Suggested answers to discussion questions
1.
Individuals that have genes that predispose them to cancer for are, on average, likely to
have fewer offspring than those that do not have these genes and therefore there will be
fewer cancer genes in the next generation. The cumulative effect of this over many
generations will be to remove genes for cancer from the population.
2.
Cancers that mainly strike old fish are likely to be less strongly influenced by natural
selection as individuals that carry these genes are likely to have had all their offspring
(and therefore passed the cancer genes on to the next generation) before the effect of
these genes is manifested.
Cancers that turn up in young breeders are likely to be more strongly influenced by
natural selection (see answer to question 1). For this reason you might expect most
cancers (at least those that have genetic causes) to have their effects later in life as
these genes are less likely to be removed from the population by natural selection.
Indeed, this does appear to be the case. In fact it has been proposed that the effects of
old age in general are caused by deleterious genes that take effect only after
reproduction has ceased and are therefore not removed from the population by natural
selection. This raises the interesting possibility that the human life-span could be
increased if everyone delayed reproduction, although it would take many generations for
this to take effect.
3.
Natural selection occurs when a particular gene influences either (a) the likelihood of
surviving long enough to pass the gene on to the offspring or (b) the ability to produce as
many offspring as other individuals in the population, i.e. the effects of natural selection
are mediated by differential survival and reproduction of individuals that carry different
genes.
Sexual selection occurs when a particular gene influences the likelihood of (a) attracting
a mate (intersexual mate choice) or (b) being able to outcompete sexual for the
attentions of a mate rivals (intrasexual competition), i.e. the effects of sexual selection
are mediated by differential access to sexual partners.
In this case natural selection is expected to decrease the frequency of the Xmrk gene as
it will kill some of the individuals that carry it before they have the opportunity to
reproduce, but sexual selection will increase the frequency of the Xmrk gene as males
that do survive will be more attractive to females and therefore will pass on more copies
of the Xmrk gene. The frequency of the Xmrk gene in the population will be determined
by the balance of these two opposing processes.
4.
One possibility is that spots may help to camouflage the fish, making it harder for them
to be detected by predators. Females that choose males with spots will have offspring
that have spots and are therefore better protected against predators. This is a good
genes hypothesis as females are choosing males with particularly good genes. Of
course, this protective effect would have to be stronger than the negative effect of the
increased risk of cancer otherwise Xmrk would not be a good gene.
Another possibility is that spots may make predators better able to see the fish! This
sounds paradoxical the idea is that if a male is able to survive despite this handicap then
he must be a particularly fit individual, with lots of good genes. This is known as the
handicap principle.
Another possibility is that there is no particular benefit of having spots other than the fact
that females prefer them. Females choose males with spots because their sons will also
have spots and therefore be more attractive to females, and produce more offspring.
This is called the sexy sons hypothesis.
5.
This rather depends on the reason for female preference for spotted males. Perhaps
larger spots are more efficient for camouflage (good genes), or make males more
conspicuous to predators (handicap principle).
It has been suggested that female prefer spotted males as this means that their female
offspring will inherit spots of pigment that may protect developing embryos in their
oviducts from damage by ultra-violet radiation in sunlight. If this is the case then larger
spots could provide greater protection.
6.
Xmrk imposes an evolutionary cost by causing individuals to die of cancer before they
get the chance to reproduce. This is true for both male and female fish that carry this
gene.
7.
Sexual selection erases this cost by making males more attractive to females. This
means that males that carry the Xmrk gene will be more likely to pass on their genes,
including Xmrk, to the next generation. This is only a direct benefit to males. Females
still have to bear the cost of increased risk of cancer with out any direct benefit, although
if their sons inherit Xmrk from them then they will benefit from increased attractiveness.
8.
As natural selection and sexual selection are balanced from an evolutionary perspective
neither is better. On one hand carriers of Xmrk are more likely to die of cancer but this is
offset by increased attractiveness. On the other hand males that do not carry the Xmrk
gene will be less attractive to females, but this will be offset by the decreased risk of
dying of cancer. In personal terms, your answer will depend of whether you place more
importance on reproduction or longevity.
9.
One possible example is risky behaviour in men. It is possible that this is attractive to
women but this is likely to be offset by increased risk of accidental death.
10. See the answer to question 10 above. Can you think of any other possibilities?
Blogs about ecology, evolution and genetics
Aetiology - http://scienceblogs.com/aetiology/
Afarensis - http://scienceblogs.com/afarensis/
The Barcode of Life blog - http://phe.rockefeller.edu/barcode/blog/
A Blog Around The Clock - http://scienceblogs.com/clock/
ecoLincNZ - http://www.ecolincnz.blogspot.com/
evolgen - http://scienceblogs.com/evolgen/
Evolving Thoughts - http://scienceblogs.com/evolvingthoughts/
Gene Expression - http://scienceblogs.com/gnxp/
john hawks weblog - http://johnhawks.net/weblog
Laelaps - http://scienceblogs.com/laelaps/
The Loom - http://blogs.discovermagazine.com/
New Zealand Ecology Reviews - http://www.newzealandecology.org/nzecologyblog/
The Panda’s Thumb - http://pandasthumb.org/
Pharyngula - http://scienceblogs.com/pharyngula/
Sandwalk - http://sandwalk.blogspot.com/
Science Made Cool - http://www.sciencemadecool.com/
Sex, Genes and Evolution - http://johnlogsdon.blogspot.com/
Small Things Considered - http://schaechter.asmblog.org/schaechter/
The Wild Side - http://judson.blogs.nytimes.com/
This Week in Evolution - http://blog.lib.umn.edu/denis036/thisweekinevolution/