the way genes function (Coen, 1999; Mayr, 2001; Stearns & Hoekstra, 2005).
(3) Third, if variation is not inherited, there is no evolution. Rocks erode and
new rocks are formed in geological history, yet the latter do not inherit any
traits from the former. Haircuts are not inherited genetically—however, they
can be inherited culturally. (4) Fourth, our definition is neutral with regard to
the origin of variation. In the neo-Darwinian theory,“random variation”is an
essential criterion of evolution. However, the inheritance of acquired traits
does play a role in so-called epigenetic evolution (see below in this section and
section 2.3). In cultural evolution, innovation (that is also passed on to future
generations) is often not random but purposeful. These questions have
recently led tofierce debates about the definition and scope of evolution.
(5) Fifth, we did not mention any struggle for life, competition for resources,
survival of thefittest, or even selection, although all of these were important for
Darwin as he developed his theory. Inheritance of variation usually leads to the
frequency of some variation of a trait increasing and others decreasing. The
reasons for such a change of proportions can be manifold (Stearns & Hoekstra,
2005, pp. 54–69): randomfluctuation can accumulate and completely erase
some variant while favoring another. Populations can be split or joined by other
populations. In biological evolution, family size can differ, slight differences
leading to important changes especially in a small population. Natural selection
is only one (although probably the most interesting) among the causes of change
in evolution. Yet even the concept of natural selection does not involve any
struggle. If a variant of a trait enhances the chance of it being inherited in greater
numbers (in a given environment), it will be selected for. (6) Sixth, improvement
and development are not parts of the definition of evolution. Evolution is free of
value: whatever traits become widespread in evolution must be good enough to
be selected for, but do not have to be regarded as superior to other traits or
optimal in any sense. This is not to say that evolution does not produce
astonishingly complex and well-adapted organisms. (7) Finally,fitnessin evo-
lutionary theory means reproductivefitness, that is, the ability to leave offspring
(that will also leave offspring). Note thatfitness does not have to do with being
in“good shape”as such.
The concept ofnatural selectionis arguably the most important element of
evolutionary theory, due to its great explanatory power. Natural selection is
also the main interest of this book whenever evolutionary theory is mentioned.
We have summarized the essence of this concept above, but a more detailed
definition will be useful. Natural selection means that there is a consistent
relationship between a heritable state of a trait, on the one hand, and whether
or not the individual carrying it has more or fewer offspring throughout their
lifetime that survive and reproduce, on the other hand (Stearns & Hoekstra,
2005, p. 28). In other words, natural selection presupposes a correlation
between the state of a trait (or a gene) and lifetime reproductive success. For
example, if the length of the beak in a bird species is both heritable and shows
Evolution 25