Biology Reference
In-Depth Information
in water independently of each other. The two groups are morpho-
logically similar; however, this is not because of common ancestry,
but because they were both subject to the same physical constraints
of an aquatic environment. Cetaceans and sirenians thus show con-
vergence homoplasy with respect to their roughly similar hydrody-
namic morphology. This being said, when compared to fish, a
vertebrate group with which both sirenians and cetaceans share a
very ancient ancestor, these two mammalian lineages each show
reversion homoplasy in relation to their hydrodynamic morphology.
A phylogeneticist who inappropriately compares true wings in birds
and in chiropterans and furthermore bases his analysis on homoiothermy
(warm-bloodedness) might well create an evolutionary tree where birds
are descendants of some sort of ancestral bat: his analysis would indicate
that some ancient reptile evolved into mammals, that certain mammals
specialized for active fight, and that some of these developed feathers and
other apomorphies to become birds. This analysis has been trapped by
the pitfall of homoplasy. In reality, an analysis based on a wider selection
of characters, e.g. egg laying,
i
establishes that homoiothermy and real
wings are two characters which have independently evolved in the same
direction in mammals and birds (convergence homoplasy).
Not all cases of homoplasy are as obvious as the examples given
above. It is especially difficult to detect homoplasy in molecular data. The
relatively high mutation rate and the small alphabet of nucleotides both
make the likelihood of random homoplasies, through convergence or
reversion, far greater than with morphological characters. To further
complicate matters, proteins are highly modular: there are domains com-
mon to a wide variety of proteins. It is not simple to resolve the branches
of an evolutionary tree of slowly evolving proteins when the structural
constraints on a domain are such that only a few amino acids are truly
free to mutate. This example leads us again to a notion that needs to be
i
Egg laying outside water is common to most reptiles, all birds, and only two genera
of mammals (the echidna and the platypus).
