Species

Reproductive Isolation Concepts 235

The novel elements here include the “entification”^30 of evospecies in place of the
population stipulation, to accommodate asexual organisms,^31 and the inclusion of
time and space to indicate the processual nature of the concept. However, one thing
that all the evolutionary concepts fail to do adequately is to specify what counts as
“independence.” If a parasitical species coevolves with its host, which is commonly
the case, are they still independent? And of course, there is a metaphorical problem
with “fate,” given the universal view that evolution is not predetermined, but we can
assume that Wiley and his colleagues understand that; this merely specifies that the
outcomes of evolution for any one species are unique to that species. In a sense, this
is a matter of evolving a unique set of traits.
Evolutionary species concepts have been assumed by some critics to imply
a gradual and constant rate of evolution.^32 This need not be the case, however, as
Simpson’s own classical work on evolutionary rates indicates,^33 but it is also some-
times held that species change over the entire course of their duration. Opposing this,
the punctuated equilibrium theorists have argued that species tend to remain stable
once evolved.^34 If correct, or rather when correct, for it is now accepted to be the case
for many species if not all, the “fate” of the species involves the stasis of the unique
set of traits once achieved.
Evolutionary species concepts, as with some phylogenetic species concepts, tend
to adopt the metaphysical species-as-individuals thesis of Ghiselin and Hull.^35 As
Wiley and Mayden note,

Evolutionary species are logical individuals with origins, existence, and ends.^36

However, if species are correctly thought of as logical individuals, they still need
not be historical individuals. There are three different notions under the term “indi-
vidual” in the species concept literature, and they ought to be kept distinct:

1. Individuals as metaphysical particulars (i.e., not universals or natural
   k i nds)


2. Individuals as coherent functional objects like organisms
   3. Individuals as clusterings of properties in a phenomenally salient manner

I shall expand on this later.

Evospecies, as we might call these entities, are something of a hybrid notion, to

my mind. They are phyletic objects, but they are often presented as achieving grades

of organization, and often run with evolutionary systematics (itself a hybrid system,

conjoining phylogenetic and adaptive conceptions of classification).

(^30) I owe this useful term to Henry Plotkin (pers. comm.).
(^31) Wiley and Mayden 2000.
(^32) Eldredge and Gould 1972, Ayala 1982, Gould 1982, Ghiselin 1988.
(^33) Simpson 1944.
(^34) Eldredge and Gould 1972, Gould and Eldredge 1977, Eldredge 1985, Eldredge et al. 1997, Gould
2002.
(^35) Ghiselin 1974, Hull 1976, 1978, 1981, Ghiselin 1988, Hull 1992, Ghiselin 1997.
(^36) Wiley and Mayden 2000, 74.