Last week’s post on The Hardy-Weinberg Equlibrium and its Implications covered the five external forces that shift that equilibrium to cause a change in gene and genotypic frequencies: selection; gene mutations; migration in and out of a population; random genetic drift; and non-random matings.
Three of these are controllable by breeders and routinely applied in breeding programmes: selection; migration in and out; and non-random matings.
The other two — gene mutations and random genetic drift — are completely random forces beyond anyone’s control, and patterns of inheritance are ultimately down to the sheer chance of gene segregation during meiosis and ‘luck of the draw’.
Gene mutations are extremely rare events and, should they even occur at all, are as likely to have good, bad or indifferent effects . Any mutation that does appear is most likely to be of a single allele, as the chance of two or more mutations occurring simultaneously, much less being inherited together, is even more remote. Such an allele, or any other rare allele for that matter, may then be subject to that other random force: random genetic drift.
Random genetic drift (also known as genetic drift or allelic drift) is an interesting phenomenon well worth being aware of. Genetic drift is where a particular allele increases or decreases in a population over time purely by chance. It can happen in all populations to any allele, but is more likely to occur in small populations, and with rarer alleles. Once it begins, genetic drift only ends when the allele either disappears completely from the population, or becomes the only allele at that locus. This can happen over any number of generations, and either outcome leads to a reduction in the gene pool for that population.
The following may help explain this concept:
In this diagram, each circle represents an individual with any of two alleles, ‘red’ and ‘blue. ‘Blue’ occurs at a lower frequency than ‘red’. Assume matings are random in this small population. After one generation, purely by chance, most of the gametes (haploid egg and sperm cells) that became zygotes (diploid fertilised eggs) happened to carry the ‘red’ allele. After the second round of matings no ‘blue’ alleles had made it into the zygotes at all. ‘Blue’ alleles would still have been produced during regular gene segregation in meiosis, but through sheer chance not a single ‘blue’ egg cell managed to become fertilised and not a single ‘blue’ sperm managed to do any fertilising. Thus were no zygotes containing so much as one ‘blue’ allele created, and this allele subsequently disappeared from the population simply by chance.
Often the frequency of an allele will fluctuate up and down over a few more generations than here, but irrespectively, this allelic drift will only end once the frequency reaches either 0 or 1. It is still possible for drift to end very quickly — in the above example, were all five ‘blue’ carriers in the first generation, by sheer chance, removed as breeders for whatever reason (death, disease) that allele would have disappeared even sooner.
Here’s a diagram whereby the ‘blue’ allele drifts in the opposite direction, to reach a frequency of 1:
Again this is an extreme example, and again allelic drift may fluctuate up and down over more generations. But in this example, it was possible that not every animal mated in this particular population, and that the ones that did were, by sheer chance, all carriers of the ‘blue’ allele. And that by the third round of matings, by sheer chance, no ‘red’ eggs were fertilised nor any ‘red’ sperm fertilising to create ‘red’-containing zygotes.
Random genetic drift is usually discussed in the context of natural evolution, but as a breeding programme is a smaller, artificial version of evolution, it’s well-worth bearing potential consequences of random drift in mind. Two specific events that can trigger random drift are mentioned below.
Bottleneck Effect. This is where an extreme event like a natural disaster drastically and suddenly reduces a population’s numbers. Imagine a thin-necked bottle of marbles. Upend it and let fall out as many marbles as possible. A few will fall out, but the bulk will remain clogged behind the bottle’s neck. The neck represents the event and the few marbles that fell out represent the surviving population. Those marbles remaining in the bottle are the removed population.
Let’s represent this visually:
Here it can be seen that the original population had a very high frequency of the ‘grey’ allele. The ‘green’, ‘red’, ‘cyan’ and ‘purple’ alleles were of lower, but similar frequencies to each other. After the bottleneck effect, by sheer chance, these frequencies shifted significantly. The ‘grey’ allele barely has the highest frequency still, while the frequency of the ‘red’ allele has increased so much it is almost as high. The ‘purple’ allele has completely disappeared from the new population and the gene pool has reduced significantly. If the ‘purple’ allele was closely linked to an ‘orange’ allele (not shown here), that ‘orange’ allele too has disappeared and the gene pool lowered still further.
In the context of evolution amongst wild populations, events such as droughts or over-hunting can decimate numbers. In extreme cases the remaining genetic diversity becomes so low that a species doesn’t have the variety of genes needed to adapt to environmental changes. Inbreeding depression can also arise. A real-life example is that of the cheetah population — it is amongst the most inbred one on the planet, to the point that extinction is an always-hovering threat. Dramatic changes to its environment or some new disease could be enough to wipe the species out, as there is very little genetic variety left to drive adaptations. It is believed that a bottleneck effect 12,000 years ago (during the Late Pleistocene, a time when many large mammals were becoming extinct worldwide) removed much of the cheetah’s genetic variation.
In the context of a breeding programme, an elite and exclusive stud herd (typically smaller in number than commercial herds) may be elite and exclusive owing to a high number of alleles that reached fixation (100%). This is a major boost to its breeding value, but it can go the other way. That small herd could succumb to a disease and breeding advances be set back by years or never recover. One example is the blackface budgerigar, an exciting colour mutation that appeared in the Netherlands in the 1990s. The breeder managed to breed a small flock of them, but kept them together instead of spreading them amongst other breeders. Every single bird was lost to a disease which entered the flock, and the mutation has not reappeared anywhere else since. (Mutations are such rare events that that specific mutation may never appear again, though a different one with the same effect could.)
Founder Effect. This is when a small subgroup of a population splinters off to form a new group with no further contact with the original. Again through sheer chance this ‘founder’ group may have increased or decreased gene frequencies of particular alleles compared to the larger, original population. These alleles may undergo genetic drift and either disappear or predominate accordingly in the new group.
The founder effect can be represented here:
where a small ‘founding’ group, that by sheer chance did not carry the ‘red’ allele, separated from the original group to form a new population with changed allelic frequencies.
With respect to natural evolution, the founder effect with time, isolation and changed environment inputs can lead to new species. With respect to a breeding programme, the founder effect can, with time and care, lead to new breeds.
The bottleneck effect mentioned above on the cheetah population 12,000 years ago is actually believed to be a second event — a much earlier founder effect is thought to have occurred 100,000 years ago. (The original population from which cheetahs split appears to have gone on to evolve further into the North American puma and the South American jaguarundi.) This first, founder, event would have reduced the gene pool enough that the second, bottleneck, event would have an even more deleterious effect that plagues cheetahs to this day.
With respect to a breeding programme, many famous studs worldwide began with a foundation herd that was eventually ‘closed’ (isolated) and carefully improved on over years. The Peppin Merino, a very important breed in the Australian wool industry today, came about from a founding herd of 300 ewes and at least six rams back in 1858 and which became closed to outside animals some years later.
A good breeding programme will readily use the three controllable factors of selections, migration in and out, and matings to effect gene and genotypic changes in a population and improve it continually over time. Even so, plenty of breeders still hope for Lady Luck to shine their way and bring forth a truly unique and stunning animal that becomes a major influence on the breed forevermore. Such an event could arise through some rare once-in-a-lifetime genetic mutation, but if the breeder doesn’t compensate for that other random event — genetic drift — that rare opportunity may slip away for good.