29 November 2006

Predators and natural selection in lizards

"Evolutionary biology is by its nature an historical science, but the combination of microevolutionary experimentation and macroevolutionary historical analysis can provide a rich understanding about the genesis of biological diversity."
Losos et al.1

Anolis sagrei is a common Caribbean lizard that is often found on the ground in the absence of terrestrial predators, but begins to spend more time on trees when a larger and entirely terrestrial predatory lizard, Leiocephalus carinatus, invades its territory.

A recent paper by Losos et al.1 demonstrates how natural selection driven by predators can influence prey phenotype over a short period of only a year, or within a single generation.

The authors introduced L. carinatus to six small Bahamian islands that naturally contained A. sagrei and as controls, chose six other islands that had A. sagrei without the predator. They predicted that: (1) initially when A. sagrei spent most of its time on the ground, individuals with relatively longer legs, being faster, would be better able to elude the predators; (2) gradually, in the presence of the predator A. sagrei would become more arboreal; (3) as A. sagrei became more arboreal, those with shorter limbs, being better able to move on tree trunks, would be selected.

The results, only after a year, confirmed their predictions. For example, as shown in the figure below, the lizards on the islands with introduced predators (filled symbols) became more arboreal, while the habitat preferences of those on control islands (open symbols) remained unchanged.

Moreover, on islands with introduced predators the lengths of the hind legs2 of A. sagrei changed as predicted as the numbers extracted from Table S1 in the supplement to the paper show:

Died (1st 6 months): 0.82
Lived (1st 6 months): 1.48
Died (2nd 6 months): 2.44
Lived (12 months): -0.06

What happened was that during the first 6 months when most A. sagrei were still on the ground, those with relatively longer hindlegs were better able to escape from the predators, but during the next 6 months after they started moving up the trees, the situation was reversed and individuals with relatively shorter hindlegs were better able to survive. As the authors note, the direction of natural selection, first favoring longer and then shorter legs, was reversed during the course of one year. This is shown on the next figure.

The magnitude of the selection gradient3 reflects the intensity of selection on hindlimb lengths. The selection gradient is positive for the lizards on the islands with introduced predators (filled symbols) during the first 6 months and then it becomes negative as the direction of natural selection reverses.

It is fascinating that we are now seeing demonstrations of how evolutionary theory can be used to make and test predictions and hypotheses about the course of evolution. This is something no pseudoscientific theory of biological evolution can accomplish.

1. Jonathan B. Losos, Thomas W. Schoener, R. Brian Langerhans, and David A. Spiller. Rapid Temporal Reversal in Predator-Driven Natural Selection. Science 17 November 2006: 1111. Abstract
2. The listed numbers are actually mean relative hindlimb lengths. Relative hindlimb length for a lizard was calculated as the residual of logarithmically transformed hindlimb length versus snout-vent length. Although not stated in the paper, I am assuming that logarithmic transformation was applied to normalize the distributions of lengths and the residuals from the regression of hindlimb length versus snout-vent length were used to correct for the dependence of hindlimb length on overall body size. A negative value for the surviving lizards after 1 year indicates that their hindlimb lengths were mostly below the regression line; in other words, their hindlimbs were getting shorter relative to their body sizes.
3. Selection gradients are coefficients obtained from regression analyses of measured traits (in this case, hindlimb lengths) versus fitness (in this case, defined as whether a lizard survived or died).

1 comment:

Marcus said...

Haven't you heard? Evolution is to be banned!! :P