Macroevolution.

Evolution happens at a number of scales, from small shifts in population, to the rise and fall of phyla. This lecture focusses on the larger scale patterns.

Introduction

We're going to cover:

  • Adaptation – Section 1.
  • Macroevolution:
    • Concepts & Clocks – Section 2.
    • Rates – Section 3.
    • Adaptive radiations – Section 4.
    • (Palaeo)EvoDevo – Section 5.

Understanding evolution on these scales is fundamentally fascinating – or at least, I find it so. But it is also really important if we want to see how, for example, life and the planet Earth have interacted over the past several billion years. Evolution, on a large scale, dictates so much about the nature of the world around us, that it's harder to find somewhere where it has no impact, than where it does!

1 – Adaptation

Evolution is driven by adaptation (although as we find out, exactly how true this statement is, is subject to debate).

We've not really covered adaptation in this course yet, so here seems like a great place to start!

Summary

  • Adaptations help an organism survive and reproduce in its environment.
  • A common process in evolution is exaptation (~the change in function of a trait).
  • We also have to consider the origin of novelty in evolution.
  • Evolution may look different to us depending on the scale we study it.
  • Important points of debate in this realm of evolutionary biology include:
    • To what extent evolution is driven by adaptation v.s. constraints.
    • The roles and prevalence of contingency, determinism & convergence.

Something to think about

Is adaptedness the same thing as fitness? I think this is a really interesting question.

Have a think – I'll put some of my own thoughts at the bottom of this page.

2 – Concepts and clocks

The main topic of this lecture is macroevolution. What is this? Well in order to dig into that, I wanted to quickly get us on the same page with a couple of terms I'll be using a lot. These are:

  • Interspecific: existing or occurring between different species.
  • Intraspecific: existing or occurring within a species or between individuals of a single species.

I will try and pronounce each of these very clearly as they sound kind of similar!

Summary

  • It is useful to consider macroevolution as a process that sorts interspecific variation.
  • The red queen hypothesis emphasises biotic processes as drivers in evolution over abiotic ones (which are sometimes called the court jester).
  • By calibrating the rate evolution with fossil ages, we can estimate clade divergence times, and thus evolutionary rates.

Note: Look to the bonus section for the article I wrote that attempts to explain clocks in an understandable way.

3 – Rates!

Cool cool cool. So we can estimate rates. But what can these tell us? Let's find out!

Summary

  • Molecular and morphological clocks, and the trace fossil record, suggest that the Cambrian explosion may have been a short period of elevated rates of evolution.
  • The idea that evolution is gradual and of ~constant rate (phyletic gradualism) held sway for a long time.
  • An alternative – that evolution happens in bursts with stasis in between (punctuated equilibrium) – is quite hard to test.
  • It seems likely that different environments lend themselves to different variation in rates.

4 – Adaptive radiations

Things called adaptive radiations are thought to be an important factor in the origin of the biodiversity of today's ecosystems, and those of the past. Let's learn about these, then think about how they, and rates, might interact.

Summary

  • An adaptive radiation is an event in which a lineage rapidly diversifies from an ancestor.
  • Newly formed lineages evolve different adaptations.
  • Rates of evolution are thought to be elevated at the start of these events in many instances.
  • We see potential examples of this in both marine reptiles and mammals.

5 – (Palaeo) Evo Devo

Lets finish by looking how the development of an organism – its ontogeny – can help us understand its evolutionary history. And what we can tell of development in the fossil record.

Summary

  • Some elements of ontogeny – especially the genetics of development – can shed light on evolutionary history. Examples include:
    • Showing the homology between different segments of the arthropod head
    • Clarifying the origins of insect wings
  • There is evidence of the ontogeny of complex organisms preserved in the fossil record, but in places this can be challenging to interpret.

Bonus stuff!

Well done, you've reached the end of this website on macroevolution. I hope you have enjoyed learning about it as much as I enjoyed writing it! Here is some bonus material that I hope is also interesting.

In which Russell does his best to explain molecular clocks

As I mentioned above, I didn't the time to explain clocks in the detail I would ideally like – there's too much exciting science to cover, and not enough time. But I have written an explanation of how Bayesian approaches to phylogeny work, in their basic form, that you can find here:

Garwood, R.J. 2020. Patterns in Palaeontology — Deducing the tree of life. Palaeontology [online] 8(12):1-10.

If you want to know more about how they work in terms of clocks, then give me a shout, and I will happily explain!


Is adaptedness the same as fitness?

I asked you above, whether you thought that being highly adapted (showing high levels of adaptedness) was the same as fitness. I think this is a really interesting question (hence me asking).

In brief, within a given environment an adaptation – to that environment – does make an organism fitter. But being highly adapted does not always mean you will have a higher fitness. For example, think about the example I have used previously of the avocado with its giant seed. This adapted to being distributed by a megafauna that is now extinct. While being highly adapted, because that fauna has now gone extinct, the avocado is no longer particularly fit – there is nothing out there left to spread its seeds. This is one example of how these two things can become decoupled.