Turnover-pulse hypothesis

The Turnover-pulse hypothesis was constructed by paleoanthropologist Elisabeth Vrba used to gauge the rate of survival and adaptations within species. The Turnover-pulse hypothesis makes clear predictions regarding the responses and actions of species to changing ecological factors.^[1] She defined Turn-over Pulse’s as involving episodes of climatic change that 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.^[2] The theory's key factors 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 effecting a small amount of organisms and typically involves 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 plates shifting, and astronomical catastrophes to name a few.^[3]

Theory

Ecosystems periodically experience significant disruptions, these in turn result in mass extinctions and speciation. Climate changes have an effect on all groups, from bacteria to hominids.^[1]

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.^[4]

Extinctions often hurt specialists more than generalists, where the generalists will in turn thrive within the environment by utilizing new environmental opportunities, or by moving elsewhere in diaspora 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 spread out.

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 activities causing geographic isolation are unmistakable in the significance of 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 A. afarensis, the origin of Homo, and the changes in behavior ecology leading to archaeological sites.^[5]

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 reason behind the appearance of bipedalism. The hominids went from trees to living on the savannah ground. The increased aridity led to the growth and expansion of the savannah, thus requiring the hominids to traverse on two legs.

Theory Testing

Previous studies have tested the hypothesis mainly by examining temporal correlations between turnover pulses and also climatic events. It is very 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 ungulates 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 antelopes after these temperature fluctuations occurred. Formerly known only to feed as woodland browsers, the antelopes were able to make a change in eating habits, eventually becoming grassland grazers. These findings further indicate rapid adaptations made by species during this event.

Application in paleoanthropology

Support for the hypothesis comes from the concurrent split in Australopithecus afarensis and Paranthropus robustus, which each developed separate traits in separate regions around the same time.

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 time-frame.

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.^[6]
• Testing done by Vrba regarding African Antelopes helped prove 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 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 is originated in this event.
• The turnover of rodent species from the Omo Valley in Ethiopia coincide 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.^[5]
• Evolutionary trends among African mammals during the Pliocene have been attributed with climate induced turnover patterns. The origin of the genus Homo was linked to global climate cooling ^[7]

Opposition

There is some controversy within the evolutionary biology community concerning the Turnover-pulse Hypothesis, with the main opposing viewpoint to be the Red Queen's Hypothesis. see also: Red Queen hypothesis

The Red Queen's hypothesis, presented by Leigh Van Valen, argues the pulse model by favoring the concept that extinction occurs in a constant turnover.^[8]

In the 1980s paleontologist Elisabeth Vrba saw a correlation between the climatic change 2.6 million years ago and the extinction of the African antelope. Vrba suggested that a single climate-driven turnover-pulse in regards to climate change was the reason for the extinction of African antelope and also the rise to lineages that led to Homo Sapiens. This idea led to a lot of skepticism from the scientific 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 it showed a sustained shift over million of years. There was a global change but its effect was not punctuated. The Smithsonian teal 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 bur that shift took million years .^[9]

The pulse hypothesis has also been counteracted by the prolonged turn-over hypothesis. In 1997 a study was conducted by Behrensmeyer that focused on the Turjana-Omo basin due to its rich record of African fossil spanning between 3 and 1.8 million years ago. Taking samples of variations showed that there was no significance pulse, instead there was a prolonged period of turnover especially between 2.5 and 1.8 million years ago.^[10]

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.^[2]

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.^[11]

References

• Johanson, Donald; Edgar, Blake. From Lucy to Language. p. 28. Retrieved 29 October 2015.