Biology Reference
In-Depth Information
had read and grasped the significance of the work of Gregor Mendel. To be fair, no
other biologist grasped the significance of Mendel's experiments on heredity in pea
plants either, until his work was rediscovered and extended by other biologists at the
start of the twentieth century.
Mendel's discovery that heredity in pea plants is particulate in nature was
extended and confirmed in the period 1900-1940 by many more experiments with
animals and plants, especially the fruit fly Drosophila. But it was only in the lat-
ter decade of this period that Mendelian genetics was found to be consistent with
the idea of natural selection. Three biologists in particular showed by mathemati-
cally based studies that the behaviour of genes in populations could be accounted
for quantitatively by natural selection - these were Ronald Fisher, John Haldane
and Sewall Wright. The fact that these three scientists worked independently of one
another strengthened their case - you will recall my earlier point that it is an essential
feature of how science works that new claims should be confirmed by indepen-
dent researchers. The classical work of these three people demonstrated that natural
selection could work with the kinds of variation observed in nature by applying
the laws of Mendelian genetics. Darwin's theory of evolution by natural selection
has rested upon firm genetical foundations since that time. This achievement of the
human intellect is often called “The Modern Synthesis”, after the title of a topic pub-
lished in 1942 by the biologist Julian Huxley, a direct descendant of Thomas Henry
Huxley, one of Darwin's strongest supporters after his return from the voyage in
HMS Beagle.
Common Misconceptions About Natural Selection
It is important to grasp two particular aspects of natural selection because these are
often misunderstood.
Firstly, natural selection acts directly on individuals, but it is only populations
that evolve, not individuals. It is often thought incorrectly that evolution applies to
individual organisms, perhaps because changes in individuals produced during their
development are confused with evolutionary changes. This is another area of science
where precise definitions are vital! To understand how populations evolve requires
some grasp of statistics, and some people find it difficult to think in statistical terms.
Secondly, natural selection responds to pressures produced by the immediate
environment. So natural selection is not random but neither is it a directed process,
because it has no foresight, no overall progress in a particular direction. It cannot
predict the future - how could it? Any inherited variation that promotes an indi-
vidual leaving more offspring than its competitors in the present environment will
be selected for. If the environment changes, some of these variations may then be
selected against, while other variations may be favoured. For example, in 1983 the
rainfall in the Galapagos Islands was 57 times greater than in 1977, the year of the
drought. This excessive rain resulted in a plentiful supply of small, soft seeds which
the finches harvest more efficiently than large, hard seeds. So after wet years, the
smaller finches with shallow beaks reproduce more rapidly than larger birds with
deeper beaks - exactly the opposite of what happens in drought years. Evolution by
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