|Themes > Science > Paleontology / Paleozoology > Fossils And Fossilisation > Fossils and Evolution|
The fossil record corresponds to the general theory of organic evolution, and any group of plants or animals can be seen to change through the record of strata. The smallest general unit of classification is the species, recognized in the fossil record by great similarity of form. Most species are seen to have an existence that is short in terms of Earth history--usually one to several million years, more rarely tens of million years, and very rarely hundreds of million years. Most genera (groups of related species) can be traced through time spans of tens of millions of years; larger units of this classification system, or taxonomy, tend to persist through longer time spans. The majority of the classes and phyla of invertebrate animals, for example, have records beginning in the Early Paleozoic Era.
All of the larger groups of animals are seen to have evolved a wide range of life forms.
With the theory of evolution by Charles Darwin in the mid-1800's came the expectation that the fossil record would provide unbroken evolutionary sequences, in which species after species would be seen to emerge gradually from their ancestors and pass, equally gradually, into their descendants.
This caused some paleontologists to doubt Darwin's belief that evolution proceeds by the gradual accumulation of small changes. It has now been determined that evolution does not proceed steadily, in response to some mysterious internal force, but in response to new opportunities. In a stable, unchanging environment, a well-adapted species is not likely to change, whereas in a changing one it may find better opportunities by changing its way of life.
In addition, evolution does not normally occur throughout a species, but in interbreeding populations, occupying some small part of the geographic range of the species as a whole. It is such populations and adjacent populations, linked by exchange of genes, that deviate from the ancestors and from the species as a whole, to form races or subspecies, and eventually, species. At any one time, a species is a combination of such groups, diverging episodically from each other. As geography and habitat change, these groups shift about, either blending when they meet, abruptly displacing each other, or coexisting side by side in different ways of life.