Once a barrier to gene flow is established, by whatever means, the separated populations may diverge genetically through the action of the evolutionary agents we described in Chapter 23. Over many generations, differences may accumulate that reduce the probability that members of the two populations could mate and produce viable offspring. In this way, reproductive isolation can evolve as an incidental byproduct of genetic changes in allopatric populations.
Geographic isolation does not necessarily lead to reproductive incompatibility. For example, American sycamores and European sycamores (also known as plane trees) have been physically isolated from one another for at least 20 million years. Nevertheless, they are morphologically very similar (Figure 24.9), and they can form fertile hybrids, even though they never have an opportunity to do so in nature.
In other cases, however, genes that result in reproductive isolation between two evolving lineages spread quickly through populations as they diverge. In this section, we will examine the ways in which reproductive isolating mechanisms may arise. In the following section, we will explore what happens when reproductive isolation is incomplete.
24.9 Geographically Separated, Morphologically Similar Although they have been separated by the Atlantic Ocean for at least 20 million years, American and European sycamores have diverged very little in appearance.
(a) Platanus occidentalis (American sycamore) (b) Platanus hispanica (European sycamore)
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