One study that appears to support the punctuationist model was carried out on a complete and apparently continuous fossil record of freshwater snails and bivalves from the Lake Turkana basin in east Africa. The sequence represented the last 4-5 million years of evolution of a series of lake- dWelling mollusc species. For long periods in this sequence, the snails and bivalves seem to have settled into an equilibrium in which they changed hardly at all.
At two key moments in the sequence, something remarkable appeared to have taken place in the now fossilized p munity. Almost every species present had been abruptly replaced by another form another species of the same basic type. Apparently, each species simultaneously and at a stroke altered into a new one. The whole dynamic of evolutionary change appears to have been concentrated into sudden
bursts of creation rather than spread out through time. In other words, this data seems to suggest that the rate of evolution is far from constant and can vary from an almost imperceptible rate to one so fast that it appears from the fossil record to be instantaneous. continuous fossil other equally support a different records, however, interpretation. One study has examined the incredibly detailed fossil history of a microscopic protozoan in a group called the foraminifera, which are hard-shelled organisms that float in the single-celled plankton. Their calcareous shells sink to the seabed where they become an important constituent of some marine sedimentary rocks. LA The fossil sequence of these foraminifera stretched from just over Io million years ago until about the present day. For the first 4.5 million years the shells varied slightly in size and thickness but with no
obvious directional trend. It was as though slow random changes were taking place in a single species. About halfway through the sequence, some 5.5 million years ago, the shape of the shells changed smoothly. Small and thin at first, the shells gradually became much larger and thicker. Over a period of about 500,000 years, a new species had been created and has retained much the same form to the present day This example alone (and there are many others) shows that the punctuated equilibrium model does not always fit the detailed fossil record. The foraminiferan fossil sequence resembles the type of pattern expected from a species which adapted smoothly, over half a million years, to a change in its selectional environment and, in the process, produced an organism different enough to be called a new species. conclusions from the
The conflicting conclusions from the
There are two contrasting ideas about the pace of evolution. One holds that evolution is gradual a progression of slow steady change. The other sees it as long periods of inactivity, interspersed with bouts of rapid change. These two diagrams show the essential differences between the two concepts. In each, the progression of evolutionary change is visualized as a rolling wheel. The track left behind is the evolutionary history of that wheel. The lower wheel, illustrating the gradualist theory, is made up of many tiny facets. Each of these represents a single mutation, which in turn produces an adaptation. This wheel rolls gradually and smoothly by the accumulation of these small mutational changes.
The upper wheel has a few large facets. As it rolls forward it stays for a long period on each facet a period of evolutionary stasis or equilibrium. The wheel then abruptly turns to the next facet. This represents a "punctuation event"- the rapid flip from one species to another with many differences evident between the two forms.
Fossils found in the Lake Turkana fossil beds suggest punctuated patterns of evolutionary change. Of the many molluscs fossilized in these rocks the diagram details the lineages of one snail, in the genus Melanoides, and one type of bivalve. Evidence shows that major replacements of one species form by another happened twice and at the same time for each species over the period of 4.5 million years. In fact many of the other mollusc species in the beds changed at these two times. The punctuation events of these species seem to have been synchronized.
Lava mounds dotting the landscape in the Lake Turkana region of Kenya in east Africa hint at the long volcanic history of the area. Fossils embedded in layers of volcanic sedimentary rock have provided important evidence for the punctuated equilibrium theory of evolutionary change.