Evolution usually refers to the processes by which life has changed through time. The record preserved in the rock is the data base from which interpretations and models are developed to explain that change. In the case of plants, breeding experiments provide data on some of the mechanisms responsible for evolutionary change.
Darwinian Evolution: Darwin believed that the acquisition by an organism of small adaptive changes allowed this species to change slowly over time from one form to another. Hence, darwinian hypotheses inferred that these changes (affecting many future generations) would eventually result in the production of a new species. Individuals that survived must have been best adapted, and by generating new offspring, would pass those "adaptive" features to the next generation. Darwin believed that when the fossil record did not support this gradual change, that the fossil record must be incomplete. These concepts were formulated BEFORE the science of Genetics was recognized.
Modern Synthesis: Following the recognition of genetic theory, population ecology, and other biological facts, this neodarwinism guided evolutionary thought into the 1970's. It's concepts included:
This concept included the idea that species are groups of interbreeding populations occupying territory and genetically isolated from neighboring groups. When gene flow is interrupted (physical, temporal, and other barriers), local populations diverge genetically and morphologically from the parent stock. The gradual change through time has been termed microevolution.
When one species becomes extinct by gradually evolving into another, this is termed ANAGENESIS.
Punctated Equilibrium: Reexamination of the fossil record led Steve Gould and Niles Eldridge to recognize that only a few examples of speciation by gradual change existed. In contrast, the fossil record documents long intervals of time during which species undergo little or no morphological change. These intervals are punctated by the sudden appearance of new species (and higher-order taxa) as a peripheral isolate.
Under this theory:
BACKGROUND rates (characterized by low rates of evolution and extinction) are more common through Geologic Time as opposed to ADAPTIVE RADIATION (rates characterized by greater rates of evolution than extinction) and MASS EXTINCTIONS (characterized by episodes of large-scale loss of taxa).
Phyletic evolution occurs within an established lineage (almost always present because there is little change in gene frequency in a species. This constancy in the gene frequency provides for STASIS in the population.).
Evolutionary trends can be determined by evaluating the amount of change (morphometric change) per unit time. If the rate of change does not deviate greatly during time, that there is a unidirectional (linear) vector, this trend is known as ORTHOGENESIS. At times, some features change markedly while others remain relatively unchanged, a process called MOSAIC EVOLUTION and very common in the Plant Kingdom.
Phylogenetic evolution considers the rates of progressive change within the phylogeny of a higher taxon, such as at the generic or family level without regard to the details of phylogeny. The average morphologic condition within a group of closely related species is plotted against time. The trends are not easily distinguished in the fossil record. The large scale changes may be the result of species selection; taxa that speciate at high rates or last for long intervals of geologic time tend to produce more total descendant species. Also, a tendency to shift morphology to a particular direction may also produce a net trend.
These phylogenetic trends can be the result of several heterochronic (a change in timing) mechanisms. HETEROCHRONY is a term used to describe the change in the sequence of development of organs in an individual or population of organisms. The terms to describe these alterations in developmental timing are:
Plants can undergo various reproductive permutations that are quite different than most of the animal kingdom. These genetic alterations provide many other genetic combinations that may allow for both microevolutionary and macroevolutionary trends to develop.
John Alroy has compiled a History of Evolutionary Biology website in which an informal and incomplete guide to the history of evolutionary biology from about 1800 to about 1950 is presented.
The University of California's Museum of Paleontology provides additional information on Evolutionary Systematics in their Phylogenetics Laboratory.