Biology Reference
In-Depth Information
The comparative method involves comparing different species to see whether
differences in behaviour or morphology are correlated with differences in ecology. Early
comparative studies of gulls, weaverbirds, antelopes and primates identified food
availability and predation as major selective pressures influencing social organization
(breeding behaviour, group size, home range size, sexual dimorphism). Recent
comparative studies have used phylogenetic trees to identify independent evolutionary
transitions and so control for the similarity of species through common ancestry.
Various statistical methods are used, including independent contrasts (illustrated by the
correlated evolution of song repertoires and brain anatomy in birds) and weighting
current species values by the distance separating them in the phylogeny (illustrated by
testis size of bushcrickets in relation to polyandry).
Phylogenetic trees can also help illuminate the order in which traits change during
evolution. For example, in primates, the transition from a single-male to a multimale
mating system preceded the evolution of sexual swellings. Sexual swellings are likely to
be advantageous to females in multimale groups because they provide a graded signal
of fertility which enables the female to bias paternity to the dominant male while, at the
same time, enhancing opportunities of mating with subordinate males too, to reduce
the chance that they will harm her offspring.
Comparative studies are especially useful for studying broad trends in evolution and
for testing hypotheses which are not amenable to experiments. The experimental
approach involves a detailed analysis of the costs and benefits of a behaviour
pattern to an individual of a particular species. Behaviour can be viewed as having
costs and benefits and animals should be designed by natural selection to maximize
net benefit. Ultimately, the net benefit must be measured in terms of gene contribution
to future generations. This will depend on shorter-term goals, such as foraging
efficiency, mating success and efficiency of avoiding predation. Optimality models
can be used to predict which particular trade-offs between costs and benefits give
maximum net benefit.
Further reading
The comparative method has been reviewed by Harvey & Purvis (1991). Ridley (1989)
provides a lucid summary of why species should not be used as independent data points,
even for evolutionarily labile traits. The statistical methods for modern comparative
analyses are beyond the scope of this topic; for recent introductions into the literature
see Freckleton and Harvey (2006), Pagel and Meade (2006), Felsenstein (2008) and
Hadfield and Nakagawa (2010). Freckleton (2009) reviews the seven deadly sins of
comparative analysis.
Further examples of comparative studies can be found in Fitzpatrick et al . (2009)
(female promiscuity promotes the evolution of faster sperm in cichlid fish) and
Kazancioglu and Alonzo (2010) (evolution of sex change in fish). Höglund's (1989)
study of size and plumage dimorphism in birds with different mating systems provides
an example of how taking account of phylogeny can change conclusions.
Huchard et al . (2009) suggest that once sexual swellings in primates have evolved as
fertility signals, they might then be further selected to signal female quality, and they
present supporting evidence from a study of wild chacma baboons in Namibia.
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