Hibridization is the reproductive crossing between two different species that they can produce Individual viable, that can be or no fertile. Although the animal hibridization can be a process natural, most frequent it is than he is associate to alterations caused by the introduction of new species by the man ever since Cattle ranch discovered .
Between plants the hibridization is an extremely common phenomenon. The introgresión is the penetration of genes of a species in another one by means of the hibridization. The hybrids must to derive, in the successive generations, towards the characters of one of the parental ones, but in the process it can be determined the transference of some genes from the other species.
The cladogénesis or bifurcation is the mechanism of more important especiación. from a species due to barriers takes place by reproductive isolation of different populations that can be prezygotic or postzygotic.
prezygotic barriers are mechanisms of isolation that before take place or during the fertilization, which they limit, act before the gamético interchange. It can be by ecological, etológico or mechanical isolation.
postzygotic barriers are all that concern to the viability of the produced individuals, through spontaneous abortions, sterility of the hybrid, premature death, weak and sickly hybrids, etc.
Also call geographic especiación or vicariante, is the gradual especiación that takes place when a species occupies a great geographic area that does not allow that the individuals that are very remote they can be crossed, due to geographic barriers as Seas Desert Mountains or the space separation of two populations of a species for a long period of time gives rise to the appearance of evolutionary new features in one or the two populations because Environment is different in the different geographic zones; pauses genetic Flow between populations.
It is the gradual especiación that happens when a species in spite of occupying a same geographic territory is diversified in two under-populations due to the appearance of Mechanisms of Reproductive Isolation (M.) that prevent the crossing eg:
ecological Isolation: within a same geographic zone Habitats characterized by temperature differences, light, humidity, etc. can exist different that make difficult the mating.
seasonal Isolation: the organisms can sexually mature in different stations or hours from the day.
etológico Isolation: is based on differences of Behavior during the courtship and the mating like signals of attraction or pacification that if they fail causes the flight or the attack.
mechanical Isolation: the copula is sometimes impossible between individuals of different species, or by the incompatible size of its genitals, or by variations in the floral structure.
sexual Isolation: is mechanisms that prevent Copula or Fertilization like the morphologic differences of reproductive organs or Gametos
genetic Isolation: takes place in Chromosomes and therefore in the genetic information. It can be of two types:
Sterility of the hybrids: when two different species mate, the descendants can be viable, but sterile.
Weakness of the hybrids: when the descendants of two different species are not viable and are eliminated before arriving at the sexual maturity by natural Selection .
It is understood continuously by parapátrica especiación to the evolution of the reproductive isolation in populations distributed in the space, but between which the interchange or genetic Flow is modest, which originates divergence and a later reproductive isolation.
Especiación by autopoliploidía
It is the spontaneous especiación in which a single species that undergoes an alteration of Meiosis, with an increase of the chromosomes takes part Poliploidía . The multiples of two in the number of chromosomes are only viable (4n, 6n, 8n) and is very frequent in vegetables with the appearance of species of great size.
Especiación by alopoliploidía
It is another spontaneous especiación in which two species similar with the same number of chromosomes take part giving rise to sterile descendants because the chromosomes are not homologous. If during Gametogénesis of the descendants took place some Mutation in the meiosis giving rise to one poliploidía, they would obtain Gametos with diploide number of chromosomes that will only be able to be fertilized with other hybrids that have undergone the same mutation or with themselves.