Patterns of Evolution
We now need to look more closely at the evolutionary patterns that we wish to explain. Again, we can use the study of patterns to discover and define important problems to be solved. The methods available for the study of patterns depend on the focal level of the mechanism with which one is concerned. If our interest is in individual variation, naturally we need extensive sampling and laboratory experimentation, looking at individuals within populations. In order to study the biology of populations we must work at the population level, and at such phenomena as gene and character frequencies in the field and laboratory. It is at this level that workers have most readily been able to measure selection coefficients and other quantitative elements of evolutionary science. If our interest is in the processes of speciation we must look as closely as possible at examples of the process in action—at sibling species, at a whole range of hybridization patterns in the wild and laboratory, at the distribution of closely related species in space and time. (It will be noted that between population-level work and species-level work there is an unfortunate gap; one can look at prespeciation and postspeciation situations but it is rare indeed to be confident that one is looking at speciation in flagrante delicto.) We can also look beyond the species level at phenomena of species distribution within higher taxa. Here we progress to using the methods of systematic biology and comparative morphology and paleontology. Each of these different approaches gives a different view of the evolutionary process and, therefore, in terms of the questions discussed in this book, gives us a different view of the role of mechanisms acting at the developmental level within evolutionary mechanisms. Obviously the most immediate element of causality in the origin of adaptive structures resulting from developmental properties is in the matter of individual phenotypic variation, but some of the most interesting questions concern the consequences of developmental properties for higher focal levels—their upwardly causing properties. To examine these we need to look at the species level and beyond (what has often been termed croevolution).