DunnySaze
June 3rd 2003, 03:37 PM
There seems to be considerable confusion on these boards as to what is and is not a species. Part of this is undoubtedly due to the fact that there is considerable debate within the biological community itself as to what a species is.
The word ‘species’ derives from the Latin specere, meaning to look at, to see, or to behold. In this sense it conforms to what might be called the common-sense definition.
Species are those organisms that show reproductive compatibility and continuity.
Central to the common sense definition is the notion that for example, dogs will always mate with dogs to make more dogs. Trout will always mate with trout to make more trout, etc. There is a distinct obvious common sense dividing line between dogs and trout. We recognize in this definition that all organisms need not look identical to be in the same species. In some cases, like dogs, these differences are apparently large, but in most other cases, like trout, one pretty much looks like another unless you look very carefully. This concept can be summarized as ‘we know a species when we see one’.
Since such a concept lacks a certain rigor and utility in science. There a more precise and quantifiable definition is preferable. The most precise and quantifiable definition there is the biological species concept as defined by Mayr, E., 1942. Systematics and the Origin of Species from the Viewpoint of a Zoologist.
"... groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups."
This definition has the advantage of being very useful in determining when you have a new species under controlled conditions, such as those one might find in a lab working with fruit flies. It’s also useful in distinguishing animal species (especially vertebrates) and most insects. The reason being that obligate sexual reproduction is the predominate mode of offspring generation in these groups.
But there are several problems associated with this concept.
1. In organisms where obligate sexual reproduction is not the predominate mode of offspring generation, it is of little use. These include asexually reproducing organisms (e.g. prokaryotes and some eukaryotes); and many plants that show hybridization.
2. In practice it’s difficult if not impossible to use in many cases. In a situation like a fruit fly culture, it might have some utility in determining speciation, since the system is highly constrained. However, in nature, its use is far less straightforward. The only way to tell if any two organisms are different species using this definition is to do cross reproduction studies. Such studies are seldom done, and even if done, the fact that two groups may not reproduce in the lab doesn’t mean they can’t do so in Nature.
3. It’s of no use at all in determining speciation between groups separate in time (e.g. extinct organisms represented by fossilized forms).
For these reasons and others, even the strongest adherents of the biological species concept rarely use it, even though it is very useful in certain restricted situations where it can be practically applied. Instead they compare the similarities and differences using various techniques between two groups. If they are similar enough, they are considered the same species unless some specific barrier to speciation is put forward. It sounds very arbitrary, but keep in mind definitions of other divisions have an even less basis in a quantifiable entity.
One way to think of the speciation as the sum of similarities and differences is to use the definition of Davis and Heywood, Davis, P. H., and V. H. Heywood, 1963. Principles of Angiosperm Taxonomy:
“assemblages of individuals with morphological features in common and separable from other such assemblages by correlated morphological discontinuities in a number of features”
It sounds simple enough, but relies heavily on subjective interpretations of taxonomists. The ‘lumpers’ will tend to focus on the similarities and the ‘splitters’ on the differences. To eliminate some of the arbitrariness some workers have proposed statistical approaches by giving characteristics a numerical weight and determining speciation based on a certain numerical value, the so-called numerical or phonetic taxonomy. It’s basically an attempt to formalize and quantify the methods. However, it’s still subjected to criticism because where the magic cut-off correlation is still a subjective determination. Plus, the use assigning equal statistical weight to all characteristics is also criticized.
To circumvent the problems inherent in the above concepts, various authors have proposed broader definitions based on the evolutionary species concept. Here, species are defined not by sexual isolation but on ‘evolutionary’ isolation, of which sexual isolation is but one component. The definition is given by Simpson, G.G., 1961. Principles of Animal Taxonomy:
“An evolutionary species is a lineage (an ancestor-descendant sequence of populations) evolving separately from others and with its own unitary evolutionary role and tendencies”
This definition has the advantage that the species concept includes change (evolution) and at the same time lays the ground-work those changes resulting from competition and interaction between species.
However, again we run into the problem of practically applying this definition. How else can an intrepid taxonomist facing a large variety of organisms on her table classify them except by morphology? Also, since speciation is a process, it’s difficult to know when the process is complete in practice. What is the defining point of complete separation? It’s hard to say. Nevertheless, this definition does allow one to include ecological, behavioral, genetic, and morphological evidence to reflect evolutionary closeness or distance.
In the end, the difficulties in taxonomy with regards to species are inherent in the process itself. The differences are hard to classify precisely because species do evolve, and evolution is a morphologically gradual, not saltational process. Different forms show different degrees of separation in various characteristics. Some of these degrees of separation are so great in some organism so as to preclude successful reproduction. That’s a handy way to tell if a new species arises, but it isn’t useful every time a new species arises. In other words, evolution doesn’t make the taxonomist’s job easy, but it does tell him why it’s so hard.
Finally, keep in mind that the concept of species, like genera or order or class, is man-made to try and keep track of things. Or in the words of John Locke, 1689, An Essay Concerning Human Understanding: “the boundaries of species, whereby men sort them, are made by men”.
