Chapter Six

Was the diversification of mammals due to luck?

Why study the mammalian radiation?

The diversification of mammals is often considered a classic example of ecological opportunity prompting evolutionary radiation. The fossil record shows a dramatic increase in the diversity of placental mammals as they evolved to occupy the niches left vacant by the extinction of the dinosaurs. But date estimates based on DNA analysis have been used to question the assumed timing of this radiation, suggesting that the major placental mammal lineages diverged from each other long before the dinosaurs’ demise. Given that different molecular dating analyses give very different estimates for the timing of the mammalian radiation, how reliable are these molecular dates? Can we estimate the timing of divergence events from DNA sequences when rates of change vary between lineages?

What are the main points?

• Molecular dates are a powerful tool in macroevolution because they can reveal the timing of diversification events.
• Genetic differences accumulate in evolving lineages, so the amount of genetic difference between two lineages can give an indication of how long it has been since they last shared a common ancestor
• Rates of molecular evolution vary between species, complicating the inference of dates of divergence from molecular data.
• Molecular dates rely on a large number of assumptions about the evolutionary process and changing the assumptions can influence the date estimates obtained

What techniques are covered?

• Molecular evolution: to understand how the genome records evolutionary history, we need to understand the mechanisms of DNA sequence change over time.
• Molecular dating: comparisons between DNA sequences from different species can be used to infer an evolutionary timescale.

What case studies will be included?

• Bayesian molecular dating: timing of the radiation of placental mammal orders