Measures of central tendencyThe mode Most distributions of phenotypes lookroughly like those in Figure 20-3: a single mode is lo-cated near the middle of the distribution, with frequen-cies decreasing on either side. There are exceptions tothis pattern, however. Figure 20-18a shows the veryasymmetrical distribution of seed weights in the plantCrinum longifolium.Figure 20-18b shows a bimodal(two-mode) distributionof larval survival probabili-ties for different second-chromosome homozygotes inDrosophila willistoni.A bimodal distribution may indicate that the popu-lation being studied could be better considered a mix-ture of two populations, each with its own mode. In Fig-ure 20-18b, the left-hand mode probably represents asubpopulation of severe single-locus mutations that areextremely deleterious when homozygous but whose ef-fects are not felt in the heterozygous state in which theyusually exist in natural populations. The right-handmode is part of the distribution of “normal” viabilitymodiﬁers of small effect.
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