Evolution__3rd_Edition

64 PART 1 / Introduction
Age (Myr ago)
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
1.0 2.0 3.0 4.0 5.0 6.0
The fossil record is complete
enough in some cases to illustrate
continuous evolutionary
transformations
Height of hyaline area (µm)
Figure 3.11
Evolution of the diatom Rhizosolenia. The form of the diatom is
measured by the height of the hyaline (glass-like) area of the cell
wall. Closed circles indicate forms classified as R. praebergonii,
open circles indicate R. bergonii. Bars indicate the range of forms
at each time. Redrawn, by permission of the publisher, from
Cronin & Schneider (1990).
3.10 Fossil evidence exists for the transformation of species
Diatoms are single-celled, photosynthetic organisms that float in the plankton. Many
species grow beautiful glass-like cell walls, and these can be preserved as fossils.
Figure 3.11 illustrates the fossil record for the diatom Rhizosolenia between 3.3 and
1.7 million years ago. About 3 million years ago, a single ancestral species split into two;
and there is a comprehensive fossil record of the change at the time of the split.
The diatoms in Figure 3.11 show that the fossil record can be complete enough to
reveal the origin of a new species; but examples as good as this are rare. In other cases,
the fossil record is less complete and there are large gaps between successive samples
(Section 21.4, p. 602). There is then only less direct evidence of smooth transitions
between species. The gaps are usually long, however (maybe 25,000 years in a good
case, and millions of years in less complete records). There is enough time within one
of the gaps for large evolutionary changes, and no one need be surprised that fossil
samples from either side of a gap in the record show large changes.
In other respects, as we saw at the beginning of the chapter (Section 3.1), the fossil
record provides important evidence for evolution. Against alternatives other than separate
creation and transformism, the fossil record is valuable because it shows that the
living world has not always been like it is now. The existence alone of fossils shows that
there has been some kind of change, though it does not have to have been change in the
sense of descent with modification.
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