Hybrid Vigour and Inbreeding
Most of our nuclear DNA is involved with the basic things of life like development, growth, respiration, healing etc.: things we share with other life forms. This is why we share so much of our DNA, even with very simple forms of life. But we are told no two individual humans (or any other species) have exactly the same nuclear DNA. This makes each of us different. The variation in DNA is usually accepted as evidence to identify individuals in paternity or criminal cases. The difference between individuals in the human population is put at about 0.02% or so. It is the regional variations in just this 0.02% the gene maps used in my story actually represent. The difference in DNA between species is much greater. It is usually claimed humans and chimpanzees share at least 98% of their DNA (see for example Stringer and McKie 1996). Therefore the difference between these two species is as much as a hundred times greater, about 2%. The difference becomes greater still when we compare humans in turn with mice, birds, fungi, plants or bacteria.
In fact the pairs of chromosomes and their DNA seem to work more efficiently if they are not exactly the same as each other. Up to a point the more heterozygous “Bb” or “bB” gene combinations you get compared to the number of homozygous genes (double recessive or double dominant, bb or BB) the more you have what is called “heterosis” or “hybrid vigour”: extra fitness and ability to survive. For most species this often also means more breeding success.
Breeding success is actually a product of many factors: the act of fertilization itself, abortion during pregnancy, the death of the mother during the birth process, weak or deformed offspring, etc. For any given population it is usually possible to measure breeding success in some way but as Charles Darwin said, “for all practical purposes it is most difficult to say where perfect fertility ends and sterility begins” (Jones 2000). As the chromosomes and DNA become either more the same or more different breeding success goes down. This can be shown diagrammatically, as a bell curve.
The graph’s peak indicates hybrid vigour, at least for breeding success. I’ve shown no scale but the right hand end of the graph probably varies in the steepness of its fall for different pairs of species. The development of either genes with conflicting effects or behaviours called isolating mechanisms could lead to quite abrupt falls at the right hand side of the graph. Anyway, by 2% humans are separated from chimpanzees.
As the difference between the genes on offer at fertilization increases beyond the peak there is a progressive decrease in breeding success. The genes begin to combine poorly. Some matings between parents recognised as belonging to even separate species may be capable of producing offspring but often only one sex of the offspring is fertile, e.g. yaks and cattle, or virtually none of the offspring may be fertile, e.g. horses and donkeys (mules). But any fertile offspring are capable of transferring genes between even these species.
Any further difference and fertilization itself is not possible. The parents can definitely be called separate species. But the separations between species cover nearly the whole of the right hand side of the bell curve. The last step of separation may be completed by the reversal of parts of the chromosome or by a change in the number of chromosomes by their splitting or combining. For example chimpanzees have an extra pair of chromosomes as compared to humans. Chromosome 2 in humans seems to be a combination of two chromosomes possessed by a common ancestor (Ridley 2000). It seems that neither of these changes is as definitive as appears at first glance though.
Even within a species some individuals may have parts of a chromosome reversed, and Stringer and McKie (1996) remark that many humans have chromosomes with bits missing (deletions) or added in (repetitions). They add these variations usually have “no bearing on a person’s genetic well-being”. And some hybrids of species with different chromosome numbers can be perfectly fertile. For example the Asian wild horse (Przewalskii’s horse) has 33 pairs of chromosomes but has no trouble forming fertile hybrids with the domestic horse which has 32 pairs. Also horses and donkeys have different chromosome numbers but the hybrid offspring are not always sterile. Besides many different species within a group may actually have the same number of chromosomes.
Rather than being the result of a different number of chromosomes, the lack of fertility between species appears to come about through differences in what is on those chromosomes, the genes responsible for turning on or off other genes. Genes with conflicting effects can prevent the survival or even formation of the foetus (Jones 2000). Of course if just one individual within any population has a mutated incompatible gene it is impossible he or she could leave any offspring. For a single population to develop into two separate species with mutually incompatible genes each species would have to develop by the accumulation of many small genetic changes over many generations. Speciation, the development of two separate species or kinds from one, is therefore usually gradual.
