Chapter Three

Does evolution favour increased size and complexity?

Why study the evolution of size and complexity? Evolution of life on earth has led to increased size and complexity: the biggest animal that ever lived is alive today, as is the most complex. If we go back in time a billion years, all life would be much smaller and simpler. Does this mean that evolution favours increased complexity over simplicity, and that larger animals have a selective advantage over smaller? Or is it a biased perspective: if you start with the largest and most complex species and trace back to its ancestor you will inevitably see an increase in size and complexity over time, even if most lineages either stay the same (e.g. most bacteria) or undergo simplification (e.g. many parasites). “Cope’s rule” (body size tends to increase over time) and the increase in complexity provide convenient focus points for us to examine how we interpret trends in biodiversity over time.

What are the main points?

• Trends in the fossil record can provide an informative view of evolutionary change over time, but they can often be interpreted in a number of different ways.
• The evolution of horses, which has long been a classic case of an evolutionary trend in size and specialisation, provides a convenient illustration of the influence of data incompleteness on views of evolutionary change.
• Explanations for “Cope’s rule” (increase in body size) include microevolutionary advantage to larger individuals, macroevoutionary advantage to lineages with larger size in diversifying or avoiding extinction, and stochastic (undirected) evolution of body size resulting in a consistent increase in the maximum size over time.
• Complexity is hard to define and measure. There is no clear and simple relationship between genome size, morphological size and complexity.

What techniques are covered?

• Palaeontological information: fossils are patchily distributed in time and space, so patterns need to be interpreted not only in light of the information we have, but also the data we lack.
• Null models: before we conclude that a trend is due to selection, we need to ask whether the same pattern could be due to a random walk (undirected change over time).
• Genomic complexity: evolutionary patterns in complexity depend on how you define and measure it.

What case studies will be included?

Accounting for measurement bias: Has biodiversity increased over time?