Evolution__3rd_Edition

430 PART 4 / Evolution and Diversity
Homologies are recognized by ...
. . . structural similarity ...
. . . relations between parts ...
. . . embryonic development ...
. . . and other criteria
The phenetic similarity between the wolves is homoplasious and is not due to a close
phylogenetic relationship.
15.4 Homologies can be distinguished from homoplasies by
several criteria
Homoplasies do not indicate phylogenetic relationships, and the first task is to recognize
homoplasies, as opposed to homologies. How can we recognize them? The crude
answer is that homologies are identifiably the same character in two species, but homoplasies
differ in some way that suggests the character has evolved independently in the
species that possess it. Research therefore starts with a character that shows some similarity
in two (or more) species, and then examines the character in detail to find out
whether it really is the same in all the species.
First, if a character is homologous it is likely to have the same fundamental structure.
The wings of birds and bats, for example, are superficially similar; but they are constructed
from different materials and supported by different limb digits (Figure 15.3c).
The differences suggest that the bird and bat wings are homoplasies, and evolved independently
from a common ancestor that lacked wings.
Second, homologies usually have the same relations to surrounding characters.
Homologous bones, for example, are usually connected in a similar way with their surrounding
bones.
Third, the character is likely to have the same embryonic development in different
groups. A character that looks similar in the adult forms, but develops by a different
series of stages, is unlikely to be homologous. One example, which we meet again in
Chapter 16, is the relation between a barnacle, a mollusk such as a limpet, and a crab
(Figure 16.1, p. 474). At least superficially, the adult form of a barnacle is more like a
limpet than a crab. The relations of barnacles had been uncertain for centuries until
John Vaughan Thompson discovered their larvae in 1830. The barnacle larva is very
like the larva of several groups of Crustacea, and unlike those of mollusks. Barnacles
therefore share a more recent common ancestor with crabs than with limpets. The
similarities between the adult barnacle and limpet, such as their hard external armour,
attachment to rocks, and feeding through a hole in the shell, are all homoplasious.
Finally, some other criteria can sometimes be useful. Convergence is caused by natural
selection, when organisms in different evolutionary lineages face similar functional
requirements (such as flying in birds and bats). We have grounds for suspecting that a
shared morphological structure may be homoplasious when the species that share it
clearly need it for their way of life.
The criteria in this section are not the only ones that can be used to distinguish
homologies from homoplasies. However, the criteria discussed here do illustrate that
we have techniques to analyze characters shared between species to distinguish homologies
from homoplasies. A homology can be recognized as a character that has fundamentally
the same structure, relations with surrounding parts, and development, in a
set of species. Once the homologies are (often tentatively) identified, they can be retained
in the list of evidence used to infer the phylogeny. The homoplasies are discarded.
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