evolution - Why and how does complexity usually tend to increase through time?

The question of complexity is classic in the very first lectures of evolutionary biology where the teacher usually tries to tell the students that complexity does not necessarily increase and that humans are not more complex than other organisms.

My questions are:

• Why does complexity tend to increase through evolutionary time?


• What are the different hypotheses to explain this pattern?

When writing "Mass extinction" on google image, we find many graphs displaying the number of families (or other taxa) through evolutionary times with the five mass extinctions. What would it look like to draw such graph replacing the family richness in the y-axis by :

• Mean complexity among all living things?


• Complexity of the most complex taxon?

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I suppose that anyone who wishes to answer to this post will necessarily need to define the words "complexity". He or she might define it in terms of number of genes, number of metabolic pathways, length of DNA sequence, number of cell types, some kind of index taken from information theory. When asking my questions I had in mind a definition close to "number of genes" or "number of metabolic pathways".

Answer

This is actually a very interesting yet difficult question to give a single precise answer to. I will try and summarize for you a "meta answer":

Complexity Science Some consider complexity not to be a Biological topic as such, since it is a property that accumulates in non-biological systems e.g. economics, technology, music, language - in fact anything that "evolves" through time. This new field of science is called "Complexity Science" or "Complex Systems" and is primarily a field of mathematics or information theory: Complex systems

Complexity in Biology What I can say is that these kinds of questions have started an almost new field in Biology, called "diversity evolution", here is a very interesting paper: Diversity Evolution.

Defining Complexity You were right to mention Complexity first needs to be defined, and as an early field of science this is where much focus has been recently. There are quite a few definitions but it is perhaps too controversial to list any particulars... HERE is a whole UCL lecture dedicated to defining Complexity!

Topics of Complexity Science I have written enough, so perhaps this lovely diagram of the topics of Complexity Science will be of some help: