Evolutionary History of the Grasses

Abstract

While some scientists have been working to sequence and describe the human genome, with increasingly dramatic results, another set of scientists has been quietly providing a map of evolutionary history, a time line that shows how life has evolved. Unlike the genome projects, which accumulate megabases of sequence from many genes in one organism, evolutionary projects accumulate megabases of sequence from the same handful of genes in many organisms. The scientists who investigate the pattern of evolutionary change are predominantly systematists, meaning, literally, those who study natural systems. Their work has three major goals. The first is to decipher the evolutionary history, or phylogeny, commonly drawn as a cladogram or branching diagram. Once that is accomplished, the second goal is to determine for each speciation event what sorts of changes must have occurred. The phylogeny allows us to define, for any point in time, what characteristics were ancestral (analogous to “wild type”) and which were derived (analogous to “mutant”). If two species have a particular characteristic, such as white flowers or hairy leaves, then their ancestor is assumed to have had the same characteristic. If two species had different characteristics, then we look to their next closest relative to help determine the ancestral condition. This sort of deduction is based on assumptions about the likelihood of change and provides a hypothesis of evolutionary pattern, which can in some cases then be tested experimentally. The third goal of systematics is to create a formal classification that reflects history. There are many ways to convert an evolutionary tree into a hierarchical classification. The only hard and fast rule is that any named group should include all the descendants of a particular ancestor, i.e. should be a monophyletic group. This distinction between determining history and producing a classification is relatively recent. Until the last part of the 20th century, classifications were assumed to represent history, and the two investigations were conflated (Stevens, 1994).