A population of birds, or any organism, can speciate if isolated from
its neighbors. In his 1942 book, Systematics and the Origin of Species, Mayr argued that the most
significant way to cut off a population is by geographical isolation (see illustration at right). For example, a glacier may thrust
down a valley, creating two separate populations, one on either side of the glacier. A rising ocean may
turn a peninsula into a chain of islands, stranding the beetles on each of them. This sort of isolation
doesnt have to last forever; it needs only form a barrier long enough to let the isolated population
become genetically incompatible with the rest of its species. Once the glacier melts, or the ocean drops
and turns the islands back into a peninsula, the animals will be unable to interbreed. They will live side
by side, but follow separate evolutionary fates.
Other Modes of Speciation
Today, scientists studying the origin of species can compare not
just the bodies of species, but their genes
as well. Geographic isolation remains a crucial element in forming new species, but a number of biologists
now argue that the formation of species can take several different paths. It may be possible, for example,
for a population to continue breeding with other members of its speciesand trading geneswhile
still diverging into a distinct group. All that may be required is that a few of its genes diverge, thanks
to strong natural selection. If
the conditions are right, this genetically distinct population may then become a new species.
Others argue that organisms can diverge into genetically distinct populations even if they are living
side by side. For example, females may be born with different preferences for mates, and those preferences
may get strengthened over time into reproductive isolation. Even as biologys understanding of species
formation evolves, Mayrs work remains hugely important to the understanding of how the millions of
species on Earth came to be.