Evolution__3rd_Edition

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Figure 15.4
Convergence in marsupial
and placental carnivores.
(a) The reconstructed bodies
and skulls of Thylacosmilus,
a saber-toothed marsupial
carnivore that lived in South
America in the Pliocene and
of Smilodon, a saber-toothed
placental carnivore from the
Pleistocene in North America.
(b) Prothylacynus patagonicus,
a borhyaenid marsupial from
the early Miocene in Argentina;
Thylacinus cynocephalus, the
extinct marsupial Tasmanian
wolf; and Canis lupus, the
modern placental wolf. From
Strickberger (1990). © 1990
Jones & Bartlett Publishers.
Convergent evolution produces
homoplasies
CHAPTER 15 / The Reconstruction of Phylogeny 429
cause homoplasy; the most important cause is convergent evolution, when the same
selection pressure has operated in two lineages. A classic example of convergence is
seen in the two major groups of mammals, the marsupials and placentals (Figure 15.4).
The marsupial and placental saber-toothed carnivores both evolved long, gashing
canine teeth and there are also striking similarities in skull shape and body form in the
marsupial and placental wolves. If we inferred the phylogeny of the marsupial wolf,
placental wolf, and kangaroo from the pattern of phenetic similarity, we would obtain
the wrong answer. The two wolves are phenetically more similar, even though the
marsupial wolf is phylogenetically closer to the kangaroo than to the placental wolf.