Charles Darwin is rightly celebrated for providing, in The Origin of Species, the first workable scientific theory to explain the stunning diversity and complexity of life on earth. His theory of evolution by natural selection comprised three principles. First, individuals vary in their characteristics (the principle of variation). For example, individual finches, to take one of Darwin's favourite species, may vary in the size and shape of their beaks. Second, not all individuals are equally likely to survive and reproduce, and their chances of survival and reproduction are tied to some extent to their characteristics (the principle of competition, or in modern biological parlance differential fitness). For example, finches with large beaks might be more likely to survive and reproduce because they can open a wider range of seeds, and thus obtain more food, than small beaked finches. Third, characteristics are inherited from parent to offspring (the principle of inheritance). Large-beaked finches tend to give birth to large-beaked offspring, and small-beaked finches tend to give birth to small-beaked offspring. Combining these three principles, Darwin saw that traits which increase an individual's chances of survival and reproduction will gradually spread in a population (large beaks, in our finch example), that different populations inhabiting different environments may diversify over time (as seen on the Galapagos Islands, where finches on different islands have evolved differently shaped beaks to feed on different sized seeds), and that complex adaptations may gradually evolve as small incrementally beneficial modifications are built up over successive generations (explaining the emergence of beaks, or eyes, or wings, in the first place). It is perhaps less well known that shortly after Darwin came up with his theory of evolution as applied to biological species, he applied it to another domain: human culture. Specifically, Darwin drew a parallel between the preservation of favoured individuals within a species and the spread of words within languages: "The formation of different languages and of distinct species, and the proofs that both have been developed through a gradual process, are curiously parallel. . . The survival or preservation of certain favoured words in the struggle for existence is natural selection." (Darwin, 1871, p.90-91). My book "Cultural Evolution" provides an overview of how scholars since Darwin have built on his suggestion that evolution can be applied to cultural change: not just changes in words over time but also changes in beliefs, ideas, knowledge, customs, technology and social institutions. Essentially, anything that is inherited: not inherited genetically, like biological characteristics such as beaks, but inherited socially, via imitation, teaching, and other forms of non-genetic transmission. As well as inheritance, cultural traits also exhibit Darwin's other two principles. They vary: there are, for example, approximately 10,000 different religions currently practiced in the world, almost 7,000 different languages spoken, each one of which containing around half a million words, and 7.7 million patented items of technology in the United States alone. And because no one could plausibly learn 7000 languages, know how to make 7.7 million bits of technology, or belong to more than even a single religion at any one time, there is also often competition between these beliefs, ideas, languages and inventions. Over time, successful cultural variants (ideas, beliefs, tools etc.) spread, different societies diversify culturally as different variants take hold, and complex cultural adaptations may evolve, such as hammers, cameras and aeroplanes, rivalling genetic adaptations such as beaks, eyes and wings. This is not to say that biological and cultural evolution are identical. As just noted, inheritance takes a clearly different form: whereas genetic inheritance occurs exclusively from one's parents, one may acquire cultural traits from potentially any member of society, as well as via books and the internet and other mass media. There may also be important differences in where the variation comes from - blind genetic mutation in the biological case; occasionally insightful and intentional invention in the cultural case - and in the reasons why variants spread at the expense of others - cultural variants may often spread because they fulfil purely social needs, such as dialects or body decorations that serve to identify group membership, rather than fulfilling objective or functional needs related to survival or reproduction as in the biological case. But these differences just make studying cultural evolution more interesting. The challenge is to figure out how biological and cultural evolution are similar enough to often generate similar solutions to the same problems (e.g. submarine sonar and bat echolocation) as well being different enough to allow humans to colonise virtually every terrestrial environment on the planet, and even travel beyond our planet, in a way that species lacking cultural evolution have not. There are many benefits of viewing cultural change as an evolutionary process. It means that social scientists can borrow the extremely powerful quantitative tools that biologists have developed over the past century, which were originally developed to explain patterns of biological evolution, and use them to address equivalent questions regarding cultural evolution. For example, biologists use what are called "phylogenetic methods" to reconstruct the evolutionary history of species, coming up with evolutionary trees that best explain the current distribution of traits (e.g. the number of toes that different animals have) parsimoniously assuming the minimum number of evolutionary changes along the branches of the tree. Many linguists and archaeologists have begun to use the same