The stochastic nature of genetic drift
(rules of chance – text chapter 7)
1. Neutral alleles will inevitably go extinct or become fixed over the short time (i.e., if mutation is not considered; see collared lizard example).
2. Some rare neutral alleles will inevitably drift to high frequency or fixation (law of large numbers; e.g., PV92 + allele in north and eastern Asian human populations).
3. Populations with a history of isolation and genetic bottlenecks will become genetically distinct. Loa et al. (2008. Current Biology 18, 1241–1248) present evidence for the genetic legacy of the Mormon founder event. Although the Utah Mormon population today stands at about 10,000,000, the original size by 1890 after the last immigrations stood at about 100,000people. The effective population size (Ne) is the harmonic mean, 1/Ne = 1/t *(1/N1 + 1/N2 + --- + 1/Nt), or 1/2(1/100,000 + 1/10,000,000) = 1/2(0.00001 +0.0000001) = 0.00000505, Ne = 1/0.00000505 ≈ 200,000. Assume that on average 1 man has had perhaps two wives during early Mormon history. An Ne of 200,000 would put breeding males at the equivalent of about 70,000 and breeding females at about 130,000. This would render an effective population size of Ne= 4(Nf*Nm)/(Nf+Nm) (page 245), which is derived from 1/Ne = 1/4[(1/Nf) +(1/Nm)], where Nf = number of breeding females, and Nm = number of breeding males. For Mormons, Ne = 4(130,000*70,000)/200,000 = 182,000. Such population bottlenecking has resulted in the genetic homogeneity of Caucasian Utah Mormons, the pinpointing of their European source area, and explain their genetic disease prevalence (e.g., hereditary nephritis or fumarase deficiency occurring much higher frequency in Mormon dominated Utah populations than in other populations in Utah and elsewhere).
The Pinglapese (page 238) are derived from 20 survivors of a 1775 typhoon and today are 3,000 in number. 1/Ne = [1/2(1/20 + 1/3,000)] = 1/2(0.05 +0.00033) = 0.0252, Ne = 1/0.0252 ≈ 40. On average, 1 man has had twenty wives during Pinglapese history, or 2 males for 38 females. Thus, Ne = 4(38*2)/40 = 8. This explains the genetic distinction of Pinglapese and their genetic diseases (e.g., achromatopsia is 1,000 times higher in Pinglapese than in other human populations).
Eugenics and inbreeding. Deleterious recessive alleles are neutral when they originate and persist in low frequency (e.g., typically up to around 1%). This means healthy outbreeding populations will accumulate disease-causing alleles. Positive eugenics efforts(selective breeding for a favored race) will enhance the homozygous expression of deleterious alleles. Negative eugenics efforts (selective breeding against diseased individuals) will have no effect on the frequency of deleterious recessives, which are harbored mostly in the undetectable heterozygous condition. Inbreeding, exemplified in chapter 7 with the greater prairie chicken example, is a problem for humans and certain of their domesticate (e.g., dogs) and managed (e.g., game) species. Inbreeding is not a problem for crop and livestock species or from an evolutionary perspective or in any instance where diseased individuals are readily culled without ethical consideration. Such culling leaves populations of homozygous individuals that have been purged of deleterious recessive alleles.
Chapter 7: answers to questions most
relevant to genetic neutrality and drift.
1.Corridors
maintain genetic diversity in otherwise isolated populations(isolated
population are thus not so prone to drift).
2.Humans
have female-biased migration (gene flow), on average.
4.Use
lizards from different glades (of course).
5.Human
populations descending from ancestral populations that have experienced a
genetic bottleneck due to isolation, migration, skewed sex ratios of breeding
adults, or population crashes, will inevitably have a unique genetic disease,
if not several of them. The reason genetic disease is inevitable in these
situations is that neutral to slightly deleterious mutations are the most
abundant kinds of mutation. The reason genetic diseases are unique to a
specific population is because mutation and drift cannot possibly play out in
the same manner among genetically isolated populations.
6.Remote
oceanic islands can only receive populations via founder events. This ensures that
they will harbor genetically distinctive lineages.
7.Pseudogenes
are non-functional, so all substitutions at such loci are strictly neutral.
8.A
genes that has more non-synonymous than synonymous substitutions among closely
related species (or distantly related species) can only be explained by natural
selection. This observation is not an expectation of neutrality.
10.The
finding of specific genes that, when compared among related species, have a
dN/dS ratio of greater than 1 can only be explained by positive selection.
12. Inbreeding is the only known mechanism that can
purge a population of deleterious recessive alleles. Culling diseased
individuals during the breeding of domesticated animal and plant lines is not
an issue, ethical or otherwise. Inbreeding has very important consequences,
however, in the fields of human medicine and management of fish and game
species, and generally charismatic species.