Ornithology, Evolution, and Philosophy 123

370 11 History and Philosophy of Biology—Mayr’s Third Synthesis
The Ontological Status of Species
Since the 1970s a controversy is raging in the literature whether a species taxon as
a whole is an ontological individual or a class. At first Mayr accepted the notion
that a species taxon is a causal individual (1976h) but later changed his position
somewhat and argued that a species is a variable biopopulation leaning toward its
interpretation as a group and not an individual (1984g, 1987d, 1996e, 2004a). He
stated that a biopopulation has the spatio-temporal properties, internal cohesion,
and potential for change of a historical individual, but he preferred the designation
‘population’ because this term conveys the impression of the multiplicity and
composite nature of a species. What Mayr evidently preferred was an ontological
category for species somewhat intermediate between an ontological individual
and a typological class. This is more or less the alternative proposition of those
authors who oppose the interpretation of species as ontological individuals. They
point out that species are neither typological classes nor ontological individuals
but non-typological classes possessing family resemblance (Bock 1986, 2000):
I. Essentialism (refers to classes or groups)
1. Typological essentialism (Plato) assumes the reality of immutable essences
underlying species and other objects.
2. Family group essentialism (Aristotle) refers to sets (e.g., species) of varying
characteristics. The individual members of such a class (species taxon) share
many, but not always all, or even most, characteristics. Under this view species
taxa are classes possessing family group resemblances. The intrinsic isolating
mechanisms of a species taxon constitute the family group essence of that class.
Family resemblance essences vary geographically and over time. A species
taxon does not have the type and degree of organization of an individual. It
can be destroyed only when a large part of it is removed or almost all of its
members.
II. Individualism (refers to ontological individuals)
Individuals are so organized that removal of certain very small portions will
result in their destruction (e.g., the heart or the pituitary gland in the case of
a human individual).
TheseviewsprobablycomeclosetoMayr’sthinking,althoughinnopaperdidhe
ever clearly accept them. The question of whether species are classes or ontological
individuals has relevance for the discussion of whether micro- and macroevolution
are “decoupled,” that is whether evolutionary theory is hierarchically structured
or not. Causes can act only on ontological individuals, not on groups. Hence
a hierarchically structured causal evolutionary theory requires that species and
higher taxa are real ontological individuals (which, however, does not appear to
be the case; Bock 2000).