Evolution__3rd_Edition

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Figure 3.12
(a) Anatomic analysis of
modern forms indicates that
amphibians and reptiles are
evolutionarily intermediate
between fish and mammals.
This order fits with (b) the
geological succession of the
major vertebrate groups. The
width of each group indicates
the diverity of the group at that
time. Redrawn, by permission
of the publisher, from Simpson
(1949).
Groups of animals can be arranged
in a series according to their
similarity
(b)
(a)
Correct order
Incorrect order
Recent
Tertiary
Cretaceous
Jurassic
Triassic
Cartilage
Permian
fishes
Pennsylvanian
Mississippian
Placo-
Devonian derms
Jawless
Silurian
fishes
Ordovician
Bony
fishes
CHAPTER 3 / The Evidence for Evolution 65
Fish Amphibians Reptiles Mammals
Fish Mammals Amphibians Reptiles
Amphibians
Reptiles
Birds Mammals
3.11 The order of the main groups in the fossil record
suggests they have evolutionary relationships
The main subgroups of vertebrates, on a conventional classification, are: fish, amphibians,
reptiles, birds, and mammals. It is possible to deduce that their order of evolution
must have been fish then amphibia then reptiles then mammals; and not, for example,
fish then mammals then amphibia then reptiles (Figure 3.12a). The deduction follows
from the observation that an amphibian, such as a frog, or a reptile, such as an alligator,
is intermediate in form between a fish and a mammal. Amphibians, for instance, have
gills as fish do, but have four legs, like reptiles and mammals, and not fins. If fish had
evolved into mammals, and then mammals had evolved into amphibians, the gills
would have been lost in the evolution of mammals and then regained in the evolution
of amphibia. This is much less likely than that amphibia evolved from fish, retaining
their gills, and the gills were then lost in the origin of mammals. (Chapter 15 discusses
these arguments more fully.) Gills and legs are just two examples: the full list of characters
putting amphibians (and reptiles, by analogous arguments) between fish and
mammals would be long indeed. The forms of modern vertebrates alone, therefore,
enable us to deduce the order in which they evolved. 2
2 Strictly speaking, on the argument given here, it could also be that mammals came first and evolved into
reptiles, the reptiles evolved into amphibia, and the amphibia into fish. However, we can extend the argument
by including more groups of animals, back to a single-celled stage; the fish would then be revealed in turn as an
intermediate stage between amphibians and simpler animals.