Hosken, David J.

Stockley, Paula

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Sexual conflict 

remate at lower frequencies.
Further research on variation in
both reproductive morphology
and life history traits is essential.
What determines male mating
rate? New research is highlighting
the potential importance of
variation in reproductive organ size
in determining the limits of male
mating frequency and reproductive
success. The accessory glands
provide the material required to
synthesise the spermatophore in
which the sperm, produced by the
testes, are packaged. Male mating
rate is both phenotypically and
genetically correlated with the size
of male accessory glands, but not
of the testes. Furthermore, mating
causes a significant short-term
decrease specifically in accessory
gland size. In other Diptera, the
accessory glands produce
substances that alter female
behaviour and physiology, as well
as structural proteins that may
correspond to those forming
spermatophores. Current research
with stalk-eyed flies is focussed on
identification of accessory gland
products and evaluating their role
in mediating potential conflicts of
interest between the sexes.
Where can I find out more?
David, P., Bjorksten, T., Fowler, K., and
Pomiankowski, A. (2000). Condition
dependent signalling of genetic
variation in stalk-eyed flies. Nature
406, 186–188.
Hurley, I., Fowler, K., Pomiankowski,
A., and Smith, H. (2001).
Conservation of the expression of
Dll, en and wg in the eye-antennal
imaginal disc of stalk-eyed flies.
Evol. Dev. 6, 408–414.
Wilkinson, G.S., and Dodson, G.N.
(1997). Function and evolution of
antlers and eye stalks in flies. In
The Evolution of Mating Systems in
Insects and Arachnids. (J. Choe
and B. Crespi, eds.) Cambridge:
Cambridge University Press.
pp. 310–327.
Wilkinson, G.S., and Reillo, P.R. (1994).
Female choice response to artificial
selection. on an exaggerated male
trait in a stalk-eyed fly. Proc. R.
Soc. Lond. B. Biol. Sci. 255, 1–6.
Wilkinson, G.S., Presgraves, D.C., and
Crymes, L. (1998). Male eyespan in
stalk-eyed flies indicates genetic
quality by meiotic drive
suppression. Nature 391, 276–279.
Department of Biology, University
College London, Gower Street, London
WC1E 6BT, UK.
E-mail: t.chapman@ucl.ac.uk
Magazine    
R535
Sexual conflict
David J. Hosken1 and 
Paula Stockley2
What is sexual conflict?
Sexually reproducing organisms
are under evolutionary selection
pressure to maximise their
Darwinian fitness, but adaptations
that function to increase fitness in
one sex can reduce or constrain
fitness in the other. Sexual conflict
is the conflict that exists as a
result of the divergent
evolutionary interests of males
and females.
Is it common? Sexual conflict is
inevitable and ubiquitous because
the evolutionary interests of males
and females never exactly
coincide. This has identifiable
consequences at genomic and
whole organism levels. At an intra-
locus level, genes may be
expressed in both males and
females that may be beneficial
only to one sex. Ultimately, this
favours sex-limited gene
expression, but because this may
take time to evolve, the adaptive
evolution of each sex can, at least
in the short term, be impeded by
counter-selection in the other sex. 
Other conflicts are mediated by
genes at different loci that can
evolve independently in the two
sexes. These ‘inter-locus’ conflicts
arise over diverse aspects of
reproduction, from mating
decisions (how often and with
whom mating should occur), to
levels of parental investment (how
much each parent expends on
offspring production). For
example, males can typically
achieve high fitness by mating
with many females, but females
are generally unwilling to mate
indiscriminately as such behaviour
is not in their best interests.
Similarly, where both parents care
for their offspring, each could
potentially achieve higher fitness if
their partner were to provide more
care, providing the other parent
with opportunities to divert
‘saved’ investment into additional
reproductive attempts. In each
case, individuals could potentially
achieve higher fitness if their
prospective or actual mating
partners were to ‘agree’ with their
strategy for doing so, but the
divergent evolutionary interests of
the sexes means that their optimal
strategies for maximising fitness
rarely coincide.
So who wins? Theory suggests
several broad scenarios: male win,
female win, compromise or
endless evolutionary chases.
Predicting outcomes can be
complicated, however, and in
optimality models they depend on
relative power and benefits of
winning. Power in these models is
the relative cost of a unit of
escalation: if it is relatively
cheaper for a female to overcome
a male adaptation than it is for a
male to counter-adapt, then
females have an evolutionary
advantage in the conflict, and this
may frequently be the case. 
The benefits of winning are
perhaps even more difficult to
assess, but one may expect that
this part of the equation is
weighted more heavily in favour of
males, as for them the issue will
frequently be whether or not they
reproduce at all, while for females
it may only be an issue of who
they mate with. Individuals of
either sex may also attempt to
shift the balance of conflict in their
favour by coercion or
manipulation. 
An interesting example of this
phenomenon is found among
poeciliid fish, where males have
evolved a sneak-mating strategy
that completely circumvents
active female mate choice. Rather
than actively court females to gain
female consent to copulate, males
sneak up on females, insert their
modified anal fin that acts as an
intromittent organ, and transfer
sperm. This tactic is the means by
which almost all insemination
occurs in the mosquito fish. 
Where adaptations in one sex
generate counter-adaptations in
the other sex, such as increased
resistance to manipulation, sexual
conflict may result in prolonged or
continuous evolutionary chases.
Such evolutionary chases or
