- Biology and Biochemistry

Research
Matthew Wills
 Matthew Wills

Research profiles: Evolutionary and Mathematical Biology

Matthew Wills

Macroevolution, fossils and phylogeny

Biography

bssmaw@bath.ac.uk

Current Research

Why study fossils?

Biologists studying extant organisms have a huge battery of methods at their disposal. Morphology can be observed and dissected in great detail, physiology and biochemistry can be made the subject of experiment, behaviour can be observed, and heritable changes within populations can be studied from generation to generation. Information is available at tremendously high temporal resolution (the "ecological" time scale). But life has a history spanning something in the region of 3.5 billion years, with Metazoa originating at least 540 million years ago. The extant biota is just one time slice of this history. However great our understanding of living animals, we could never have predicted the existence of dinosaurs from looking at today’s birds and reptiles, or giant 5m long sea "scorpions" (eurypterids) from studying spiders and mites. Fossils provide us with less detailed morphological and palaeobiogeographical information, but over vastly longer (geological) time scales.

Should we use fossils to reconstruct the "Tree of Life"?

Evolutionary biologists try to reconstruct rapid and deep evolutionary branching events that happened many tens or hundreds of millions of years ago. Unlike living species, fossils offer ancient snapshots of life forms that were around at the time those branching events occurred. Moreover, living species have millions of years ‘worth’ of change piled on top of this, which can often bury the important signals we need to understand. Detractors have claimed that because fossil data are often less complete, usually just bones, shells and other hard parts, they are likely to muddy the water and make it difficult to find a robust evolutionary tree. Work on groups as diverse as plants, insects and vertebrates demonstrate that this is not the case. We are also documenting cases where adding just one fossil to an analysis can result in a radically different picture of that group's evolutionary history. The trees constructed without fossils may be oversimplifications, and far from the truth.

Why is the evolution of arthropods so difficult to reconstruct?

Arthropods are the most abundant and diverse of all the multicelled animal phyla. From their rapid radiation in the Cambrian they evolved to fill virtually every habitat and exploit almost all imaginable lifestyles. Their radiation has been the focus of debates concerning the overall pattern of the diversification of life, and their internal relationships remain problematic despite a century of study. Data from molecules, morphology and the fossil record frequently suggest conflicting patterns in arthropod evolution. Controversy now centres on the relative merits of these different sources of data, appropriate methods of analysis, and how to reconcile conflicts. The Malacostraca (shrimps, crabs, woodlice and their relatives) are my primary focus at present.

Do phylogeny and the temporal sequence of fossils agree?

Biologists routinely compare inferences about the order of evolutionary branching (phylogeny) with the order in which groups appear in the fossil record (stratigraphy). Where they conflict, ghost ranges are inferred: intervals of geological time where a fossil lineage should exist, but for which there is no direct evidence. The presence of very numerous and/or extensive ghost ranges is often believed to imply spurious phylogenies or a misleadingly patchy fossil record, or both. Some major taxa have cladograms with a better fit to the fossil record than others. Arthropods appear to be especially bad, whereas mammals and dinosaurs are especially good, for example. There are also trends through time. It has usually be assumed that the frequency of ghost ranges should increase with the age of rocks. While this is true, there is also a marked increase in ghost range frequency from the late Mesozoic to the present day.

Selected publications