Biology Reference
In-Depth Information
Cooley et al. (2001)
carry the story forward and predict that the newly
described
M. neotredecim
inhabits midwestern habitat that will be unsuit-
able for its survival during the next “ice age.” They argue that the most south-
ern populations of
neotredecim
have the greatest “likelihood of colonizing a
refugium during the next glacial cycle
…
then after the next glacial retreat all
undisplaced
M. neotredecim
will have gone extinct, and the pattern of character
displacement linking
M. neotredecim
to an allochronic speciation event in the
M. septemdecim
lineage will have been erased.” Thus, the
Magicicada tredecim-
septemdecim-neotredecim
story supports the hypothesis that instantaneous spe-
ciation can occur by shifts in reproductive timing (allochrony), although the precise
mechanism by which the 4-year shift occurred remains unknown.
Cooley et al.
(2003)
hypothesize that the 2.5% difference in mitochondrial gene sequences
between
N. tredicim
and
M. neotredecim/M. septemdecim
lineages suggest that
this degree of genetic distance is correlated with
≈
1 million years of separation.
Nariai et al. (2011)
asked the question how life-cycle switching could occur
in small numbers of these cicadas and how the population could increase. In
theory, small numbers could fail to reproduce or be destroyed by predators. The
Allee effect was invoked to explain the change in life cycles. The Allee effect is
“a positive relationship between any component of individual fitness and either
numbers or density of conspecifics” (
Stephens et al. 1999
).
Nariai et al. (2011)
evaluated the possibility that life-cycle switching between 13- and 17-year cycles
can be explained by introducing a few individuals into an isolated population
with the other cycle, without the need for hybridization or introgression in
hybrid zones. Their model assumed that the life cycle is controlled by alleles at
a Mendelian locus, with one cycle dominant over the other and neither allele
has a selective advantage. The simulation models were started with different
proportions of two pure populations with alleles for either the 13- or 17-year
cycles. In the model, if there were no Allee effects, the mixed population exhib-
its the genetically dominant life cycle (either 17- or 13-years). If moderate-to-
strong levels of Allee effects are present, a population of a recessive cycle “may
shift entirely to the genetically dominant cycle after the introduction of a few
individuals with the dominant alleles” and “all broods of the recessive cycle dis-
appear.”
Nariai et al. (2011)
conclude, “Our model makes an intriguing predic-
tion. Life cycle switching by gene introduction appears to be possible under a
moderate level of Allee effects
…
the direction of switching depends only on
genetic dominance: switching from recessive to dominant cycles.” The model
relied on several assumptions: no mating preferences, neither cycle alleles have
a selective disadvantage, and the net reproductive rate per generation is the
same for both types.