phylogenetic methods to reconstruct the evolutionary history of both languages and prehistoric artifacts, which, like species, show gradual change and diversification over many generations. One interesting result, for example, has been the demonstration that many major language families, from the Indo-European to the Austronesian, originally spread by piggybacking with agriculture several thousand years ago. While this had been previously suspected, it was only with quantitative evolutionary methods that this hypothesis could be formally tested and confirmed. Other evolutionary-minded researchers have used mathematical models and computer simulations to probe the underlying "micro-evolutionary" processes - who people learn from within their social group - that generate large-scale "macro-evolutionary" patterns and trends observed by anthropologists, sociologists and other social scientists. For example, while sociologists have typically assumed that technological innovations, from tractors to iPods, spread through societies because people independently discover the merits of those innovations, evolutionary models suggest that the spread is much more social than this: most people adopt a new technology not because it is objectively better than what has gone before (although it may well be) but rather because lots of other people are using it. We know this because mathematical models assuming strong social influence tend to generate S-shaped diffusion curves: a slow initial uptake, then a rapid spread, then a slowdown. Independent adoption, in contrast, generates r-shaped diffusion curves: a rapid initial increase followed by a slowdown (without the initial slow period). Real-life diffusion is almost always S-shaped, not r-shaped. And this is consistent with decades of research in psychology demonstrating how people are highly influenced by the behaviour of other people, even when social influence directly contradicts our private opinion. Other evolutionary models have shown that some cultural traits fit the statistical signature of what biologists call "neutral drift". This is where gene frequencies change purely at random without any intrinsic fitness differences (a temporary absence of Darwin's second principle, competition). Exactly the same mathematics can be applied to cultural evolution, to test whether different cultural variants are functionally equivalent and are changing purely at random, or whether there is instead some directional change. Researchers have tested many cultural traits in this way. Perhaps unsurprisingly, traits that we think of as subject to random fads or social whims, such as modern-day pop music or prehistoric pottery decorations, fit the neutral drift model. But so do first names, which is surprising given that most parents probably think that they put a lot of effort into naming their newborn baby. Academic buzzwords, such as "hermeneutic" or "habitus", also show random fluctuations as if they were intrinsically functionless, perhaps worryingly for the social scientists who use them (although no such random fluctuation is observed for terminology from physics). In contradicting our intuitions in this way, quantitative evolutionary tools can provide a more rigorous footing for the study of culture. Another advantage of adopting an evolutionary approach to culture is that it can solve the thorny "micro-macro" problem that has beset the social sciences for so long, and help to bridge links between social science disciplines that have traditionally been isolated from one another. On the one hand there are "micro" disciplines that focus on the behaviour of single individuals or individuals within small groups, such as psychology. On the other hand there are "macro" disciplines that focus on long-term or multi-regional patterns and trends, such as archaeology and anthropology. The problem is that scholars in these different disciplines seldom interact and seldom exchange ideas or findings. Sometimes one even finds a micro-macro divide within a single discipline, such as micro- and macro-economics. This creates explanations at one level that are inconsistent with the other, or at least unlikely. The aforementioned assumption in sociology that innovations are adopted independently rather than socially is an example of this, inconsistent as it is with evidence from social psychology. Ever since the "evolutionary synthesis" that occurred in the 1930s and 1940s, biologists have realised that micro and macro are complementary, and have developed tools to link the two levels. Palaeontologists studying macroevolutionary trends over millions of years share the same theoretical framework - Darwinian evolution - as molecular biologists studying the intricacies of genetic inheritance. In the same way, an evolutionary approach to culture can serve to unify the social sciences. Archaeologists studying macroevolutionary cultural trends over hundreds or thousands of years, and psychologists studying the decisions that people make during their lifetimes regarding who to copy when acquiring a new skill or when solving a problem, would realise that they are studying the same phenomenon but at different scales, one macro, and one micro. The result would be a rigorous, quantitative and synthetic evolutionary science of culture. Such a science would be able to explain not only how our species has managed to adapt - not genetically, like other species, but culturally - to virtually every terrestrial environment on the planet through agriculture, technology and socio-economic institutions. It may also help to solve some of the challenges we still face, which are often cultural in origin, such as how to prevent the spread of poisonous anti-democratic ideologies, how to change beliefs concerning climate change or energy consumption to prevent environmental damage, or how to make economic institutions more stable and less exploitative.