Now, if we go back to look at the other extreme we find that as the parents become genetically closer breeding success can also become close to, or actually, zero for many species. This is called inbreeding depression.
The bell curve’s left-hand end represents inbreeding depression or loss of heterosis. In a population of fixed size genes will be eliminated over generations either randomly or by selection. The opportunity for two different genes to meet is gradually reduced. With less opportunity for variation the members of a population become more similar to each other over time, both genetically and in appearance, and the level of heterosis falls.
It is most likely each one of us carry one or more genes that would be lethal if we had a double dose (Steve Jones 2000). Because of this it has been suggested a human population of less than 400 to 600 is not viable in the long term (Rhys Jones quoted in Flood 1988). Interestingly this seems to be the average size of tribes of hunter-gatherers (Cavalli-Sforza 1995). This fact also has implications for the preservation of threatened species (Tudge 1996). Of course if intense selection within a population has already eliminated any lethal genes inbreeding is not a problem.
But more usually as the level of inbreeding increases the chance of two of these lethal (or at least disadvantageous) recessive genes coming together is greatly increased. Any dominant gene useful for survival is usually already widely spread. Any recessive gene that confers an advantage has also usually already replaced any other gene and so inbreeding usually brings out recessive genes that limit survival and breeding success. Inbreeding can be a big problem for animal breeders. Most breeds by definition have a fairly limited genetic variety to start with.
Many people involved in dairy cattle breeding are worried about inbreeding. The use of artificial insemination for many generations has meant the world’s dairy herds are becoming more closely related through the continued breeding to the best bulls available. This has already resulted in a loss of fertility in the international dairy herd, although selection only for production at the expense of fertility has probably been the main factor in this case.
The human genome is well studied and I guess we know as much about human genes as any genes. But for the study of population genetics the national dairy herd can’t be beaten. The ancestry of 85% of the New Zealand herd is on computer. When the technician arrives to inseminate a cow it is possible in many cases to tell what the theoretical loss of production through inbreeding for that particular combination of bull and cow will be.
This level of information is not readily available for humans. Several genes have been discovered in cattle that lead to the death of the foetus if it finishes up with a double recessive. In many cases the mutation that causes this has been traced through breeding records to the individual animal that introduced it to the New Zealand dairy herd. This is often also the individual in which the mutation occurred.
When two inbred populations meet, mix and breed some level of heterosis is restored in their offspring. This is called hybrid vigour. D. S. Falconer (1964) says, “the phenomenon of heterosis is simply inbreeding depression in reverse”.
Wave Theory of Evolution
Large stable populations are, by definition, successful and largely in balance with their environment. Selection is therefore towards the average for that population. The bell curve for any characteristic is steep on either side. But individuals can carry a recessive gene that doesn’t show up in the individual’s observed characteristics. Half the offspring of any individual with a mutated gene will, on average, carry that gene. Any new gene that forms through mutation may become widely dispersed but unable to form a double recessive, and so subject to selection either for or against, for many generations. A double recessive will only be possible when two individuals with the recessive gene are themselves able to mate and produce offspring. Therefore double recessives usually come about through inbreeding. On average one quarter of the offspring of two single recessive parents will be double recessive and express the genotype.
Of course if individuals with the double recessive are at a disadvantage selection will keep the gene at a low level. Otherwise these double recessive individuals will occasionally meet and mate with other individuals in the population that have the single recessive mutation. This time half their offspring will be double recessive. Selection for or against the mutation can then get seriously underway. If individuals with the double recessive leave many more descendants the change in the population could be very rapid as the gene spreads through a population containing many single recessives. The bell curve will widen, flatten out and then shift its centre.
