Evolution__3rd_Edition

..
. . . leading to the development of
theories of directed variation
Weismann was a rare early
supporter of the theory of natural
selection
Mendel’s ideas were rediscovered
around 1900
CHAPTER 1 / The Rise of Evolutionary Biology 13
A second objection was that gaps exist between forms in nature a gaps that could not
be crossed if evolution was powered by natural selection alone. The anatomist St
George Jackson Mivart (1827–1900), for instance, in his book The Genesis of Species
(1871), listed a number of organs that would not (he thought) be advantageous in their
initial stages. In Darwin’s theory, organs evolve gradually, and each successive stage has
to be advantageous in order that it can be favored by natural selection. Mivart retorted
that although for a bird a fully formed wing, for example, is advantageous, the first
evolutionary stage a of a tiny protowing a might not be.
Biologists who accepted the criticism sought to get round the difficulty by imagining
processes other than selection that could work in the early stages of a new organ’s
evolution. Most of these processes belong to the class of theories of “directed mutation,”
or directed variation. These theories suggest that the offspring, for some unspecified
reason to do with the hereditary mechanism, consistently tend to differ from their
parents in a certain direction. In the case of wings, the explanation by directed variation
would say that the wingless ancestors of birds somehow tended to produce offspring
with protowings, even though there was no advantage to it. (Chapter 10 deals with this
general question, and Chapter 4 discusses variation.)
Lamarckian inheritance was the most popular theory of directed variation. Variation
is “directed” in this theory because the offspring tend to differ from their parents in the
direction of characteristics acquired by their parents. If the parental giraffes all have
short necks and acquire longer necks by stretching, their offspring have longer necks to
begin with, before any elongation by stretching. Darwin accepted that acquired characters
can be inherited. He even produced a theory of heredity (“my much abused
hypothesis of pangenesis,” as he called it) that incorporated the idea. In Darwin’s time,
the debate was about the relative importance of natural selection and the inheritance of
acquired characteristics; but by the 1880s the debate moved into a new stage. The
German biologist August Weismann (1833–1914) then produced strong evidence and
theoretical arguments that acquired characteristics are not inherited. After Weismann,
the question became whether Lamarckian inheritance had any influence in evolution at
all. Weismann initially suggested that practically all evolution was driven by natural
selection, but he later retreated from this position.
Around the turn of the century, Weismann was a highly influential figure, but few
biologists shared his belief in natural selection. Some, such as the British entomologist
Edward Bagnall Poulton, were studying natural selection. However, the majority view
was that natural selection needed to be supplemented by other processes. An influential
history of biology written by Erik Nordenskiöld in 1929 could even take it for granted
that Darwin’s theory was wrong. About natural selection, he concluded “that it does
not operate in the form imagined by Darwin must certainly be taken as proved;” the
only remaining question, for Nordenskiöld, was “does it exist at all?”
By this time, Mendel’s theory of heredity had been rediscovered. Mendelism
(Chapter 2) has been the generally accepted theory of heredity since the 1920s, and is
the basis of all modern genetics. Mendelism eventually allowed a revival of Darwin’s
theory, but its initial effect (around 1900–20) was the exact opposite. The early
Mendelians, such as Hugo de Vries and William Bateson, all opposed Darwin’s theory
of natural selection. They mainly did research on the inheritance of large differences
between organisms, and generalized their findings to evolution as a whole. They