Reproductive Isolating Mechanisms

The key to species differences under the biological species concept is reproductive isolation—biological characteristics that prevent genes from being exchanged between different species. Any biological characteristic or mechanism that prevents genes from being exchanged with members of other species is termed a reproductive isolating mechanism.

Some species are separated by prezygotic reproductive isolating mechanisms, which prevent gametes from two different species from fusing and forming a hybrid zygote. This type of reproductive isolation can arise in a number of different ways (Table 18.3). Other species are separated by postzygotic reproductive isolating mechanisms, in which gametes of two species may fuse and form a zygote, but there is no gene flow between the two species, either because the resulting hybrids are inviable or sterile or because reproduction breaks down in subsequent generations (see Table 18.3).

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TABLE 18.3 Types of reproductive isolating mechanisms
Type Characteristic
Prezygotic Mechanisms that act before a zygote has formed
Ecological Differences in habitat; individuals do not meet
Temporal Reproduction takes place at different times
Mechanical Anatomical differences prevent copulation
Behavioral Differences in mating behavior prevent mating
Gametic Gametes are incompatible or not attracted to each other
Postzygotic Mechanisms that act after a zygote has formed
Hybrid inviability Hybrid zygote does not survive to reproduction
Hybrid sterility Hybrid is sterile
Hybrid breakdown F1 hybrids are viable and fertile, but F2 are inviable or sterile

CONCEPTS

The biological species concept defines a species as a group of potentially interbreeding organisms that are reproductively isolated from the members of other species. Under this concept, species are separated by prezygotic reproductive isolating mechanisms or postzygotic reproductive isolating mechanisms.

image CONCEPT CHECK 6

Which of the following is an example of postzygotic reproductive isolation?

  1. Sperm of species A dies in the oviduct of species B before fertilization can take place.

  2. Zygotes that are hybrids between species A and B are spontaneously aborted early in development.

  3. The mating seasons of species A and B do not overlap.

  4. Males of species A are not attracted to the pheromones produced by the females of species B.

b