Biology Reference
In-Depth Information
T. conica is obtained. These species differ in spicule length as follows:
approximately 55, 120, 40, 120 mm. It seems that species with the greatest
spatial overlap are reproductively segregated by copulatory spicules of
different size. The situation in paramucosal species is not as clear:
M. uncinata, M. microstoma, T. robusta, M. stylosa, have spicules of 100,
100, 100, 550 mm length. However, in addition to spicule length there
are differences in body length and in genital papillae around the male
gonopore which may lead to reproductive isolation; and the possibility
must also be considered that unstudied chemical factors may contribute.
Evidence, although not conclusive, shows that the interpretation by Schad
(
1963
) of segregation as the result of competition is at least doubtful.
Species of Drosophila and related genera have been used in many field
and laboratory experiments, some concerned with the question of niche
segregation and its causes. Barker (
1983
) has reviewed some of these
studies and concluded that, although the idea of interspecific competition
leading to niche segregation and differential adaptation is attractive, and
although studies have demonstrated niche segregation, they provide no
evidence for the mechanism that has led to segregation.
Importantly, random selection of microhabitats in largely empty niche
space may also lead to niche segregation, even if interspecific effects have
never occurred in evolutionary time and are not occurring now (Rohde
1977b
).
If competition has the effect of limiting the similarity between species
living in the same habitat, it might lead to a reduction in the co-occurrence
of congeneric species in sympatric habitats. Is there evidence for this
assumption?
Reduction in the number of sympatric congeners as evidence
for competition
A reduction in the number of sympatric congeners relative to the number
expected, if congeners were randomly acquired, has been suggested as an
indicator of the evolutionary significance of interspecific competition
(Pianka
1983
). Elton (
1946
) appears to have been the first to make this
suggestion. He counted the numbers of congeneric species in many well
defined habitats and compared them with the total number of congeners
in the various genera. He found a much smaller percentage of such species
in the various habitats than in the genera, and concluded that interspecific
competition was responsible for the discrepancy. Concerning parasites,
related species supposedly compete more strongly than unrelated ones
