Evolutionary Psychology                                                                                          John Horn

Dr. Mills                                                                                                                James Reid Summary                                                                                                       Palmer Thornton



The Environment of Human Evolutionary Adaptation


R. Thornhill. (1997). Characterizing human psychological adaptations.  In Ciba  Foundation Symposium, Characterizing human psychological adaptations:  The  concept of evolved Adaptation (pp. 4-13). England: John Wiley & Sons.



The term adaptation has many meanings in biology.  One of the most widely accepted definitions refers to changes in an individual that occur during its lifetime in order to solve some type of environmental problem that individual is facing.  These types of adaptations develop from a characteristic that all individuals within that species possess, but which only some activate, depending on their specific environment.  An example of one such environmental adaptation is the ability for humans to develop calluses on their hands or feet.  All humans have this ability, but not all humans activate it.  Humans who live in an environment where calluses benefit them on an everyday basis will obviously develop this trait to a greater extent than those who live in an environment where calluses are not necessary.  The key thing to remember about environmental adaptations, however, is the fact that they have the capacity to develop in all individuals of a given species, but only do so when they benefit the individual by providing some type of solution to an environmental problem.

 The environment of evolutionary adaptation (EEA) is based on the premise that an organism becomes modified in order to better exist under the conditions of its environment.  In order to further explain EEA, Thornhill uses a specific type of adaptation, Darwinian..  These adaptations are related more to the organisms’ initial design, as opposed to the environmental adaptation’s relationship to development.  These adaptations are the result of the process of past selection.  In other words, characteristics that worked well for a species are passed on, whereas characteristics that did not help a species are eliminated.  Darwinian Adaptations are important because they can demonstrate important links between a species and its environment.  Because of the process of selection, it is difficult to blame functionally purposeful characteristics to random chance.  In other words, if a species has a characteristic, and this characteristic serves a specific purpose that benefits the species, it is not by accident.  Selection has rewarded those in the species with this characteristic and allowed them to pass it on to the next generations.

When there is a characteristic of an individual that cant be considered a Darwinian Adaptation (that is it seems to serve no functional purpose), it can be classified a number of different ways.  The first possibility is that it is an incidental effect of an adaptation.  Incidental effects refer to characteristics that develop as a result of a Darwinian Adaptation, but do not carry the same functional importance. 

Another possibility is that a specific characteristic is the result of Phylogenetic legacy.  This classification refers to all characteristics of an organism that were at one point a Darwinian Adaptation for a given species, but no longer provide any functional purpose.  This raises the question of why a species would continue to possess characteristics it does not use or benefit from.  The best answer to that question lies in the idea that if the entire species possessed a certain characteristic (which it would have when it was considered functionally purposeful) that trait would be continuously passed from generation to generation, that is until that characteristic becomes costly and hurtful to the species in some way.  In that situation, the individuals that were lucky enough to deviate from the common genetic make-up would be most successful and pass their new genes down through the generations, without that specific characteristic. 

A third major classification for characteristics that do not carry the same functional purpose of the Darwinian Adaptations are basically mutations known as drifts.  Drifts are characteristics similar to those of the previous generation, but slightly, randomly, different in some aspect.   If the characteristics that result from mutations negatively impact selection, eventually they become eliminated from the species.  These are referred to as deleterious genes.  However, if by chance the random mutations positively affect the selection process, they become Darwinian Adaptations in that they serve a purpose and contribute positively to a species.

It is difficult to completely understand the environment of evolutionary adaptation without having an understanding for the process of selection.  Selection refers to nature’s way of eliminating all individuals in a species who possess characteristics that limit reproductive success, and thereby not allowing their genes to pass from generation to generation.  The reproductive success of an individual is based on what is called its “fitness.”  The fitness for all species is different depending on what characteristics will benefit that particular species the most.  Darwinian Adaptations are created in order to increase the fitness of a species.

Through a current study of selection, it is possible to identify all characteristics that are currently increasing the reproductive success of a species.  One study argues that through the use of Teleonomy, it is possible to examine and better understand the environment of a species through a historical perspective.  By examining current characteristics, it is possible to understand how selection has factored into the design of a species.  However, everyone does not support this idea.

It has been argued that measuring fitness in EEA is impossible but Thornhill strongly supports the idea saying, “A species EEA is the set of environmental features which generated the selection that made the species’ adaptations.  The EEA is stamped in the functional design of adaptation. (10)” By using Teleonomy we can determine levels of fitness and functionality.   While Thornhill stands by Teleonomy and its ability with EEA to find fitness he does warn of possible pitfalls if the ideas are improperly used.  While using EEA it is imperative that organisms and subjects are only compared within their species and not across species groups.  It is also important to remember that organisms of the same species can be viewed as historical documents for phenotypes.  Another important aspect to remember is that Teleonomy functions best on problem specific design for discovering traits.  The most important point when using EEA is “in order for researchers to know the evolutionary history of their study species, they must obtain data outcomes or phenotypic stamps of that history. (12)”



I.                    Key terms and definitions

a.       Adaptation- “modification of an organism or its parts that makes it more fit for existence under the conditions of its environment. (Merriam Webster’s)”

b.      Darwinian Adaptation- “the long term outcome of evolution by Darwinian selection. (5)”

c.       Fitness- “an individual’s reproductive success due to its fit to/design for an environmental problem. (5)”

d.      Phenotype- an organisms characteristics established by its genes and environment.

e.       Drift (genetic drift)- Individual differences occurring by chance that are not functionally designed. (Mutation)

f.        Functional Design- Adaptive qualities derived by Darwinian evolution and not by genetic drift.

g.       Teleonomy- “the quality of apparent purposefulness in living organisms that derives from their evolutionary adaptation. (Merriam Webster’s)”

h.       Phylogenetic Legacy- Characteristics of a species which remain from when they were once Darwinian Adaptations

II.                 Evolved adaptation vs. Environmental adaptation

a.       Evolved adaptation is the result of genetic differences between those with and those without an adaptation.

                                                               i.      Also known as Darwinian adaptation.

Ex.  Long necked giraffes having more offspring because they are healthier and thus flooding the genetic pool with that adaptation.

b.      Environmental adaptation refers to an organism’s ability to change throughout its life to gain advantage over an environmental problem.

Ex. A monkey forming a callous on his hand because the trees he climbs has rough bark.

III.               Darwinian Adaptation is interrelated with other ideas of adaptation.

a.       All adaptations submit to a subjects design against environmental problems.

b.      Teleonomy as a study

                                                               i.      Can we really discover what environmental aspects initiated adaptations to occur?

IV.              Current selection: what is causing it?

a.       Phylogenetic inertia

                                                               i.      If a trait is prominent throughout a species than it will continue to be until an equally stronger environmental force meets it

b.      Ecological pressures

                                                               i.      The environmental forces which meet phylogenetic inertia.

V.                 Humans EEA is useful in measuring Fitness.

a.       By using Teleonomy we can determine levels of fitness, but we must be carefull how we go about it.

                                                               i.      What to remember

1.      Can’t compare other species

2.      Organisms of the same species can be viewed as historical documents

3.      teleonomy focuses on problem specific design