While hopes for finding clock-like evolution of DNA sequences and analytical methods that assume clock-like evolution have pervaded the literature since the beginning of molecular systematics in the 1960’s, molecular evolution is generally heterogeneous over the tree of life. Furthermore, rates of molecular evolution within lineages are not consistent through time. Amongst the diversity of rates found across the tree of life, a few lineages stand out by having consistently slow rates of molecular evolution through time relative to nearly all other lineages; these are the so-called “molecular living fossils,” in many cases the same lineages described as “living fossils” on morphological grounds alone.
Adaptive radiation has been the source of considerable study in all groups of organisms with the hope of isolating factors that drive elevated rates of diversification. Knowledge of these factors can help us more fully understand the process and history of evolution by characterizing the effect of selective forces on phenotypic and genomic evolution and by explaining mitigation of constraints on evolution that may otherwise be present. Studies of living fossil lineages may prove to be similarly informative by approaching some of the same issues from the opposite angle: instead of studying factors that promote rapid evolution, factors contributing to whole genome conservation and constraints on molecular and phenotypic evolution would be the focus of such a study.
For my doctoral dissertation research, the marattioid ferns are proposed as a model system in plants in which to study whole genome evolution and the “molecular living fossil” phenomenon. The benefits of this group are a) the age of the group (~354 myr), b) the position on the tree of life (one of the earliest branches of the ferns), c) the well-documented fossil record which shows a high degree of morphological stasis, and d) previous evidence of highly reduced rates of molecular evolution (Soltis et al. 2002). Additionally, this research will produce the first molecular phylogeny of the group, which will coincide with the completion of the sequencing of the Angiopteris chloroplast and mitochondrial genomes (a marattioid fern) by the Joint Genome Institute. The major objectives of this research are:
- Construct a multi-gene phylogeny of the marattioid ferns and discuss biogeographic implications and evidence for recent diversification;
- Characterize rates of molecular evolution within the marattioid ferns and isolate potential causal factors for the apparent reduced rates of both molecular and phenotypic evolution;
- Resolve deep-level relationships and rooting of the group using DNA sequence, morphological, genomic, and fossil evidence.
Please email me for more detail and results so far if you are interested – murdock at berkeley.edu