Evolution of Polymorphism in Bryozoa

Colonial animals are composed of genetically identical clones produced via asexual budding. In many groups, like corals, the clones look identical and carry out the same tasks. In contrast, some colonial animals have clones that are physically specialized for different functions. For example, many hydrozoan colonies have different types of clones for feeding, reproduction, and defense. Dividing tasks between clones is known as division of labor. Division of labor is thought to increase efficiency, particularly when combined with physical specialization of clones (referred to as polymorphism). However, if division of labor increases efficiency, why do many colonial organisms lack polymorphism? This is not an easy question to answer directly, so we have focused on four subquestions: 1) Are polymorphic lineages more successful than non-polymorphic ones? From an evolutionary perspective, a successful lineage has many descendant species (high species richness), a high speciation rate, and a low extinction rate. We hypothesize that increased efficiency from polymorphism will make polymorphic lineages more successful. 2) What factors promote the evolution of polymorphism? One hypothesis suggests that an external factor, environmental stability, could promote the evolution of polymorphism. Other hypotheses suggest that a variety of internal factors (larger colony size, increased physical variation between clones, and decreased physical integration between clones) may be important instead. We will test each of these hypotheses independently to determine which best fits the data. 3) Is increased physical specialization of polymorphic clones linked to increased extinction risk? Increased physical specialization may have diminishing returns in efficiency. For example, a super specialized defensive clone that only protects the colony from one particular predator may only be useful on a few occasions. In addition, extreme physical traits are often associated with increased extinction risk. Therefore, we hypothesize that lineages with extreme polymorphism are more extinction prone. 4) Do developmental biases break down during the evolution of polymorphism? A developmental bias limits the amount of physical variability that a developmental system can produce. Developmental biases are typically measured in terms of the relationship between traits (e.g. how arm length is related to leg length), and can predict the direction of evolution even over very long time scales (>60 million years). We hypothesize that the evolution of polymorphism will also be constrained by developmental biases. This project will combine phylogeny with modern and fossil specimens to create the most thorough investigation into the evolution of polymorphism to date. We will use cheilostome bryozoans as a model organism. These colonial animals have an excellent fossil record and a wide range of polymorphism, making them an ideal choice for this project.