Prezygotic Barriers
Two species that occupy different habitats within the same area may encounter each other rarely, if at all, even though they are not isolated by obvious physical barriers, such as mountain ranges.
Habitat Isolation
The genes of different species may interact in ways that impair the hybrids development or survival in its environment.
Reduced Hybrid Viability
Characterizes a species by body shape and other structural features.
Morphological Species Concept
Accident during cell division that results in extra sets of chromosomes.
Polyploidy
Courtship rituals that attract mates and other behaviors unique to a species are effective reproductive barriers, even between closely related species. Such behavior rituals enable mate recognition- a way to identify potential mates of the same species.
Behavioral Isolation
Even if hybrids are vigorous, they may be sterile. If the chromosomes of the two parent species differ in number or structure, meiosis in the hybrids may fail to produce normal gametes. Since the infertile hybrids cannot produce offspring when they mate with either parent species, genes cannot flow freely between species.
Reduced Hybrid Fertility
Views a species in terms of its ecological niche, the sum of how members of the species interact with the nonliving and living parts of the environment.
Ecological Species Concept
Can occur when two species interbreed and produce hybrid offspring. Most such species are sterile because the set of chromosomes from one species cannot pair during meiosis with the set of chromosomes from the other species. However, an infertile hybrid may be able to propagate itself asexually. In subsequent generation, various mechanisms can change a sterile hybrid into a fertile polyploid.
Allopolyploid
Mating is attempted, but morphological differences prevent its successful completion.
Mechanical Isolation
Some first-generation hybrids are viable and fertile, but when they mate with one another or with either parent species, offspring of the next generation are feeble or sterile.
Hybrid Breakdown
Defines a species as the smallest group of individuals that share a common ancestor, forming one branch on the tree of life.
Phylogenetic Species Concept
Sympatric speciation can also occur when genetic factors enable a subpopulation to exploit a habitat or resource used by the parent population.
Habitat Differentiation
Species that breed during different times of the day, different seasons, or different years cannot mix their gametes.
Temporal Isolation
Gene flow is interrupted when a population is divided into geographically isolated subpopulations.
Allopatric Speciation
Selection where (typically) females select males based on appearance.
Sexual Selection
Sperm of one species may not be able to fertilize the eggs of another species. For Instance, sperm may not be able to survive in the reproductive tract of females of the other species, or biochemical mechanisms may prevent the sperm from penetrating the membrane surrounding the other species eggs.
Gametic Isolation
Speciation occurs in populations that live in the same geographic area.
Sympatric Speciation