Evolution__3rd_Edition

48 PART 1 / Introduction
Tooth quality (time in days to develop caries)
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Generations
Living creatures are classified into
species, and higher taxa
Resistants
Susceptibles
Figure 3.4
Selection for better and worse teeth in rats. Hunt et al. (1955)
selectively bred each successive generation of rats from parental
rats that developed caries later (resistants) or earlier (susceptibles)
in life. The age (in days) at which their descendants developed
caries was measured.
Artificial selection can produce dramatic change, if continued for long enough. A
kind of artificial selection, for example, has generated almost all our agricultural crops
and domestic pets. No doubt the artificial selection in these cases a begun thousands of
years ago in some cases a employed less formal techniques than would a modern
breeder. However, the longer timespan has led to some striking results. Darwin (1859)
was impressed by the varieties of domestic pigeons, and chapter 1 of On the Origin of
Species begins with a discussion of those birds. The point here of these, and similar,
examples is to illustrate further how, on a small scale, species can be shown experimentally
not to be fixed in form.
3.4 Interbreeding and phenotypic similarity provide two
concepts of species
We are now close to the stage in the argument when we can consider evidence for the
evolution of new species. Most of the evidence so far has been for small-scale change
within a species. The amounts of artificially selected change in pigeons and other
domestic animals borders on the species level, but to decide whether the species barrier
has been crossed we need a concept of what a biological species is.
All living creatures are classified into a Linnaean hierarchy. The species is the lowest
important level in the hierarchy. Species, in turn, are grouped into genera, genera into
families, and so on up through a series of levels. Figure 3.5 gives a fairly complete
Linnaean classification of the wolf, as an example. If all life has descended from a single
common ancestor, evolution must be capable of producing new groups at all levels in
the hierarchy, from species to kingdom. We shall be looking at the evidence in the rest
of this chapter. Here, however, we are at the species stage. What does it mean to say a
new species has evolved?
The question unfortunately lacks a simple answer that would satisfy all biologists.
We shall discuss the topic fully in Chapter 13, and we shall see that there are several
concepts of species. What we can do here is to take two of the most important species
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