Geology Reference
In-Depth Information
of heredity known as
genes
cannot be altered by any effort
by an organism during its lifetime. Despite many attempts
to demonstrate inheritance of acquired characteristics, none
have been successful (see Geo-Focus on page 474).
plants and animals so produced made Darwin wonder whether
a process selecting among variant types in nature could also
bring about change. He came to fully appreciate the power of
selection when he read in Malthus's essay that far more ani-
mals are born than reach maturity, yet the adult populations
remain rather constant. Malthus reasoned that competition for
resources resulted in a high infant mortality rate, thus limiting
population size.
In 1858, Darwin received a letter from Alfred Russel
Wallace (1823-1913), a naturalist working in southern Asia,
who had also read Malthus's essay and came to exactly the
same conclusion that a natural process was selecting only a
few individuals for survival. Darwin and Wallace presented
their idea, called
natural selection
, simultaneously in 1859 to
the Linnaean Society in London.
and Alfred Russel Wallace
In 1859, Charles Robert Darwin (1809-1882) (
Figure 18.2)
published
On the Origin of Species
, in which he detailed his
ideas on evolution and proposed a mechanism whereby evo-
lution could take place. Although 1859 marks the beginning
of modern evolutionary thought, Darwin actually formu-
lated his ideas more than 20 years earlier but, being aware of
the furor they would cause, was reluctant to publish them.
Darwin had concluded during his 1831-1836 voyage
aboard the
Beagle
that species are not immutable and fi xed
(see the Introduction), but he had no idea what might bring
about change in organisms through time. However, his ob-
servations of selection practiced by plant and animal breed-
ers and a chance reading of Thomas Malthus's essay on
population gave him the ideas necessary to formulate his
theory.
Plant and animal breeders practice
artifi cial selection
by
selecting those traits they deem desirable and then breeding
plants and animals with those traits, thereby bringing about a
great amount of change (
◗
The salient points of
natural selection
, a mechanism that
accounts for evolution, are summarized as follows:
1. Organisms in all populations possess heritable
variations—size, speed, agility, visual acuity, digestive
enzymes, color, and so forth.
2. Some variations are more favorable than others; that is,
some variant types have a competitive edge in acquiring
resources and/or avoiding predators.
3. Those with favorable variations are
more likely
to
survive to reproductive maturity and pass on their
favorable variations.
◗
Figure 18.3). The fantastic variety of
In colloquial usage, natural selection is sometimes expressed
as “survival of the fittest,” which is misleading because it
reduces to “the fittest are those that survive and are thus
the fi ttest.” But it actually involves differential rates of sur-
vival and reproduction. Therefore, it is largely a matter of
reproductive success. Having favorable variations does not
guarantee survival for an individual, but in a population of
perhaps thousands, those with favorable variations are more
likely to survive and reproduce.
Natural selection works on the existing variation in a
population, thus simply giving a competitive edge to some
individuals. So evolution by natural selection and evolution
by inheritance of acquired characteristics are both testable,
but evidence supports the former, whereas attempts to verify
the latter have failed (see Geo-Focus on page 474).
Darwin was not unaware of potential problems for his
theory of natural selection. In fact, in
On the Origin of Species
,
he said:
If it could be demonstrated that any complex organ
existed which could not possibly have been formed
by numerous, successive, slight modifi cations, my
theory would absolutely break down. (p. 171)
Image not available due to copyright restrictions
Critics of natural selection have cited vertebrate eyes and
birds' wings that they claim could not have evolved, because,
according to them, anything less than eyes and wings as they
exist now would be useless. All eyes, no matter if they are
Search WWH ::
Custom Search