Inbreeding vs Mutations

Question

Small populations of organisms may be exposed to the dangers of inbreeding. But if a small population manages to survive for many thousands or millions of years, would mutations eventually diversify the gene pool, rescuing the species from adverse genetic impacts?

For example, if the cheetah - a classic example of a genetic "bottleneck" - survived for a million years, would we expect the cheetahs of the future to be more diverse?

If you know of any studies that expand on this, I'd be interested in knowing about them. For example, what is the minimum number of breeding age individuals of a mammal species that would have to wash up on an isolated island to survive, and approximately how long would it be before the population is free of problems related to a lack of genetic diversity?

Answer 1

There really is not a definite answer to your question, because there are a lot of factors that can affect the outcomes. Here is a very simplified but extreme example: Suppose there are just two individuals on an island. Adam's genome comprises gene pairs, one of each pair from each parent. Let's say that every one of Adam's gene pairs consists of two different alleles of the corresponding gene. The genes are packaged in 23 chromosomes with hundreds to thousands of genes per chromosome. That means that Adam can contribute any one of 2^23 different combinations of chromosomes to his offspring. Same is true of Eve, so there are 2^46 different combinations of chromosomes available to the offspring of Adam and Eve. Allow just one crossover per chromosome, and the number of possible genetic combinations goes up to 2^69. Those are gigantic numbers.

Now imagine instead that Bernie and Francine have genes are nearly identical. Now instead of 2^69 combinations, there might be only 2^4 or so possible unique combinations. Clearly Adam and Eve have vastly more potential to spawn a viable, genetically diverse and healthy population that Bernie and Francine do.

In reality, any two individuals in a species will be much more genetically similar than the hypothetical Adam and Eve above. The number of individuals needed to initiate a genetically healthy population will be the number needed to provide all the different alleles needed for a genetically healthy population; and the reproductive dynamics in the population will need to be such that all the needed alleles remain in the population.

In order to obtain a really reliable calculation of the number of individuals needed, it would be necessary to know the genetic profiles of the individuals. And, it would be necessary to know (and this is NOT known) exactly which alleles are needed within the population in order to ensure a healthy population. We only know this for a very small subset of the genes in any species.

You also asked if a small population can be expected to become more genetically diverse over time. Unless the population is large enough to contain all the important alleles, and unless the reproductive dynamics is such that the genetic mix is kept really stirred up, the population is most likely to become less diverse over time. If the population survives, it will be because it has chanced upon a relatively small selection of alleles that produce phenotypes that are adapted to the environment. However, any change in environment is likely to drive the population to extinction because its genetic adaptability will be very low (due to the small number of different allele combinations available to each offspring).

Of course, mutations can add diversity to the genetics of a population. However, the vast majority of mutations that aren't deleterious are are neutral. Crossover and sexual recombination provide genetic variation that has much, much more likelihood of producing adaptive genetic change than mutation - because crossover and sexual recombination shuffle viable alleles rather than inflicting damage that on very rare occasions turns out to be a useful change.

Answer 2

There are two major factors that will affect the development of diversity, given the same starting point: The mutation frequency, and the strength of selection.

Mutation frequency is unlikely to change significantly, so you're left with the strength of selection as the deciding factor for how much diversity arises and how rapidly.

"Selection" means that less-fit individuals are removed from the population, so if the selection is strong enough, you're left with the race between evolution and extinction. With a small starting population and strong selection, the most likely scenario is extinction, but the right level of selection can lead to diversity.

An oversimplified view of selection is that a population will evolve toward some Platonic ideal. If long necks give more fitness than short necks, then the population will evolve toward having longer necks. The population changes, but isn't necessarily more diverse.

But there are several modes of evolution that drive a population toward increased diversity. In theory, you could imagine that the population does better with a 50/50 mix of long and short necks than either an all-short or an all-long necked population. Examples of drives toward population diversity include positive and negative frequency-dependent selection (also known as "balancing selection"), and overdominance.

