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Evolution. The theory of evolution is an explanatory framework for the diversity and form of organisms in the natural world. Darwin developed the theory in the nineteenth century. He argued that the many species observed by naturalists could have all diverged from a common ancestor by the gradual accumulation of small hereditary differences, each of which increased the bearer's chances of survival and reproduction in their immediate environment. This was in sharp distinction to the traditional orthodoxy, which held the each species had been separately created, perfect and immutable, by God. Darwin's theory, as he formulated it, was incomplete, as he had no grasp of how variations were passed from one generation to the next. \nThe synthesis of Darwin's hypotheses with the new and independently discovered science of genetics did not happen until the twentieth century, but once done it established Darwinian evolution as the foundation stone of biology. It is often alleged that evolution is just a theory, since it can never be directly observed, but this is not in fact the case. Most bacteria today show resistance to one or more types of anti-biotic drug. This is because they have evolved under our very eyes since anti-biotics were invented some fifty years ago. When a new anti-biotic is introduced in a particular area, the subsequent changes in the genetic makeup of the local bacteria populations give us an insight into evolution in action. The debate in biology today about evolution largely concerns how much of the development of organisms can be explained by adaptation to the environment, and how much of it arises as a byproduct of other factors. This debate is particularly difficult to resolve when it comes to the application of evolutionary analysis to the behaviour of contemporary humans. 

Evolutionary. As the name suggests, evolutionary psychology tries to make sense of behaviours by looking at their possible evolutionary basis, i.e. how a particular behaviour might have helped humans adapt to their original prehistoric environments. This is an example of a functional explanation, as it seeks to understand a behaviour by seeing the possible function it may have played, within the evolutionary framework of Darwinian theory. The central idea is that modern humans will have brain structures, behaviours and motivations that evolved in prehistoric times but still exist today as genetically-transmitted, biologically based predispositions, passed down because of their adaptive value. A key point to understand is that evolutionary adaption (at least for humans) acts on long timescales, typically over tens or hundreds of thousands of years, or even longer. However, human culture changes dramatically over much shorter timescales – think of the changes over just the last century or so. In fact, it is only a few thousand years since most humans were still living essentially 'prehistoric' hunter-gather lifestyles. So although the environment humans live in is dramatically different in practically every way from our original prehistoric one, our brains and nervous systems will not have had time to adapt to any great extent, if at all. The extent to which our behaviours have changed is the crucial empirical question for which evidence has to be collected.\n\nThis perspective is multidisciplinary as it draws on a mixture of existing data, evidence and methods from psychological disciplines such as cognitive psychology, neuropsychology and group behaviour, as well as disciplines outside psychology, for example: • genetics • social anthropology: observations of current-day tribes living in hunter-gatherer groups • paleoanthropology: the collecting and documenting of ancient artefacts and tools, also prehistoric cave pictures and other art • primatology • ethology. Although it is clearly not possible to devise experiments to study the behaviours and social organisation of prehistoric humanity, evolutionary psychologists draw on data from the above disciplines to formulate possible hypotheses about different aspects of human behaviour. It may then be possible to examine actual human behaviour, to see if it fits in with the prediction, for example on the kinds of gender differences that would be expected to be found in male and female experiences of jealousy, based on Darwinian ideas about reproductive strategy. \n\nAnother area to which evolutionary psychology has been applied is in-group and out-group differences, i.e. the strong tendency to favour members of the 'in-group' to which one belongs, while being indifferent or even hostile to outsiders from an 'out-group'. This can be explained in evolutionary terms by seeing the potential 'adaptive value' this could have had for our ancestors living in small tribal groups. Group members who supported and co-operated with others within their group but were prepared to compete with outsiders for resources could plausibly be argued to have a survival advantage. Their superior access to resources would have enhanced their chances of survival, and hence reproduction. This would therefore have meant they had a greater chance of passing on their genes to future generations. So those genetic combinations which led to greater in-group/out-group differentiation would have been more likely to have been perpetuated (this is essentially what the 'adaptive value' of a particular behaviour refers to). While such behaviour towards in-groups and out-groups may well have been 'adaptive' for prehistoric hunter-gatherer communities, it is likely to be a good deal less fruitful under modern day conditions. For example, an evolutionary psychologist might try to analyze the conflict in Northern Ireland between Nationalists and Unionists in 'tribal' terms, as an expression of in-group/out-group behaviour. This might help understand some of the (to outsiders) apparently irrational and self-destructive aspects of many of the social interactions involved, If such social behaviours are indeed at least partly rooted in our evolutionary past, this understanding may suggest some contributions evolutionary psychology could make towards possible solutions. Explanations of social behaviour in evolutionary terms tend to be opposed by those psychologists who are influenced more by sociological viewpoints than biological, such as SOCIAL CONSTRUCTIONISTS (see the section on this). The latter argue that the major reasons for such conflicts are better seen in terms of socio-cultural forces, and the kinds of language in which they are framed. Unfortunately, there is rarely much attempt made in psychology to integrate such differing viewpoints. It is on the one hand important to be on guard against the kind of 'theoretical laziness' which ignores the profound conceptual and methodological conflict between such perspectives, in pursuit of a superficial eclecticism. However, it is perhaps another kind of 'laziness' which prevents proponents of one approach from even trying to get deeply inside alternative perspectives, with the aim of developing a genuinely multiple perspective viewpoint (even if this involves acknowledging some unresolved contradictions and disagreements). Humans are complex creatures, rarely explainable in terms of a single type of influence. This arguably requires psychologists to draw together as many perspectives as possible.