Field of Science

Evolution does mean better and more complex

ResearchBlogging.orgThe American Society of Human Genetics has a quick little quiz on evolution. Unfortunately they get one question wrong:

I am well aware that it is a common objection that evolved does not mean 'better', and that evolution is not synonymous with 'progress'.

I am also quite familiar with the examples of loss of for example eyesight in many species, and this is often taken to mean that the animal becomes more simple.

However, as living things evolve, they do in fact become better. Better at what? Better at reproducing. Better than who? Better than the previous generations. Natural selection favors those organisms that reproduce more, and so over time the population as a whole will become better at reproducing. But is this always so?

No, it isn't always true that a population evolves to reproduce more. There are exceptions, in nature and in theory. These are well understood. For example, Wilke et al. (2001) found that a population favors a genotype with lower fitness if the neighboring genotypes are equal in fitness (a plateau), compared to a higher fitness genotype with neighbors that have even lower fitness (a sharp peak). Thus, less reproduction, but fewer low fitness offspring saves the day.

However, on average, those that are better at reproducing are the ones that have the most descendants, and thus living things do evolve to become 'better' in this sense. I suspect that the reason many people have gone away from this kind of thinking is that it seems like an endorsement of the view that humans are more evolved and thus better than other species. Well, this is just not what it means in terms of evolution. Other species are more or less as evolved as humans, and a comparison between species is not really possible the way I have here compared organisms within a population. When two populations live in different niches, they aren't really competing with each other to make the most offspring. They have become different species.

Secondly, the quiz does not state what is meant by 'more complex'. This is highly important, as there are many measures of complexity. A quantitative measure is required if we are to stringently compare different organisms or species. A vague notion that 'more complex' means bigger, more civilized, more intelligent, more diverse, and/or a larger genome just does not work. If any of those are meant, then that's what people should say instead of 'complex'.

There are many definitions of complexity, and I suspect most researchers think of structural complexity when they don't specify which kind they are talking about:
Structural complexity is generally what we mean when we consider animals, but this seems to be the hardest measure to define. McShea(6) has studied several measures of structural complexity, based on number of cell types, different limb-pair types, and even the fractal dimension of sutures in ammonoids. [Adami, 2002]
Given this problem of measuring structural complexity, Adami proposes that physical complexity is the best quantitative measure, and that this kind of complexity does increase in evolution:
Arguments for or against a trend in the evolution of complexity are weakened by the lack of an unambiguous definition of complexity. Such definitions abound for both dynamical systems and biological organisms, but have drawbacks of either a conceptual or a practical nature. Physical complexity, a measure based on automata theory and information theory, is a simple and intuitive measure of the amount of information that an organism stores, in its genome, about the environment in which it evolves. It is argued that physical complexity must increase in molecular evolution of asexual organisms in a single niche if the environment does not change, due to natural selection. It is possible that complexity decreases in co-evolving systems as well as at high mutation rates, in sexual populations, and in time-dependent landscapes. However, it is reasoned that these factors usually help, rather than hinder, the evolution of complexity, and that a theory of physical complexity for coevolving species will reveal an overall trend towards higher complexity in biological evolution. [my emphasis.]
The answer to the quiz question is given like this:
Living things do not have to be perfect to survive, just good enough for a given environment. Insects evolved mouth parts adapted for chewing (A), lapping (B), and siphoning (C), all of which are different but not necessarily better. All originated from a common ancestor, driven in part by the evolution of plants. However, other organisms such as sharks have changed very little over millions of years. Also, species can evolve simplicity too. While it is true that all species originally evolved from simple cells, as time passed “less” became more beneficial. For example, humans no longer have the tails of our distant ancestors, and blind mole rats lost the vision of their ancestors.
Who said anything about perfection? And the fact that sharks and many other species have evolved very little for millions of years doesn't even address the question asked. Blind mole rats (and many other blind species) have lost vision for a reason. They certainly didn't become 'simpler'. On the contrary the evidence suggest that blind cave fish lost vision because of pleiotropic constraints. Some of the genes that are involved in making eyes are also involved in jaw development. PZ Myers wrote a readable article in Seed about this loss of vision:
What we have is a perfect example of an evolutionary tradeoff. Because hedgehog and pax6 are negatively coupled to one another, one can be expanded only at the expense of the other, and what is going on in the blind cavefish is not selection for an economical reduction of the eyes, nor the accidental loss of an organ that has no effect: It is positive selection for a feature that is only indirectly related to the eyes.
It's frankly a little sad that ASHG didn't make a better evolution quiz than this. As we celebrate Darwin's 200th birthday this year, we will see and hear a lot about evolution, and that's going to be exciting. However, let's strive to get it right, and when there isn't a clear consensus, make that clear as well.

Claus O. Wilke, Jia Lan Wang, Charles Ofria, Richard E. Lenski, Christoph Adami (2001). Evolution of digital organisms at high mutation rates leads to survival of the flattest Nature, 412 (6844), 331-333 DOI: 10.1038/35085569

Christoph Adami (2002). What is complexity? BioEssays, 24 (12), 1085-1094 DOI: 10.1002/bies.10192


  1. Re: "However, as living things evolve, they do in fact become better. Better at what? Better at reproducing. Better than who? Better than the previous generations."

    Some years ago I heard about a Danish study of the reproduction capabilities in men (in the military). The study investigated the sperm count of these individuals and found that it was severely reduced for those whose parents had received help to conceive children.

  2. Evolution doesn't care how reproduction takes place. If people conceive artificially, then that might affect the average sperm count in the population negatively. When the day arrives that this technology isn't available anymore, then the average fitness will probably plummet as a result.

  3. I think "better" is simply too "vague", and the alternative version of this post might say: "Yes, in many, if not most, cases organisms do evolve to be more physically and structural complex, yet the principle of natural selection favors only the ones most suited for survival and reproduction and there are other ways of achieving that besides increased complexity."

  4. But the point is that even when dumping eyes, for example, it is not done by lowering complexity (i.e. any measure thereof). Increasing your fitness will result in increased complexity in most cases (for genetic and physical complexity).

  5. Great post.

    One question it brings up in my mind is how genetic remnants of previous "complexity" play into the definitions of complexity.

    For instance (just as a random example), when the blind cavefish lost its eyes, it no doubt left behind lots of small genetic regions (such as regulatory domains) that no longer serve a purpose. Thus over time, even if it is gaining complexity in its jaw, while losing complexity in its eye, the genomic underpinnings of both developmental processes are accumulating mutational changes (whether advantageous or neutral).

    I guess I was just curious how much even things like pseudogenes or leftover unused regulatory regions can be considered increased complexity from a genomic standpoint.

    For the record, I'm pretty ignorant on the definitions of complexity and such from an evolutionary biologist perspective. So forgive me if this is a stupid question...

  6. I think the point about the loss of vision on the cavefish is that the changes to the jaw (which are beneficial, meaning better in that environment) are contingent upon the loss of vision. The pleiotropic constraints require it as the edges in the regulatory network happens to be wired. Thus, the overall complexity increases.

    Physical complexity also increases as vision is lost and jaws change, I would say, but I will make sure to ask Chris Adami (who's my prof) what he has to say...


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