The species or sub-species most different to the rest of their kind (including other species in the genus) are usually found at the geographic extremities of the range of that collection of species. As I mentioned in “The Human Star” [A Map] this fact has been recognised for a long time (Mayr and Diamond 2001). Environmental conditions at the geographical extremities of a population’s distribution are more likely to be marginal for the survival of that population leading to smaller numbers and isolation. This increases the likelihood of either extinction or selection in a particular direction through inbreeding. Genetic mutations in the central area of the distribution may also either fail to reach the margins or become subject to selection only once they do.
The fact that chromosomes and DNA are subject to change means populations of a species that have become genetically isolated or had different selection pressures acting on them will develop a different gene pool and usually appearance. Any species, including the human species, which inhabits a large geographical area, varies over that area. As an extreme example Tim Flannery (2001) mentions the grizzly bear of North America. It was at one time divided into a ridiculous seventy-four different species.
These species actually show much less physical variation than do the different modern breeds of dog though, presumably all descended from a single species of wolf. The different kinds of the many species of animals, birds and plants that vary markedly over their range are usually quite capable of forming fertile hybrids when brought together. The kinds are similar to human “races”.
But sometimes even what are definitely recognised as being separate kinds or species show what are called “hybrid zones”, regions where a proportion of the individuals have a mix of characteristics of the two species. Occasionally the hybrid population has been classified as a third species until the mistake has been realised (Gill 1998). Hybrid zones between pairs of species are usually narrow because of selection against the hybrids. If the hybrid zone is wide it means genes flow freely between the populations. The two types grade gradually into each other and they form a cline, a series of subspecies.
A new characteristic in a species could arise, and probably usually does, through inbreeding in an isolated population (Tudge 1996). Isolation is probably usually geographic but it can be tribal. Again the development of any new characteristic may lead to recognition of a separate subspecies. But if there are not many individuals in any particular population its survival will depend on contact with another group before the inbred population becomes extinct.
If some new characteristic in an inbred population is advantageous to the species as a whole the genes will be able to spread into any incoming population by selection in the hybrid zone. A hybrid zone of just one fertile hybrid individual is sufficient to transfer genes of course. In some cases the incoming group may be inbred as well. The spread of the characteristic could then be helped by restoration of heterosis in hybrids between the inbred population and the incoming group.
I have just pointed out that individuals with a single recessive gene could become widely but thinly spread in a population before selection begins to operate on double recessives. It has probably been the sudden expansion of recessive genes that has led to the pattern of sudden change observed in the fossil record called “punctuated equilibrium” (Tattersall and Schwartz 2000). The balance of nature is very fluid and so are the hybrid zones (Gill 1998).
Extinction or survival is a function of the numbers in the population. This is often a function of the size of the area available to it. Rises in sea level at the end of ice ages resulted in a reduction of the size of islands in most places. The consequent drop in population numbers leading to inbreeding and local extinction has been offered as an explanation for the gaps in the distribution of many bird species through the islands of Northern Melanesia (Mayr and Diamond 2001). There has also been differential extinction of tigers, elephants, rhinoceros and orangutans through Island Southeast Asia with rising sea level. The same thing seems to have happened with humans in that region in prehistoric times.
In fact reduction in the size of individuals often occurs in populations confined to areas of limited extent such as islands (Tudge 1996). The Shetland Islands, north of Scotland, provide a well-known example in the popular Shetland pony although it may be a product of human breeding. But there are other examples from other islands. Smaller size means individuals use fewer resources therefore more numbers can occupy the same space (Jones 2000). This makes inbreeding less likely. If an isolated population is able to reduce the size of individuals rapidly enough, it will survive.
Human induced changes in animal breeds is usually achieved by inbreeding, called “line breeding” when it is done in a controlled manner. Change in a breed’s body size, shape or colour can be very rapid when this method is used. But reducing population numbers through intense selection also leads to problems with inbreeding.
Change can also be rapid in nature (Tudge 1996). Ernst Mayr and Jared Diamond (2001) point out that several bird species that can have re-occupied Long Island in Northern Melanesia only since a volcanic eruption destroyed life there in the seventeenth century have already begun to differentiate from the nearby populations that are presumably their ancestors. They provide three explanations for this: they are “character displacement”, “founder effect” and “formation of a stabilised hybrid”. “Character displacement” results from the same process as human breeding of domestic animals, the changing of a characteristic through selection in a particular direction.
