At what point does a mutation become a separate species? Also, from a biological standpoint, is there a useful distinction between the terms “mutation” and “variation”?
The classical definition of a “species” is that mating between two of the same species produces a fertile offspring. If two not of the same species mate, either no offspring or no fertile offspring will be produced. This is usually because of he mismatching of chromosomes, such as the horse and donkey. However, in a recent thread, Colibri announced that in captivity fertile offsprings between two different species are possible. Now I don’t know what makes captivity special. Either it is possible or not possible. Usually different species is determined by matching the DNA.
A variation, or sub-species, is a member of the species and mating between sub-species produces fertile offspring, which is common in the bird family. A sub-species is sometimes called a “variation.”
A mutation can either be a good or bad mutation. If it is bad, the product will not produce further in that particular mutation. “Survival of the fitter” stuff. If it produces offspring with some variations, it will further that line. For example, the people that lived in the malarial areas of the world developed a blood cell (sickle) that did not foster the life cycle of the mosquito. Those people have sickle-cell anemia, but this developed because it offered advantages that outweighed the disadvantages for those living in those areas. That is an overall beneficial mutation, but those persons are not “variations.”
A mutation doesn’t become a separate species (nor does a mutant, for that matter). At its most basic, a mutation is simply an alteration in the genetic code of an organism. There are some mutations which produce no phentoypic effects at all (many amino acids can be formed by multiple combinations of DNA base pairs, for example. So swapping out a base pair can sometimes still result in the same protein being produced), while others can result in an unviable embryo.
Those mutations which do have phenotypic effects result in variation in individuals. Variation can also result from environmental influences during development (e.g., height differences between individuals, or sex in some reptiles).
Over time, the build of heritable variation from generation to generation can lead to a population slowly transforming over time. If part of a parent populaton is separated (as in, the populations no longer exchange genetic material, for whatever reason) from the rest, the two resulting sub-populations will begin to diverge from one another. At some point, the two populations may become so divergent that even if brought back together again, the two cannot produce viable offspring (or simply do not view one another as a source of potential mates). At this point, they are then typically deemd to be separate species.
Also, from a biological standpoint, is there a useful distinction between the terms “mutation” and “variation”?
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As mentioned, a mutation is an alteration of genetic material, while variation can be the result of a mutation or of non-heritable environmental factors.
It’s worth noting that species is a completely artificial concept. It’s a distinction that we, as scientists, imposed upon the natural world, because we really really like to have things classified. A bird doesn’t look at another bird and think, “Hmm. I wonder if our genomes are similar enough that we could manage to produce a viable offspring.” It thinks, “Hey! Mating time!”
That said, as barbitu8 said, if two animals can produce viable offspring, we consider them to be the same species. Within a species, there can be a lot of variation, consisting of lots of different mutations (or polymorphisms, but let’s not bring more vocabulary into this - the distinction is unimportant).
There are many things that can prevent two animals from successfully mating - different mating calls, different mating seasons, or even something as simple as two populations that are on separate sides of the mountain and never bump into each other, so they evolve separately. As barb said, there are times when animals that don’t mate in the wild can mate in captivity. That doesn’t mean they’re the same species, because the criteria is whether they can succesfully mate in the wild.
I’m not sure what you mean by the “classical” definition of species. Species definitions before the Twentieth Century were often “typological,” that is, based on a species’ morphological characteristics, and did not explicitly recognize a reproductive component.
However, this is emphatically not the essence of the Biological Species Concept (BSC), which was developed in the 1940s and which has been the predominant definition for at least the past 50 years. The mere ability to produce fertile offspring is a necessary condition for being part of the same species, but it is by no means a sufficient one.
It drives me nuts that this misconception is so widespread, because it speaks of how poor science education is in the U.S. In virtually every thread of this kind, someone will come in and say, “Hey! I thought that being members of the different species meant they can’t produce fertile offspring.” I’m thinking of writing up a standard response that I can post any thread that asks about species and hybridization, since I end up having to write this over and over.
Here’s more on species concepts
(Note that this page does not describe the Phylogenetic Species Concept, which has become increasingly popular in recent years, at least in certain circles.)
Note that lack of hybrids or hybrid sterility (due to genetic incompatibility or other factors) is only one of several different mechanism that can reproductively isolate populations. Species can also be prevented from interbreeding by behavioral (e.g. courtship behavior) or ecological (e.g. different seasonality) characteristics.
If two organisms cannot produce fertile offspring under any circumstances, then they are certainly not members of the same species. However, according to the BSC, even if they can produce fertile offspring under unusual circumstances (due to not having other mates available in captivity, or where one of the two species is rare, or due to various kinds of ecological or behavioral distruption), as long as they do not regularly produce hybrids in the wild, they are considered separate species.
Scientists 'see new species born (June 9, 2004)