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There are certain organisms that you hear about a lot in evolutionary biology. In some cases, like Drosophila flies or E. coli bacteria, that’s because the organisms are easy to use in experimental studies. Other organisms, like Hawaiian silversword plants or Galapagos finches, come up frequently because they’re fantastic examples of evolution happening out in the “real world”. And then there are those rare cases where an organism is both a fantastic example of evolution in the field, and a convenient organism to work with in more controlled circumstances. The three-spined stickleback (Gasterosteus aculeatus) is one of those doubly-convenient organisms.
There are populations of three-spined sticklebacks in the ocean and in many freshwater streams and ponds. The oceanic populations have been around for a long time, but the freshwater populations are all relatively recent in evolutionary terms – they’re found in bodies of freshwater that were formed after the ice sheets retreated about 12,000 years ago. These populations appear to have evolved independently of each other, but they share a number of similar traits.
One of the more notable of the traits concerns the bony “armor” along the sides of the fish. The marine populations typically have a line of over 30 bony plates along their sides. The freshwater populations typically have only 6-9 of these plates. Why this is the case is a classic evolutionary biology question: do the freshwater populations lose the armor because there is a real advantage to losing the plates, or do they just lose them because there’s no real disadvantage to losing them.
Casey Luskin cites a news story about a recent scientific paper to support his view that the loss of the armor is just the result of the freshwater populations not facing the selective pressure seen in the oceans:
A scientific study published a few months ago reports that the marine stickleback (the ones with the armor plates) came before freshwater sticklebacks (the ones without armor-plating), meaning that this is not an example of the evolution of a new function, but an example of loss-of-function, or what one might term devolution. As a Science Daily press release on the paper stated, this evolution entailed “[s]hedding some genetically induced excess baggage”:
Shedding some genetically induced excess baggage may have helped a tiny fish thrive in freshwater and outsize its marine ancestors. Measuring three to 10 centimetres long, stickleback fish originated in the ocean but began populating freshwater lakes and streams following the last ice age. Over the past 20,000 years – a relatively short time span in evolutionary terms – freshwater sticklebacks have lost their bony lateral plates, or “armour,” in these new environments. “Scientists have identified a mutant form of a gene, or allele, that prohibits the growth of armour,” says UBC Zoology PhD candidate Rowan Barrett. Found in fewer than one per cent of marine sticklebacks, this allele is very common in freshwater populations.
Alas, Collins’ example, which is intended to break down the distinction between macroevolution and microevolution, really only provides evidence that populations of organisms can lose unique and complex features when selection pressure is relaxed. …
The scientific paper itself, however, reaches a very different conclusion:
Our results highlight the utility of direct measurements of natural selection on genes for understanding the genetic basis of adaptation by enabling us to test a mechanism favoring reduction of lateral plates in freshwater environments. Many of our results are consistent with selection against high plate number, although they do not rule out the possibility that selection is also occurring on genes tightly linked to Eda. Our results also expose opposing selection on Eda early in life similar in magnitude to the measured advantage of the low allele later in life. This demonstrates not only that countervailing selection pressures diminish the advantage of the low allele over the whole life span but also that the overall fitness effects of Eda do not seem to be determined solely by differences in lateral plate number. Along with the fluctuating dominance in fitness at the Eda locus, these results indicate that there may be additional pleiotropic effects of this gene. This work underscores the need for a synthesis of population biology and genomics, to determine the genetic basis of fitness differences in natural populations.
The two different opinions raise a key question: how can we tell the difference between selection in action and a lack of selection in action? The Science paper in question provides us with a simple answer: you do a hell of a lot of hard work to conduct a series of fairly simple experiments.
Dolph Schluter’s lab at the University of British Columbia has been working with the sticklebacks for a while, and one of his grad students, Rowan Barrett, took a look at the armor question. The experimental design was very simple: he went out and trapped a large number of sticklebacks in the ocean. He took a genetic sample from each of them, and checked to see if the fish in question had a copy of a gene that is known to cause a drop in armor plates (this gene is found in about 1% of the oceanic fish). He selected wild-caught fish that had one copy of the gene in question (heterozygotes), and placed similar numbers of the fish into each of four experimental freshwater ponds. The fish were allowed to breed, then Barrett sampled the offspring repeatedly during the following year to follow any changes in the gene frequency over time.
The easiest way to show you what Barrett found is just to show you one of the figures from his paper:
(Modified from Figure 2 of Barrett et al 2008)
That figure shows the percentage of alleles on the x-axis and time on the y-axis. The four lines on the graph represent the four separate experimental ponds.
If there’s one thing that stands out on that graph, it’s the agreement among the lines. At least three of the four are always doing the same thing at the same time. The pattern is not remotely random-looking. If the changes in gene frequency were simply the result of a selective pressure disappearing, we would not at all expect to see a graph like that one. We’d expect to see each of the ponds doing its own thing, and the gene frequency remaining more or less stable over time. It’s also clear from the graph that whatever is going on is not simple. There’s clearly a strong selective pressure favoring the armor gene early in the lifespan of the organisms, which is then followed by a strong selective pressure against the gene. It’s going to be interesting to see what the Schluter lab turns up as they continue to study these organisms.
This experiment does clearly illustrate one thing, though: talking about “devolution” or “loss of function” doesn’t make much sense at all when we’re talking about evolutionary biology. At least as far as these fish are concerned, not having armor isn’t a lack of a trait. It is a trait. A lack of armor has its own set of advantages and disadvantages, and is – as Barrett illustrated – subject to both positive and negative selection. Saying that freshwater sticklebacks simply lack the armor trait doesn’t make any more sense than it would to say that the saltwater sticklebacks lack the “lack of armor” trait.
It’s just another example of something that’s easy to lose sight of: organisms do not exist to manufacture complex traits. Complex traits exist because – and when – they help manufacture more complex organisms.
R. D. H. Barrett, S. M. Rogers, D. Schluter (2008). Natural Selection on a Major Armor Gene in Threespine Stickleback Science, 322 (5899), 255-257 DOI: 10.1126/science.1159978
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