Page 141 - Introduction to Paleobiology and The Fossil Record

128 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

20
15
33 1 13 6 7 4
8
33 29 1 2 3 13 5 7 4
0 30 9 31 6 18 19 20
29 8
1 15 27
10 11 28 21 32 22 23
2
Time (Ma) 3 34 12 16 17
30
25
4
24
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6
Impalas Gnus and hartebeests

Figure 5.7 Reconstructed phylogeny of African antelopes. Two lineages diverged 6–7 Ma, the slowly
evolving impalas and the rapidly speciating gnus and hartebeests. The second group could be said to be

evolutionarily more successful than the first, and this might be interpreted as a result of species
selection of species-level characters – the rate of speciation. However, the gnus and hartebeests have
more specialized ecological preferences than do the species of impalas: perhaps selection has occurred
at the individual level (natural selection), and this has had an effect at the species level. Species numbers
14 and 26 are omitted in this study. (Based on Vrba 1984.)

where their ecological tolerances determine scale questions like “Which species of ape is
their evolutionary rates, and produce a super- closest to humans – the gorilla or chimp?”,
ficial appearance of species selection. large teams of researchers are hastening to put

Is evolution hierarchical? And, if so, was together complete trees of all species of
Darwin wrong? The case has been overstated mammals, angiosperms, amphibians, spiders
by critics: evolution occurs by natural selec- and many other groups. As each complete
tion, as Darwin said in 1859. Many proposed tree, that is, a tree containing all species, is
examples of species selection can be explained published, systematists are getting closer to
by natural selection, coupled with rapid asym- Darwin’s ideal of understanding the shape of
metric geographic speciation and the effect the whole tree of life.
hypothesis. Nonetheless, species selection is a It is important to distinguish trees from
possibility, and convincing examples may be ladders. Many people think that all plants and
found in the future. animals are arranged in a series from simple
to complex, or “lower” to higher”. The
pattern of evolution is then like a ladder, a
THE TREE OF LIFE single long line of progression from one species
to the next, an idea that was popular 200
Tree thinking
years ago and termed the Scala naturae (see
As noted at the start of this chapter, Darwin p. 13). But all the evidence shows that evolu-
was the first to picture evolution as a great tion is a process of splitting and so the tree is

branching tree, and to point out that all the correct analogy, not the ladder.
species had evolved from a single ancestor at Fossils offer fundamental information on
the base of the tree. The idea of the tree of the history of life and on large-scale patterns
life has come to the fore recently, with a of evolution. There has been a revolution in
massive effort by biologists and paleontolo- the ways in which paleontologists interpret
gists to discover the whole tree. From small- evolutionary aspects of the fossil record, and