Inbreeding over time

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A number of people have commented on a recent paper showing an increase in heterozygosity in human populations over time, presumably due to increased outbreeding (though Dienekes suggests some of this effect may be due to more homozygous individuals living longer, my feeling is that the results associating homozygosity and lifespan are more likely to be artifacts due to increased outbreeding over time, rather than vice versa).

This is an interesting result, and seems plausible, but the figure in the paper is difficult to judge–I wondered why the authors chose not to show their actual data, but rather only the fitted regression line. The answer is that the data itself looks much less impressive than the pretty lines in the main text (see right).

This isn’t to say that the result isn’t correct (I assume the authors made sure their results are robust to the few outliers in that plot), but the relationship between homozygosity and time is certainly more noisy than implied by the figure.

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6 Comments

  1. I wonder why the referees let them get off with not showing the data. In large part the science is the data; the tales we spin about data come and go.

  2. Yuck… that’ll teach me to not check out the supplementary data before posting on an article.  
     
    Like you say, the result seems plausible (in fact I’d be shocked if there wasn’t SOME decrease in autozygosity over time), but it’s a good example of highly selective reporting of data. I’m a little surprised it got past the referees.

  3. Actually, considering the spreads one normally gets in population studies, the data are compact about the regression line. Of course the trend is very small, and the r2s pathetic, but the p values are good. 
     
    Outbreeding is a very recent phenomena everywhere in the Old World and South America, but it is very common in North America. This may be yet another instance of American researchers confusing the peculiarities of American populations with the world. America is the great outlier.

  4. Actually, considering the spreads one normally gets in population studies, the data are compact about the regression line. Of course the trend is very small, and the r2s pathetic, but the p values are good. 
     
    sure, i’m not really disputing the result, though it’s tough to really see where the density lies in plots like this. just pointing out that the figure in the main text is quite a bit more visually striking than the actual data.

  5. gc’s rule 
     
    a paper that reports a pvalue or regression line without an accompanying scatterplot, boxplot, or contingency table is probably not a real result

  6. “Outbreeding is a very recent phenomena everywhere in the Old World and South America” 
     
    It depends what you mean by ‘very recent’. In Western Europe, populations were mixed up by urbanisation in the 19th century. In Australia and New Zealand, the white population was formed by immigration from all over the British Isles. In these areas, the breakdown of traditional local inbreeding would have occurred more than a century ago. Of course, there would have been various departures from strict panmixia, but the same is true today.

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