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The turnover-pulse hypothesis was constructed by paleoanthropologist Elisabeth Vrba, and is used to gauge the rate of survival and adaptation within species. The turnover-pulse hypothesis makes clear predictions regarding the responses of species to changing ecological factors. She defined turnover pulses as involving episodes of climatic change which caused geographic isolation in various taxa; the isolation subsequently spurred extinction and speciation in several different clades. The turnover pulse hypothesis is significant because it extends the geographic radiation concept from a single to a multi-clade context. The theory's key features are based on the sequence of species in the palaeontology of related genera, and environmental aspects in adaptation, survival and extinction. The majority of turnover pulses that occur result in small peaks affecting a small number of species, and typically involve a small geographic area. It is important to note that, based on Vrba and the turnover-pulse hypothesis, evolutionary change is caused by physical changes in the environment, such as climate change, tectonic plate shifting, and astronomical catastrophes, among other things.
Ecosystems periodically experience significant disruptions; these in turn result in mass extinctions and speciation. This would imply that climate changes have an effect on all groups, from bacteria to hominids.
Modern studies of individual populations have shown large cyclical shifts in phenotype/genotype that correlate with climatic variations. These studies, in turn, enable scientists to create a better turnover model.
Extinctions often hurt specialists more than generalists, wherein the generalists will thrive within the environment by exploiting new environmental opportunities, or by moving elsewhere in diasporas to take advantage of other environments. The specialists will experience more extinctions, and a "pulse" of positive and random speciation within their groups.
These two events lead to more specialists in isolated areas whereas the generalists will become more ubiquitous.
This geographic isolation is a common thread in evolution. It is a key factor in the evolution of species on a grander scale. Thus, the events causing geographic isolation are unmistakable in their significance as evolutionary mechanisms.
This hypothesis was developed to explain the different patterns of evolution seen in African antelopes. Later, it was used in an attempt to explain the speciation and distribution that lead to early hominins and subsequently Homo sapiens.
Paleoecologists largely agree that global climatic changes in conjunction with tectonic activity are viable explanatory factors in the extinction of Australopithecus afarensis, the origin of Homo, and the changes in behavior ecology leading to archaeological finds.
This hypothesis has coincided with several theories, including the savannah hypothesis, which was later renamed the aridity hypothesis. This environmental theory is used to extend the scope of our understanding of the appearance of bipedalism. The hominins went from trees to living upon the savannah. Increasing aridity led to the growth and expansion of the savannah, thus requiring the hominins to ambulate on two legs.
Previous studies have tested the hypothesis, mainly by examining temporal correlations between turnover pulses and climatic events. It is difficult to observe such correlations and study turnover as it is occurring, or to predict how different environments will respond to climate change. Thus, the idea that climate change drives faunal turnover in the manner predicted by the turnover-pulse hypothesis remains unclear. However, in one study, the underlying mechanics of the turnover-pulse are tested using well-dated Quaternary ungulate records from southern Africa's Cape Floristic Region. Changes in sea level, vegetation, and topographic barriers across glacial-interglacial transitions in southern Africa caused shifts in habitat size and configuration, allowing scientists to generate specific predictions concerning the responses of ungulate species characterized by different feeding habits and habitat preferences.
The 2.5 Million Year Event
A well-known example is the "2.5 million year event", in which a mass fluctuation of temperature occurred 2.5 million years BP, causing a rapid burst of speciation. It was during this event, so the hypothesis states, that many species attempted to move from their now uninhabitable habitats and later developed different adaptations in their new environments, evolving into different species. An example of this is seen in African antelopes after these temperature fluctuations occurred. Formerly known only to feed as woodland browsers, the antelopes subsequently made a change in eating habits, eventually becoming grassland grazers. These findings further indicate rapid adaptations made by species during this event.
Application in paleoanthropology
Counter-evidence points to the presence of Homo habilis and the lack of evidence that would support significant mutations occurring within that species in that same period.
Data Confirming The Turnover-pulse Hypothesis
- It is argued that the mammalian fauna of East Africa experienced a pulse of extinction and speciation from approx. 2.9Ma to 2.5Ma.
- Testing done by Vrba regarding African antelopes helped demonstrate the validity of the turnover-pulse hypothesis. During the 2.9-2.5 million year event, a rupture of extinction and speciation occurred, due to the expansion of the savannah.
- The rupture of extinction and speciation during the 2.9-2.5 million year event also is responsible for the turnover of the hominins; the Homo lineage originated in this event.
- The turnover of rodent species from the Omo Valley in Ethiopia coincides with the climate change in the 2.9-2.5 million year event.
- New mammalian fossils were found in the Busidima Formation in the Hadar study area of the Afar region, Ethiopia. This helps to illustrate the significant faunal turnover of the region.
- Evolutionary trends among African mammals during the Pliocene have been attributed to climate induced turnover patterns. The origin of the genus Homo was linked to global climate cooling.
There is some controversy within the evolutionary biology community concerning the turnover-pulse hypothesis, with the main opposing viewpoint being the Red Queen's hypothesis.
The Red Queen's hypothesis, presented by Leigh Van Valen, argues against the pulse model by favoring the concept that extinction occurs in a constant turnover.
In the 1980s, paleontologist Elisabeth Vrba saw a correlation between the climatic change 2.6 million years ago and the extinction pattern of the African antelope. Vrba suggested that a single climate-driven turnover-pulse resulting from climate change was the cause of the extinction of African antelope species and also for the rise of hominin lineages that led to Homo Sapiens. This idea produced much skepticism from the paeleontology community. The North American Paleontological Convention was the first of many to raise doubts about this hypothesis. Some of the oldest and richest fossils found had no sign of a "turnover-pulse;" instead, they showed a sustained shift over million of years. There was a global change, but its effect was not punctuated. A Smithsonian team plotted the rate of evolution in the Turkana fauna (2.5 million years ago) and also found that the "turnover-pulse" did not hold up. Even though there was a dramatic shift, that shift took millions of years .
The pulse hypothesis has also been countered by the prolonged turn-over hypothesis. In 1997, a study was conducted by Behrensmeyer that focused on the Turkana-Omo basin, due to its rich record of African fossils spanning between 3 and 1.8 million years ago. Taking samples of variations showed that there was no significant pulse. Instead, there was a prolonged period of turnover, especially between 2.5 and 1.8 million years ago.
Studies that have used the Turnover-pulse Hypothesis
Adaptive Radiations in the Context of Macroevolutionary Theory: A Paleontological Perspective
Here, the turnover-pulse hypothesis and other concepts are integrated into the theory of evolutionary radiations in general, and adaptive radiations in particular, and different types of evolutionary radiations are identified, including geographic radiations. Showing that several separate clades are radiating during a commonly experienced episode of climate change is powerful evidence that it is not the individualistic, biotic characteristics of each of these clades that is spurring the diversification, but rather the common Earth history factors. Indeed, this signifies the important connection between the turnover-pulse hypothesis and phylogenetic biogeography.
Constant extinction, constrained diversification, and uncoordinated stasis in North American mammals
The "coordinated stasis" model has far-reaching implications. Among them are three important predictions concerning diversity dynamics that I test here against the Cenozoic fossil record of terrestrial North American mammals. First, origination and extinction rates should be correlated; second, turnover should be a composite function of very low background rates and occasional, dramatic turnover pulses; and finally, stasis should result from ecological (niche) incumbency, with the domains of incumbent species being defined by ecological similarity, which in the case of mammals corresponds closely with taxonomic affinity.
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