Biology Now, 2e

Fast Lizards, Slow Corals ■ 237

around or swim extremely long distances. There

are few physical barriers yet many separate,

unique species. How do new species form when

populations are free to mix and mingle?

Caribbean Corals

Carlos Prada did not spend much of his time at

the University of Puerto Rico in a classroom.

Instead, he spent it in the ocean, studying corals,

a type of marine invertebrate. On regular dives

onto the Caribbean reef, the graduate student

documented the morphology of a species of coral

named Eunicea flexuosa, commonly called “sea

fans.” At 12 different locations along the Puerto

Rican coast, Prada observed different morphol-

ogies of the coral at different depths. In shallow

areas, less than 15 feet from the surface, the sea

fans were wide, with a broad network of thick

branches like a bush. In deeper areas, between

15 and 55 feet from the surface, the sea fans were

much taller and spindly, resembling trees with

networks of thin branches.

“They’re supposed to be the same species,

but they really looked different at different

depths,” says Prada. Curious about whether

the morphologies were due to genetics or a

result of the surrounding environment shap-

ing the animals as they grew, Prada began

Time

A single plant species is

distributed over a broad

geographic range.

The sea level rises and isolates plant

populations from one another. The

populations may adapt to different

environments on opposite sides of the

barrier, indirectly causing genetic changes

that reduce their ability to interbreed.

When the barrier is removed,

the plants recolonize the

intervening area and mingle,

but do not interbreed.

Range of overlap

Time

Figure 13.7

Physical barriers can produce allopatric

speciation by blocking gene flow

Allopatric speciation can occur when populations

are separated by a geographic barrier, such as a

rising sea.

Q1: What factors must be present for

allopatric speciation to occur?

Q2: If a geographic barrier is removed

and the two reunited populations

intermingle and breed, what attributes

must the offspring have in order for the

two populations, according to the biological

species concept, to be considered still the

same species?

Q3: If the two populations in question 2 are

determined to still be the same species, did

allopatric speciation occur?