This is a thought experiment:

If we form a population with only a single founder pair, can this population survive? What would happen? Would this inbreeding cause the population to go extinct? Could such a population continue to exist indefinitely?

  • $\begingroup$ There is only a probabilistic answer to this question. However, if you exclude external factors such as environmental stochasticity, predation pressure etc., and only care about the genetic component, there should be formulae to estimate this probability. You would still have to make some assumptions, e.g. the degree of heterozygosity of the initial pair... $\endgroup$ – Hav0k Feb 19 '16 at 13:52
  • $\begingroup$ If we're talking about lab mice, this is routinely done, since pure strains of lab mice are all genetically identical (barring the low level of spontaneous mutation). If we're speaking more generally, inbreeding is generally bad and may well lead to extinction, but it's not inevitable; the presence of introduced species on e.g. islands originally populated with only a handful of individuals shows that it can be overcome. $\endgroup$ – iayork Feb 19 '16 at 16:58

Populations founded by a single pair are quite common in the lab

The Drosophila Genetic Reference Panel, one such example of high artificial inbreeding, is a series of inbred lines where the researchers mated brother and sister flies for 20 generations to form the lines, which means they will be highly inbred. When you do these though you often lose many of the initial lines because of inbreeding depression. Inbreeding depression is reduced fitness as a result of increased homozygosity in the lines - deleterious mutations are more frequently expressed. I have tried to find the number of initial pairs they used to make the ~200 DGRP lines but come up unsuccessful, but I've heard it was around 1500 pairs, although that may be well off the mark*.

What would happen to your mice?

Just like the DGRP lines, there is a high risk that the population would collapse. However, if they do get through a few generations of inbreeding, and are kept in steady conditions (i.e. selection doesn't change once the lines are formed), then they have an ok chance of persisting. If you started with multiple founder pairs then there's a better chance you will still have some (very inbred) mice in a few generations. The success rate will depend on the frequency of deleterious mutations and strength of their effects.

What is inbreeding?

Inbreeding is increased homozygosity in the population than would be expected under random mating. For example, if the frequency of two alleles ($A_1$ and $A_2$) at a gene are $p = 0.1$ and $q = 0.9$ respectively, the expected frequency of $A_1A_1$ homozygotes is

$p^2 = 0.1^2 = 0.01$

the expected frequency of $A_2A_2$ homozygotes is

$q^2 = 0.9^2 = 0.81$

and the expected frequency of heterozygotes is

$A_1A_2$ is $2pq = 2 \times 0.1 \times 0.9 = 0.18$

If the true frequency of heterozygotes ($A_1A_2$) is less than 0.18, then there is inbreeding.

Is there a benefit to inbreeding?

Well inbreeding can be a good thing. Imagine all of those lines that did not collapse. It is possible that a population formed out of those would have higher fitness than the original population, because deleterious mutations may have been purged (lines carrying deleterious mutations are more likely to collapse). So a highly inbred population can prosper. However, that population would also have lower genetic variance. Genetic variance places limits on the potential for adaptation, low genetic variance means low potential to respond to selection. So, if selection is not constant, then inbred populations are more likely to go extinct as they are unable to adapt to novel scenarios.

Inbreeding in nature

One classical example of high inbreeding in nature is in cheetahs, where it is beleived they went through a strong genetic bottleneck ~10,000 years ago. The problems faced by cheetahs today really highlights the problems of high inbreeding because they are struggling to adapt. Genetic bottlenecking is a serious long-term problem for many species including the Javan rhino, the Kakapo, Mountain Gorillas, and others; even if we can stop poaching and habitat loss wiping out these species in the short-term, they will struggle to adapt to changes in the climate etc..

"As a species, cheetahs have famously low levels of genetic variation. This can probably be attributed to a population bottleneck they experienced around 10,000 years ago, barely avoiding extinction at the end of the last ice age. However, the situation has worsened in modern times. Habitat encroachment and poaching have further reduce cheetah numbers, consequently snuffing out even more genetic variation and leaving cheetahs even more vulnerable to extinction."

