behavioral isolation is not a beneficial trait of the species but is a process required for the existence of that species in the first place. Other mechanisms of reproductive isolation such as geographical isolation, mechanical isolation or temporal isolation can also lead to the formation of new species by separating populations of organisms and reducing their potential gene pool. Over time, these isolated groups will develop a distinct genetic profile and will be unable to mate with members of the population they originally diverged from. The main point is that these mechanisms of reproductive isolation act before the formation of a new species and continue to act as the species develops. Without some form or reproductive isolation, species would not divide into distinct genetic mating pools.
So in summation, behavioral isolation refers to the different behaviors of organisms that serve to reproductively isolate them from other species. Behavioral isolation works to limit the potential mating pool of specimens to those that can perform the requisite behaviors. By doing so, behavioral isolation contributes to the viability of offspring by ensuring that two parents will be of the same species. Behavioral isolation is an important biological concept because it in part explains why populations of organisms develop into separate species in the first place. Without the mechanisms of behavioral isolation and other mechanisms of reproductive isolation, evolution could not happen in the first place, and so the concept of behavioral isolation helps form the backbone of modern evolutionary theory.