Thursday, November 24, 2011

What William the Conqueror's Companions Teach Us about Effective Population Size


















  1. After reading this and "The Logic of Chance", I think it may be time to put Lynch on the reading list.

    I do have a bit of a quibble of with the example though... from what I recall of my population genetics, even if N = Ne, you'd still expect to see inter-individual variation in the number of offspring; each individual should produce off-spring following a Poisson distribution; that means that you'd expect occasional lineage extinction even when N = Ne. It also means you can get Ne > N, if you reduce inter-individual variance ( obligatory Wikipedia reference).

  2. Going by the article you may find this interesting though you will have to copy and paste it into your address bar.

  3. "The effective population size is always less than the actual population (Ne < N)"

    This is not true. You can have instances when Ne > N.

  4. Just as a follow up. As pointed out by Eric underdispersion in the Poisson model can lead to Ne > N. In addition, there are several structured population models that can also lead to Ne > N. The traditional one from Wright is when there are independent demes that are equally productive in offspring numbers (i.e., local population regulation). This leads to no among-group variance at the demic level and thus drift cannot act at this level with the basic result that Ne > N (usually, this is accompanied, however, by an increase in homozygosity). With interdemic selection (differences in reproductive output among demes) you get N > Ne.

    Other structured models that incorporate sex-biased dispersal can also produce Ne > N.

    Under the Poisson model of family size, any factor that can reduce the variance in relation to the mean can produce higher Ne than N. For example, if country imposed and could enforce a two-child-policy in family size this would elevate Ne > N.

  5. Re the illustration caption, the good Bishop Odo probably used his mace too, as he also appears to have unabashedly fathered illegitimate children, something else that clerics weren't supposed to do (and yet there is a long history of them doing).


    I'm a bit wary of arguments promoting the causal role of population size - particularly when comparing prokaryotes, say, with eukaryotes. I think the concept of an effective population size for a clonal, LGT-dominant organism such as a bacterium is fraught, and in any case I don't think junk scales with population size within these groupings.

    Direct mechanistic constraints and immediate neighbours seem a more pressing issue than the selection-drift balance imposed by membership of a wider 'population' of a particular size.

    A prokaryote has one or two origins of replication, an energetic constraint determined by its surface area, a reliance upon diffusional transfer of metabolites.

    A eukaryote has multiple origins, a much greater ratio of energetic membrane to cell volume, engulfment of food and active transport, and, crucially, meiosis.

    For a multicellular eukaryote there is the addition of a largely quiescent germline phase - for most of the life cycle, DNA replication time and cost is a constraint on somatic cells alone.

    These factors, to me, have far more bearing upon the relative impact of junk upon DNA replication than does population size.

  7. Thanks for mention of the Augustan Society lineage group, "Order of the Conqueror's Companions". Sadly, the link above no longer works. This is the result of this group merging with the "Order of the Conqueror".

    The new group is called the "Society of Descendants of the Conquest", and the correct web page is ... something the system apparently won't let me post. You should be able to Google it, and I'll try to set up forwarding to the link above.

    Membership in this group is open to all who can document descent from either the Conqueror or his known Companions. We welcome your inquiries.

    Bruce A. Metcalf
    SDC Administrator
    The Augustan Society