special case of hybrid sterility termed hybrid breakdown ,
-----Preventing gene exchange
Sometimes the differences that produce prezygotic isolation involve mechanical isolation . That is , mating is physically impossible between members of different species because genitals of males and females are structurally incompatible or because molecules on the surfaces of sperm and egg fail to bind . A final type of prezygotic mechanism is temporal isolation , in which time-related environmental cues that trigger reproductive processes are different for related species.
the second and subsequent generations after a cross show reduced reproductive success .
-----Preventing gene exchange
In postzygotic isolating mechanisms mating occurs, but the resulting hybrid organism is inviable or sterile. In a
2. Preventing gene exchange
Two general types of mechanisms operate to block the exchange of gene . The first general type is made up of prezygotic isolating mechanisms - mechanisms that prevent the formation of zygotes . Prezygotic isolation falls into two categories: ecological and behavioral . In the first case, two related group may become adapted to slightly different environments _ perhaps varying soil types or food sources. Over time , these genetic differences become so great that successful cross-fertilization can no longer take place . In behavioral isolation , related groups evolve differing behaviors-such as species mating rituals-that restrict the exchange of genes to members of the same group.
cyclic
Lesson 9
The origin of species
1.The biologists define a species
Modern biologists generally define a species as group of actually or potentially interbreeding populations that is reproductively isolated from other such groups . Members of species can interbreed with each other , but they cannot breed with organisms belonging to another species . One advantage of the standard of reproductive isolation is that it is very precise . Notice ,however , that it can only be applied to organisms that reproduce sexually . Asexual reproducers, including most prokaryotes, many plants, and some animals, must be classified into species on the basis of physical (biochemical or morphological) traits.
In postzygotic isolating mechanisms mating occurs, but the resulting hybrid organism is inviable or sterile. In a
special case of hybrid sterility termed hybrid breakdown ,