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LTHOUGH EVOLUTIONARY SUCCESS IS A difficult if not im-possible concept to define, we believe we know it when we see it. This is the
 

Summary: 1
1
A
LTHOUGH EVOLUTIONARY SUCCESS IS A difficult if not im-
possible concept to define, we believe we know it when we see it. This is the
case with the extant placentals, the clade of mammals to which we belong.
Living placentals include only 1,050 genera and some 4,400 species (Wilson and
Reeder, 1993)--compare this to just one of the other tetrapod clades, the birds, which
has more than 9,000 species. Nevertheless, placentals range tremendously in their
ecological diversity, from tiny shrews to the gargantuan blue whale, from pinnipeds
swimming the frigid high-latitude oceans to the golden moles swimming the hot
sands of southern Africa.
The evolutionary success of mammals is one of the few in evolutionary history
for which we can offer an explanation. First, there is what Gould (1989) popularized
as historical contingencies. For placental mammals, these were mode of reproduction,
level of metabolism, and an ancestral, generalized quadrupedal stance (Archibald,
2001). Euviviparity, which includes lengthy in utero development of the embryo,
requires that all support and sustenance come from the mother through the chorio-
allantoic placenta. This mode of reproduction is unique to placentals. It allows the
mother to continue normal activities while pregnant. Placentals, like other mam-

  

Source: Archibald, J. David - Department of Biology, San Diego State University

 

Collections: Biology and Medicine; Geosciences