So we hear frequently that inbreeding is bad, full stop. But is this the case? What are the real implications behind close sibling pairing, and is inbreeding necessary?

Well, firstly I guess its best to define the term inbreeding, so the easiest place to start is probably: breeding between close relatives. But what exactly is wrong with this. Well, statistically speaking, full siblings will share 50% of their genes (or more accurately the alleles present at each loci). This can be both a positive and a negative factor then? Both positive and negative phenotypes (observed morphological characteristics i.e. colouration) can be created through being homozygous at a specific loci (two copies of the same allele at a single site). Examples of this are clear in captive morphs, albinism for example.

How then does this happen? The explanation for this is genetic fixation, and it hinges on a number of assumptions:

• Population size – Generally speaking, must be small or inbreeding will not be prevalent (law of averages).
• Selection, the driving force behind species development, must not be acting negatively (this is not strictly true, selection can not act anthropomorphically) but, simply put, phenotypic traits that have negative effects on the offspring, cannot be selected for in a specific environment.

So what does this mean? If you can’t select for a negative trait, then how does it happen? Well, a trait that is not under selection in a specific niche (habitat) can show negative characteristics when removed from that habitat. Or that the negative trait has a late onset, frequently after reproduction.

                                                            

Ok, so with the aid of the simplified tree above, I will attempt to explain how inbred homozygous traits are fixed in the population. First off, the squares represent the males, thus circles are females. Red is the negative trait. In small populations (here a single pair) a number of offspring are produced, each with a 50 percent chance of receiving the negative red trait. When these breed, if no advantage or disadvantage is given, then mate choice will be based on other characteristics. As you can see here, random male - female pairings will rapidly lead to all red (negative trait) pairings, which will lead to all the offspring carrying this trait. Through genetic drift this allele can reach 100% in the population and become fixed (i.e. each subsequent generation will contain this allele, as it is the only one possible at that loci).

So when does this become a problem? Well, in truth, it never has to be a bad thing, in the given niche in which it was fixed. A problem can occur when, the species (or individual) change there environment, and the gene is under selection.

From this it becomes clear that, inbreeding can in fact offer positive benefits, if the gene you become homozygous for, confers a positive benefit, then it will be selected for and the speed of fixation will increase. Simple examples of this would be characteristics like camouflage, limb length etc…

So, how frequently does inbreeding occur in wild populations, and how important is it? Well the simple answer to this is very frequently, and it has huge evolutionary impact. Speciation relies heavily on breeding between individuals with a new mutation that confers a positive advantage. Without this, the trait would not be seen in subsequent generations, and species would not evolve.

Traits that confer a negative weighting however should not be selected (as they have negative effects on fitness). These effects can take many forms, some simple examples here would be reduction in camouflage, reduced foraging ability, or depressed reproductive output. These traits tend to reduce the likelihood of reaching reproductive age, or the ability to compete for a mate. This is purging, when a deleterious phenotype (and genotype) is removed from the gene pool. This is important, as breeding these individuals will reduce the overall fitness of the population.

So where does captive breeding enter into this, and what positives and negatives can it produce? Well, the main concern here is that fitness is no longer under selection, or not as we have come to recognise it from the “wild” situations. Selection is now based on human choice, rather than inferred fitness of a specific characteristic. Environmental pressure is removed to a large extent, and so negative traits can build in the genome, without challenge. The converse to that is that specific traits (which offer no positive or negative other the aesthetics) can be selected and intensified. “So lets not breed the weak” I hear you say, is it that simple? Well the boat is out, what constitutes a negative characteristic, and can we assess how it is linked to the gene we want to express. Well, the fact is that we simply cannot predict which genes are linked, especially if in the heterozygous state no specific phenotype is expressed, and that’s the problem. Breeding two carries which do not show symptoms CAN produce homozygotes that do. So outbreed. is that the answer. Again, the boat is out, in some cases it is, if the gene that confers the problem is not linked to the gene that confers the desired trait. If it is, then out breeding is of no use. Unfortunately we know too little about dominance relationships of specific gene families, and how inheritance of characteristics is affected by these.

Again this can be affected by traits being polygenic…

So what is the ultimate answer? Is there one? Short of genetic testing, what can we do to prevent the spread of deleterious alleles? Is out breeding the answer, or is purging the only suitable strategy?

Well, there are probably arguments for both, and I’m sure both have compelling aspects to be discussed, but in my opinion purging is the best way to avoid inbreeding depression (negative fitness traits associated with a specific genotype). I certainly do not mean that animals should be euthanised, however a colleague of mine is currently advising European zoos that this is the best way forward for a certain species (due to space constraints and overall animal health). I just think that the trait should be removed from the genepool.