Bio Basics: Convergent and Divergent Evolution

By now, you know I’m a [mathematical] biologist. You may not know that I used to be a biology education content writer or that evolution is my absolute favorite thing to talk about.

Why do I love talking about evolution?

There is so much complexity behind evolution. There is so much to learn about it from the origins of different species, to what speciation itself means, to the very molecular bases of genetics, to mathematical studies of evolution.

And what’s probably an unpopular opinion for a biologist: I think debates and discussions about the truthfulness of theories of evolution are worth having! There is bound to be something we’ve taken for granted about our knowledge of life that we discover to be wrong or only partly right.

Finally, I just love the complex simplicity of evolution. As we understand it, it’s a game with easy-to-follow rules that lead to myriad results. It’s a game that is never stagnant, where winners are only winners for a generally unpredictable amount of time.

What is evolution?

Basically, evolution is changes in groups of organisms over time. In other, more scientific, words, evolution is “descent with modification from preexisting species cumulative inherited change in a population of organisms through time leading to the appearance of new forms the process by which new species or populations of living things develop from preexisting forms through successive generations” (Merriam Webster).

A graphic I made for Expii

Let’s break that definition down with some simple examples.

“Descent with Modification”

Sexually produced organisms are not clones of their parents, and even asexually reproduced offspring can have genetic differences from the parent organism due to mutation. Offspring inherit genes from their parent or parents, and those genes build the organism’s phenotype, or traits.

So, if a clownfish is immune to the stinging of an anemone due to some piece of its DNA, and that clownfish has offspring, at least some of it’s offspring will very likely also be immune to the stinging of an anemone. Meanwhile other clownfish and their offspring may not have the ability to resist anemone stings.

It’s also fully possibly for a mutation to arise that doesn’t come from either parent. That’s not a possibility we think about often, but of course DNA can mutate – that’s how new genotypes and phenotypes come about in the first place.

“Cumulative Inherited Change”

Now, you can imagine that clownfish who are immune to anemone stings are able to hide out in anemones better than those fish who get stung. If predators can’t get into the anemones, then the clownfish who can hide there will be better at surviving. Over time, more of the sting-able clownfish will be eaten by predators than will the immune ones.

And so, over time and generations, you’d expect the proportion of sting-immune clownfish to increase in the population. Those are the fish that survive to pass on their genes, and the genes they pass on endow their babies with sting immunity, which allows them a higher chance of surviving to pass on those genes again.

“Appearance of New Forms”

You can imagine that generations and generations went by without any clownfish that were immune to anemone stings. Then, a gene or two mutated or a new mix of genes came about and now a variant has appeared that has such immunity. Before that variant existed, natural selection couldn’t ‘select’ for fish that are immune to anemone stings because there just weren’t any in existence. As the immune phenotype is selected for and passed down through generations, a popular new form of clownfish emerges.

“New Species…Develop from Preexisting Forms”

New species don’t just blink into existence out of nowhere. Eventually, non-immune clownfish and immune clownfish may no longer reproduce together or, if they do, they may not produce viable offspring. The two groups might become more than just variants of one species: they could form two wholly separate species.

Be careful not to mix evolution up with adaptation: adaptation is smaller-scale; adaptations and natural selection support evolution. Adaptations are those “inherited change[s]” that can accumulate into the evolution of populations. For example, the ability to withstand the sting of an anemone is an adaptation, while the accumulation of variant clownfish with such an adaptation, possibly to the point of speciation, is evolution.

Now, there are all kinds of patterns and causes and kinds of evolution, including convergent and divergent evolution.

What is Convergent Evolution?

In convergent evolution, organisms without a recent common ancestor develop the same or similar traits. An incredible example is insects and birds: birds and bugs are only very distantly related yet both have developed wings and flight.

Convergence is where evolution gets interesting, at least to me. Evolutionary processes are complex in that there are many variables, and yet it’s possible for a physical structure or behavioral trait to evolve separately in relatively unrelated species.

Think about that. Think about two groups of organisms facing similar selective pressures. Think about all the myriad possibilities, ones we can’t even imagine, that could arise to give those organisms a leg up against that selective pressure. Some species do develop different strategies to deal with the same issues, but somehow some species evolve the same adaptation(s) separately. Like I said, evolution is complex.

What is Divergent Evolution?

This is probably more similar to the ‘evolution’ you know. Divergent evolution is when individual organisms begin, over time, to seriously differ from one another and may eventually form separate species.

Okay, so what’s the difference between divergent evolution and speciation? Speciation refers to the specific “moment” of two groups of organisms becoming so different from one another that they no longer reproduce viable offspring together; meanwhile, divergent evolution can be thought of as the process by which two groups of organisms begin to differ from a common ancestor.

Analogous vs. Homologous Structures

And now, a favorite sub-topic of mine: analogous and homologous physical structures. These types of physical features correspond to convergent and divergent evolution, respectively.

Think about complex eyes. Plenty of species have complex eyes that may be built and behave in similar ways, and yet not all of those species have a recent common ancestor. Complex eyes have evolved separately many times, making the complex eye an incredible example of an analogous structure. Analogous structures are similar to one another and serve the same function across species.

To me analogous structures raise an important question about evolution: How, and why, do extremely similar structures, traits, behaviors, etc. arise separately more than once? I mean, couldn’t there be other solutions to selective pressures requiring enhanced senses besides eyeballs? Or is there something special behind the eye? Is there some sort of physical law – of geometry, of genetics, of nature – that will generally lead toward a complex eye, for example?

Credit Wikipedia

And then we have homologous structures. These are often used as evidence for the theory of evolution – these are anatomical structures across species that have been passed down and adapted from a recent common ancestor. The formations of bone across human arms, dog legs, bird wings, and whale flippers are homologs.

Here’s some of my old work @ expii as a content writer, if you’re interested:

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