A species is a group of organisms that interbreed and produce fertile offspring. Typically, individuals of the same species appear similar and share common characteristics due to their highly similar genomes. However, not all organisms that look alike are members of the same species. Various mechanisms keep most species discrete. While some mechanisms prevent reproductive behavior and fertilization (pre-zygotic isolation), others prevent the production of fertile offspring after mating has occurred (post-zygotic isolation).
Although individuals of a single species typically look very similar, variation in coloration or morphology is not uncommon. For example, the common wall lizard, Podarcis muralis has six distinct morphs which vary by the color of their throat and underbelly.
Conversely, the monarch and viceroy butterflies are two genetically distinct species that appear similar, inhabit common habitats, and are both unpalatable to predators. This is an evolutionary strategy known as Müllerian mimicry—the species share warning signals, such as their bright orange and black appearance to educate common predators more effectively of their toxicity.
Phenomena that prevent mating or fertilization from occurring are referred to as pre-zygotic isolation mechanisms. Geographic isolation and ecological isolation are two examples of environmental barriers. In the case of geographic isolation, two bird species might live in different forests on either side of an impassable mountain range; ecological isolation could be one insect species living solely in the treetops while another inhabits the soil.
There may also be behavioral reasons a species remains isolated. Examples are differences in mating rituals or communication, like birds-of-paradise, where males of different species have novel dances to attract female conspecifics. In temporal isolation species are kept apart by different timing of daily routines or breeding seasons. For example, the red-legged frog Rana aurora has a breeding season from January to March, but its close relative the yellow-legged frog, Rana boylii, breeds from late March through May.
The physical characteristics of different species may isolate them pre-zygotically. Insects can display an array of different genital morphologies that means they can only physically mate with members of their species—referred to as mechanical isolation. Finally, prevention of gamete fusion is often the last pre-zygotic barrier which may be controlled by mechanisms like the prevention of pollen tube growth in plants, or an inability to physically fuse with or penetrate the outer layers of an egg cell.
When individuals of different species overcome pre-zygotic isolation, post-zygotic isolation can prevent the resulting offspring from either surviving or reproducing. In some cases, a zygote may be formed but the embryo is not viable because of incompatible genetic material and thus dies. This is referred to as hybrid inviability.
However, some hybrid embryos may survive to reach fertility. For instance, horses and donkeys can be crossed to produce hinnies or mules. But because horses and donkeys have different chromosome numbers (64 and 62, respectively), their hybrid offspring have an odd number of chromosomes (63) that cannot be sorted equally into gametes, rending the hybrids infertile. This post-zygotic barrier is also observed in crosses between zebras and horses or donkeys crosses.
In cases where hybrids can survive to adulthood and copulate, non-genetic factors can prevent procreation. Hybrids of the sister species Drosophila pavani and D. gaucha produce viable gametes. However, the hybrids are unable to create offspring: the sperm of hybrid males cannot survive in the female semen receptors of either parent species or another hybrid, nor is the sperm of either parent line able to survive in a hybrid female.
Other instances of post-zygotic factors may affect the viability of hybrids. Hybrid genomes contain material from two truly distinct species and can, therefore, harbor different genes and chromosomes that do not act harmoniously in the offspring, resulting in obvious fitness costs. Plants are an exception in some cases. Overall, pre- and post-zygotic isolation mechanisms cause most species to remain distinct.
