THE COLONY AS AN EVOLUTIONARY UNIT

Bryozoan colonies grow by asexual propagation of members (“zooids”) within a colony. Many bryozoans produce calcified skeletons that allow for straightforward morphometric analysis of their biology after death. One such genus of bryozoans, Stylopoma­, has a wide range of morphological variation in zooids within a colony. Previous studies on the morphometrics of members within colonies have shown that there are no heritable features passed between individuals. This pattern limits the amount of evolution that occurs within a colony, since phenotypic changes don’t accumulate along specific lineages. However, different Stylopoma species are morphometrically distinct, showing that even though these same attributes cannot evolve within colonies, they can and have evolved across different species. Therefore, the pattern of evolution within colonies must be reconciled with the pattern of evolution between species. To study this evolutionary process, we test the idea that aggregate values of traits present in colony members are heritable between colonies, rather than between individuals. Furthermore, we propose that colonies in Stylopoma, and not individuals, serve as the primary unit of evolutionary change.

To test this hypothesis, we studied pairs of parent and offspring colonies raised in a common garden breeding experiment with known maternity, wherein we measured and analyzed 5 different member-level traits. The results suggest that the median value for each of these traits is heritable between generations of colonies, with varying levels of heritability for the interquartile ranges and the median absolute deviations. These findings indicate that aggregate member-level traits serve as heritable features at the colony level of organization in these animals. Therefore, the unit of evolutionary change in Stylopoma appears to be the colony. Our results provide evidence of a novel evolutionary transition present in Stylopoma; inheritance does not follow the individual, but rather a higher hierarchical level: the colony. Additionally, this result suggests that group-level inheritance of aggregate traits is possible, and that further study of modular colonial animals could shed more light on how widespread group-level inheritance may be in the animal kingdom.