Classification: Habitat Compulsion
Mnemonic: Variability Selection
Specific Model: Variability Selection (VS) Hypothesis
Original Proponent(s): Potts (1996)    
Basic Summary: Potts argues that adaptations to increasing climate instability during Miocene Plio-Pleistocene was the chief causal factor in human evolution. He argues against habitat-specific models, such as those that espouse savannas or forest/woodlands, but suggests that large disparities in environmental conditions, both geographically and temporally were responsible for important episodes of adaptive evolution    
Discussion: Richard Potts, in a series of papers in the mid-late 1990s, made a strong case that increased climatic variability since the Miocene was a major causal factor in human evolution generally and the adoption of bipedalism in particular.
His argument was largely contra the specific paradigm in paleoanthropology based upon a braod savannah context. He showed that the evidence indicated savannah habitats were not a constant factor in human evolution and instead suggested that it was the increasing variability in climates that was the key factor.
It is my opinion that Potts' evidence supports waterside hypotheses of human evolution because climate variability translates directly into "wet dry" cycles which would most prfoundly affect those habitats at the water's edge.
 Strengths: The VS Hypothesis is based on evidence from paleohabitats associated with hominins over long periods of time. Potts makes a strong case that no single, stable, habitat could have been responsible for the adaptive shifts we see in human evolution and that some aspect of climaate variability itself is a more likely cause.    
Weaknesses: Potts is rather vague about a mechanism by which increased habitaat variability could manifest itself into increased bipedality.
The list of habitats evaluated was very short. Almost all of Potts (1998) critique was directed at savannah-based models. Woodland/forest habitats and seasonally varying riparian habitats were briefly considered too, but waterside habitats were not considered at all.
1.1 Survival Value 4 (Fair) The VS hypothesis argues that increased variability in climate resulted in hominins needing to survive changing habitats, both temporally and geographically but it is not clear how this translates into bipedalism or why doing so would make them better able to survive.    
1.2 Sexual Selection 5 (Fair) The VS hypothesis appears to be neutral on this point.    
1.3 Not Teleological 7 (Good) Potts' hypothesis is based on sound contemporary evidence.    
2.1 Improved Food Acquisition 5 (Fair) The VS Hypothesis appears to be neutral on this point.    
2.2 Accounts for Predation 4 (Fair) It could be argued that arguing that hominins were exposed to ever changing habitats over the past 5Ma or so would make them more vulnerable to predation. Potts' argument that it was the very changing nature of their habitats that they became adapted to is hard to translate into plausible realisstic mechanisms of predator avoidance.    
2.3 Why Apes are not Bipedal 7 (Good) Assuming the VS Hypothesis' premise that increased variability of climate and habitat could account for bipedalism and other human atributes, this must be seen as a factor in its favour.    
2.4 Extant Analogues 2 (Poor) If the VS hypothesis were true, we'd expect to see greater bipedality amongst apes that are more genralist in nature. However the most bipedal of the non-human apes, the gibbons, are probably the most specialist clade of all.    
2.5 Applies to Both Sexes 9 (Good) As with most models, the VS hypothesis applies equally to both sexes.    
3.1 Hominid Anomalies 2 (Poor) Potts does not refer to any anatomical anomalies of the australopithecines or other early hominins.    
3.2 Fits Paleoecological Record 9 (Good) Potts VS hypothesis was conceived in order to explain the paleoecological record.    
3.3 Precursor to Strider and knuckle Walker 2 (Poor) The VS hypothesis does not propose any precursor to bipedalism.    
4.1 Extended Explanatory Power 6 (Fair) As humans are undoubtedly environmental generalists today, the VS hypothesis ceratinly explains more than just bipedal origins.    
4.2 Complimentary 4 (Fair) As the VS hypothesis aims to supplant those hypothesis based on specific savannah and woodland/forest habitats any such model of bipedalism based on such a habitat assumption must be regarded as contradictory.
4.3 Falsifiable or Testable 8 (Good) Unlike most model of bipedal origins, Potts uses a good scientific framework for his hypothesis and it is phrased very much in terms of falsifiable predictions.    
References Potts, R. Evolution and Climate Variability. Science 273:922-923, (1996).
Potts, R. Environmental Hypotheses of Hominin Evolution. Yearbook of Physical Anthropology 41:93-136, (1998).