Biology Reference
In-Depth Information
they described the “new” 13-year population as a previously undescribed spe-
cies called
Magicicada neotredecim
.
Magicicada neotredecim
and
M. tredecim
,
both 13-year cicadas, overlap geographically and, since 1868, their broods over-
lap chronologically. The
tredecim
and
neotredecim
populations differ in abdo-
men coloration and mtDNA. Such traits in
M. neotredecim
are not consistently
different from the sympatric populations of the 17-year
M. septemdecim
and
thus
neotredecim
appears most closely related to this geographically adjacent
population with a 17-year life cycle.
Marshall and Cooley (2000)
suggest this is
evidence that speciation in
Magicicada
involves temporal isolation.
Simon et al. (2000)
, in a companion article, supported the conclusion that
M. neotredecim
is a new cryptic species, and provided mt DNA data to support
the conclusion that a life-cycle switch occurred to produce two overlapping
13-year cicada lineages. Furthermore, the genetic evidence suggests that assor-
tative mating is taking place in the area where the two populations of 13-year
cicadas overlap. Thus,
Simon et al. (2000)
propose two possible scenarios for the
evolution of the
septemdecim-tredecim-neotredecim
species (
Figure 13.3
). In
both models, the ancestor is a cicada with a 13-year life cycle, which gave rise
to a 13-year lineage (
tredecim
lineage) and to a new lineage that had a 4-year
extension (17-year
septemdecim
lineage). In one scenario,
neotredecim
evolved
from
septemdecim
via a single 4-year life cycle reversion (instantaneous specia-
tion) (
Figure 13.3A
). In scenario two, the
septemdecim
lineage gave rise to two
independent 17- to 13-year life-cycle reversions, resulting in two
neotredecim
broods (
Figure 13.3B
). There were no genetic or behavioral data to distinguish
between these two hypotheses.
Simon et al. (2000)
addressed the question as to whether the 13- and 17-year
cicadas should be called different species. The concept of “species” varies
among different systematists, but
Simon et al. (2000)
support the thesis that spe-
cies should be designated as soon as they are distinguishable if it is likely that
they will remain extant and isolated long enough for reproductive isolation to
develop.
Marshall and Cooley (2000)
presented behavioral data and field obser-
vations to suggest that important differences do exist in male song and female
response, supporting the separation of
neotredecim
and
tredecim
as species.
They argue that
neotredecim
is unlikely to revert to a life cycle that is synchro-
nized with any extant 17-year brood. Despite behavioral and genetic similarities
between the 17- and 13-year broods, it is likely that reproductive isolation will
be achieved between
neotredecim
and
septemdecim
due to the differences in
brood timing.
Simon et al. (2000)
summarize their hypothesis as follows: “an ini-
tial allochronic event separated lineage A and B periodical cicadas and a second
allochronic event placed them in secondary contact.”
