A History of Head Bobs

If you were to cast lizards as characters in a remake of The Great Gatsby, anoles, of course, would have to be cast as protagonist Jay Gatsby. “~~What~~ Why Gatsby?” you might ask. Well, like Gatsby, anoles have gaudy, aggressive displays that show fitness and define territory. Also like Gatsby, (SPOILER ALERT) anoles cannot seem to escape their past (for the headbob part of their display, at least).

In a recently published paper, Terry Ord investigated whether past ancestry explains variation in Anolis diplays better than evolution by natural selection. To begin to ask this question, Terry compiled data on the average time of headbob and dewlap bouts for species for a Western Caribbean Anolis lineage (Jamaica, Cayman Islands, Cuba) and for an Eastern Caribbean lineage (Puerto Rico and Hispaniola). The data and phylogenetic relationships are shown in Figure 1.

Fig. 1 Variation in headbob and dewlap duration contrasted with phylogenetic relationships among taxa. Error bars are 95% confidence intervals and indicate taxa for which data were compiled from display videos. Data for taxa lacking error bars were compiled from the literature (‘nd’ refers to no data). The tree is based on the mitochondrial phylogeny developed by Nicholson et al. (2005) and reproduced by Losos (2009) with branch lengths scaled proportional to time. Divergence among replicate populations of the same species was not represented in this species tree and was set here using the minimum level of population divergence reported among Jamaican species by Jackman et al. (2002). – Caption was quoted directly from the paper. Figure and caption Copyright 2012 The Author and European Society for Evolutionary Biology.

Then, Terry generated three hypotheses for how Anolis displays may have been influenced by selection or history.
(1) Contingency. Under this hypothesis, evolutionary lineages that radiate from a common ancestor should share many characteristics of their display because of their shared evolutionary history. This predicts that Anolis species from Puerto Rico and Hispaniola, which both belong to the Eastern Caribbean radiation, should be more similar to each other in display behavior than they are to lizards from Jamaica, Cuba, and the Cayman Islands in the Western radiation.
(2) Selection Regime. Under this hypothesis, species found on different islands will have experienced unique selection regimes that have led to independent evolutionary differences in display patterns such that closely related lineages will not necessarily share the same displays.
(3) Random Change. This is the null model describing stochastic divergence in display behavior where variance in display behavior would be randomly distributed among species and lineages.

Terry used phylogenetic comparative methods to test these different models against one another. He found that headbob duration was best explained by a model of historical contingency (hypothesis 1). That is, display behavior tended to be conserved within radiations and Western and Eastern Caribbean radiations had headbob display durations that were “highly divergent, nonrandom, and independent of island origin.” (Dewlap display was better explained by a random change model.)

What does Terry conclude about the evolution of Anolis displays? Well, this study shows that the remarkable divergence in headbob behavior at wide phylogenetic scales can be explained by historical contingency. Interestingly, and somewhat in contrast, previous work (Ord et al. 2010) showed that Anolis headbob displays are often dependent on the environment and shaped by natural selection. Terry resolves this conflict by noting that the actions of natural selection seem to be restricted to smaller phylogenetic scales (i.e. influencing the differences between species within islands rather than broad patterns). In fact, Figure 1 shows quite a bit of variance between species within a lineage; these differences presumably arise from natural selection molding individual species displays to specific environments.

In conclusion, it appears that historical contingency acts at broad scales and can constrain the directions that evolution by natural selection takes within distantly related lineages.

T.J. Ord (2012). Historical contingency and behavioural divergence in territorial Anolis lizards Journal of Evolutionary Biology DOI: 10.1111/j.1420-9101.2012.02582.x

TJ Ord, JA Stamps, and JB Losos. (2010). Adaptation and plasticity of animal communication in fluctuating environments. Evolution DOI: 10.1111/j.1558-5646.2010.01056.x

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Yoel Stuart

I am interested in whether, how, and why ecology shapes evolution (and evolution shapes ecology) through time, with an emphasis on microevolutionary pattern and process, adaptation, and field experiments. I completed my Ph.D. on Anolis lizards in the Department of Organismic and Evolutionary Biology at Harvard University. I am currently a post-doctoral researcher at the University of Texas, Austin studying threespine stickleback. They're not anoles, but they're cool too.