What is a species? This is not an easy question to answer to everybody's satisfaction, except in a very non-specific way. Generally speaking, species is a word that biologists have defined to refer to specific groups of populations, in which all the members of the group are closely related. Like most definitions, it loses much of its value when the people who use it can't agree on its exact definition.
Formally stated, the definition of a species that I like is a group of organisms that can, and do, interbreed to produce offspring that can continue to breed successfully with the rest of the group.
This definition is valuable because it is definite, and leaves less room for speculation than other definitions. However, not all biologists agree with this definition. Other popular ways to define a species are based on morphological aspects, likelihood of mating, and genetic traits.
What techniques can be used to distinguish between species? With a concept that is as broad as species, all organisms must be considered of the same species until proven otherwise. Sometimes it is easy to tell that organisms are of a different species due to great levels of differentiation between them, but this is not always the case. Simple tests related to appearance can be useful for drastically different animals, such as birds and fish, but quickly lose usefulness when comparing animals that are somewhat related. The same is true for plants, a tree is easily differentiated from a fern, but when comparing plants of similar types much more care must be taken.
It must be remembered that not all samples of a population are a good representation of that population. When comparing a pituitary dwarf to a human with gigantism, it would be easy to think that you had two species, based on physical dimensions alone. It is also important to compare your organisms when they are of the same age, since many animals, and even a few plants, can be found in drastically different physical and behavior stages over their lifetime. Clearly, the numerically larger your sample (and the more diverse), the better your results.
Even with a large sample, physical appearance can fail to identify different species. Evolutionary pressures of an environment can shape two different species in a common environment so that they look and act alike, while still being separate species. In another example of evolution muddying the waters, distinctive markings of some wasps are mimicked by other insects that can't sting. By sight alone they look closely related, and possibly of the same species.
A more time consuming method, but a generally more successful one, is attempting to interbreed the organisms, to see if they produce successful offspring. (An unsuccessful offspring can be defined as not being able to successfully reproduce with the same groups of organisms that created it.) If the organisms are of different species, they must not interbreed when they are both placed in the same environment, or when they do, not produce successful offspring. The reason for inability to interbreed can be diverse for animals, such as different breeding seasons, different methods of attracting a mate, or on a simpler scale, inability to breed due to different physical anatomy. For plants, common reasons for not interbreeding are mostly related to types of pollinators being different between plants.
It is the simplicity, and the fact that the outcome is fairly easy to judge, that makes me choose interbreeding as the basis for my definition of species. By this definition, there are no murky issues related to different species that can interbreed and produce fertile offspring. If they can interbreed, with success, then they really are all part of the same species.
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