* Supporting material from the DGRP paper would suggest that it was indeed formed from 1500 isofemale lines.


There have been several studies with similar structure and background as your experiment, such as "Inbreeding Effects on Reproductive Outcome: A Study Based on a Large Sample from the Endogamous Vadde of Kolleru Lake, Andhra Pradesh, India" from 1992.

It was an experiment to investigate this hypothesis:

Sanghvi's hypothesis on long term effects of inbreeding was tested in Kotas. Kota is a numerically small tribal population in the Nilgiri district, Tamil Nadu State, India. Consanguineous marriages are common in this tribe. A total of 95 couples were taken for this study and necessary data were collected on a set proforma. Of the 95 couples, 28 (29.5%) were consanguineously related. The inbreeding coefficient for autosomal genes is 0.022 and for sex-linked genes is 0.03. Inbreeding effects on reproductive losses were examined through an exponential regression model. Although the regression coefficient B values are positive, they are insignificant, suggesting no consistent relationship between degree of consanguinity and the reproductive losses. The estimates of genetic load is 1.8 lethal equivalents per gamete and the average B/A ratio is 5. These findings empirically support the Sanghvi's contention.

The abstract of the experiment from 1992:

Inbreeding effects on reproductive outcome vis-à-vis fertility, prenatal loss and prereproductive mortality, and secondary sex ratio of live and dead children were examined in a large sample of 2078 women of the Vadde fishing population of Kolleru Lake in Andhra Pradesh, India. Demographically, this population is a single endogamous unit. By using an exponential regression model with the proportion of offspring survival as a dependent variable and the inbreeding coefficient as an independent variable, I further examined the inbreeding effects. The results were compared with results from other fishing groups and other southern Indian and non-Indian populations. The results among the Vadde were consistent with those found for other groups of Telugu-speaking fishermen and several other southern Indian populations in that the effects were neither perceptible nor significant. The average B value and the number of lethal equivalents found for the highly inbred southern Indian populations in general and for the Vadde in particular were much smaller than those from other parts of the world, providing empirical support to Sanghvi's hypothesis on long-term effects of inbreeding.

The hypothesis was also supported by another investigation, looking at The effect of inbreeding on mortality and morbidity among Telugu-speaking populations of Kharagpur, West Bengal, India.

Increased mortality and morbidity including congenital malformations among the offspring of consanguineous marriages have been widely reported in human populations from different parts of the world. However, there are few studies on the effect of the intensity of inbreeding and different degrees of inbreeding on mortality and morbidity. The present study is an attempt to examine the effects of inbreeding on mortality and morbidity including congenital disorders in different levels of inbreeding among Telugu-speaking populations of Kharagpur, West Bengal, India, based on data collected through extensive pedigrees. The study reveals that the frequency of spontaneous abortions and stillbirths is higher in the offspring of consanguineous marriages than in that of non-consanguineous marriages. A similar effect is also observed in the infant mortality rate, which is known to have a genetic component, but is not seen in the mortality rate of children and juveniles. The rate of morbidity is consistently higher in the offspring of consanguineous marriages with a sex bias in favour of inbred females. The increased morbidity rates in inbred individuals tend to be inversely correlated with the increase in average autosomal inbreeding coefficient. This appears to strengthen Sanghvi's hypothesis of a decline in the frequency of deleterious genes with intensification of inbreeding through generations. The present study also confirms an increase in genetic disorders with an increase in inbreeding in almost all populations.

Sources & Further Reading:

  • 2
    $\begingroup$ It's generally best to write up an answer in your own words. By quickly scanning the pasted excerpts, I can't find an answer to the question. $\endgroup$ – AliceD Feb 19 '16 at 14:07
  • $\begingroup$ What is "Sanghvi's hypothesis on long term effects of inbreeding"? This post doesn't answer the question at all well and simply pasting text from a paper is generally not a good approach $\endgroup$ – rg255 Feb 20 '16 at 9:17

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