Evolution__3rd_Edition

..
Evolutionary thinkers existed before
Darwin, but either lacked, ...
. . . or proposed, unsatisfactory
mechanisms to drive evolution
1.3.1 Evolution before Darwin
CHAPTER 1 / The Rise of Evolutionary Biology 7
The history of evolutionary biology really begins in 1859, with the publication of
Charles Darwin’s On the Origin of Species. However, many of Darwin’s ideas have an
older pedigree. The most immediately controversial claim in Darwin’s theory is that
species are not permanently fixed in form, but that one species evolves into another.
(“Fixed” here means unchanging.) Human ancestry, for instance, passes through a
continuous series of forms leading back to a unicellular stage. Species fixity was the
orthodox belief in Darwin’s time, though that does not mean that no one then or before
had questioned it. Naturalists and philosophers a century or two before Darwin had
often speculated about the transformation of species. The French scientist Maupertuis
discussed evolution, as did encyclopédistes such as Diderot. Charles Darwin’s grandfather,
Erasmus Darwin, is another example. However, none of these thinkers put forward
anything we would now recognize as a satisfactory theory to explain why species
change. They were mainly interested in the factual possibility that one species might
change into another.
The question was brought to an issue by the French naturalist Jean-Baptiste Lamarck
(1744–1829). The crucial work was his Philosophie Zoologique (1809), in which he
argued that species change over time into new species. The way in which he thought
species changed was importantly different from Darwin’s and our modern idea of
evolution. Historians prefer the contemporary word “transformism” to describe
Lamarck’s idea. 1
Figure 1.2 illustrates Lamarck’s conception of evolution, and how it differs from
Darwin’s and our modern concept. Lamarck supposed that lineages of species persisted
indefinitely, changing from one form into another; lineages in his system did not
branch and did not go extinct. Lamarck had a two-part explanation of why species
change. The principal mechanism was an “internal force” a some sort of unknown
mechanism within an organism causing it to produce offspring slightly different from
itself, such that when the changes had accumulated over many generations the lineage
would be visibly transformed, perhaps enough to be a new species.
Lamarck’s second (and possibly to him less important) mechanism is the one he is
now remembered for: the inheritance of acquired characters. Biologists use the word
“character” as a short-hand for “characteristic.” A character is any distinguishable
property of an organism; it does not here refer to character in the sense of personality.
As an organism develops, it acquires many individual characters, in this biological
sense, due to its particular history of accidents, diseases, and muscular exercises.
Lamarck suggested that a species could be transformed if these individually acquired
modifications were inherited by the individual’s offspring. In his famous discussion of
the giraffe’s neck, he argued that ancestral giraffes had stretched to reach leaves higher
1 The historic change in the meaning of the term “evolution” is a fascinating story in itself. Initially, it meant
something more like what we mean by development (as in growing up from an egg to an adult) than by evolution:
an unfolding of predictable forms in a preprogramed order. The course of evolution, in the modern
sense, is not preprogramed; it is unpredictable in much the same way that human history is unpredictable. The
change of meaning occurred around the time of Darwin; he did not use the word in The Origin of Species
(1859), except in the form “evolved,” which he used once as the last word in the book. However, he did use it in
The Expression of the Emotions (1872). It took a long time for the new meaning to become widespread.