Reproductive Constraints that Separate Species:
A reproductive constraint is any factor that will not allow the production of a fertile offspring. These factors can be classified as prezygotic (constraint that makes mating difficult) and postzygotic (constraint that prevents a zygote from developing).
1. Habitat Isolation. Two species may live in the same area but in different habitats. Living in these different habitats ( in water, living on land, or living in tree tops) effectively segregate these organisms from each other. Since there is little if any contact the possibility of successfully mating is drastically reduced.
2. Temporal Isolation. Two species that breed at different times of the day, season, or year cannot mix their gametes. Since the breeding times are different there is no chance of reproductive contact.
3. Behavioral Isolation. Species-specific signals and elaborate behavioral patterns are used by closely related species to insure contact with the proper mate. Birds, mammals, and insects have pre-mating rituals that attract the proper mate. These signals can be chemical or physical in nature. Other organisms pay little or no attention to these behaviors or scents.
4. Mechanical Isolation. Anatomical incompatibility may prevent sperm transfer between two closely related species. The absence of certain appendages or their modification may inhibit a male from grasping and successfully fertilizing the female. Difference in floral structure may prevent pollen from reaching the stigma of the intended flower.
5. Gametic Isolation. If for some reason foreign sperm is introduced into a female there are several preventative measures to insure that there is no union between the sperm and egg. Internal environmental conditions may cause the sperm to die. Gamete recognition sites on the sperm do not fit with the intended egg. If the two species differ in the type of fertilization (external and internal) there is no chance of the sperm ever contacting the egg.
Postzygotic Constraints: If prezygotic constraints are crossed and a hybrid zygote forms, one of several constraints will prevent development of a viable, fertile hybrid.
1. Reduced Hybrid Viability. Genetic incompatibilities between the species may abort the development of the hybrid during some stage of development. Difference in chromosome number may cause abnormal cell division. Since the chromosomes align to insure equal distribution upon cytokinesis, abnormal chromosome counts could occur based on this numerical difference.
2. Reduced Hybrid Fertility. If two species mate and produce a viable offspring, these offspring will be sterile due to the misalignment of the chromosome number. During gametogenesis the odd number of chromosomes makes it impossible for viable gametes to be produced by meiosis.
3. Hybrid Breakdown. In some cases a fertile hybrid is produced. When these hybrids mate with each other, their offspring of the next generation are feeble or sterile.
Introgression is the transplantation of genes between species. This occurs when alleles slip through all zygotic barriers. This occurs when two species hybridize and a small number of the offspring manage to mate with the general population. This occurs in plants more so than in animals.
Geographic isolation plays an important role in species development and maintenance. There are two categories of this type of isolation: allopatric speciation and sympatric speciation. These relationships deal with the contact of the new species with that of its ancestral species.
1. Allopatric Speciation. This type of speciation is produced when a physical barrier separated a species into two separate areas and does not allow any further contact. Mountain building, glacial movement, river boundary movement, etc. are examples of geographical situations that can divide up a single species into two distance areas. When the species is split, microevolution will cause changes in the species to make them different in phenotype. Small populations have a much better chance to develop into a new species than larger populations. These small populations usually occur on the edges of a larger population. These fringe populations are good candidates for speciation. Their gene pool differs from the parent population, genetic drift will continue to cause chance changes in the gene pool of small populations until a larger population is formed. Different selection pressures are at work on there peripheral populations. Adaptive radiation is the evolution of many diversely adapted species from a common ancestor. Geographical isolation lends itself to this type of new species development.
2. Sympatric Speciation. This is a type of speciation that develops within the range of the parent population. This type of speciation does not include geographical isolation. It can occur rapidly if a genetic change results in a barrier between the mutants and the parent population.
a). Autopolyploid. An organism has more than two chromosome sets. This can occur due to nondisjunction in either mitosis or meiosis or self-fertilization.
b). Allopolyploid. A polyploid hybrid resulting from contributions by two different species. This is more common than autopolyploidy. These are usually sterile hybrids, but can reproduce asexually.