With much interest in the history of human evolution and the lack of well-preserved fossils, there is considerable uncertainty as how to best study human’s history. In order to study how natural selection led to the split between humans and chimps and what adaptations make humans unique, there is a need to define ‘adaptation.’ Currently, there is a great debate among the scientific community as how to best define ‘adaptation,’ and there does not appear to be a general consensus. This is a disadvantage to the study of human evolution because without an agreement in adaptation’s definition, there will continue to be inconsistencies in the interpretations of which traits led to the uniquely characterized humans we have today. This further delays the piecing of the human evolutionary puzzle.
As a potential solution to the problem at hand, I propose that an adaptation is defined as any phenotypic trait that increases an organism or group’s reproductive fitness in comparison to a past state where that phenotypic trait did not exist or was altered.
In this paper I will first summarize the opposing definitions that exist in the scientific community, the main dilemma being the historical genesis definition or the current utility definition (Gould & Vrba, 1982). I will then argue as to why my proposed definition for adaptation is a practical solution for the problem at hand. Afterwards, I will discuss the challenges with utilizing my proposed definition when studying the past. Lastly, I will explain how my definition of adaptation allows for a better understanding of human evolutionary history.
Within the scientific literature, adaptation can be defined by two different measures: historical genesis and current utility (Gould & Vrba, 1982). Historical genesis defines adaptation as a feature that was naturally selected for the role it currently serves, and current utility defines adaptation as a feature that increases the organism’s current fitness (Gould & Vrba, 1982). Historical genesis emphasizes the historical origin in defining a feature as an adaptation while current utility does not care for it (Gould & Vrba, 1982). These two opposing definitions highlight the clear dilemma that exists in the literature. This dilemma is problematic because if the scientific community cannot agree on one definition, the findings from evolutionary history research will be inconsistent. A feature that one scientist concludes to be an adaptation may not be considered as one for a different scientist because of differing interpretations of an adaptation’s qualifications.
There also exists a definition of adaptation that extends off of the historical genesis definition where a new word called ‘exaptation’ is introduced (Gould & Vrba, 1982). ‘Exaptation’ is meant to define a trait that initially rose as an adaptation but has now “coopted” a new function (Gould & Vrba, 1982). Gould and Vrba’s definition of adaptation requires a look into the history of the trait in order conclude whether or not it is an adaptation. It is only if that trait is still currently serving the same function that it had originally been created for, can that trait be labeled as an adaptation (Gould & Vrba, 1982). However, if that trait serves a different function than it did in the past, then it would be labeled as an exaptation (Gould & Vrba, 1982). While this appears to be a valid approach, it has its limitations. One challenge is poor fossil preservation, which makes it difficult to create an extensive phylogeny of a trait to investigate what its primary function was (Reeve & Sherman, 1993). Additionally, Reeve and Sherman find fault in the fact that whether a trait is identified as an exaptation or adaptation depends on what the individual establishes as the historical origin—this makes it possible for any trait to easily be considered as an exaptation or an adaptation (1993).
While the split between historical genesis and current utility remains the main theoretical divide that exists, there are other proposed definitions of adaptations. One definition states that a feature is an adaptation only if it is so complex that this complexity could not have had arisen through random processes (Thornhill & Alcock, 1990). This definition suggests that an adaptation must have had come about due to natural selection where components of the feature built upon each other over time, increasing its complexity. Several other scientists support this notion that a characteristic is an adaptation only when it is spread through natural selection (Sober, 1984 & West-Eberhard, 1992). There is also a “derived trait” definition where the feature in question must have been derived relative to its history (Reeve & Sherman. 1993). Numerous definitions of adaptation exist in the scientific literature, which can lead to much confusion when characterizing an adaptation. The potential for confusion emphasizes the urgency that must be taken in order to clarify the perplexity that seems to engulf what defines a feature as an adaptation.