A unifying theory.

Everywhere we look in the natural world, the hallmark of evolution is present. It creates and maintains the diversity of life on Earth. Here we're going to cover some of the underlying principles. How did this field of science develop? Where are we now? How does evolution (and natural selection) scale from mutations, through to populations and species? We cover all this and more here. This material forms the basis from which other Evolution and Palaeobiology websites build.


This week, we're going to cover:

  • Evolution, a history – Section 1.
  • Evolution is a theory – Section 2.
  • Evolution is true – Section 3.
  • The scale of evolution – Section 4.
  • Species – Section 5.
  • Origin of species – Section 6.

There are, of course, whole text books covering evolution. I link to some further, free, reading at the bottom of this page.

1 – Evolution, a history

It's always a good idea to think about the process of discovery by which we got to where we are now. We don't have much time for this in this course, but here is a brief overview!


  • We're going to be covering morphology and general principles of evolution today. For the evolution of DNA, look to the bonus materials!
  • We've covered the basics elsewhere. If you would like to catch up on those, you can do so in Evolution 101.
  • The roots of evolutionary thought run deep.
  • Key strides towards our modern understanding of evolution can be traced to the enlightenment, and its move towards evidence-based approaches.
  • Natural selection was proposed by Darwin and Wallace in the 1850s.

That passage that I mentioned, in which James Hutton struck upon some ideas close to natural selection, but which didn't catch on at the time, can be found in his 1794 work An Investigation of the Principles of Knowledge. In this tome (and it is a big book) he wrote:

..if an organised body is not in the situation and circumstances best adapted to its sustenance and propagation, then, in conceiving an indefinite variety among the individuals of that species, we must be assured, that, on the one hand, those which depart most from the best adapted constitution, will be the most liable to perish, while, on the other hand, those organised bodies, which most approach to the best constitution for the present circumstances, will be best adapted to continue, in preserving themselves and multiplying the individuals of their race.


This, and the quote I mentioned from the Linnean Society president after Darwin and Wallace's paper, just goes to show that sometimes, in science, it is hard to immediately identify important ideas.

2 – Evolution is a theory

That title just above. It has some words. But what do those words mean? Given that I am creating this website around bonfire night, that question brings to mind a quote from Alan Moore's V for Vendetta:


Words offer the means to meaning, and for those who will listen, the enunciation of truth.

But that truth depends on the context of the words, and the word theory means different things scientific language to what it means in our day to day discussion. In this video, we'll learn where we are today, in terms of evolution, and explore properly what the word theory means in science.


  • Today, we study evolution in many different ways.
  • In science, if something is called a theory it summarises a body of knowledge comprising facts, hypotheses and laws.
  • A theory is as well established as anything can be within an evidence based framework.
  • Modern science operates on falsifiability, and theories can be falsified.

3 – Evolution is true

True is a tricky word in science. But many us are most comfortable using this concept to talk about something as well substantiated as evolution. Let's learn why that is.


  • Evolution is true.
  • Evidence supporting evolution is drawn from diverse fields, including:
    • The fossil record
    • DNA sequences
    • Imperfections
    • Experiments
    • Biogeography
    • Selective breeding
  • Consilience occurs when many independent lines of evidence agree on a conclusion.
  • This is the case with evolution, however, for a range of reasons, some people remain resistant to the idea.

4 – The scale of evolution

Evolution scales from molecular scales, to whole animals, and from seconds to millions of years. Let's explore this more!


  • In evolution, changes start with DNA (genotype), but are then linked to changes in morphology (phenotype) through development (ontogeny).
  • At a molecular (DNA) level, things tend to be a bit random.
  • We can consider fitness – the ability to leave viable offspring – for an individual, or consider also an individual's kin.
  • Individuals belong to populations, which can be subject to directional or stabilising selection.
  • When a population changes over time, we can see anagenesis (change within a lineage) or cladogenesis (the splitting of a lineage).
  • Whether large scale evolutionary change is just a sum of all the small scale dynamics remains debated.

5 – Species

A useful concept in understanding the living world is that of the species. But how do we define a species? What are the strengths and weaknesses in how we do so? In this section we look into just this question when meeting two important species concepts.


  • The biological species concept, based on reproductive isolation, is useful for living groups.
  • There are however, exceptions, such as ring species, and circumstances in which we can't apply this definition (asexual organisms; fossils).
  • In the world of fossils, we use morphological or phenetic species concepts.
  • Whilst we have developed a number of conventions to try and maximise the rigour of this approach, it can still be challenging for fossils.

I said I would embed that model of Lucy's skull in the website. Behold:

Lucy was a member of an extinct hominin species that lived from ~3.9 to 2.9 million years ago in Eastern Africa.

6 – Origin of species

So, we have learned about species. But how do they evolve. Let's find out!


  • Speciation is the formation of species.
  • Geography is an important consideration in speciation, which can occur with overlapping, semi-overlapping and non-overlapping populations.
  • There are a wealth of mechanisms by which reproductive isolation can occur.
  • The fossil record is unique in allowing us to see speciations play out over longer periods of time than we can observe in studies today.

Bonus stuff!

Nice one, you have completed Evolution 201. Here are some bonus materials if you're interested.

Want to know more?

As I said at the start, there is a wealth of research on – and exciting stuff to learn about – evolution. There is also, sadly, a lot of disinformation in the world and on on the internet regarding the topic. This is, in part, driven by creationists in the US. One organisation fighting the good fight against this movement is the National Centre for Science Education. The reason I mention them, beyond the brilliant work that they do, is that they have made a wide range of trustworthy reading material available for free on the website. If you want to read more, this is an excellent place to start: NCSE – Free evolution and climate change book downloads.

How did Lucy die?

I used Lucy, the famous Australopithecus fossil, several times as an example. If you're interested in learning more about this particular fossil, the latest research, which I have linked below, actually goes as far as trying to figure out how this individual died:

Kappelman, J., Ketcham, R.A., Pearce, S., Todd, L., Akins, W., Colbert, M.W., Feseha, M., Maisano, J.A. and Witzel, A., 2016. Perimortem fractures in Lucy suggest mortality from fall out of tall tree. Nature, 537(7621), 503-507.

Do, however, bear in mind, that as with everything in palaeontology (and, indeed life), we can't be certain - and not everyone agrees. See for comparison:

Charlier, P., Coppens, Y., Augias, A., Deo, S., Froesch, P. and Huynh-Charlier, I., 2018. Mudslide and/or animal attack are more plausible causes and circumstances of death for AL 288 (‘Lucy'): A forensic anthropology analysis. Medico-Legal Journal, 86(3), 139-142.

If you're interested in a more general overview of the importance of this species when it comes to our own origins, this provides a summary, as of 2009:

Kimbel, W.H. and Delezene, L.K., 2009. “Lucy” redux: A review of research on Australopithecus afarensis. American journal of physical anthropology, 140(S49), 2-48.