Estimated fitnesses of *TM2/+* and *TM1/+*, relative to *TM1/TM2*, for those cases where *TM2* invaded successfully. These estimates are made assuming constant fitnesses. In each part, the top heavy curve shows the threshold for elimination of *TM1*, and the bottom heavy curve shows the threshold for invasion of *TM2*; thus, wild-type chromosomes with fitnesses between the two curves can remain polymorphic with both *TM1* and *TM2* present. Pairs of replicates are linked by lines. Top left, the 15 chromosomes for which +/+ was lethal. In six cases, the estimated *TM1*/*TM2* fitness was extremely low. These are shown by pairs of circles superimposed at top right. Top right, the 4 chromosomes for which +/+ was viable. Bottom left, the 12 chromosomes from . (Note the narrower range of fitnesses.) The pair of shaded circles shows line 52, in which *TM2* invaded but did not displace TM1. Bottom right, comparison between all three classes. For clarity, only the geometric means of replicate estimates are shown, so that pairs of points are replaced by a single point midway between. Solid circles, +/+ lethal; dark-shaded circles, from ; light-shaded circles, +/+ viable. The dashed contours show the initial rate of increase of *TM2* per generation (steeper curves) and the final rate of elimination of *TM1* (shallower curves); contours are for increase by 10^{0.2}, 10^{0.4}, … 10 per generation.

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