3 The New York Years (1931–1953)

350 11 History and Philosophy of Biology—Mayr’s Third Synthesis
Fig.11.1. Darwin’s explanatory model of evolution through natural selection (from Mayr
1997b: 190)
◮
Mayr had conceptualized the difference between Darwin’s population thinking
and typological essentialism during the late 1940s (p. 361) and had explained it
in a short paper (1959b) on “Darwin and the evolutionary theory in biology.”
For many years this was the most widely quoted reference by other authors to
these so drastically different ways of viewing the world. Population thinking takes
into consideration the uniqueness of each individual in a population of organisms
and unlimited variation which, in conjunction with natural selection, may
lead to further evolutionary change. On the other hand, typologists assume that
the unchanging “essence” of each species determines the extent of its variation.
For typologists, the presumably fixed limits of variation preclude evolution from
occurring (except through saltations).
Darwin’s inference from the first three facts in Fig. 11.1 was the “struggle for
existence” and resulted from his reading Malthus (l.c.) on September 28, 1838.
Mayr felt strongly that the origin of Darwin’s “population thinking” was from
his reading the literature of animal and plant breeders during the six months
before the Malthus event. Darwin had learned that every individual in the herd
was different from every other one and extreme care had to be used in selecting
the sires and dams from which to breed the next generation. All that Darwin
needed to know was that individual variation occurred (step 1 of natural selection).
A correct theory of the origin of variation (non-existing at the time) was not
a prerequisite for the establishment of the thesis of natural selection. Selection
of individuals with particular heritable qualities (step 2), continued over many
generations, automatically leads to evolution (inference 3). This is natural selection
(selective demands) as a cause or mechanism. Natural selection as an outcome is
an a posteriori phenomenon of probabilistic nature. It is the survival of those
individuals that have attributes giving them superiority in the particular context.
Natural selection is the nonrandom retention (survival) of some of the new genetic
variants. Darwin said: “The preservation of favorable variations and the rejection
of injurious variations, I call Natural Selection” (1859: 81), natural selection as
an outcome. Although natural selection is an opportunistic optimization process,
it does not necessarily lead to perfection. There are numerous constraints. Also,
selection as a process obviously was unable to prevent the extinction of the majority
of species which originated during the history of life. 3
Immediate support for Darwin’s concept of natural selection came only from
several naturalists, especially Wallace, Bates, Fritz Müller, Weismann, Alfred Newton,
and initially Tristram (who interpreted the substrate adaptations of North
African larks as due to natural selection in 1859 before the publication of the Origin,
but regretted this interpretation after the publication of the Origin of Species).
3 See Bock (1993) for a discussion of natural selection as (1) a cause or mechanism,
(2) a process, or (3) an outcome.