ESTIMATING THE EVOLUTIONARY POTENTIAL OF COLONY-LEVEL TRAITS IN THE BRYOZOAN STYLOPOMA

We understand how natural selection works in solitary animals to produce evolutionary changes in structure and function. But for colonial animals, this process is complicated by the fact that two levels of selection, selection among zooids (members) and among colonies, are superimposed onto each other. It is an open question which level of selection dominates phenotypic evolution in bryozoans and also whether or not the zooid and colony levels of selection interact in any way. Here I use a colonies of the cheilostome genus Stylopoma grown in a breeding experiment to estimate the evolutionary potential of colony and zooid-level traits. I directly estimate the heritability of zooid and colony level traits using quantitative genetic approaches. The fact that these colonies where propagated clonal and sexually allows me to measure the ability of traits to response to zooid and colony level selection by differences in the G-matrices and heritabilities between the two lines of propagation. I find significant evolutionary potential for all colony-level and some zooid-level characters to evolve by colony-levels selection. Beyond the occurrence of heritable variation, the key factor that allows colony-level selection to operate on a particular trait is its negative covariation with the relative frequency of ovicells.