Page 132 - Introduction to Paleobiology and The Fossil Record

MACROEVOLUTION AND THE TREE OF LIFE 119
• He had seen evidence for relationships in
time and space – in South America he saw
the bones of giant extinct ground sloths
and armadillos, obviously close relatives
of living forms; and as he went from island
to island in the Galápagos and elsewhere,
he saw close similarities between species
of plants, reptiles and birds.
• He was aware of the record of fossils in
the rocks, and that fossils changed
through time, and seemed to progress
from simple forms in the oldest rocks
towards modern forms in the Pliocene and
Pleistocene.
• Thomas Malthus argued in his book An
Essay on the Principle of Population
(1798) that human populations tend to
grow far faster than their food supply, and
Darwin transferred this concept to the
natural world, seeing that reproductive
rates are higher than they need to be.
Thus, by September 1838, Darwin under-
stood the concept of evolution, a view that
had been expressed by many thinkers before,
and that claimed that life had not been static
forever, but that species changed and never
stopped changing. He had a rich understand-
ing of modern geographic variation. Why, he
asked, does every island in the Galápagos
archipelago have a different set of species of
small birds when the same set would do per-
fectly well throughout? Further, why did the
bird species on neighboring islands look more
similar to each other than those on distant
islands?
So, Darwin’s first key insight was that life
is more diverse than it ought to be if it had
been created and his second was that all
species living and extinct can be linked in a
single great evolutionary tree that shows their
relationships and that tracks back to a single
ancestor. These are descriptive observations
of pattern.
But Darwin is remembered most for his
third insight, and this was the principle of
natural selection, a process that explains the
diversity of life and its branching history of
relationships: only the organisms best adapted
to their environment tend to survive and
transmit their genetic characteristics in increas- and wherever opportunity offers, at the
ing numbers to succeeding generations while
those less adapted tend to be eliminated.
Darwin made the case with remorseless logic,
and this can be dissected into a series of clear
statements:
1 Nearly all species produce far more young
than can survive to adulthood (Malthus’
principle).
2 The young that survive tend to be those
best adapted to survive (larger at birth,
faster growing, noisier in the nest, faster
to escape predation, less disease, etc.).
3 Characters are inherited from parent to
offspring, so the characters that ensure
survival (size, aggressiveness, speed, free-
dom from disease, etc.) will tend to be
passed on.
4 These survival characteristics will increase
generation by generation. The changes are
not inexorable, so cheetahs run fast enough
to catch their prey, not at 2000 km per
hour, because they do not have to and their
bodies would fall to bits if they tried.
Each of these observations can be supported
by huge numbers of observations. For point
1, note that most plants and animals produce
hundreds, thousands or millions of offspring;
if every melon seed grew into a melon plant
or every cod egg became an adult, melons and
codfish would soon cover the surface of the
Earth to a depth of hundreds of meters. For
point 2, observe any litter of puppies or nest
of fl edgling birds and see how siblings compete
with each other for their parents’ attention.
For point 3, observe your parents or children
and see the evidence for inherited characters.
For point 4, consider how this emerges from
points 1–3. Evolution by natural selection is
on the one hand rather simple, but also rather
complex, and it is frequently misunderstood
or misrepresented (Box 5.2).
Darwin’s Origin (1859) said it all, and he
said it so well. In conclusion of this section,
Darwin described natural selection in action:
It may be said that natural selection is
daily and hourly scrutinising, throughout
the world, every variation, even the
slightest; rejecting that which is bad,
preserving and adding up all that is good;
silently and insensibly working, whenever
improvement of each organic being in
relation to its organic and inorganic
conditions of life.