CROSSING THE SPECIES BOUNDARY

Speciation (one species evolving into two) is usually a slow process. It is generally accepted that different species usually cannot mate and reproduce - this is called "reproductive isolation". The exception was closely related species which can produce hybrids, although those hybrids have reduced fertility. The more easily two species form hybrids, the more closely the species are related in evolutionary terms. However, nature defies human attempts to compartmentalize creatures into static species. Hybridization is turning out to be more common than previously realised.

One way reproductive isolation occurs is changes in genes due to mutation. One group of animals might be geographically isolated from others of the same species. Each group undergoes slightly different mutations over many generations - some genes affect appearance, others affect behaviour. Many generations later, the two groups become different enough that even if they can mate, they can't produce fully fertile offspring.

Sometimes, one species can split into two through behavioural isolation. Some individuals develop behaviour patterns which limit their choice of mates e.g. they might be attracted to certain colours or might be active at different times of day. Though they are fully capable of interbreeding with the other group, their different behaviours keep them apart. If their habitat became permanently overcast, those behaviour barriers would break down and they would interbreed freely; their hybrids might become new species.

Another way reproductive isolation occurs is when fragments of DNA accidentally jump from one chromosome to another in an individual (chromosomal translocation) The mutant individuals cannot reproduce except with other mutant individuals - not much good unless the individual has mutant siblings to mate with! There are also "master genes" which govern general body plan (Hox genes) and those which switch other genes on and off. A small mutation to a master gene can mean a sudden big change to the individuals that inherit that mutation. Sometimes, those radical mutations can "undo" generations of evolution so that two unrelated species can mate with each other and produce fertile young (so far, this has only been seen in micro-organisms).

Hybridisation is usually considered a dead end because the hybrids are not fully fertile; if they are fertile, the hybrids are usually absorbed back into the population of one or other parent species and most of the alien genes are bred out. More rarely, hybrids can become new species or new sub-species. In the hands of breeders, some domestic/wildcat hybrids can become breeds; these are not new species because the wildcat genes are largely bred out by crossing with domestic cats, until only the wildcat pattern remains.

In some species, hybridisation plays an important role in evolutionary biology. Most hybrids face handicaps as a result of genetic incompatibility, but the fittest survive, regardless of species boundaries and may contain a combination of traits which allows them to exploit new habitats or to succeed in a marginal habitat where the two parent species have trouble surviving (seen in some sunflowers). Unlike mutation, hybridisation creates variations in many genes or gene combinations simultaneously. Some successful hybrids could evolve into new species within 50-60 generations. Life may be a genetic continuum rather than a series of self-contained species.

Usually, where there are two closely related species living in the same area, less than 1 in 1000 individuals will be hybrids because animals rarely choose a mate from a different species. Otherwise, genetic leaks would cause species boundaries to break down altogether. In some closely related species there are recognized "hybrid zones".

For example, in Heliconius butterflies, hybrids are common, healthy and fertile - hybrids can breed with other hybrids, or with either parent species. Genes have leaked from one species into another through regular hybridisation. However, hybrids are disadvantaged by natural selection. Pure-bred Heliconius butterflies have warning colouration recognised by predators. Hybrids have intermediate patterns which are not recognised - the predators have not yet adapted and so the hybrids are disadvantaged. In mammals, hybrid White-Tail/Mule Deer don't inherit either parent's escape strategy (White Deer dash. Mule Deer bound) and are easier prey than the pure-bred parents.

Another example is seen in Galapagos Finches. Healthy Galapagos Finch hybrids are relatively common, but their beaks are intermediate in shape and less efficient feeding tools than the specialised beaks of the parental species so they lose out in the competition for food. Following a major storm in 1983, changes to the local habitat meant new types of plant began to flourish and the hybrids had a advantage over the birds with specialised beaks - demonstrating the role of hybridization in exploiting new niches. If the change is permanent or is radical enough that the parental species cannot survive, the hybrids become the dominant form. Otherwise, the parental species will re-establish themselves when the environmental change is reversed and hybrids will remain in the minority.

Finally, what happens if two species previously kept separate by geographical boundaries suddenly meet up? The hybridisation of the native European Red Deer and the introduced Chinese Sika Deer means that pure Red Deer are being hybridized into extinction. While humans want to protect the Red Deer; evolution wants to utilise the Sika Deer genes.

Mechanisms for keeping species separate:-

Physical separation: the species live in different geographic locations or occupy different ecological niches in the same location and so never have the chance to meet each other.
Temporal isolation: the species that mate during different seasons or different time of day and cannot breed together.
Behavioral isolation: members of different species may meet each other, but do not mate because neither performs the correct mating ritual. Imprinting by fostering the young of one species on a female of the other species can overcome this in some cases.
Mechanical isolation: copulation may be impossible because of incompatible size and shape of the reproductive organs.
Morphological isolation: copulation may be impossible because of the difference in body size or shape.
Gametic isolation: the sperm and egg may not fuse and hence fertilization cannot occur; if it does occur then the embryo fails to get past the first few cell division.

ORIGINAL: gorse

Good article, bigbulls. That appears to be solid information (rather than just 'because the newspaper said so' (heresay) - which is often the line of argument here (though I am NOT saying that you do that). Way to go. Perhaps it IS possible.

now both the scientist are on the same side I see

you can say all you want about these captive animals that are breed artifitually but they are differnt because no scienctist is sticking a tube or his hand in the female its totally different

Take a male dog and your leg for example. When animals get that horney anything can happen.