Geology Reference
In-Depth Information
The continual extinction of species is referred to as
background extinction
, to clearly differentiate it from a
mass
extinction
, during which accelerated extinction rates sharply
reduce Earth's biotic diversity. Extinction is a continuous oc-
currence, but so is the evolution of new species that usually,
but not always, quickly exploit the opportunities created by
another species' extinction.
Everyone is familiar with the mass extinction of dino-
saurs and other animals at the end of the Mesozosic Era (see
Chapter 22). The greatest extinction, though, during which
perhaps more than 90% of all species died out, was at the
end of the Paleozoic Era (see Chapter 21). In some of the fol-
lowing chapters, we discuss those extinctions and their pos-
sible causes, as well as other extinctions of lesser magnitude.
Almost everything in the sciences has some kind of theo-
retical underpinning—optics, the nature of matter, the pres-
ent distribution of continents, diversity in the organic world,
and so on. Of course, no theory is ever proven in some fi nal
sense, although it might be supported by substantial evidence;
all are always open to question, to revision, and occasionally
to replacement by a more comprehensive theory. In his book
Why Darwin Matters
(pp. 1-2), Michael Shermer noted that
A 'theory' is not just someone's opinion or a wild guess
made by a scientist. A theory is a well-supported and
well-tested generalization that explains a set of obser-
vations. Science without theory is useless.
Prediction
is commonly taken to mean to foresee an
event that has not yet occurred, as in predicting the next
solar eclipse. However, not all predictions are about future
events. For example, one prediction of seafloor spreading
theory is that oceanic crust should be younger at spreading
ridges and become progressively older with increasing dis-
tance from ridges, which, in fact, it does. Likewise, the theory
of evolution allows us to make many predictions about what
we should see in the present-day world and in the fossil re-
cord if the theory is correct (Table 18.1).
If the theory of evolution is correct, closely related
species such as wolves and coyotes should be similar not
only in their overall anatomy, but also in terms of their bio-
chemistry, genetics, and embryonic development (point 4
in Table 18.1). Suppose they differed in their biochemical
mechanisms as well as embryology. Obviously, our predic-
tion would fail, and we would at least have to modify the the-
ory. If other predictions also failed—for example, mammals
appeared in the fossil record before fi shes—we would have
to abandon the theory and fi nd a better explanation for our
observations. Accordingly, the theory of evolution is truly sci-
entifi c because it can, at least in principle, “be falsifi ed”—that
is, proven wrong.
EVOLUTIONARY THEORY?
When Charles Darwin proposed his theory of evolution, he
cited supporting evidence such as classifi cation, embryology,
comparative anatomy, geographic distribution, and, to a lim-
ited extent, the fossil record. He had little knowledge of the
mechanism of inheritance, and both biochemistry and molec-
ular biology were unknown during his time. Studies in these
areas, coupled with a more complete and much better under-
stood fossil record, have convinced scientists that the theory
is as well supported by evidence as any other major theory.
Of course, scientists disagree on many details, but the central
claims of the theory are well established and widely accepted.
But is the theory of evolution truly scientifi c? That is, can
testable predictive statements be made from it? First, we must
be clear on what a theory is and what we mean by “predictive.”
Scientists propose hypotheses to explain natural phenomena,
test them, and, in some cases, raise them to the status of a
theory—an explanation of some natural phenomenon well sup-
ported by evidence from experiments, observations, or both.
TABLE 18.1
Some Predictions from the Theory of Evolution
1. If evolution has taken place, the oldest fossil-bearing rocks should have remains of organisms very different from those existing now,
and more recent rocks should have fossils more similar to today's organisms.
2. If evolution by natural selection actually occurred, Earth must be very old, perhaps many millions of years old.
3. If today's organisms descended with modifi cation from ones in the past, there should be fossils showing characteristics that connect
orders, classes, and so on.
4. If evolution is true, closely related species should be similar not only in details of their anatomy but also in their biochemistry,
genetics, and embryonic development, whereas distantly related species should show fewer similarities.
5. If the theory of evolution is correct—that is, living organisms descended from a common ancestor—classifi cation of organisms
should show a nested pattern of similarities.
6. If evolution actually took place, we would expect cave-dwelling plants and animals to most closely resemble those immediately
outside their respective caves rather than being most similar to those in caves elsewhere.
7. If evolution actually took place, we would expect land-dwelling organisms on oceanic islands to most closely resemble those of
nearby continents rather than those on other distant islands.
8. If evolution has taken place, a mechanism should exist that accounts for the evolution of one species to another.
9. If evolution occurred, we would expect mammals to appear in the fossil record long after the appearance of the fi rst fi sh. Likewise,
we would expect reptiles to appear before the fi rst mammals or birds.
10. If we examine the fossil record of presumably related organisms such as horses and rhinoceroses, we should fi nd that they were
quite similar when they diverged from a common ancestor but became increasingly different as their divergence continued.
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