Question by Stephen Wise, answer by Theresa Mecklenborg
"It's great to find someone who offers to answer questions. I have lots of questions relating to lots of different fields. In the realm of biology, I am curious about species: how real are they? i.e. do species really exist in nature, or are they just a convenient fiction invented by biologists? If two species can cross-breed, what is the basis for calling them two different species?
One thing that makes me a bit skeptical is the idea that of all the traits that survival of the fittest can foster in a species, the most valuable one is adaptability. So we should expect to find a single species taking on different appearances under different conditions. I wonder whether biologists have systematically tested the DNA of defined species to confirm that they really are unique, which I guess means whether the difference between large samples from each population is significantly greater than the individual variation within each population."
There are three main definitions of a species. The one that sees the most use is the biological species concept, which defines a population of interbreeding or potentially interbreeding organisms as a species. (Ernst Mayr came up with this one -- you may want to look for some of his work if you're interested.) There's also the morphological species concept, which defines species based on physical characteristics (if it looks like a duck, it's a duck, basically). It was used before people understood genetics well. It's also the definition used for determining the species of dinosaurs or other extinct animals, since we can't know which other animals they were breeding with. A third definition, the phylogenetic species concept, defines a species as a group of organisms descended from a common ancestor. The phylogenetic definition is a bit hard to use in the field, but a lot of bacterial species are defined this way, since they reproduce asexually and thus don't interbreed (more or less; a great number of bacterial species can exchange genes across species boundaries, but it's not really what you'd call sex, and it's not reproduction).
For the most part, these definitions will all get you the same results -- a population of interbreeding organisms that all descended from a common ancestor and look more or less the same. However, there are some issues with each definition. I'll save the biological species concept until last, since that's kind of the big one and I want to go into more detail. The big problem with the morphological species concept is subjectivity. It's a bit arbitrary to say "okay, all the black-and-white ducks with tufts on their heads are Species X, and all the black-and-white ducks without tufts are Species Y," so this definition is usually used in concert with one of the other definitions. Once you've determined that, say, the ducks with tufts interbreed with each other but not with the tuftless ducks, it's more reasonable to go to a pond in the next county and assume the ducks there that look just like the two species you already know are those same species.
The problem with the phylogenetic concept is that you can really only tell if the group is descended from a common ancestor by watching it happen. Most vertebrate species evolved much too long ago for that to have been observed, but there have been some laboratory experiments with short-lived creatures like fruit flies and bacteria that demonstrate the process of speciation. (I'll see if I can find some references.)
The problem with the biological species concept is defining what "potentially interbreeding" means. It's possible to tell if organisms *are* interbreeding, if you can observe it, but it's not necessarily possible to tell if two populations that live in different habitats or geographical locations can interbreed, since they never meet. It's possible to experimentally bring individuals from different areas together in a laboratory or other captive environment, but this doesn't necessarily tell you whether or not they would breed in the wild, since laboratory conditions are often very different from the natural environment and can really throw off normal mating behaviors. Of course, this isn't much of an issue with plants, but plants have relatively loose species boundaries anyway. Even experimental matings aren't always possible given financial constraints, difficulty of sustaining large or rare animals in captivity, quarantines, and so on.
Okay. So that's what a species is, to a biologist. Now, do these definitions reflect groups found in nature? That is, are they real, useful divisions? The answer is yes, pretty much.
Species are divided by a variety of reproductive isolating mechanisms. They can be divided by "pre-zygotic" mechanisms, which include geographical, temporal, and habitat separations, as well as behavioral or physical isolating mechanisms. A geographical separation is pretty straightforward -- if the two species never meet, they can't breed. Temporal separations means the species are active (or at least interested in mating) at different times. A habitat separation can be as small as one species of insect living in pears and another in apples, but as long as they don't come into contact with each other, they can't mate. Behavioral isolating mechanisms include things like mating dances and other courtship rituals which must be completed properly before mating will occur. Physical isolating mechanisms include massive size differences and incompatible genitalia (some insects have ridiculously complex genitalia; I tried to find you some pictures, but can't seem to find a good collection online). In some cases, the organisms can mate just fine, but the sperm cannot reach or fertilize the egg.