sexual arms races can have
important evolutionary
consequences, and have been the
subject of much recent
investigation. However, theory
predicts that unresolvable sexual
arms races may be infrequent,
and that it is more usual for one
sex or the other to gain an
evolutionary advantage, halting
antagonistic co-evolution, even
though the underlying conflict
may not be resolved, or that a
balance in ‘arms levels’ results.
Exceptions may be common at
the molecular level — for
example, involving seminal
proteins and receptors — where
costs of escalation are probably
relatively low.
What’s the big deal? Sexual
conflict has been implicated as
the driving force behind many
important biological phenomena,
including the rapid molecular
evolution of reproductive genes
and proteins, genome evolution
and genomic imprinting. Conflict
over reproductive investment may
even explain the evolution of the
sexes: proto-males may have
parasitized the larger gametes of
proto-females. Recently, sexual
conflict has been implicated in
speciation. In any evolutionary
arms race between the sexes,
there are potentially many
different ways individuals of
either sex could evolve to
enhance their fitness. Any
adaptation then counter-selects
on the opposite sex, which again
has many ways it could counter-
adapt, and so on. 
In any sexual arms race, it is
genes involved in reproduction
that are the focus of conflict, and
molecular evidence suggests that
many reproductive genes evolve
extremely rapidly. As a result,
populations can, in principle,
evolve differences in almost any
reproductive trait extremely
rapidly through sexual conflict,
and it is largely this feature that
has prompted the promotion of
sexual conflict as a speciation
agent. It is important to note,
however, that more traditional
mechanisms of sexual selection
can also generate rapid evolution
of reproductive characters, and
that distinguishing between the
different selection pressures that
can potentially lead to
reproductive isolation is likely to
be difficult.
Sexual conflict, sexual
selection, what’s the
difference? The two concepts are
intimately related. Sexual
selection, selection in exclusive
relation to securing matings (more
strictly fertilizations), inherently
generates sexual conflict because,
for example, females will mate with
some males, but not with others.
Additionally, sexual conflict can be
thought of as a sexual selection
mechanism, as adaptation to win
sexual conflicts can increase
variance in reproductive success.
Traditionally, sexual selection via
female choice was seen to be
generated by either direct or
indirect benefits to females. In
contrast, recent formulations of
sexual selection generated by
sexual conflict give primacy to
direct costs, and female mating
decisions are viewed as an
attempt to minimise costs rather
than maximise benefits. The
importance of various mechanisms
is currently being debated, but it
seems likely that all play some role
in sexual selection, although their
relative importance may vary.
Where can I find out more?
Trivers, R.L. (1972). Parental
investment and sexual selection. In
Sexual Selection and the Descent
of Man, 1871-1971. (B. Cambell,
ed.) Chicago: Aldine-Atherton.
pp 136-179. 
Parker, G.A. (1979). Sexual selection
and sexual conflict. In Sexual
Selection and Reproductive
Competition in Insects. (M.S. Blum
& N.A. Blum, eds.) New York:
Academic Press. pp 123-166. 
Rice, W.R., and Holland, B. (1997). The
enemies within: intergenomic
conflict, interlocus contest
evolution (ICE), and the
intraspecific Red Queen. Behav.
Ecol. Sociobiol. 41, 1–10.
Chapman, T., Arnqvist, G., Bangham,
J., and Rowe, L. (2003). Sexual
conflict. Trends Ecol. Evol. 18,
41–47.
Hosken, D., and Snook, R. (2005). How
important is sexual conflict? Am.
Nat. 165 Suppl. 5, S1–S4.
1Centre for Ecology & Conservation,
University of Exeter in Cornwall,
Tremough, Penryn, Cornwall TR10 9EZ,
UK. 2Animal Behaviour Group, Faculty
of Veterinary Science, University of
Liverpool, Leahurst Veterinary Field
Station, Chester High Road, Neston
CH64 7TE, UK.
E-mail: 1D.J.Hosken@ex.ac.uk,
2p.stockley@liv.ac.uk
Current Biology Vol 15 No 14
R536
Primer
Fidelity in protein
synthesis
Luisa Cochella and Rachel Green
The flow of genetic information
from DNA to RNA to protein
constitutes the basis for cellular
life. DNA replication, transcription
and translation, the processes
through which information
transfer occurs, are the result of
millions of years of evolution
during which they have achieved
levels of accuracy and speed that
make modern life possible. All
three processes have base
complementarity at the core of
their mechanisms. DNA
replication and transcription both
depend on complementarity of
the incoming nucleotide to the
DNA template, whereas
translation depends on the
complementarity of the anticodon
of the incoming transfer RNA
(tRNA) to the codon in the
template messenger RNA
(mRNA). Fidelity of genetic
information transfer thus relies
heavily on discrimination between
complementary, Watson-Crick
(and in a few cases wobble) base
pairs and non-complementary
ones. 
To ensure high selectivity, the
macromolecular machines that
carry out replication, transcription
and translation — DNA
polymerase, RNA polymerase and
the ribosome, respectively — have
evolved specific substrate
recognition strategies. These
strategies exploit the stability
arising not only from the
hydrogen-bonding and stacking
capacity of Watson-Crick base
pairs but, more importantly, from
their distinct geometry. Both
polymerases and the ribosome
have chemical groups that directly
monitor the geometry of the
template-substrate base pair. In
the case of DNA polymerases, this
‘geometric selection’ is estimated
to contribute three orders of
magnitude or more to selectivity,
while hydrogen bonding only
provides 7–40-fold selectivity.


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Sexual conflict

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