Philosophy of Macroevolution
Microevolution is used to refer to changes in the gene pool of a population over time which result in relatively small changes to the organisms in the population--changes which would not result in the newer organisms being considered as different species. Macroevolution, in contrast, is used to refer to changes in organisms which are significant enough that, over time, the newer organisms would be considered an entirely new species. In other words, the new organisms would be unable to mate with their ancestors, assuming we were able to bring them together. Creationists often argue that they accept microevolution but not macroevolution--one common way to put it is to say that dogs may change to become bigger or smaller, but they never become cats. Therefore, microevolution may occur within the dog species, but macroevolution never will. There are a few problems with these terms, especially in the manner that creationists use them.
Evolutionary Biology. Approaches to macroevolution require integration of its two fundamental components, i. Macroevolution occurs in multiple currencies that are only loosely correlated, notably taxonomic diversity, morphological disparity, and functional variety. The origin of variation within this conceptual framework is increasingly understood in developmental terms, with the semi-hierarchical structure of gene regulatory networks GRNs, used here in a broad sense incorporating not just the genetic circuitry per se but the factors controlling the timing and location of gene expression and repression , the non-linear relation between magnitude of genetic change and the phenotypic results, the evolutionary potential of co-opting existing GRNs, and developmental responsiveness to nongenetic signals i. The developmental factors underlying macroevolution create anisotropic probabilities—i.
Macroevolution refers most of the time, in practice to evolutionary patterns and processes above the species level. It is usually contrasted with microevolution, or evolutionary change within populations. Some evolutionary processes, such as the spread of a trait from one population to another, might count as within-species processes but not within-population processes. Population genetics see entry , which emerged during the modern synthesis of the early- to mid-twentieth century, explains within-population microevolutionary change in terms of natural selection , genetic drift , mutation, and migration. One question that looms over philosophical work on macroevolutionary theory is how macroevolution and microevolution are related. One view, which is closely associated with the modern synthesis, is that macroevolutionary patterns are fully explicable in terms of microevolutionary processes.