This essay is the third of a series authored by Dave Wisker, Graduate Student in Molecular Ecology at the University of Central Missouri.
This series of essays counters common Intelligent Design/Creationist arguments against the fixation of the fusion that produced human chromosome 2. To recap briefly, ID/C’s argue that the fusion would have resulted in a chromosome with two centromeres, which would then be torn apart during meiosis. In “The Dicentric Problem” I explained how the presence of two centromeres does not necessarily interfere with proper segregation. ID/C’s also claim that heterozygotes for the fusion suffer from greatly reduced fertility, preventing the fusion from ever becoming fixed. I explained how this is not true for centric fusions in “The Fertility Problem” . Using realistic values for human populations, “Fixation Within a Deme” showed that the fixation probability of the fusion within a local breeding population, or deme, may be between 4.5 and 10 percent. This final essay will look at the probability of fixation for the fusion in the species as a whole.
Lande (1979) looked at the problem of fixation for chromosomal rearrangements with heterozygote disadvantage. He constructed a model with populations subdivided into numerous, semi-isolated subpopulations, or demes, in which spontaneous chromosome rearrangements with heterozygote disadvantage occur. In addition, the model included random extinction and colonization of demes. Lande did not include mechanisms like meiotic drive (see “Fixation Within a Deme” for explanation) in his model, letting genetic drift (i.e., chance) be the primary force for fixation within a deme. He found that the biggest obstacle to overall fixation was establishment of the fusion within one deme. The overall fixation probability turned out to be the probability of fixation within that first deme:
Once established in a deme, a negatively heterotic [one with a selective disadvantage for heterozygotes–DW] gene arrangement can spread in homozygous form through a subdivided population by random local extinction and colonization. For this process, the fixation rate in a species composed of many semi-isolated demes is approximately equal to the rate of establishment of new gene arrangements in a single one of its demes.
In “Fixation Within a Deme”, I showed that meiotic drive plays a major role in influencing fixation of centric fusions in humans. In addition, the probable structure of early human populations is very similar to that used in Lande’s model. It is therefore reasonable to suggest the overall fixation probability for the human chromosome 2 fusion is at least the value of 4.5 -10% determined earlier.
Interestingly, there may be another factor that influenced the rise in frequency of human chromosome 2. Hedrick and Levin (1984) suggested a mechanism that greatly increases the probability of fixation for negatively heterotic chromosome rearrangements during migration and colonization of other demes. They pointed out that the process of colonization often involves small groups of individuals, so genetic drift in the form of the “founder effect” plays an important role. Furthermore, the individuals in these founder groups may be closely related. So, if a chromosome rearrangement becomes established in a deme, then any founder groups derived from it will already have the rearrangement at a very high frequency to start with. If the group establishes itself in an unoccupied area, fixation would be instantaneous for that deme. If the group joins another existing group, given that most demes are small, the newcomers may compose a significant percentage of the combined deme. Therefore, the initial frequency of the rearrangement could already be high enough to exceed the unstable equilibrium frequency (see “Fixation Within a Deme” for discussion of this) and thus drive the rearrangement to fixation there as well. Hedrick and Levin called this “kin-founding”. They note that kin-founding is common in many groups of organisms. Anthropological data confirms this in humans (Fix, 1978). Since Lande’s model does not incorporate mechanisms that influence fixation such as meiotic drive and kin-founding, there is reason to believe that the overall fixation probability for human chromosome 2 may be higher than the base calculation for the deme.
In conclusion, this series of essays effectively counters every ID/C claim about the probability of fixation of human chromosome 2, using actual data from human populations as well as population models based on plausible assumptions. It is up to the ID/C community to present data that specifically supports their contentions.
Many thanks to Art Hunt for discussions and publishing help, and the PT crew for encouragement. Many thanks also to the ID proponent ‘Ilion’ (wherever he may be) for getting me mad enough to actually put it all together.
Fix AG (1978). The role of kin-structured migration in genetic microdifferentiation. Ann. Hum. Genet. Lond. 41: 329-339.
Hedrick PW & DA Levin (1984). Kin-founding and the fixation of chromosomal variants. Amer. Nat. 124(6): 789-787
Lande R (1979). Effective deme sizes during long-term evolution estimated from rates of chromosomal rearrangement. Evolution 33(1): 234-251.