Biology Reference
In-Depth Information
Some differences between animals may be like this. Sheep use horns for fighting and
deer use antlers. Horns are derived from skin while antlers are derived from bone. The
differences between horns and antlers need not necessarily reflect ecological differences; it
may simply be a case of evolution working with different raw materials to produce the
same functional end. The problem with non-adaptive explanations is that they are
hypotheses of the last resort. Further scientific enquiry is stifled. Maybe there is an adaptive
explanation for the difference but we just haven't discovered it yet. For example, antlers
are dropped and then renewed each year whereas horns are not. Perhaps this difference is
related to the extent of seasonal variation in mating competition and food supply?
Some differences
between species
may reflect
different solutions
to the same
problem
(e)
Statistical analysis and independent data
We need statistical analysis to tell us how confident we can be in our conclusions. To do
this, we need to think carefully about what constitutes independent data points. For
example, in Crook's analysis of the weaver birds (Table 2.2), 14 of the 16 grassland
species belong to one genus,
Euplectes
. Can we consider all these as the outcomes of
independent 'evolutionary experiments'? Congeneric species may often have similar
behaviour simply through common ancestry. In this case, analysis of species data will
be statistically biased by those genera containing large numbers of species.
What are the
independent
data?
Summary
These criticisms are important, but they certainly do not mean that the comparative
method is a failure. On the contrary, the approach is impressive in the way it brings
together such a wide diversity of behavioural and morphological traits within the same
ecological framework. Crook's study of the weaver birds and Jarman's work on the
antelopes have served as models for ecological work on other groups of species. However,
the most recent comparative studies have attempted to control for these various problems,
and we will now discuss other examples, bearing the criticisms in mind, to illustrate how
changes in methodology have made comparison between species a more rigorous exercise.
Comparative approach to primate
ecology and behaviour
As with the weaver birds and antelopes, primates vary in their social organization
(Fig. 2.4). There are solitary insectivores, like tarsiers, which live in forests and are
nocturnal. There are diurnal forest monkeys, like colobus monkeys, which go around in
small groups, feeding on leaves or fruit. Other monkeys, like baboons, are terrestrial and
live in large troupes of 50 or several hundred individuals. Among the apes, the
orang-utan is solitary, the gibbon lives in pairs and small family units, while the
chimpanzee may live in bands of 50.
In the 1970s and 1980s, Tim Clutton-Brock and Paul Harvey used the comparative
approach to analyse the evolution of this diversity. Their analyses introduced three
marked improvements in methodology over the earlier comparative studies. Firstly, they
measured the various aspects of behaviour and morphology on a continuous scale (rather
than categorizing primates into groups with different traits). Secondly, they considered
alternative hypotheses, and used multivariate statistics to tease out the effects of different
Three
improvements in
comparative
studies
