Encyclopedia of Evolution.pdf - Online Reading Center

430 appendix
chapter 11. On the geological Succession of
Organic Beings
Species evolve very slowly; and even the ancient deposits
contain a few species that are very similar to those that
exist today. Once a species becomes extinct, however, it
never comes back. Species become rare before they become
extinct; to be surprised at the extinction of a species makes
no more sense than to be surprised at its rarity. The history
of life is recorded in the geological strata; each new set of
species represents not a new creation but “only an occasional
scene, taken almost at hazard, in an ever slowly changing
drama.”
The pattern of fossils in the sedimentary rocks is not random,
as one might expect if they were deposited by a great
flood. In fact, the species contained within each layer closely
resemble, and are intermediate between, those of the layer
immediately above and immediately below. These similarities,
furthermore, are found in strata of a particular age throughout
the world. It has long been known that the more ancient a
fossil deposit is, the more greatly its species differ from those
found on the Earth today. I wrote in 1839 and 1845 about
the close relationship, on each continent, between living and
extinct forms, even when the continents differ greatly among
themselves. For example, most marsupials today are found
in Australia, which is also where most of the fossil marsupials
are found. There is nothing special about the climate of
Australia that makes it particularly friendly to marsupials; in
fact, placental mammals have largely displaced them. I have
now explained why this pattern has occurred.
Groups of organisms that today appear entirely distinct
can often be linked by the fossils of species now extinct; that
is, we can find many examples of “missing links” that are no
longer missing. For example, fossilized animals have been
found intermediate between whales and terrestrial mammals.
The Archaeopteryx and Compsognathus link birds to their
dinosaurian reptile ancestors. We cannot, however, expect
the geological record to provide evidence of all the links.
“Thus, on the theory of descent with modification, the main
facts with respect to the mutual affinities of the extinct forms
of life to each other and to living forms, are explained in a
satisfactory manner. And they are wholly inexplicable in any
other way.”
Has progress occurred over evolutionary time? This
question cannot be answered, for we have no way of defining
which life-forms are higher than which other forms. The
only true experimental test would be to allow the ancient
species to compete with the modern ones, and see whether
the modern ones win; but this can never happen. We cannot
even guess the outcome. For example, who would ever have
predicted, just from studying their anatomy, that European
placental mammals would have so completely displaced the
native Australian marsupials? There has undoubtedly been
much progress over evolutionary time, but natural selection
favors only the amount of progress that is beneficial to a species;
after that point, no more progress is necessary. Therefore
the persistence of species that closely resemble very ancient
forms is not at all surprising.
chapters 12–13. geographical Distribution
The geographical distribution of organisms can be reasonably
explained only in terms of descent from a common ancestor,
followed by evolutionary modification (see biogeography).
Everyone has noticed how different are the animal and
plant species of the different continents, especially the Old
vs. New Worlds, even when they share very similar climates.
For example, the animals of South America are much more
closely related to one another than to the animals of either
Australia or Africa, despite having similar Southern Hemisphere
latitudes and range of climates. For example, South
America, Africa, and Australia all have large flightless birds,
but they are in three different groups: the rheas, the ostriches,
and the emus, respectively. There are entirely different
sets of species among the blind animals of caves in the different
continents. Just as on land, there is a similar biogeographical
pattern found in the oceans. This pattern cannot
be explained in terms of the independent creation of species
suited to each type of habitat [nor by dispersal of species
after a gigantic flood, a theory to which many modern creationists
cling but which had been nearly abandoned by the
creationists of Darwin’s time]. “On the principle of inheritance
with modification we can understand how it is that …
whole genera, and even families, are confined to the same
areas, as is so commonly and notoriously the case.” [Darwin
believed, like all scientists prior to the 20th century, that the
continents had not moved. continental drift makes an
evolutionary explanation of biogeography even more convincing.]
Dispersal, followed by descent with modification, also
explains many facts regarding island biogeography, such as
the following:
1. Nearly everywhere in the world, animal species on islands
most closely resemble the animal species on the continents
nearest to them. “Why should this be so? Why do the species
which are supposed to have been created in the Galápagos
Archipelago, and nowhere else, bear so plainly the
stamp of affinity to those created in America?” The climate
of the Galápagos, off the coast of South America, is
very similar to that of the Cape Verde Islands, off the coast
of Africa; yet their species are entirely different.
2. Islands usually have fewer species than corresponding
areas of continents. This has occurred because they have
not existed as long as the continents, and not as many species
have evolved there.
3. Islands almost always have a high proportion of endemic
species—species found nowhere else in the world. This has
occurred because the endemic species have evolved in isolation.
The islands that do not have very many endemic
species are those in which isolation has been incomplete:
in which organisms frequently disperse from the mainland
(as in the birds of Bermuda) and intermix with their native
relatives. The Galápagos Islands have their own species of
animals, for example mockingbirds; this is because each
species evolved on its own island, and then, if one of the
island species dispersed from one island to another, it lost