Evolution__3rd_Edition

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In a stepped cline, the gradient is
less smooth
Mayr distinguished population from
typological thinking
Typological thinking is often
appropriate outside biology
CHAPTER 13 / Species Concepts and Intraspecific Variation 363
Clines may be smooth or “stepped” (Figure 13.6c), depending on how suddenly gene
frequencies change in space. If the environment varies smoothly, the cline will also be
smooth. If the environment changes more suddenly, the cline may be more stepped.
The shape of the step depends on the fitness difference between the genotypes in
the two regions, the fitness of any intermediate genotypes (such as heterozygotes or
recombinants), and the amount of gene flow. A sudden change in the environment is
called an “ecotone” (Section 13.7.2 below contains an example from the grass Agrostis).
However, ecotones are not the only explanation for stepped clines. Stepped clines may
also result when the ranges of two formerly separate populations expand and the two
populations meet up (Section 17.4, p. 500). Or they may result from genetic drift.
When biologists see a stepped cline, they are interested to know whether it corresponds
with an ecotone or has some other explanation. The main point here, however, is that
geographic variation often takes the form of a cline. Clinal variation contrasts with
a case such as that of the mice of Madeira, where the local populations do not show a
gradient of variation.
13.5 “Population thinking” and “typological thinking” are
two ways of thinking about biological diversity
Species show variation, both between individuals at any one place (often this has the
form of a “bell curve” or normal variation) and geographic variation between individuals
from different places. This variation has been thought about in two main ways:
“population thinking” and “typological thinking” (Mayr 1976). We have already met
the typological species concept (Section 13.2.3). A “type” specimen has to exist in order
for a species to be defined. However, variation will exist in the species with some individuals
more like the type specimen, and others less like it. By typological thinking,
Mayr meant the idea that the type individual, and other individuals like it, are in some
sense “better” examples of their species a they are more real, or more representative,
members of their species. We can see what this means if we think about the classification
of many non-biological entities.
Suppose we are classifying objects as chairs or non-chairs. Some objects will be better
specimens of chairs than others. If an object has four equal length legs and a horizontal
surface to sit on, it is a “good” chair. By calling something a good chair, or a better specimen
of a chair, we mean that it is easily recognized as a chair, not that it is morally superior
to other objects that are less easily recognized as chairs. Some other object may look
rather like a chair, but have two legs missing and a third broken, making it is less representative
of the category of chairs. Other objects may be so smashed up that we might
hesitate to call them chairs at all. The variation between objects consists of some objects
that are good chairs, and others that are less good chairs. The “less good” chairs mainly
exist because of some kind of accident or environmental error, such as an accident in
which a leg is broken off. We think to some extent typologically about chairs: some entities
are typical chairs, others are less typical because there is something wrong with them.
Creationism could give an account of biological species that is rather like the typological
account of chair classification. Each species might have a “best” form, perhaps