Species can also be divided by "post-zygotic" mechanisms, which means the sperm does fertilize the egg, but something goes wrong afterwards. The can occur before the zygote develops into an embryo, after it develops into an embryo but before it is born, after it is born, or even in subsequent generations. If an organism is born from a mixed-species mating, it is called a hybrid. Hybridization is actually very rare in animals, although more common in plants. Some hybrids are healthy and fertile, able to breed with one or both parent species, or with other hybrids. Big cat hybrids are often fertile (there are some great pictures of a variety of hybrid big cats at http://members.aol.com/jshartwell/hybrid-bigcats.html). Most hybrids, however, die soon after birth, if they were not spontaneously aborted. Many hybrids that do survive are sickly or infertile (mules, for instance, are perfectly healthy but incapable of producing offspring). In some cases, a phenomenon called "hybrid breakdown" occurs, in which the first-generation hybrid is reasonably healthy and fertile, but its offspring are unhealthy or infertile.
Pre-zygotic and post-zygotic isolating mechanisms are caused by a wide variety of genes. It is thought that post-zygotic incompatibility usually develops first, and then pre-zygotic isolating mechanisms evolve to prevent the organisms from wasting effort, time, and resources on attempting to produce offspring that aren't as successful as purebred offspring would be.
There are a few cases in which hybrids are actually as fit or more fit than their parent species. In general, if a biologist sees this happening, they'll rethink whether those parent species really ought to be considered separate species. Perhaps the groups would more accurately be described as subspecies. One important thing to remember about species is that the vast majority were first named and described in the absence of sophisticated molecular or genetic analysis, and without observing interbreeding. There are many debates about whether certain groups ought to be considered separate species or just subspecies (there is a rather hot debate about North American bears at the moment).
So, to sum up: Yes, species do exist in nature, although they can be a little fuzzy around the edges, what with mutations and things. What scientists *think* are species may not quite match up with what's really going on, so as different sorts of evidence come in, species and subspecies may be lumped together or divided in a variety of different ways. For instance, a number of vertebrates with high levels of sexual dimorphism were originally described as two species, until people realized that the males and females, which looked very different, were actually interbreeding. In answer to your question "If two species can cross-breed, what is the basis for calling them two different species?", that depends on what the offspring are like. If they are completely healthy, fertile organisms that don't display hybrid breakdown and are produced in the wild, then the reason they might be called separate species is tradition -- the parent "species" will most likely be reclassified eventually. If those healthy offspring are *not* produced in the wild, even though the organisms can meet, the parent species will still be considered separate species, since they do not interbreed naturally. If the hybrids are unfit (unhealthy or infertile), the parent species will still be considered different species.
"One thing that makes me a bit skeptical is the idea that of all the traits that survival of the fittest can foster in a species, the most valuable one is adaptability. So we should expect to find a single species taking on different appearances under different conditions."
Keep in mind that evolution acts on an individual level, not for the benefit of the species or life as a whole. It cannot think ahead to create a characteristic that would be useful later, only to preserve one that appears by random mutation and happens to be useful (or at least not harmful) now. While there are many species that are generalists (for example rats, cockroaches, and humans), they are generalists by virtue of each individual being able to respond to the conditions it finds in a variety of ways, not by virtue of producing a variety of specialized individuals. Species do vary over environmental conditions, however. Different populations can vary genetically, for instance being furrier in colder areas, without being so different as to be unable to interbreed. Individuals can also vary for non-genetic reasons. If two populations of a species occur in places that have very different environmental conditions, each population will adapt to the conditions it is in. If the two populations change enough, they will no longer be able to interbreed, and will have become different species.
In general, an organism that is specialized for the environment it is in will do better than a generalist will. Generalists usually do better when conditions change frequently. Most environments on Earth (except for the ones humans alter) are relatively stable most of the time, so most species on Earth are specialized to some extent.
"I wonder whether biologists have systematically tested the DNA of defined species to confirm that they really are unique, which I guess means whether the difference between large samples from each population is significantly greater than the individual variation within each population."
DNA sequencing is very expensive and time consuming, so I doubt a great deal of this sort of work has been done.
 A zygote is a fertilized egg.
|Go back to the biology index.||Go back to the main index.|