Change in the environment leads to the same effect in nature. The process has led to pesticide resistance in insects and anti-biotic resistance in bacteria. The “founder effect” is a special case of genetic drift. It happens when a new population doesn’t contain a completely representative sample of genes from the original population. Because the new population starts off with a different mixture of genes it is slightly different right from its origin.
This is one of the random elements in evolution. Lastly “formation of a stabilised hybrid” means the establishment of a species that combines characteristics from two other populations. Farmers find that, apart from the first generation, hybrid populations of any given pair of cattle breeds for example are very variable. In the first cross recessive genes from either breed can be obscured by dominant genes from the other and so all the first hybrids tend to look much the same.
Commercial breeders use this to advantage. New double recessive combinations are able to form only once the hybrids themselves are able to breed with each other. It is only after several more generations of breeding and selection within the hybrids that the characteristics stabilise again and the extremes are eliminated. Again, the same process happens in nature. In fact the above three methods of genetic change have all happened during our species’ evolution.
For example it’s almost certain that many times during our history groups of humans with a new technology have advanced through territory already occupied by other humans, often of quite different appearance and sometimes even classified as being different species. The advancing wave would be unlikely to contain a totally representative sampling of genes from any original population and so would demonstrate the founder effect.
As the group advanced through different environments it would have undergone character displacement and eventually would consist of small, inbred groups. They could then have formed hybrid populations with any resident inbred groups they encountered. Over generations the members of the combined population would have become more similar to each other as genes were eliminated. They could then be called a stabilised hybrid.
Inbreeding in a small isolated population can be offset if there is still a degree of genetic variation within it and, through the availability of unlimited resources, the population is suddenly able to breed up very rapidly. This shuffles the genetic variability available into as many different combinations as possible. This can happen when members of a species are able to use a so far unexploited region and selection is minimal. If some inbred population were suddenly able to expand into a new region or use the old region in a new way it could lead to the apparent sudden appearance of a new species.
A rapid buildup of population may have occurred at times during our prehistory, e.g. in the Pacific islands, the Americas, Australia and perhaps other areas in even earlier times. Possibly even during the original expansion of humans. It has certainly happened with human introductions of some animals into new environments around the world, e.g. the rabbit, starling and sparrow. In general, though, studies have shown any introduced populations of less than a hundred fails to survive (Jones 2000). The case of the Chatham Island black robin may appear at first sight to contradict this argument. It has been able to increase in numbers from a single female and five males but this has involved very intensive breeding management by humans.
Usually a population can stave off inbreeding depression only if it is large enough so that the rate of elimination of genetic diversity through inbreeding is offset by the rate of mutation in the DNA of that population. Theories that new species (or new races) usually arise from the expansion of very small populations are therefore flawed.
Cavalli-Sforza, Luigi Luca and Cavalli-Sforza, Francesco (1995) The Great Human Diasporas. Addison- Wesley
Falconer, D. S. (1964) Quantitative Genetics. Oliver and Boyd Ltd., Great Britain.
Flannery, Tim (2001) The Eternal Frontier. Text Publishing, Australia.
Flood, Josephine (1988) Archaeology of the Dreamtime. Collins, Australia.
Gill, Frank B. (1998) Hybridization in Birds. The Auk, Vol. 115 No 2 April.
Jones, Steve (2000) Almost Like a Whale. Anchor, London.
Mayr, Ernst and Diamond, Jared (2001) The Birds of Northern Melanesia. Oxford University Press, New York.
Ridley, Matt (2000) Genome. Harper Collins, New York.
Stringer, Christopher and McKie, Robin (1996) African Exodus. Random House, UK.
Tattersall, Ian and Schartz, Jeffrey H. (2000) Extinct Humans. Westview Press, New York
Tudge, Colin (1996) The Time Before History. Scribner, New York.