Biology Reference
In-Depth Information
fewmutations lead to the establishment of a species co-occurring with the
parent species, it must be reproductively isolated from the parent in order
to survive as a species. Such isolation may be spatial, or it may be by
different copulatory organs, different pheromones, etc. Whatever the
case, differences in the use of resources, if they exist, may be purely
secondary. In other words, in such cases not competition but reproduc-
tive segregation may be the driving force for speciation. Competitive
speciation can be assumed only if explicit evidence for competition as the
causative factor is given. But how can we test for such evidence?
Tests for divergence due to competition
Dissimilarity between species is often used as evidence for competition
(see pp. 52-55), but Connell (
1980
) has convincingly shown that tests for
competition are inadequate. In spite of his criticisms, many authors still
uncritically claim competitive effects without any evidence whatsoever.
Connell established very strict criteria for demonstrating divergence of
competitors: (1) there actually has been divergence in resource use between
competitors; (2) competition and not some other mechanismwas reponsible
for the divergence; and (3) divergence has a genetic basis and is not simply
phenotypic.
Concerning the first point, Connell stresses that divergence between
extant species has not been demonstrated except for some pests of crop
plants; some fossil sequences showing divergence are known but it is
impossible to say whether competition was responsible. Concerning the
other two points, field experiments are necessary to provide evidence, but
these have been performed only in part, for one case. Connell concludes
that, at present, there is little support for the coevolutionary divergence of
competitors, and that is probable only in low diversity communities.
According to Connell, point 2 can best be tested by manipulating the
distribution and abundance of one or both species of competitors in field
experiments. However, controls are difficult. Thus, if all populations that
are compared are sympatric, it is impossible to obtain data on the relevant
dimensions in multidimensional niche space, and limiting similarity is
purely theoretical and has no proven biological basis. Also, even if species
overlap in resource use, they may not compete for them. On the other
hand, if there are allopatric and sympatric populations and if they do
indeed differ, it is difficult to prove that the only difference is the absence
of the competing species in allopatric situations, and comparison with
random assemblages is very difficult.
