Evolution in mosaic
A controversial idea of how evolution works gains new support.
Mosaic evolution is a long-discussed and still controversial idea that different parts or characteristics of an organism evolve separately, in different ways and at different times. The current dogma suggests, instead, that most parts of an organism evolve together, and thus that single traits are good approximations for how all traits in an organism evolve. For example the shape of a deer's antler might change along with tooth shape, and thus one could approximate the other.
Now, a new study led by Dr. Melanie Hopkins from the Museum für Naturkunde in Berlin and Dr. Scott Lidgard of the Field Museum of Natural History, in Chicago, USA, shows that such traits do not seem to evolve together. Their study points to a role for mosaic evolution in a large number of species. Their work was published on November 26 in the journal Proceedings of the National Academy of Sciences.
The research team compiled data of how 635 morphological traits have changed over time, from a wide variety of living and fossil organisms including mammals, fish and plankton. Statistical analyses were used to explore how well three different models of evolution fitted the data. Evolutionary models included one where very little change occurs at all ("stasis"); another where random undirected evolution occurs ("random walk"); and one where evolution follows a specific direction ("random walk with trend", e.g. shells tending to get wider through time).
The results were unexpected. Which model fitted best depended on the trait, not on the organism, meaning that each trait was following its own evolutionary path, thus supporting the idea of mosaic evolution. For example shell length and head length did not evolve hand to hand. As Dr. Hopkins explains the key message of her work,
"Within individual lineages [e.g. species, populations], traits may show different patterns of change... conflicting patterns are seen in many different types of organisms at many different periods of earth history, i.e. mosaic evolution is everywhere! Thus our understanding of how species evolve depends on what [trait] we choose to measure."
While previous studies have shown some evidence for mosaic evolution, the present work is the first to present such convincing evidence. As Dr. Hopkins explains, "we analyzed all of the trends using the same analytical protocol". And coming to clear conclusions based on previous work was difficult, because each author tended to only analyze one or a few traits, and each author had their own definition for measuring different types of evolution. "[our] consistent model-selection-based method is so powerful because it gives us a way to compare all of these different studies in a consistent way".
Dr. Richard Butler, a palaeontologist at the GeoBio-Center of the LMU in Munich, agrees that the work is sound: "This is an interesting and well-designed study...The results are not necessarily all that surprising [as there has been much discussion and evidence present for mosaic evolution previously], but they provide new quantitative support for an important part of evolutionary theory".
The results seem to have been worth it, but compiling and analyzing so much data is painstaking work. "the most boring part was collecting the data" says Dr. Hopkins; "in some cases, the information we needed was ...in tables [in papers]. In many other cases, the data we needed was provided in figures, which we had to digitize". Before this an extensive literature review, via the internet and library, was needed: "a considerable amount of effort went simply towards keeping track of everything we had read!" "Analyzing the data [the last step] was the fun part".
Though the study was already very extensive, Dr. Hopkins hopes to extend her work in the future to look at geographical and altitudinal trait variation:
" [we may] get different results for the same species if we looked at different... [geographic areas]. ...it is [also] possible that some ... trends were influenced by clinal [e.g. altitude, depth] variation. More work on this problem... would be really informative".
Hopkins, M.J. & Lidgard, S. 2012. Evolutionary mode routinely varies among morphological traits within fossil species lineages. PNAS (online before print: doi: 10.1073/pnas.1209901109).
For the full article, go to: http://www.pnas.org/content/early/2012/11/21/1209901109.abstrac
