So You Think You Know Evolution?

D for Darwin followed by E for evolution is simply irresistible! All right, roll up your sleeves, here we go.

  • Evolution happens to species, not individuals
  • Individuals carry the variations that allow evolution
  • Natura non facit saltum

We are starting with these things as facts and moving forward to learning about Evo Devo. That’s right, I said FACTS. “But it’s only a theory, so you can’t have facts,” I hear someone whine.

Raise your hand if you think “theory” means any of the following:

  • opinion
  • guess
  • hypothesis
  • idea
  • unproven

You raised your hand for all them, didn’t you? If not, congratulations!

Theory: an explanation for observations of the natural world constructed using the scientific method that brings together facts from more than one researcher and more than one discipline.

For a scientist, a theory is about as opposite from the list above as you can get. I’m not here to argue IF evolution has occurred and is occuring. It’s a done deal.

Moving on, HOW does evolution happen? There have been many ideas, even from Aristotle. You already know that Darwin was the guy to come up with the enduring theory that has been tested for the last 150 years and has gotten stronger with each experiment. He said, as an echo of others, that nature does nothing in leaps (Natura non facit saltum). He saw slow and gradual change as the mechanism of evolution.

What about punctuated equilibrium? This idea of stasis (equilibrium) with intervals of rapid change (punctuation) is very appealing. Look at some the weird things that happen. Surely this is evidence of punctuated equilibrium!

A few questions:

  1. Are these things from changes in the DNA?
  2. Are these things a result of something that happened during development?
  3. Are these things heritable?

The answer isn’t as easy as you may think. The cyclops lamb, it’s a birth defect due to an herb the mother ate during gestation. The fly with legs instead of antennae? Made by a researcher, but not necessarily heritable. Six digits in humans, changes in DNA and heritable. Extra wings on dragonfly, don’t have this individual, but other insects in the wild with extra wings, not heritable. Frog with weird extra legs? Weird story that. It’s caused by a parasite. Definitely not heritable.

This is where evolutionary biology and developmental biology cross roads and we get the term Evo Devo. What if the changes in development were hereditary? Would that give rise to new species? What are the genetics of development and how does that contribute to evolution?

There is a short answer. Yes. Not very satisfying? Pick up Endless Forms Most Beautiful. Those are Darwin’s words used as a title for Sean Caroll’s book on Evo Devo.

Back to the short answer. Big changes in morphology (body shape) can lead to new species, which is one way evolutionary biologists measure evolution. What clues can developmental biology give us about these changes? It turns out quite a lot!

There are genes called homeoboxes, hox genes, that control placement and number of body parts. Like humans with six digits and flies with legs instead of antennae. Simply put, one gene can control large changes in morphology. Are these hox genes the holy grail of punctuated equilibrium? Depends on who you talk to. Nature taking a leap would be many genes changing all at once in an individual.

It only takes one change in one hox gene to cause a large change in morphology. Gradual or a leap? One gene isn’t much of a leap. Which is the measure of evolution? Morphology or genetics? As geneticist, it seems clear to me: morphology is not heritable, but changes in genes are. Ergo, punctuated equilibrium is just a dream. Darwin is still right.

Read Sean Caroll’s book. He’s a brilliant researcher, fabulous writer, and seems like a genuinely nice guy. It’s one of my favorite books. It’s written at a level for the educated non-scientist.

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