Species

The Synthesis and Species 215

In the book announcing the synthesis, Huxley later criticized Dobzhansky for

underplaying the difficulties that a simple intersterility criterion encountered, par-

ticularly in plants.

The dynamic point of view is an improvement, as is the substitution of incapacity

to exchange genes for the narrower criterion of infertility: but even so, this defini-

tion cannot hold, for it still employs the lack of interbreeding as its sole criterion.

“Interbreeding without appreciable loss of fertility” would apply to the great majority

of animals, but not to numerous plants. In plants there are many cases of very distinct

forms hybridizing quite competently even in the field. To deny many of these forms

specific rank just because they can interbreed is to force nature into a human definition,

instead of adjusting your definition to the facts of nature. Such forms are often mark-

edly distinct morphologically and do maintain themselves as discontinuous groups in

nature. If they are not to be called species, then species in plants must be deemed to

differ from species in animals in every characteristic save sterility...^29

He gives his own criteria a few pages later, after determining that single-criterion

definitions are useless:

In general, it is becoming clear that we must use a combination of several criteria in

defining species. Some of these are of limiting nature. For instance, infertility between

groups of obviously distinct mean type is a proof that they are distinct species, although

once more the converse is not true.

Thus in most cases a group can be distinguished as a species on the basis of the

following points jointly: (i) a geographical area consonant with a single origin; (ii) a

certain degree of constant morphological and presumably genetic difference from

related groups; (iii) absence of intergradation with related groups. ... Our third crite-

rion above, if translated from the terminology of the museum to that of the field, may

thus be formulated as a certain degree of biological isolation from related groups.^30

After discussing freely hybridizing groups, sympatric ecological forms, plants,

polyploidy,^31 and asexuality, Huxley says

Thus we must not expect too much of the term species. In the first place, we must not

expect a hard-and-fast definition, for since most evolution is a gradual process, border-

line cases must occur. And in the second place, we must not expect a single or a simple

basis for definition, since species arise in many different ways.^32

(^29) Huxley 1942, 162f.
(^30) Op. cit., 16 4f.
(^31) Polyploidy is the state of having three or more complete sets of chromosomes, in contrast to the usual
state of diploidy (two sets) in sexual organisms. Alloploidy occurs when hybrids are formed through
the fertilization of gametes (sex cells) across species. It is usually also polyploidy, in which case it is
called allopolyploidy. In plants, fertile individuals often result when chromosomes are duplicated and
then separate to form symmetrical diploid chromosome sets. If the hybrid is significantly different
from the parental populations, it will not interbreed with them, or will interbreed incompletely, so
that eventually the novel karyotype (chromosomal type) will breed true with itself but not with either
parental type. Speciation can be achieved in one or only a few generations this way [Grant 1975,
431ff, Ramsey and Schemske 1998].
(^32) Huxley 1942, 167.