Recognition of species taxa in sexually reproducing organisms

SINMORFICASincluded in the analysis.

The major problems facing the working systematist is how

to deal with geographic (allopatric) forms, with fossil organisms,

and with asexual organisms. These are quite different

questions and a general solution cannot be offered. Each of

these types of species taxa will be considered separately.

Asexual organisms

One of the results of recognizing species taxa in sexually

reproducing organisms is sorting out the ecological diversity

into organized units based on the fact that each species is a

separate ecological unit. These distinct ecological units of

sexual species reflect the discontinuity of ecological conditions

from which arise a discontinuity of selective demands. These

selective demands result in discontinuous distribution of

phenotypic features in the different species taxa. Presumably

the ecological conditions and ensuing selective demands acting

on asexual organisms are also discontinuous with the result

that the distribution of phenotypic features of asexual organisms

will also have gaps. Hence in most groups of asexual

organisms, phenotypic features which are adaptations to

selective demands arising from the external environment will

cluster groups of individuals together with distinct gaps

between these clusters. These clusters of individuals in asexual

organisms correspond reasonably closely to the ecological

units of species in sexually reproducing organisms. Hence the

best set of criteria for recognizing species taxa in asexual

organisms are adaptations to selective demands arising from

the external environment of these organisms. Such asexual

species taxa will be closest in meaning to the ecological aspects

of sexual species taxa.

Geographic representatives

Among sexually reproducing organisms, the largest problems

exist in the taxonomic treatment of geographically replacing

populations. Herein are included all types of problems such as

the degree and types of phenotypic differences among the

geographic populations, whether any interbreeding occurs

where they come into contact, etc. In the last century when the

subspecies concept was being developed for geographically

replacing populations, the central criteria was that interbreeding

and integration was the hallmark of subspecies. This

criterion is too simplistic as many isolated allopatric populations

exist without any signs of interbreeding, but which are

best treated as subspecies.

Basically, the major practical criterion for deciding on the

taxonomic status of geographically replacing populations is

the degree of phenotypic differences observed

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