In this module, you will be introduced to the different modes of selection that can operate in natural populations and how they can affect fitness. We will look at positive and negative selection (one allele is better/worse than the rest) first. Then we will look at the mystery of why sometimes multiple alleles are maintained in populations, with sickle-cell anaemia offering a good example of balancing selection.

We have so far tried to explain the spread of traits by considering how they increase the probability of an organism surviving, just like Darwin did initially. In this module, we look at two alternative explanations for why a trait might spread: sexual selection and genetic drift. When considering sexual selection, you will learn about the difference between intrasexual and intersexual selection and how these can lead to the evolution of traits that increase mating success despite reducing survival probability. We will also look at the idea of non-adaptive evolution and how sometimes an allele can increase in frequency despite no survival or reproductive benefits. Iris colour variation in humans is discussed as an example of genetic drift.

Behavioural adaptation is the most complex and impressive form of adaptation. In this module, we look at how the cost-benefit framework of optimally modelling, along with the concept of trade-offs, can let us understand the evolution of animal behaviour. We will look at a worked example – how high should a crow fly? The concept of proximate and ultimate causation is also discussed here, helping you to see how 'why' questions can be answered in biology.

Evolutionary biology is a fascinating science in its own right, but this final teaching week will help you see some of the practical applications of what you have been learning about. We look at evolutionary medicine (e.g., the Old Friends hypothesis, antibiotic resistance, and tumour resistance to chemotherapy) and predicting responses to climate change. We gesture towards some of the many other applications possible, from reconstructing galaxy evolution to understanding the evolution of Red Riding Hood.