Are there any examples of bottlenecked populations where one of these drivers has led to increased diversity? Yes, there are several. The best example might be the Channel Island fox:

The San Nicolas Island fox (Urocyon littoralis dickeyi) is genetically the most monomorphic sexually reproducing animal population yet reported and has no variation in hypervariable genetic markers. ... foxes colonized the three northern Channel Islands (San Miguel, Santa Rosa, and Santa Cruz) ≈16,000 years ago and, subsequently, were transported by Native Americans to the three southern Channel Islands (San Nicolas, Santa Catalina, and San Clemente) 800 to 4,300 years ago ... We examine variation of five loci within the MHC of San Nicolas Island foxes and find remarkably high levels of variation. Further, we show by simulation that genetic monomorphism at neutral loci and high MHC variation could arise only through an extreme population bottleneck of <10 individuals, ≈10–20 generations ago, accompanied by unprecedented selection coefficients of >0.5 on MHC loci. These results support the importance of balancing selection as a mechanism to maintain variation in natural populations and expose the difficulty of using neutral markers as surrogates for variation in fitness-related loci.

--High MHC diversity maintained by balancing selection in an otherwise genetically monomorphic mammal

The specific question "how long would it be before the population is free of problems related to a lack of genetic diversity?" is too broad to answer with any detail, but another point to keep in mind is that the problems of inbreeding are usually less to do with loss of diversity (although that can be one problem, as the San Nicolas foxes show) and more to do with deleterious recessives emerging as homozygotes; this can be softened by various factors, including chronic low-level inbreeding before the bottleneck occurs, which "purges" out the deleterious recessives and makes the population more resistant to inbreeding. One review: Do Plant Populations Purge Their Genetic Load? Effects of Population Size and Mating History on Inbreeding Depression

Answer 3

It would be worthwhile to read http://medicine.jrank.org/pages/2498/Meiosis-Sources-Genetic-Diversity.html. "A total of 2^23 (8.4 million) possible combinations of parental chromosomes can be produced by one person, and recombination further increases this to an almost unlimited number of genetically different gametes."

A good place to learn about how meiosis works is http://labs.russell.wisc.edu/peery/files/2011/12/Primer-in-Population-Genetics.pdf

Yes, mutation contributes to genetic diversity. But the main source of genetic variation for a small isolated population is the set of alleles available to the offspring of the initial population; and it is the multiplicity of chromosomes (plus crossover) that enables the shuffling of those alleles in producing those offspring.

For a definition of genetic diversity, see: UN (1992) Environment and Development (Terminology bulletin: 344). United Nations, New York, USA: "The variation in the amount of genetic information within and among individuals of a population, a species, an assemblage, or a community."

I should probably add that for the past 30 years I've been working with genetic algorithms, which are a software implementation of Darwinian evolution used for solving practical problems like stock portfolio optimization, machine design, and even for modeling biological evolution. The equivalent of chromosomes and crossover are, in most cases, unquestionably the key components that provide the genetic diversity necessary for finding "pretty good" solutions; while mutation is key to fine-tuning those "pretty good" solutions.

In biological evolution, chromosomes and crossover provide ways to make new combinations of alleles that are already available in the population; and mutation provides ways to create new alleles. Creation of new, useful alleles is a slow and uncertain process. Genetic recombination in meiosis involves the pairing of homologous chromosomes; crossover involves switching of blocks of genes between homologous chromosomes. In the short term (tens to hundreds of generations) those processes have vastly more adaptive potential than mutation. In the long term, other kinds of genetic rearrangement - and mutation - are necessary for big changes like creating new species.

The problem with the “standard” definition of genetic diversity is that, by that definition, it's possible for a large population to have genetic diversity of 1 and still contain only a handful of distinct genotypes. An environmental change could be a disaster, because the next generation would only have that handful of genotypes to select from regardless of the size of the population. But if the same set of alleles were randomly allocated throughout the population, every individual in the population could have a different genotype. In other words, genetic diversity according to the standard definition is not a reliable measure of the population's genetic resilience.

A better measure of the ability of a population to genetically adapt to changing selective pressures would be something that gauges how evenly the genotypes in the population are spread out over the space of possible genotypes. The maximally different hypothetical Adam and Eve would not constitute a genetically resilient population because they would represent only two genotypes. Not very many generations later, however, due to chromosomal recombination and crossover alone, without need for mutation, their descendants could comprise an extremely resilient population containing hundreds of thousands of distinct genotypes.