Comments?
The word ‘species’ derives from the Latin specere, meaning to look at, to see, or to behold. In this sense it conforms to what might be called the common-sense definition.
Species are those organisms that show reproductive compatibility and continuity.
Central to the common sense definition is the notion that for example, dogs will always mate with dogs to make more dogs. Trout will always mate with trout to make more trout, etc. There is a distinct obvious common sense dividing line between dogs and trout. We recognize in this definition that all organisms need not look identical to be in the same species. In some cases, like dogs, these differences are apparently large, but in most other cases, like trout, one pretty much looks like another unless you look very carefully. This concept can be summarized as ‘we know a species when we see one’.
Since such a concept lacks a certain rigor and utility in science. There a more precise and quantifiable definition is preferable. The most precise and quantifiable definition there is the biological species concept as defined by Mayr, E., 1942. Systematics and the Origin of Species from the Viewpoint of a Zoologist.
"... groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups."
This definition has the advantage of being very useful in determining when you have a new species under controlled conditions, such as those one might find in a lab working with fruit flies. It’s also useful in distinguishing animal species (especially vertebrates) and most insects. The reason being that obligate sexual reproduction is the predominate mode of offspring generation in these groups.
But there are several problems associated with this concept.
1. In organisms where obligate sexual reproduction is not the predominate mode of offspring generation, it is of little use. These include asexually reproducing organisms (e.g. prokaryotes and some eukaryotes); and many plants that show hybridization.
2. In practice it’s difficult if not impossible to use in many cases. In a situation like a fruit fly culture, it might have some utility in determining speciation, since the system is highly constrained. However, in nature, its use is far less straightforward. The only way to tell if any two organisms are different species using this definition is to do cross reproduction studies. Such studies are seldom done, and even if done, the fact that two groups may not reproduce in the lab doesn’t mean they can’t do so in Nature.
3. It’s of no use at all in determining speciation between groups separate in time (e.g. extinct organisms represented by fossilized forms).
For these reasons and others, even the strongest adherents of the biological species concept rarely use it, even though it is very useful in certain restricted situations where it can be practically applied. Instead they compare the similarities and differences using various techniques between two groups. If they are similar enough, they are considered the same species unless some specific barrier to speciation is put forward. It sounds very arbitrary, but keep in mind definitions of other divisions have an even less basis in a quantifiable entity.
One way to think of the speciation as the sum of similarities and differences is to use the definition of Davis and Heywood, Davis, P. H., and V. H. Heywood, 1963. Principles of Angiosperm Taxonomy:
“assemblages of individuals with morphological features in common and separable from other such assemblages by correlated morphological discontinuities in a number of features”
It sounds simple enough, but relies heavily on subjective interpretations of taxonomists. The ‘lumpers’ will tend to focus on the similarities and the ‘splitters’ on the differences. To eliminate some of the arbitrariness some workers have proposed statistical approaches by giving characteristics a numerical weight and determining speciation based on a certain numerical value, the so-called numerical or phonetic taxonomy. It’s basically an attempt to formalize and quantify the methods. However, it’s still subjected to criticism because where the magic cut-off correlation is still a subjective determination. Plus, the use assigning equal statistical weight to all characteristics is also criticized.
To circumvent the problems inherent in the above concepts, various authors have proposed broader definitions based on the evolutionary species concept. Here, species are defined not by sexual isolation but on ‘evolutionary’ isolation, of which sexual isolation is but one component. The definition is given by Simpson, G.G., 1961. Principles of Animal Taxonomy:
“An evolutionary species is a lineage (an ancestor-descendant sequence of populations) evolving separately from others and with its own unitary evolutionary role and tendencies”
This definition has the advantage that the species concept includes change (evolution) and at the same time lays the ground-work those changes resulting from competition and interaction between species.
However, again we run into the problem of practically applying this definition. How else can an intrepid taxonomist facing a large variety of organisms on her table classify them except by morphology? Also, since speciation is a process, it’s difficult to know when the process is complete in practice. What is the defining point of complete separation? It’s hard to say. Nevertheless, this definition does allow one to include ecological, behavioral, genetic, and morphological evidence to reflect evolutionary closeness or distance.
In the end, the difficulties in taxonomy with regards to species are inherent in the process itself. The differences are hard to classify precisely because species do evolve, and evolution is a morphologically gradual, not saltational process. Different forms show different degrees of separation in various characteristics. Some of these degrees of separation are so great in some organism so as to preclude successful reproduction. That’s a handy way to tell if a new species arises, but it isn’t useful every time a new species arises. In other words, evolution doesn’t make the taxonomist’s job easy, but it does tell him why it’s so hard.
Finally, keep in mind that the concept of species, like genera or order or class, is man-made to try and keep track of things. Or in the words of John Locke, 1689, An Essay Concerning Human Understanding: “the boundaries of species, whereby men sort them, are made by men”.
Comments?