Evolution__3rd_Edition

382 PART 4 / Evolution and Diversity
Reproductive isolation is the main
topic in research on speciation
Speciating populations can have
various kinds of geographic
relations
14.1 How can one species split into two reproductively
isolated groups of organisms?
The crucial event for the origin of a new species is reproductive isolation. As we saw in
Chapter 13, the members of a species usually differ genetically, ecologically, and in their
behavior and morphology (that is, phenetically) from other species, as well as in who
they will interbreed with. Some biologists prefer to define species not by reproductive
isolation but by other properties, such as genetic or ecological differences. Probably no
single property can provide a universal species definition, applicable to all animals,
plants, and microorganisms. However, many species do differ by being reproductively
isolated, and even if the evolution of reproductive isolation is not always the crucial
event in speciation, it is certainly the key event in research on speciation. The topic of
this chapter is the evolution of reproductive isolation. The aim is to understand how a
barrier to interbreeding can evolve between two populations, such that one species
evolves into two.
Reproductive isolation can be caused by many features of organisms (see Table 13.1,
p. 356). However, for most of the research in this chapter, we only need a distinction
between prezygotic and postzygotic isolation. Prezygotic isolation exists when, for
instance, two species have different courtship or mate choices, or different breeding
seasons. Postzygotic isolation exists when two species do interbreed, but their hybrid
offspring have low viability or fertility. Some of the theories of speciation apply only to
prezygotic isolation, some only to postzygotic isolation, and some to both.
14.2 A newly evolving species could theoretically have an
allopatric, parapatric, or sympatric geographic relation
with its ancestor
We can start with a distinction between different geographic conditions in the speciating
populations. If a new species evolves in geographic isolation from its ancestor, the
process is called allopatric speciation. If the new species evolves in a geographically contiguous
population, it is called parapatric speciation. If the new species evolves within
the geographic range of its ancestor, it is called sympatric speciation (Figure 14.1). The
distinctions between these three kinds of speciation can blur, but we shall begin the
chapter with the most important of the three processes: allopatric speciation. Almost
all biologists accept that allopatric speciation occurs. The importance of parapatric and
sympatric speciation are more in doubt, and we shall come on to them later.
In allopatric speciation, new species evolve when one (or more) population of a
species becomes separated from the other populations of the species, in the manner of
Figure 14.1a. This kind of event often happens in nature. For example, a species could
split into two separate populations if a physical barrier divided its geographic range.
The barrier could be something like a new mountain range, or river, cutting through
the formerly continuous population. Or the intermediate populations of a species
may be driven extinct, perhaps by a local disease outbreak, leaving the geographically
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