RISKS OF RARITY: GENETIC ISSUES -- 2
Corresponding Readings in Primack, Richard B. Essentials of Conservation Biology.
The practical applications of genetics to conservation mainly focus on management goals to keep populations genetically healthy -- avoiding the genetic costs of in-breeding, and maintaining enough genetic variation within a population to enable it to adapt to changing environments. Safeguarding the future evolutionary potential of species depends upon conserving their genetic diversity.
Measurement of Genetic Variation:
H = heterozygosity = the percent of genotypes in a population that are heterozygous for a particular gene location. In a two allele system this is simply 2pq X 100.
P = allelic diversity = percent of gene locations at which alternative alleles exist in the population. So if you surveyed 100 different gene locations, and 40 were homozygous while 60 had some heterozygotes present, P = 60%.
Thus, H looks at a particular gene location. P looks across all gene locations that can be surveyed. Both are useful.
Genetic Variation: Maintenance? Or Loss?
The Hardy-Weinberg Equilibrium principle states that, given certain assumptions, gene frequencies stay constant from generation to generation. In other words, genetic variation should not decline.
But in small populations, the assumptions of the HW Eqm are not met.
Genetic Drift: In a small population, say 100 or fewer breeding individuals, chance has much opportunity to affect the gene frequency of the subsequent generation. By chance alone (i.e., completely independent of any qualities of AA vs. aa individuals), the aa's may have a good year or a bad year, causing gene frequencies in small populations to fluctuate. And once A or a = 1 and a or A = 0, the genetic variation has been qualitatively reduced. Unless mutation or dispersal reintroduces that gene, it simply is gone. The process is called DRIFT, it results in high homozygosity and low genetic variability, and it is an important force in small populations.
Effective Population Size (Ne): This is a theoretical concept, but it also makes a lot of sense. In general Ne should be << N, because some individuals are non-reproductive juveniles, some are infertile, and if behavioral dominance exists, only a few males may perform most of the matings (e.g., elephant seal). Estimates of Ne vary depending upon the species, but value of 0.5 or less are not unreasonable. Ne is the fraction of the population actually contributing gametes to the next generation -- from a genetic viewpoint, the effective population size. You may also visualize it as the "N" of an ideal population in which all individuals are sexually mature, they mate at random, and some other assumptions are met
Estimating loss of Heterozygosity: Take on faith that loss of heterozygosity over time can be estimated with the following equation.
Ht = Ho (1 - 1 ) t
To retain 90% of initial genetic variation after 100 years, Ne must equal 500, and N must equal 1,000 to 2,000 individuals.
Interestingly, body size is again important, because of its effect on generation length. Since loss of genetic variation is associated with episodes of genetic recombination, slow-reproducing species are slower to lose heterozygosity than are fast-reproducing species.
Captive populations of large vertebrates often descend from a handful of individuals: for the Siberian tiger, 25; for the Mongolian wild horse, 13; an African gazelle began with one male and three females. Such populations face a high risk of inbreeding, which means mating with a close relative -- parent-offspring, siblings, cousins etc. Most species rarely inbreed in nature, because of larger population sizes and various behavioral factors, hence this consequence of rarity and captivity is unusual.
Comparison of inbred vs. non-inbred captive mammal populations from zoos shows that reproduction and survival are reduced by inbreeding. This effect, termed inbreeding depression occurs because inbreeding results in increased homozygosity, which in turn increases the likelihood that deleterious recessive genes will be expressed. Normally, harmful, recessive alleles are so rare that they almost never occur in the homozygous state.
Inbreeding increases the probability increases that two alleles present at a location are identical due to common descent. If a son mates with his mother, not too unlikely in a small population, the offspring might be "aa" for some trait. Furthermore, the offspring may well receive both those "a's" from the female who is simultaneously the mother and grandmother. In small populations, all individuals quickly become related, and as a consequence are more homozygous than non-inbred individuals and have lower levels of genetic diversity.
Although drift and inbreeding both result in increased homozygosity, the mechanism and the consequences are different.
Drift increases homozygosity by random loss of alleles, and harms the populations long-term capacity to adapt. Rare, harmful, recessive genes are most likely to be lost, because they already are so rare.
Inbreeding increases homozygosity by descent (individuals receive the identical gene through each parent), and facilitates the appearance of harmful, recessive alleles in the homozygous state. Individuals are less healthy as a consequence, and the effect is shorter-term.
By a similar equation, it can be calculated that an Ne of 50 is large enough to keep the in-breeding coefficient at 1% per generation. Animal husbandry studies show this inbreeding rate to be as high as can be maintained in artificial selection.
You might think it unlikely that our captive population will contain even one "Aa" individual. True enough, for that gene location. But there are 104 - 106 gene locations, and many lethal and deleterious genes are found in normal healthy populations, such as the NRE 220 class.
Sometimes, during captive breeding, in-breeding will be "forced" in order to "purge" the population of harmful alleles.
The 50/500 Rule:
Managers like hard goals "save them all" and "save all you can" are dissatisfying. Hence the appeal of the 50/500 rule:
"Managers should maintain an Ne of > 50 to ensure short-term survival and minimize the risks of inbreeding. "Managers should maintain an Ne of > 500 to ensure long-term survival thropugh the ability to adapt to environmental change."
And, N = 2 to 4 times Ne
Today, most conservation biologists feel that demographic risks outweigh genetic risks, and these hard numbers give a false sense of certainty. They live on in statements such as, "minimum population sizes should exceed 2,000 individuals".