Species

The Development of the Philosophy of Species 319

thing the not-eukaryotes have in common is a lack of a nuclear membrane, allowing

the transfer of genetic material between genomes. While the term “microbe” refers to

single-celled organisms, whether eukaryotic or not, most of the discussion that follows

can be regarded as covering the prokaryotic domain, and particularly the bacteria.

The Problem of Cohesion

In ... an asexual group, systematic classification would not be impossible, for groups

of related forms would exist which had arisen by divergence from a common ances-

tor. Species, properly speaking, we could hardly expect to find, for each individual

genotype would have an equal right to be regarded as specifically distinct, and no

natural groups would exist bound together like species by a constant interchange of

their germ-plasm.

The groups most nearly corresponding to species would be those adapted to fill so

similar a place in nature that any one individual could replace another.

R. A. Fisher^108

Many bacterial and archaeal species either do not exchange genes to reproduce or they

can but do not need to. The question is sometimes raised whether these organisms form

species at all, or if there is some replacement term or concept for groups of them.

It took a long time for some to even accept that there were asexual organisms. Darwin

discussed hermaphroditic species, but they still had mating types or genders; it was just

that a single individual had both kinds. It was long recognized that some plants could

propagate vegetatively, but the notion that there were obligately asexual organisms was

doubted, for instance, by Fisher as late as 1958, saying that “organisms in which sexual

reproduction was entirely unknown” could not with certainty be ascribed to any known

group.^109 In general, early members of the Modern Synthesis of genetics and Darwinian

evolution had real problems with asexual taxa. Theodosius Dobzhansky, the famous joint

architect of the synthesis and first author of the “biological” species concept, simply

denied that asexuals formed species. Call them something else, he said.^110

As pointed out by Babcock and Stebbins... , “The species, in the case of a sexual

group, is an actuality as well as a human concept; in an agamic complex it ceases to

be an actuality.”^111

And while many modern writers, such as Templeton^112 or Coyne and Orr^113 now,

rather grudgingly accept the reality of asexual species, others, such as Meier and

Willmann, continue to deny species rank to asexual taxa, instead calling them

“agamotaxa.”^114

(^108) Fisher 1958, 135.
(^109) Loc. cit.
(^110) Dobzhansky 1941, 320f.
(^111) Dobzhansky 1951, 275.
(^112) Tem plet on 19 98.
(^113) Coyne and Orr 2004.
(^114) Meier and Willmann 2000. Other terms include binoms [Grant 1957], or agamospecies [Cain 1954].