Evolution__3rd_Edition

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Globin evolution in “living fossil”
sharks illustrates the molecular
clock
CHAPTER 7 / Natural Selection and Random Drift 167
Table 7.3
Amino acid differences between the a- and b-hemoglobins for three species pairs.
Adapted, by permission of the publisher, from Kimura (1983).
Species pairs Number of amino acid differences
Human a vs human b 147
Carp a vs human b 149
Shark a vs shark b 150
morphological evolution. The Port Jackson shark Heterodontus portusjacksoni is a
living fossil a a species that closely resembles its fossil ancestors (some over 300 million
years old). Its molecules have been evolving very differently from its morphology.
Hemoglobin duplicated into α and β forms before the ancestor of mammals and
sharks, at the beginning of the chordate radiation. We can count the amino acid differences
between α- and β-hemoglobin as a measure of the rate of molecular evolution
in the lineages leading to the modern species. Table 7.3 reveals that changes have accumulated
in the Port Jackson shark lineage at the same rate as the human lineage. The
rates of molecular evolution in the two lineages are roughly equal.
The constancy of molecular evolution in the shark and human lineages for the past
300 million years is in marked contrast with their rates of morphological evolution.
The lineage leading to the modern Port Jackson shark has hardly had any change at all.
But the lineage leading to humans has passed from an initial fish-like stage, through
amphibian, reptilian, and several mammalian stages, before evolving into modern
humans. Moreover, as Table 7.3 shows, human β-globin is as different from human αglobin
as it is from carp α-globin. This is despite the fact that human α- and β-globin
will have shared much more similar external selective pressures, as they have been
locked in the same kind of organisms throughout evolution, than have human βglobin
and carp α-globin.
The result suggests that the α- and β-globin molecules have been accumulating
changes independently, at roughly constant rates, regardless of the external selective
circumstances of the molecule. This in turn suggests that most of the evolutionary
changes in the globin molecule have been neutral shifts among equivalent forms, of
equal adaptive utility. While the rates of morphological change vary greatly among the
various evolutionary lineages of vertebrates, the rates of molecular evolution all seem
to have been more similar.
7.4 The molecular clock shows a generation time effect
The molecular clock seems to support the neutral theory of molecular evolution.
However, when we examine the evidence in more detail, the support becomes less