Hybrids can only occur where the species are closely enough related enough for the egg and sperm to result in a viable embryo. Differences in chromosome number are not always a barrier to producing viable hybrids; it depends on the gene combinations in the hybrid and whether these allow an embryo to develop. However, large differences in chromosome number make female hybrids poorly fertile and male hybrids sterile.

Where the two species are very closely related, the hybrids may even be partially or fully fertile. In the laboratory, vole species are sometimes hybridised during research into genetic traits. Some hybrids are bred for curiosity or public display, others are bred by researchers involved in genetic researcher and a few occur naturally - usually where the animals are housed together or where a same-species mate is not available.

Hybrids are not always reliably reported. In "Mammalian Hybrids" by Annie P Gray (foreword by Osman Hill), Hill wrote "In the older literature, the claim that an animal is a hybrid has sometimes had no more solid foundation than that a possibly mutant type happens to have characters which are a mixture of those of two different species, or that an animal of one species is found suckling young which bear a strong resemblance to the young of some other species."

Chimeras are not the same as hybrids (some online sources erroneously define chimera as a hybrid). Hybrids have intermediate features and each cell is a mix of chromosomes from the parental species. Chimeras are a mix of genetically different cells to form a mosaic animal. Details on chimeras can be found at the end of this article.

Chimeras are not really hybrids, but are organisms containing cells from different "parents". The Geep (mentioned earlier) was made by merging a sheep embryo with a goat embryo. Each population of cells keeps its own character and the animals is a mosaic of mis-matched parts. An analogy is two jigsaw puzzles cut using an identical cutter, but with different pictures. You can make a single puzzle out of the mis-matched parts, but the completed puzzle will show parts of both different pictures.

Interspecies chimeras are made in the laboratory. There have been rat/mouse chimeras and recently a rabbit/human mix (it was not allowed to develop beyond a few days). Like hybrids, the parent species must be closely enough related if the jigsaw-puzzle offspring is to be born alive and relatively healthy. The chimeras have either 4 parents (2 fertilized eggs are fused together) or 3 parents (a fertilized egg is fused with an unfertilized egg or a fertilized egg is fused with an extra sperm).

In nature, chimeras sometimes form when twin embryos fuse together in the womb during pregnancy. This is not detected unless the offspring has visible abnormalities (e.g. some tortoiseshell male cats or ambiguous sex organs) or behavioural abnormalities (e.g. confused gender behaviour). Recent studies of tortoiseshell male cats and unusually coloured cats (containing a mix of coloured patches considered genetically impossible) suggest that natural chimerism is more common than previously realised and that it frequently goes undetected.

In April 2005 , scientists at the University of California created a human-cat chimera. Far from creating an "anthro-cat", they fused the feline Fel d 1 protein (the protein that triggers the allergic reaction in cat allergy sufferers) with a human protein known to suppress allergic reactions. When tested in mice, the chimeric protein stifled cat allergy. The feline part of the protein binds to the specific immune cells that generate the allergic reaction to Fel d 1. The human part of the protein also binds to the immune cells and tells them to stop reacting. Because the human part is more dominant, the allergic reaction is halted. Chimeric proteins could be used to desensitise allergy sufferers by retraining their immune system.

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.

Haldane's Rule states that in animal species whose gender is determined by sex chromosomes, when in the first cross offspring of two different animal species, one of the sexes is absent, rare or sterile, that sex is the heterogametic sex. The "heterogametic sex" is the one with two different sex chromosomes (e.g. X and Y); usually the male. The "homogametic sex" has two copies of one type of sex chromosome (e.g. X and X) and is usually the female.

Haldane's Rule for Hybrid Sterility states that a race of animals could diverge enough to be considered separate species, but could still mate to produce healthy hybrid offspring in a normal ratio of males and females. If any of the hybrid offspring were sterile, the sterile offspring would be the heterogametic offspring (males). If the heterogametic offspring was fertile, it produced the normal 50:50 ratio of X and Y sperm.

Haldane's Rule for Hybrid Inviability states that if the divergence between the species became large enough to generate genic differences, but not to prevent mating, then parental gene products may fail to co-operate during development of the embryo, resulting in hybrid inviability (the hybrids are aborted, stillborn or don't survive to maturity). In this case, the male to female ratio of hybrid offspring is skewed with more homogametic offspring while the heterogametic offspring (males) are absent or rare.

Haldane considered the speciation process (i.e the "growing apart" of one species into two species) to occur in stages. The first stage of the speciation process was complete if the two species could mate and produce healthy but sterile hybrids. As the species continued to diverge, they became genetically less compatible. These incompatibilities prevented hybrids from being formed or caused them to die before maturity i.e. it didn't matter whether or not they were sterile since they would not survive to breeding age. These are called "post-zygotic barriers" because a zygote (fertilized egg) is formed, but the offspring (particularly the males) do not breed.

As species differentiation progresses even further, it results in anatomical (body shape), physiological (body function e.g. mismatched pregnancy periods) or psychological (behavioural) differences which prevent the two species mating with each other. Haldane called these "pre-zygotic barriers" because they prevent offspring from being conceived in the first place.

Speciation can involve big jumps as well as gradual shifts and fertile hybrids are more common than Haldane could have realised. The following examples show that some pre-zygotic barriers can be overcome and that there are intermediate stages in post-zygotic barriers. The species involved may have been kept separate by other means e.g. physical separation.

Male Jackals only mate with domestic bitches if the Jackal pups are raised by a domestic bitch (to become imprinted on dogs). There is a psychological barrier, but the offspring are fertile (pre-zygotic barrier, but no post-zygotic barrier). Lions and Tigers must overcome behavioural (courtship) barriers, but produce fertile female offspring and sterile male offspring (pre-zygotic and post-zygotic barriers). Lions and leopards have some physical barriers (size), but these are overcome if the lioness lies on her side to let the leopard mount her; the male Leopons are sterile, though female offspring are fertile (pre-zygotic and post-zygotic barriers). In these cases, pre-zygotic barriers are overcome by rearing the two species together (in whales and dolphins this occurs naturally).

Some cases seem to need additional rules! In Beefalo, Domestic cows may have an immune response against Bison/Cow hybrid calves - this is a physiological barrier, but does not prevent conception. Bison cows don't have this immune response against hybrid calves and hybrid Beefalo males can be fertile. In some hybrids of domestic cats with small wildcats, a proportion of hybrid males are claimed to be partially fertile (incomplete post-zygotic barrier?) and though the hybrid females are fertile they may not successfully raise their young - a psychological barrier, but one which does not prevent mating/conception.

In addition to Haldane's Rules, the viability and fertility of hybrid offspring can depend on which species is the male parent and which is the female parent since some embryo developmental effects come into play depending on which genes come from which parent (e.g. giantism in ligers, but not in tigons).

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