Species

The Synthesis and Species 219

to the parental species), self fertilization, parthenogenesis, and so on, but says this is

“apparently rare, even where it is hypothetically possible.”^55

In later publications, Mayr introduced the notion of species being “non-dimensional”:

Noninterbreeding between populations is manifested by a gap. It is this gap between

populations that coexist (are sympatric) at a single location at a given time that delimits

the species recognized by a naturalist. Whether one studies birds, mammals, butter-

flies, or snails near one’s home town, one finds species clearly delimited and sharply

separated from all other species. This demarcation is sometimes referred to as the

species delimitation in a non-dimensional system (a system without the dimensions of

space and time).^56

He also adds the distinction between the “species category” and the “species

taxon”—the former “designates a given rank in a hierarchic classification,”^57 while

the taxon is

... the concrete object of classification. Any such group of populations is called a

taxon if it is considered sufficiently distinct to be worthy of being formally assigned

to a definite category in the hierarchical classification. A taxon is a taxonomic group

of any rank that is sufficiently distinct to be worthy of being assigned to a definite

category.^58

By the 1963 version, the biological species as defined by Mayr has become a

reproductive community, an ecological unit, and a genetic unit:

... species are reproductive communities. The individuals of a species of animals rec-

ognize each other as potential mates and seek each other for the purpose of repro-

duction. A multitude of devices insure intraspecific reproduction in all organisms ....

The species is also an ecological unit that, regardless of the individuals composing it,

interacts as a unit with other species with which it shares the environment. The species,

finally, is a genetic unit consisting of a large, intercommunicating gene pool, whereas

the individual is merely a temporary vessel holding a small portion of the contents of

the gene pool for a short time.^59

and by 1970 the definition has changed to read:

(^55) Op. cit., 192. Again, later work has found sufficient examples, mostly in plants and other gamete
broadcasters, to establish this as a real process [Dowling and Secor 1997, Aldasoro et al. 1998,
Chepurnov et al. 2002].
(^56) Mayr 1970, 14f, italics original. According to Chung 2003, 285, Mayr first began to discuss the dimen-
sionality of species in an address in 1946 [Mayr 1946], where he described the Linnaean conception
as having no dimensions. Otherwise his tone is, as Chung remarks, fairly neutral on the difference
between the “morphological” species concept and the “biological,” “polytypic” species concept at
that time. Chung traces Mayr’s emerging view of species concepts as differing in their typology and
populational nature from 1953 [Mayr et al. 1953] through to 1959 [Mayr 1959] and concludes that he
discovered the typological aspect of the prior conceptions at around this time, especially in his 1955.
(^57) Mayr 1970, 13.
(^58) Mayr 1970, 14, italics original.
(^59) Mayr 1963, 21.