Evolution__3rd_Edition

428 PART 4 / Evolution and Diversity
Figure 15.3
(a) A homology is a character
state shared between two
species that was present in
their common ancestor.
(b) A homoplasy is a character
state shared between two
species that was not present
in their common ancestor. A
and A′ are two character states.
(c) The wings of birds and bats
are an example of a homoplasy.
They are structurally different
as the bird wing is supported by
digit number 2, and the bat
wing by digits 2–5. The bird
wing is also covered with
feathers, the bat’s with skin.
We distinguish homologies from
homoplasies
(a) Homology (b) Homoplasy
Character state
Species
Common ancestor
(c) Homoplasy example: bird and bat wings
A A A A
1 2 1 2
A A'
Bird Bat
shared between two or more species that was not present in their common ancestor
(Figure 15.3). Thus we start with a character that is similar in two species. We then trace
all the way back to their most recent common ancestor. If the common ancestor had
that same character, then the character in the two descendant species is similar by
common evolutionary descent and is a homology. If the common ancestor had some
different character state, then the character in the two descendant species evolved
independently and is a homoplasy. The distinction matters because homologies may
reveal phylogenetic relationships, whereas homoplasies do not.
A homologous character such as the heart, or lungs, of a human and a chimpanzee is
easily recognized as the same character, presumably shared from a common ancestor
that also possessed that character. In other cases, the similarity is less obvious. The fivedigit
limb of tetrapods is homologous even though its form varies (Figure 3.6, p. 58),
and in extreme cases homologies are so subtle that it takes clever detective work to
reveal them. The ear bones of mammals, for example, do not superficially resemble the
skull and jaw bones of reptiles. But a classic piece of comparative anatomic research in
the nineteenth century traced a series of intermediates that can be found between three
skull and jaw bones of reptiles and three ear bones of mammals. The bones also have a
common embryonic origin. A homologous character does not have to be the same in all
the species possessing it a there only has to be some shared morphological information
among them.
Homoplasies can arise for a number of reasons. In DNA evidence, as we discuss later,
homoplasy can easily arise by chance. In morphological evidence, chance is unlikely to
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