Encyclopedia of Evolution.pdf - Online Reading Center

Traits can reappear after even hundreds of generations,
available for natural selection, and providing evidence for
evolutionary ancestry. Ancestral characteristics are frequently
seen in pigeons, and among species of the horse genus. To say
that God put those characteristics into each species, to make it
look as if they had a common ancestor when in fact they did
not, “makes the works of God a mockery and a deception.”
chapter 6. Difficulties of the Theory
By this point, many difficulties of the theory must have
occurred to the reader. They used to bother me, too, until I
carefully thought them through, and I am now prepared to
answer them.
Transitional forms. If all species have arisen by gradual
changes from common ancestors, why do we not see innumerable
intergradations among organisms today? In fact, we
see distinct species instead—and this is also what we see in
the fossil record. But consider these facts. First, the crust of
the earth is a vast fossil museum—but a very imperfect one
(see below). Second, when a new variety begins to form, it
will be in direct competition with the parental stock, or with
other varieties forming alongside it from the same parent
stock. Competition is strongest between and among the most
closely related forms. This will lead to the diminution, even
extinction, of most of the closely related forms and result in
what I have previously called divergence of character. The
rarity of intermediate forms does not disprove natural selection;
it is the result of natural selection. This is in fact what
we see; when varieties within species form even today, the
intermediate forms are less common than those that differ the
most from one another.
Complex adaptations. Many people have pointed out
[and continue to do so; see intelligent design] that in complex
adaptations, the incipient forms would have experienced
no benefit; of what use is a partway-evolved adaptation? Of
what use is a part of a wing, or a part of an eye? But, as a
matter of fact, throughout the animal kingdom, we see some
very good examples of intermediate forms that benefit from
their seemingly imperfect adaptations. Some single-celled
animals have simple eyespots that allow them only to distinguish
light from darkness, and to orient themselves toward
the light; they cannot, in the strict sense of the word, see,
but they benefit immensely from their ability to detect light.
Flying fish do not, in the strict sense of the word, fly, but
they can launch themselves above the waves long enough to
escape from predators. Flying squirrels cannot really fly, but
their ability to glide can and often does save their lives, when
pursued by a predator or when they fall out of a tree. While
we cannot demonstrate, from living animals or from fossils,
a complete series of transitional forms within a given lineage,
we can find examples of intermediate forms of the general
class of adaptations—which shows us of what natural selection
is capable.
Occasionally, an animal’s habits can evolve without
a corresponding change in anatomy. But the habit is just as
much a part of its adaptation as is its anatomy (see behavior,
evolution of; gene-culture coevolution). While it
appendix 425
may seem incredible to us that something as complex as the
vertebrate eye, or the flight of birds, could evolve by means
of natural selection, we have the evidence to demonstrate
that such a thing is possible. [One cannot use what one evolutionary
biologist (see Dawkins, Richard) has called the
“argument from personal incredulity,” which implies that
if one cannot imagine it to have happened, it cannot have
happened.] If there were any example of an adaptation that
could not have been formed by small steps in natural selection,
“my theory would utterly break down; but I can find no
such case.”
There is another pattern that makes sense only from the
viewpoint of evolution. Complex eyes are found not just in
vertebrates but in invertebrates as well—the eyes, for example,
of cephalopods such as the squid, and of arthropods
such as insects. But in each case, the eye has a fundamentally
different structure. In fact, there are at least three different
organs, all of which we call by the same name, “eye.”
Insects have compound eyes. Vertebrates have eyes in which
the nerves come out in front of the light sensors and converge
in the optic nerve. Cephalopods have eyes in which the nerves
come out in back of the light sensors. It appears that these
“eyes” have evolved along different pathways, and converged
upon similar but not identical structures: structures that still
retain vestiges of their different evolutionary pasts. Other
examples of convergence include organs for the production of
electricity in various aquatic animals, and the wings of birds
vs. bats vs. insects.
In all cases of beings, far removed from each other in the
scale of organization, which are furnished with similar and
peculiar organs, it will be found that although the general
appearance and function of the organs may be the same,
yet fundamental differences between them can always be
detected … natural selection would have had different
materials or variations to work on, in order to arrive at
the same functional result; and the structures thus acquired
would almost necessarily have differed. On the hypothesis
of separate acts of creation the whole case remains unintelligible.
A new adaptation does not have to arise from scratch; it
can arise from the modified use of a previously existing organ.
Lungs, for example, develop in the same location as the swim
bladders that fish use for flotation and have many structural
similarities to them; lungs could therefore have evolved from
swim bladders. [It is now considered more likely that swim
bladders evolved from lungs; see fishes, evolution of.] In
some cases, the corresponding structures, which had been
present in the adult or juvenile forms, might have been lost if
the adult or juvenile phase of the life cycle had been lost (as
has happened in some animals), making it look as if the structure
had appeared out of nowhere (see neoteny).
I have been astonished how rarely an organ can be named,
toward which no transitional grade is known to lead …
Nature is prodigal in variety, but [stingy] in innovation.