Evidence for the evolution of behaviour
Evidence for the evolution of behaviour
The thing that sets Darwin's theory - explaining the origins of life - apart from the rest is the wealth of evidence that supports it. Much of this evidence is in the fossil record (palaeontology) and the similarities in living organisms (comparative anatomy).
The oldest rocks contain the simplest fossils and, in some cases, it is possible to observe a gradual change in the fossils over time. One problem is that fossils only provide a record of hard structures (for example, bone and shells), so behaviours can only be guessed at from the structures seen.
Another problem is that the record is incomplete because special conditions are needed for fossil formation and we have not yet found them all! For masses of information about fossils, geology and evolution, visit the University of California's Museum of Palaeontology online.
Comparative anatomy: This shows us that the same structures appear in a variety of different organisms, for example, the pentadactyl (or five-fingered) limb is common to all terrestrial vertebrates (with some changes over time).
Comparisons of developing embryos of different species demonstrate huge similarities (all vertebrate embryos develop gill slits, yet only fish have gills!). These facts provide evidence of common ancestry, and consequently, evolution.
As psychologists, however, we are interested in behaviours.
We want to find out what evidence there is that behaviours have evolved. This is where the nature-nuture debate comes in again.
In other words:
To what extent is a behaviour instinctive or learned (genetic or environmental)?
What do you think?
Try this exercise:
Which of the following statements is correct?
Only inherited characteristics can experience natural selection. Therefore, we have to prove that behaviours have a genetic component if we are to say they can evolve.
It is evident that genetic differences can result in differences in behaviour: Producing genetic mutants using radiation or chemicals (for example, Benzer, 1973) and artificial selection experiments (for example, Manning, 1961) show that behaviours can change according to genetic factors. The best evidence, however, comes from the comparative approach.
If the evolutionary connections between species are known using anatomical physiological or genetic techniques, inferences can be made about whether a behaviour occurs due to common ancestry (homologous behaviours) or common environments (analogous behaviours).
Convergent evolution occurs when unrelated species evolve similar behaviours usually because of similar environmental conditions.
Closely related species may evolve different behaviours (perhaps to avoid confusion in courtship!). This is known as divergent evolution.
These two phenomena help us to understand the origin of behaviours in different species and, combined, can can teach us about the evolution of a particular behaviour. This approach has been used by the likes of Konrad Lorenz (1958).
Termites (top) and ants (bottom) exhibit co-operative care. They are not closely related, so this analogous behaviour has evolved as a result of convergent evolution.
The bowerbirds' constructions are an example of a behaviour arising through sexual selection. In other words, the behaviour is adaptive becuase it leads to the attraction of a mate and, consequently, reproductive success.
To attract females, male bowerbirds build elaborate structures called bowers. These bowerbirds are closely related and so their bower-building behaviour is homologous. However, there has been some divergent evolution as the Vogelkop bowerbird builds a different shaped bower from the Satin bowerbird and decorates it with green moss, red berres and silver objects, whereas the Satin bowerbird exclusively favours blue objects.
Darwin had an idea about the mechanisms of sexual selection (male rivalry and female choice) but could not explain the precise reasons for these sex differences (Darwin, 1871).
One issue that Darwin could not work out was the apparent altruism displayed by a number of species, including the termites and ants above.