The Origin and Evolution of Mammals - Moodle

276 THE ORIGIN AND EVOLUTION OF MAMMALS
82
81
78
77
Cetartiodactyla
76
Eulipotyphla
Chiroptera
Rodentia
55
56
57
58
50
51
Primates
49
Xenarthra
65
66
Figure 7.25 Divergence of major supraorders of placentals. Molecular based dates of Springer et al. (2003).
which is indeed the palaeontologist’s commonest
response. Benton (1999), in a typical example of
such an argument, suggested that the high rate of
speciation and morphological evolution during a
phase of rapid lineage splitting, may be associated
with an increased rate of selection at the molecular
level. This would lead to greater molecular differences
between groups, and therefore earlier estimated
dates of divergence if it was falsely assumed
that molecular evolution had been clock-like.
The most recent molecular studies use methods
that not do not depend on a single, simple molecular
clock model, for instance the Bayesian approach,
used by Springer et al. (2003). Their study also compared
the divergence dates generated by different
categories of the molecular data, namely only
nuclear genes, only mitochondrial genes, only
exons, or only third codon positions, and found relatively
little difference amongst them. It would be
68
67
69 Perissodactyla
62
74
63
60
Carnivora
59
Pholidota
80
75
73
71
72
Lagomorpha
Scandentia
Dermoptera
52
79
Afrosoricida
45
46
47
Tubulidentata
48
Sirenia
42 Hyracoidea
43 Proboscidea
110 100 90 80 70 60 50 40 30 20 10 0
Cretaceous
K-T boundary
61
53
Cenozoic
64
Macroscelidea
70
44
54
whale
dolphin
hippo
ruminant
pig
llama
rhino
tapir
horse
cat
caniform
pangolin
flying fox
rousette fruit bat*
false vampire bat*
phyllostomid bat*
free tailed bat*
hedgehog*
shrew*
mole*
mouse*
rat*
hystricid
caviomorph
scurid*
rabbit
pika
flying lemur*
tree shrew*
strepsirrhine
human
sloth
L
aurasiatheria
E
u
a
r
c
h
o
n
t
o
g
l
i
r
e
s
B
O
RE
O
EUT
H
E
R
I
A
anteater
armadillo
tenrec*
golden mole*
s.e.elephant shrew*
Xenarthra
i.e.elephant shrew*
aardvark
sirenian
hyrax
elephant
Afrotheria
very surprising for all these categories to have
exactly the same pattern of variability of evolutionary
rate under the influence of varying intensities
of molecular-level selection. The authors also calculated
that forcing the molecular differences
between the groups onto a tree constructed on the
fossil-based divergence dates implies extremely
variable rates of molecular evolution amongst the
orders and subordinal groups. The highest rates,
between certain diverging orders, would have to be
up to almost 700 times the lowest rates found
within certain subordinal groups. This level of rate
variation seems far-fetched, even for the most committed
of molecular selectionist views.
The Garden of Eden hypothesis. An explanation for the
discrepancy between the fossil and molecular dates,
which allows that both the fossil and the molecular
data are correct, was discussed by Foote et al. (1999).