230 Species
constitution”) and the nominal essence (that is, how we know the species, describe
it, define it, and apply a name to it).^5 It is not clear that “essence” in these cases plays
a definitional role—the Real Essence is not, by definition, definable.
Modern views of species are widely known and discussed, so we shall be brief
in covering them, except for the ways that the most well-known reproductive isola-
tion concept, the class of which we shall call for brevity “isolation concepts” (see
Table 8.1), which is of course Ernst Mayr’s biological species concept, or biospecies,
developed. Also, there is considerable confusion and ambiguity in the phylogenetic
concepts, so this will need discussion. I have attempted to be comprehensive, though,
and provide all the current species concepts on offer since Dobzhansky introduced
the issue into the Synthetic debate^7 (see Append ix B).
(^5) Hull noted (in correspondence) that it is ahistorical and Whiggist in turn for historians such as myself
to apply current standards to the Received Historians of the 1960s; he, Mayr, and Cain had what
scholarly resources were then available. In large part due to their work, later research has identified
the “missing links,” and I am certain later work will overturn some of the claims made in this and
other modern work too. It should not be thought that I am criticizing him or Mayr, etc., for failing to
take into account later scholarship. That would be Whiggist.
(^6) From Littlejohn 1969, 461.
(^7) Building on Mayden 1997.
TABLE 8.1
A Classification of Reproductive Isolating Mechanisms (RIMs)^6
- Reduction of contact
(a) temporal
(b) ecological - Reduction of mating frequency
(c) ethological
(d) morphological
PREMATING
—
POSTMATING - Reduction of zygote formation
(e) gametic and reproductive tract incompatibility
PREZYGOTIC
—
POSTZYGOTIC - Reduction of hybrid survival
(f) hybrid inviability - Reduction of gene flow through hybrids
(g) hybrid ethological isolation
(h) hybrid sterility
(i) hybrid breakdown
Note: That the barriers here are not absolute—the RIMs only produce a reduced frequency of successful
breeding.