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The Evolution of Body Size

Lecture by Gavin Hanke, April 14, 1998

Rapporteur: Grant McIntyre

Cope's Rule states that body size increases during the evolutionary history of a lineage. This may be true from certain perspectives but is no longer generally accepted. The lecture examined several studies on the evolution of body size that shed light on the prevalence of exceptions to Cope's Rule and concluded with a discussion of reasons for the apparent support for the rule.

The focal paper (Pianka 1995) used Felsenstein's method of independent contrasts to evaluate the evolution of body size of varanid (monitor) lizards. This method requires a phylogeny which was developed in this case using immunological distances determine by microcomplement fixation. The phylogeny matched species distributions fairly well but was not rooted since no available outgroup cross-reacted immunologically with the ingroup. Within varanids there are examples of the evolution of both larger and smaller size. The main problem with the paper was that lack of an outgroup precluded determination of the polarity of size evolution. The inclusion of fossil and habitat data would have greatly improved the paper. Fossil data would have improved the phylogeny and perhaps provided an outgroup. Habitat data could have been used to investigate the role of ecology in the evolution of varanid body size.

The second example was a comparison of recent fossil and extant anole lizards by Pregill (1986). Extant anoles are smaller than recent fossils anoles, belying Cope's Rule. It was concluded that large forms in this lineage have either become smaller or extinct. It is possible that some modern lizards have evolved smaller bodies due to pressures from introduced predators and habitat destruction.

Two studies of mammoths have shown that body size can evolve very rapidly and can also oscillate. Mammoths isolated on Wrangel Island decreased >30% in size in only 5,000 years (Vartanyan et al. 1993). Carroll (1997) says that in a 35000 year period mammoth body size doubled and then returned to "normal". Both of these are counter-examples to Cope's Rule.

A final example involved three separate adaptive radiations within the Foraminifera (reviewed in Gould 1996). In each case larger species were found at the end of the radiation than at the beginning. Gould criticized this result on the basis that only largest and mean species sizes were included in the analysis; smallest species were ignored as was overall variation in size. Gould suggested that the mode was a better indicator of central tendency than the mean. Only one of the three radiations showed any trend for modal size increase. Size changes from parent species to offspring species were plotted and appeared normally distributed with a mean of zero further supporting the absence of a trend in body size. These criticisms suggest that the increase in size in the foraminiferan radiations is more apparent than real.

Gould (1996) presented a model called the Drunkard's Walk to explain the apparent increase in size within lineages. The model is based on the random walk of a scientist drunk along a wide sidewalk with the drunk beginning at the wall which bounds one side of the walk. The average path taken will tend to move away from the wall. If the paths represent lineages in a taxon, the wall is the lower size limit, and the gutter the upper bound, it appears that size increases within the taxon but what really increases is the variation in size. It is unclear that most taxa have definable lower size limits but otherwise the model is appealing and can be practiced at home.

In light of these examples Cope's Rule is not generally valid and apparent trends in body size evolution must be carefully examined to distinguish appearance from reality.


Discussion

Discussants: Yazdan Kievany and Rene Polziehn

Certain aspects of the Drunkard's Walk model are suspect. It is unclear why the basal taxon defines a lower size limit to a lineage and there may be disadvantages to being at or near the lower size limit for a taxon. The advantage of this model is that it demonstrates a need to carefully examine the size variation within a lineage including at least mean, mode, min, max, and a measure of distribution.

Varanids are a good system for examining body size evolution since the amount of variation within the genus is very large (for vertebrates). But the phylogeny used was inadequate and Pianka should have waited for the new improved phylogeny promised in his paper. Microcomplement fixation was seen as a poor method for determining phylogeny, especially in this case since it eliminated the possible outgroup. Microcomplements can also vary in their evolutionary rates among lineages so it is a poor choice of clock for calibrating a phylogeny. Some methods of making trees do not assume constant evolutionary rates among lineages and could be used to circumvent this problem.

The use of averages to determine the size of the ancestral taxa was unsatisfactory. However, no better method was suggested.

Are there differences in the evolutionary rates of specialized and generalized organisms? Does this apply to the varanid example? This is impossible to answer without defining specialized and generalized. This challenge was dropped.

Figure 3 from Pianka 1995 was interpreted as a plot of coefficient of variation versus standard deviation. This was useful to indicate when larger or faster than normal size changes had occurred within the varanids. It was not clear how standard deviation was calculated in this instance.

Pianka should have considered habitat. An examination of size/habitat relationships might have revealed some ecological correlates of size change. This could have been accomplished using Felsenstein's method.

The discussion concluded with a frustrated consensus that Pianka's paper finished with the same question it began with: Why is there so much size variation in Varanids?

References:

Carroll, R.L. 1997. Patterns and Processes of Vertebrate Evolution. Cambridge University Press. 448 p.

Gould, S.J. 1996. Full House. Harmony Books, New York. 244 p.

Pianka, E.R. 1995. Evolution of body Size: Varanid lizards as a model system. Am. Nat. 146:298-414. (Focal Paper)

Pregill, G. 1986. Body size of insular lizards: a pattern of Holocene dwarfism. Evolution 40(5): 997-1008.

Vartanyan, S.L., V.E. Garutt and A.V. Sher. 1993. Holocene dwarf mammoths from Wrangel Island in the Siberian Arctic. Nature, 362: 337-340.

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