posted
When I was in school, I was told that the definition of a species was a group that could interbreed viably, and that members of two separate species cannot produce viable offspring.
I know know that this is false. There are many species that can interbreed viably. Wolves can interbreed coyotes, even though they generally don't. Polar and brown bears are still close enough to each other that they can produce viable offspring.
So that brings me to my question -- what is the definition of a species? Does it really mean anything, or is it just a way of separating creatures that, for whatever reason, we deem separate enough to deem different names?
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I'm no bio buff, but it appears to depend on which species definition you choose. The morphological (shape-based) species definition is how most mammals were originally divided up into species, but it doesn't take into account genetics and breeding, leading to contradictions with other, more rigorous species definitions. another forum discussion: "what is a species?"
quote: Definitions of species: The definition of a species given above as taken from Mayr, is somewhat idealistic. Biologists frequently do not know whether two morphologically similar groups of organisms are "potentially" capable of interbreeding. Further, there is considerable variation in the degree to which hybridization may succeed under natural and experimental conditions, or even in the degree to which some organisms use sexual reproduction between individuals to breed. Consequently, several lines of thought in the definition of species exist:
A morphological species is a group of organisms that have a distinctive form: for example, we can distinguish between a chicken and a duck because they have different shaped bills and the duck has webbed feet. Species have been defined in this way since well before the beginning of recorded history. Although much criticised, the concept of morphological species remains the single most widely used species concept in everyday life, and still retains an important place within the biological sciences, particularly in the case of plants.
The biological species or isolation species concept identifies a species as a set of actually or potentially interbreeding organisms. This is generally the most useful formulation for scientists working with living examples of the higher taxa like mammals, fish, and birds, but meaningless for organisms that do not reproduce sexually. It distinguishes between the theoretical possibility of interbreeding and the actual likelihood of gene flow between populations. For example, it is possible to cross a horse with a donkey and produce offspring, however they remain separate species?in this case for two different reasons: first because horses and donkeys do not normally interbreed in the wild, and second because the fruit of the union is rarely fertile. The key to defining a biological species is that there is no significant cross-flow of genetic material between the two populations.
A mate-recognition species is defined as a group of organisms that are known to recognise one another as potential mates. Like the isolation species concept above, it applies only to organisms that reproduce sexually.
A phylogenetic or evolutionary or Darwinian species is a group of organisms that shares an ancestor; a lineage that maintains its integrity with respect to other lineages through both time and space. At some point in the progress of such a group, members may diverge from one another: when such a divergence becomes sufficiently clear, the two populations are regarded as separate species.
See also microspecies under apomixis, for species that reproduce without meiosis or mitosis so that each generation is genetically identical to the previous generation.
In practice, these definitions often coincide, and the differences between them are more a matter of emphasis than of outright contradiction. Nevertheless, no species concept yet proposed is entirely objective, or can be applied in all cases without resorting to judgement.
posted
It's also true that different taxonomists never agree 100% on which groups are distinct species and which are varieties or races of a single species.
The line between species is a blurry one. Darwin talks about this a great deal in "The Origin of Species", I believe.
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posted
Most evolutionary biologists use the phylogenetic species definition given in Morbo's citation. Paleontologists, on the other hand, primarily rely on the old morphological definition, as you can't tell much about a species' mating habits or genetic variation from fossils. By and large, however, most biologists stick with the classic "can't interbreed successfully" as a working definition, with the usual caveat that it doesn't apply universally.
As sequencing of complete genomes becomes more and more common, I think the phylogenetic definition will become the standard. Unlike the other definitions, genetic divergence over time is something that can be quantified, and thus allows much more rigorous categorization.
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posted
I think it's safe to say that the interbreeding thing is a fairly unidirectional criterion. If two closely related critters cannot interbreed and produce viable offspring, they are definitely different species.
Thinking about it, though, the fuzziness of the line between species is exactly what the theory of Evolution would predict. The kinds of things that are thought to cause speciation in nature (selection pressure, geographic isolation, behavioral isolation)even when sudden (like a rift valley opening suddenly between two halves of a breeding population, result in gradual genetic drift...
Anyway, I had this idea that, if I'd stayed in NYC, I was going to pursue looking at genetic relatedness among sub-populations of squirrels throughout the area. There were probably some interesting changes one could trace as populations within each of the city's parks became more and more isolated from one another as the city grew and it became harder and harder for squirrels to move from one greenspace to another.
The size & shape of Central park would provide an interesting test case to show that mere physical distance wasn't enough to predict the magnitude of genetic differences. Squirrels from areas bordering the park, even at its extreme north and south ends should be more similar than squirrels from parks on the east and west sides of Manhattan in areas that were above or below Central Park (thus lacking that intermediate safe zone.
Squirrels in parks close to one side or the other of bridges should be more similar than squirrels from either side of busy highways that are merely 100's of feet across.
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