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Figure 1. The evolution of relative brain size in groups of vertebrates over the past 200 million years (adapted and updated from Jerison, 1976, p. 96; Jerison, 1991, fig. 17). This plot seems to show an evolutionary trend towards increasing relative brain size (=E.Q. = Encephalization Quotient) and is probably the most definitive evidence for such a trend. Average living mammal E.Q. is defined as 1. The broken lines indicate gaps in the fossil record. Variation within groups is not shown. The lineage that led to humans is drawn thicker than the other lineages.

the trend seen in our lineage to be a general trend representative of any other lineages. If you choose a feature because of its current extreme nature, it is no surprise that it had to get that way and that its evolution will display a trend. But this trend has no claims to being representative of life in general.

Another example may make selection bias more obvious. Elephants have longer noses than their living relatives. Thus, when we focus on this unique feature and plot the sizes of the noses of its living relatives and of their evolutionary ancestors (Fig. 2), we find that in the series of progressively earlier ancestors, noses get progressively shorter. This is a selection effect that has nothing to do with a general tendency that can be extrapolated to the rest of biology. Increasing nose size is not a general feature of evolution. It is something that occurred in the lineage that led to elephants. After diverging from this lineage the N.Q. (nasalization quotient) of most groups stayed constant.

Another example: Humans have big brains with extremely small olfactory lobes. A similar analysis of the evolution of our olfactory lobe size would lead to the conclusion that the shrinkage of olfactory lobes is a generic trend. Such a conclusion would be misguided. By selecting an extreme outlying feature of an extant organism (whether big or small, short or long, hard or soft - anything as long as it is an outlier) the evolution of that feature in that organism's ancestors is very likely to show a monotonic trend that leads up to the outlier (Fig. 3).

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