THIRD QUESTION SET, Spring 2008
ANSWER FOUR OF THE FOLLOWING FIVE:
(DUE MONDAY 12 MAY)
1. When apple trees set a very heavy crop, a large number of apples
will be dropped, spontaneously, when they are very small.
Orchardists, however, will thin the crop even further, knowing that
otherwise the mature apples will be small (and, from marketing
perspective, fewer big apples are worth more than more little
ones). Discuss what’s going on here from fitness
‘perspective’ of both parent tree and offspring (embryos in the seeds
in the apple) and what that means for their interaction. (Don’t
worry about the fitness of the orchardist.). (One piece of
potentially useful information; allocation of resources to developing
apples/seeds is clearly through the parent/tree, but, just as a human
embryo can physiologically influence parent, so can plant embryo
produce hormones that might influence tree.) Remember that the
‘apple’ is really there as a ‘reward’ (or pay-off) to induce animals to
consume and so disperse the seed; you can assume that the bigger the
apple the more likely
the seed will be dispersed. It also turns out that there’s a
strong correlation between amount of reserves in seed and size of apple.
2. In many species with internal fertilization, effectiveness of
fertilization is a function of the length of copulation and copulatory
bouts can be quite extended (up to hours). For example, when females
carry many eggs, the number fertilized by a particular male is often
proportional to the length of copulation. There doesn’t seem to
be any inherent reason why this must be so (i.e., it should be possible
to insert sperm quickly, and, in some species, fertilization is fully
effected with very brief copulation). Consider selective forces
that might be at work here for each sex (are there possible fitness
advantages/disadvantages to either sex of extending copulatory period?
Of making it as brief as possible?). ALSO, consider whether
mating system would make a difference in these arguments (i.e., whether
individuals tend to be monogamous or promiscuous).
3. Mimicry is common in nature. A
simple mimicry system might involve a tasty species that
looks very similar to another species which is distasteful or even
toxic; the palatable species is
the mimic, the unpalatable one the model. Usually, both have very
obvious, even flamboyant
markings or coloration. In such instances, predators may learn to avoid
the model for obvious
reasons, and subsequently avoid the mimic as well.
It is a strong generalization that
individuals of the 'model' species are much more abundant than individuals of the 'mimic'
species (monarchs are much more common than viceroys. Develop a
selective argument for why this should be expected. Imagine that,
within the mimic species, there are two possible
heritable phenotypes; the 'mimic' form, and a 'cryptic' form that does
not look like the model
species but is instead camouflaged; consider the fitnesses of these
two types as the relative
abundance of model and mimic change.
4. Periodical cicadas are herbivorous insects who spend most of their
long lives (13 or 17 years depending on area) underground as nymphs
feeding on sap drawn from the xylem of tree roots. They are
extremely numerous. Ultimately, they emerge as flying adults, who
have no digestive system and live only a few days or weeks. All
of the cicadas in a particular region emerge in the same year and in
the same season (late spring/early summer), so that they may be present
in astounding numbers. Several species may be present in these
regional ‘broods’, and all are mutually synchronized. It is
thought that such simultaneous emergence has the effect of suddenly
‘saturating’ predators, so that only a small proportion of the cicadas
can be eaten (even though they are large, tasty, and extremely noisy
and so easy to find) before they complete their reproductive process
and die anyhow.
This simultaneous emergence behavior, of course,
‘benefits’ the species in terms of reducing mortality rates from
predation. However, its origin must ultimately be explained by
selection working on differences in fitness among individuals within
populations (and it must act independently to produce same result in
each sympatric population where there are several species).
Construct such a selective
hypothesis/scenario. It may help to imagine an initial condition
where emergence is not synchronized, but some individuals in a
population emerge every year (but some years may have more individuals
emerging than other years).
In a normal, synchronized ‘brood’, some individuals
will still emerge a year early or a year late. Consider
their likely fitness. Is synchronized emergence likely to be an
ESS?
5. Wolbachia is a genus of
bacteria found as intracellular parasites in many species of
insects. Wolbachia is
transmitted, almost exclusively, through maternal inheritance, being carried within the
cytoplasm of the egg (it is not transmitted in the sperm). Wolbachia may exist in several
genetically distinct types within a particular host population.
The effects of the parasite are diverse; it rarely causes the host to
become 'sick, but it often induces odd reproductive
dynamics. Two common effects are:
a) 'male-killing', where infected male offspring are
selectively aborted early in development (or male eggs do not hatch);
b) 'unidirectional cytoplasmic incompatibility ',
where infected males produce sperm that will produce viable zygotes only if the male mates with a
female carrying the same strain of Wolbachia.
Matings with other females (uninfected with Wolbachia, or carrying a different
strain) produce no viable zygotes.
Offer a selective argument for these two Wolbachia-induced reproductive
anomalies based on their effects on the fitness of the parasite.
Suggest some possible coevolutionary responses on the part of the host
(i.e., what new traits might enhance hostfitness).