It's a bit like a character in a movie going off and having adventures that change him so drastically that
when he returns, the folks in his hometown no longer recognize the way he looks and behaves. The
biological equivalent is "allopatric speciation," an evolutionary process in which one species divides into
two because the original homogenous population has become separated and both groups diverge from
each other.
In their separate niches, the two groups go their own evolutionary ways, accumulating different gene
mutations , being subjected to different selective pressures, experiencing different historical events,
finally becoming incapable of interbreeding should they ever come together again. For many years this
has been regarded as the main process by which new species arise.
Often this type of speciation occurs in three steps. First, the populations become physically separated,
often by a long, slow geological process like an uplift of land, the movement of a glacier, or formation of
a body of water. Next, the separated populations diverge, through changes in mating tactics or use of
their habitat. Third, they become reproductively separated such that they cannot interbreed and
exchange genes.
Under normal conditions, genes in a given population are exchanged through breeding, so that even if
some variation occurs, it is limited by this "gene flow." But gene flow is interrupted if the population
becomes divided into two groups. One way this happens is by "vicariance," geographical change that can
be slow or rapid.
An example of vicariance is the separation of marine creatures on either side of Central America when
the Isthmus of Panama closed about 3 million years ago, creating a land bridge between North and
South America. Nancy Knowlton of the Smithsonian Tropical Research Institute in Panama has been
studying this geological event and its effects on populations of snapping shrimp. She and her colleagues
found that shrimp on one side of the isthmus appeared almost identical to those on the other side -having once been members of the same population.
But when she put males and females from different sides of the isthmus together, they snapped
aggressively instead of courting. They had become separate species, just as the theory would predict.
In your groups, discuss the following:
 Why does the geological history of the Isthmus of Panama make it a good place to look
for evidence of allopatric speciation in marine organisms?
 What kind of life history strategy (mating patterns, number of offspring, degree of
mobility of larvae and adults, etc) do you think might encourage allopatric speciation of
populations along the isthmus?
 What life history strategies might encourage, not only allopatric speciation, but the kind
of speciation that Schneider and his colleagues hypothesize among Ecuadorian
hummingbirds?