A good way to start an argument among biologists is to ask them what seems a simple question: "What is a species?"
To evolutionary theorists, a species is a fundamental unit, and transitions between species mark the flow of evolution. But in fact, scientists and philosophers have debated for centuries the precise definition of a species.
In high school biology we are told that species are communities of individuals that can interbreed successfully. Two creatures that cannot interbreed, even if they resemble each other, must belong to different species.
As is often the case in evolution, the rules of speciation are by no means ironclad.
Lions and tigers are two different species. They look different, they have different lifestyles, they vocalize differently, and they generally live on different continents. Yet when they are brought together artificially, they can interbreed. Such hybrids are called tions and ligers.
Do they challenge the definition of species? Not really. The key words are "interbreed successfully"; tigons and ligers generally are sterile and short-lived -- an evolutionary dead end.
If species exist, why? And what maintains them over time? Biologists are still wrestling with these questions, which some call the "species problem" or the "species crisis."
Biologist Ernst Mayr sees the species as a small gene pool protected from too much variability by a reproductive barrier. In other words, the species is a population adapted to a certain niche, and if the members of different species could interbreed with each other, too much genetic variability would occur, reducing the success of the adaptation. "The basic biological purpose of the species," says Mayr, "is the protection of a harmonious gene pool." To bolster his argument, he points out that hybrids between species are usually less successful and are often sterile.
Another definition -- one not necessarily at odds with Mayr's -- is that a species is a classification containing organisms that are physically similar.
What keeps a species coherent over time? "Gene flow" between the different populations of a species is one main explanation. Because members of a species usually interbreed only amongst themselves, the same shared genes get passed on to successive generations.
Another unifying factor is the rule of natural selection, which continues to act on species that have adapted to an environmental niche. A combination of the two -- natural selection and gene flow between its members -- serves as a force that maintains the integrity of a species.