Inbreeding depression and outbreeding depression
Inbreeding depression refers to reduced (depressed) biological fitness in the offspring of genetically very closely related individuals, while outbreeding depression refers to reduced biological fitness in the offspring of distantly related individuals. More generally, it refers to reduced biological fitness in whole populations due to such effects.
In particular outbreeding depression is an important genetic consequence to consider regarding the effects of race mixing. However, it is very rarely mentioned, with the focus often instead being on the supposed benefits of a reduction of inbreeding depression, despite the population sizes needed to avoid inbreeding depression being very small and even smaller if there is even a small genetic exchange with neighboring groups from the same race.
The main explanation for inbreeding depression is that this increases the risk of two harmful recessive alleles at the same locus, which may be negative (for example, both alleles coding for dysfunctional proteins).
Incest between siblings and parents-children is and has always been almost universally forbidden in all cultures ("incest taboo"). Exceptions include some royal dynasties who were seen as gods. A similar avoidance is seen in many animal species. This has been argued to be caused by a genetically determined instinct against sex between persons who live in close domestic proximity during the first few years of their lives. This effect is called the Westermarck effect, named after the Finnish scientist Edvard Westermarck. The effect has received empirical support in studies. Smell and other characteristics may also be involved in determining if another individual is very closely genetically related or not.
In contrast, Sigmund Freud argued that the incest taboo is merely a social construct and that children actually want to have sexual relations with the parent of the opposite sex ("Oedipus complex"). Steven Pinker has argued that "The idea that boys want to sleep with their mothers strikes most men as the silliest thing they have ever heard. Obviously, it did not seem so to Freud, who wrote that as a boy he once had an erotic reaction to watching his mother dressing. But Freud had a wet-nurse, and may not have experienced the early intimacy that would have tipped off his perceptual system that Mrs. Freud was his mother. The Westermarck theory has out-Freuded Freud."
Avoiding inbreeding depression
The population number needed to avoid inbreeding depression has been studied in biology, in part due to many species dramatically decreasing in population sizes, due to the effects of humans on the environment.
An often cited rule is the 50/500 rule, which states that for wild animals a minimum of 50 individuals is needed to avoid inbreeding depression due to recessive alleles.
Furthermore, a minimum of 500 individuals is needed in order to avoid decreasing genetic variability within the population. This since otherwise the number of new mutations will be lower than the effects of random genetic drift which decrease genetic variability. It has been questioned how important a high genetic variability is. For example, there are species of albatrosses which have survived for nearly a million year, despite extremely low genetic variation. A high genetic variability may mainly be important if the environment should dramatically change.
The 50/500 rule assumes conditions that seldom exist in practice. This may be due to factors such as only a few females living long enough to reproduce, only a few males reproducing, or large fluctuations in population size (for example, due to periodic droughts). It has therefore been estimated that for many species, the numbers may be 10 times as large. This number varies greatly for different species and may be much lower for species with a low fertility, such as humans.
On the other hand, selective breeding has been performed with considerably less than 50 individuals without any obvious negative effects. Another aspect of selective breeding is breeding between very closely related individuals in order to "fix" desired characteristics. This may however be accompanied by negative side-effects.
The 50/500 rule assumes a completely isolated group. In practice, many groups of wild animals have some genetic exchange with neighboring groups. Even only a very small such exchange with neighboring groups dramatically reduces the numbers needed in order to avoid inbreeding depression.
Contrary to popular belief, a population that has been inbreeding for a long time may have reduced risk of inbreeding depression, due to the harmful recessive alleles increasingly being removed from the population by natural selection.
A 2010 article stated that couples related as second cousins or closer and their descendants accounted for an estimated 10.4% of the global population. Such marriages were most common in Africa, the Middle East, and west, central, and south Asia. Also, in these regions, even couples who regard themselves as unrelated may exhibit high levels of genetic relatedness, because of a long tradition of marriages within clan, tribe, or caste boundaries.
Such marriages, in the particular those between closer relatives, may cause problems due to inbreeding depression. However, such marriages could be avoided while still marrying within the same ethnicity/race. Race mixing in not necessary in order to avoid such marriages.
There are several genetic mechanisms which may cause outbreeding depression:
- Two identical alleles at the same locus can in some cases be the alternative with the highest fitness ("homozygote advantage"/"underdominance"). This is the opposite of the effect which for other loci causes inbreeding depression.
- Disruption of beneficial, synergistic interactions between loci ("coadapted gene complexes"). This is a more important cause of outbreeding depression than homozygote advantage. This effect is partly insidious, since it will likely increase in strength over future generations, as new genetic recombinations in each new generation break increasing numbers of such beneficial interactions.
- The two parental populations may be adapted to different environments. The offspring risks not being well-adapted to either environment.
- Reproduction between distantly related individuals may increase the risk of immunological incompatibility between mother and offspring, which may cause stillbirth and other problems. This has been suggested as one explanation for why a study 2008 study in Iceland found that third cousins have the highest number of children and grandchildren.
The interactions between inbreeding depression, outbreeding depression, and the local environment may create complicated effects that differ for different populations. Thus, a suggested explanation for a high prevalence of cousin marriages in areas with a high prevalence of malaria is that this increases average genetic protection against malaria and that this benefit is so great that it outweighs negative genetic effects.
The interactions may also cause changing effects of over time. For example, farmers may use hybrid plants (seeds) created from two purebred plants strains, which may cause overall positive effects during the first generation ("hybrid vigor"), due to reduced inbreeding depression, but may avoid using plants (seeds) created from such hybrids, due to the above mentioned insidious negative effects from outbreeding depression becoming increasingly important over further generations.
Avoiding outbreeding depression
The risk of outbreeding depression roughly increases with increased distance (genetic or geographic) between populations.
Species in risk of extinction may also be threatened by inbreeding depression. Outbreeding between different populations within such species is often suggested in order to avoid this. However, this instead risks causing outbreeding depression. A 2011 study recommended (based on both theoretical analysis of population genetics as well as empirical evidence) avoiding or be very cautious regarding crosses between two populations with characteristics such as being isolated from one another for more than 500 years or living in different environments.
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