Biology Reference
In-Depth Information
4.0
3.5
3.0
2.5
2.0
1.5
1.0
5.5
6.0
6.5
7.0
7.5
In (body mass)
Fig. 2.10
Relationship between testis mass and body mass in bushcricket species
(Tettigoniidae) with low (filled circles) and high (open circles) degrees of polyandry.
Phylogenetic information was incorporated into the statistical analysis by weighting
current species values by the distance separating them in the phylogeny. The lines, fitted
from a phylogenetic model, are for low (solid line) and high (dashed line) degrees of
polyandry. From Vahed
et al
. (2010).
clade had larger testes, relative to body weight, confirming the original study's
conclusion (Harcourt
et al
., 1995). A comparative analysis has revealed that increased
sperm competition also leads to larger testes in bushcrickets (Fig. 2.10).
Discrete variables and the order of change during evolution
Characters such as brain size and testis size are continuous variables. Others we
might want to consider in a comparative analysis are discrete variables, for example
diet (insect versus seeds) or mating system (single versus multimale). Statistical
methods have been developed to determine likely ancestor states and which order of
character change is most likely in a tree (Pagel, 1994; Pagel & Meade, 2006). For
example, in Crook's weaverbirds, did an evolutionary change to a seed diet lead to an
increase in flocking, or was it a change to flocking that favoured the evolution of a
seed diet? If we could determine this, then we could answer the cause-effect question
we raised earlier.
We now turn to another primate example to show how the analysis of discrete traits
in a phylogeny can help to identify both independent evolutionary transitions and the
order of change.
… and the order
of changes
