Encyclopedia of Evolution.pdf - Online Reading Center

422 appendix
with those (admittedly few) instances in which Darwin was in
error.
The following summary is not meant as a replacement
for the original, any more than a tract replaces the Bible. The
author hopes that this summary will inspire many to read
Origin of Species in its entirety.
Charles Darwin may be the most fascinating person you
never met. When you read Origin of Species, you will also get
to know Charles Darwin as a person—passionate for truth,
humble in response to his critics, continually in awe of the
natural world.
The author has used the following format to summarize
Origin of Species:
• The author uses the same chapters and chapter titles as
Origin of Species.
• The author has condensed Darwin’s text but maintained
much of the style and terminology. This summary largely
restricts itself to the concepts and knowledge that Darwin
had. Occasionally it has been necessary to insert updated
material, in brackets. The author has retained Darwin’s use
of the first person in this summary.
• The exact quotations from the sixth edition of Origin of
Species are contained within quotation marks or, for longer
passages, in indented blocks.
• The author has inserted cross-references to entries in this
encyclopedia.
chapter 1. Variation under Domestication
Perhaps the best way to see what evolution can do is to study
domesticated plant and animal species (see artificial selection).
First, artificial selection demonstrates that wild populations,
from which crops and livestock were domesticated,
contain a tremendous amount of heritable variation. Breeders
can still improve old varieties of, and produce new varieties
from, even the oldest cultivated plant species and domesticated
animal species. Breeders sometimes try to maintain the
purity of the stock by breeding only the best animals in their
herds. Even while they are trying to prevent change from
occurring, they end up causing it to occur: These superior
stocks become even better over the decades as a result. There
is no end in sight for heritable variation in these species.
Second, plant and animal breeding shows what selection
is capable of doing. Consider the pigeon. There are many
breeds that are astonishingly different from one another in
anatomy and behavior. Yet they all came from the wild rock
dove. Scientists know this because if one crosses these breeds
with one another, the offspring after two generations once
again look like rock doves. The characteristics of the wild
rock doves still persist in their highly modified descendants.
These breeds are so different from one another that if one
encountered them in the wild one would consider them different
species or even different genera. They are known to be
varieties of one species only because breeders produced them
within historical times. Nature provides the raw material of
variation; human selection adds them up in a direction that
humans find useful. Some of this selection is intentional; but
some of it is unintentional. When breeders select for one trait,
other traits will be selected along with them.
This is artificial selection in the sense that humans select
the characteristics in the plants and animals that are most
favorable to human purposes, even to the detriment of the
plants and animals: Many domesticated plants and animals
can no longer survive in a natural state. Artificial selection
demonstrates that wild populations contain a tremendous
amount of variation; and it is an example of what selection
can do.
chapter 2. Variation under nature
Natural populations contain a great deal of heritable variation
(see population genetics). It is difficult even for
experts to decide which groups of organisms constitute species,
and which constitute mere varieties; species delineations
are often arbitrary. Because of this, there is no reason to
believe that varieties evolved but species did not.
Large genera are experiencing rapid evolution, and varieties
are on their way to evolving into separate species. This
explains (1) why the species within large genera are more
similar to one another than are species in small genera, and
(2) why the species within large genera have more varieties
than do species within the smaller genera.
chapter 3. Struggle for existence
Natural populations experience competition. There are
hardly ever enough resources to allow all of the offspring of
any species to survive and reproduce. “We behold the face
of nature bright with gladness,” for example, the singing
birds, not realizing that these birds survive only because of
the destruction of the seeds and insects that they eat, and that
at frequent intervals these birds themselves suffer starvation
and death.
Malthus explained how human populations grow exponentially
(see Malthus, Thomas; population). This is
also true of every plant and animal species. Some species are
increasing in number, but not all can do so, “for the world
would not be able to hold them.” Populations will, therefore,
always outrun resources, unless and until some disaster
reduces their numbers. Even a single pair of elephants,
with perhaps the slowest reproductive rate known, could
produce millions of offspring in a few hundred years, if all
survived. “Lighten any check to population growth, mitigate
the destruction of offspring ever so little,” and the population
size of any species will quickly increase.
This demonstrates that it is the struggle for existence
rather than the rate of reproduction that determines the size
of a natural population. The population size of a species
does not depend upon how many offspring its individuals
can produce; one species of fly may produce a thousand eggs
at a time, another just one, but both are equally abundant.
Parasitic flies can make some livestock species incapable of
surviving in certain regions. Rare plants are abundant in the
few places in which they live. Even in a desert or tundra,
organisms compete for the scarce resources that are available.
When a plant produces a thousand seeds, these seeds
go out into a world already thickly clothed with plants, and