278 Speciesthe subject have also been published.^6 However, the best overview of which I know of
the entire philosophical issues, as well as the scientific ones, is Frank Zachos’ book,
which is written for non-philosophers. I strongly recommend it.^7 For those who are
new to the philosophical issues, Peter Godfrey-Smith’s introduction has a worthwhile
chapter, and Kim Sterelny’s and Paul Griffiths’ now dated but still worthwhile book
offers a more extensive discussion.^8 Matt Slater has an extensive and clear argument for
a view he calls Populationism, and discusses the philosophical aspects of essentialism.^9The Three Species Problems
The species problem is actually a number of problems that biologists have dealt with
since the term was first applied to biological organisms by Aristotle. I call the three
main problems the grouping problem, the ranking problem, and the commensura-
bility problem. It will benefit us to clearly distinguish these at the beginning of our
discussion and to bear them in mind as we consider the philosophy of species.
The Grouping Problem
Our first problem is, How are members of species to be grouped together and excluded
from other groups? No matter whether species are constructs and arbitrary, or natural
and objective, the issue remains how we can include some and exclude other organisms
from a species. During the Middle Ages, the notions of differentiae and relata were
brought to bear. A species was defined by characters, and those that differentiated
organisms in some relevant way enabled classifiers to exclude some character bearers
from a species. Likewise, characters that related organisms were thought to be suffi-
cient to include organisms within species despite their individual differences. In short,
the grouping problem is one primarily of operationality. But not entirely—organisms
can be seen as self-classifiers, especially in the reproductive isolation concepts of spe-
cies. So long as organisms are able, it is said, to differentiate between themselves and
find related enough organisms to mate with, then they form species even if we humans
cannot identify what it is about them that the organisms themselves find salient in the
process. Maynard Smith, for example,^10 notes that one reason we have so little trouble
identifying some kinds of species (e.g., birds) is that they are of the same general size as
we are, and use much the same criteria in self-differentiating: sound, appearance, and
behavior. It follows, some think, that other groups equally salient to us must also have
some causally significant differentiae and relata. Likewise, it follows that organisms
that are (reproductively) distinct but are otherwise indistinguishable by us must have
some differentiae and relata that we cannot recognize (e.g., biochemical differences)
without specialist equipment. So our grouping problem reduces to the issue: what is
appropriately seen (by us or by organisms themselves) as grounds for distinguishing
some and grouping others into species?
(^6) Most recently, Pavlinov 2013, an online open source book.
(^7) Zachos 2016. Previous works include Claridge et al. 1997, Hey 2001.
(^8) Sterelny and Griffiths 1999, Godfrey-Smith 2014.
(^9) Slater 2016.
(^10) Maynard Smith 1975.