Species

250 Species

He then noted that this means that subspecies are “by definition, unobservable

and undefineable,” since they have no such apomorphies. Subsequently, Nelson and

Platnick proposed in passing that in their book on systematics and biogeography that

they would treat species as

... simply the smallest detected samples of self-perpetuating organisms that have

unique sets of characters.^45

They, too, noted that this meant that diagnosable “subspecies” (groups identifi-

able by unique sets of characters) were thus species. This was not intended to be a

complete definition but rather a description of their current practice (Nelson, pers.

comm.). Even so, it was very influential on subsequent discussion. Independently, and

almost contemporaneously, Eldredge and Cracraft defined a species as

... a diagnosable cluster of individuals within which there is a parental pattern of

ancestry and descent, beyond which there is not, and which exhibits a pattern of phy-

logenetic ancestry and descent among units of like kind.^46

Cracraft subsequently revised his formulation by removing mention of reproduc-

tive cohesion (“like kind”) to read

... the smallest diagnosable cluster of individuals within which there is a pattern of

ancestry and descent.^47

Quentin (not Ward) Wheeler and Platnick base their definition on this tradition as

well.^48 They define species thus:

We define species as the smallest aggregation of (sexual) populations or (asexual) lin-

eages diagnosable by a unique combination of character traits. This concept represents

a unit concept.^49

This concept is prior to a cladistic analysis,^50 and so unlike Nelson’s earlier
note that species are taxa like any other level of a phylogenetic tree,^51 Wheeler and
Platnick do not worry about apomorphies and homologies when recognizing species,
only characters. Species are found wherever characters are fixed and constant across
all samples, while traits may be variable.^52 Their willingness to handle and include
asexual taxa within their species concept marks them out from most other concep-
tions, and they bite the bullet on recognizing clones of asexual lineages as species:

(^45) Nelson and Platnick 1981, 12 quoted in Wheeler and Meier 2000, 56.
(^46) Eldredge, 1980, 92 quoted in Wheeler and Meier 2000, 55–56.
(^47) Cracraft, 1983, 170 quoted in Wheeler and Meier 2000, 56, and see discussion there.
(^48) Wheeler 1999, Wheeler and Platnick 2000.
(^49) Wheeler and Platnick 2000, 58.
(^50) Op. cit., 59.
(^51) Nelson 1989.
(^52) See figure 5.1 in Wheeler and Platnick 2000, 58.