Olivia Judson: A Natural Selection

July 22, 2008, 7:44 pm
A Natural Selection
(The fourth part in a series celebrating Charles Darwin.)
Last week, I discussed how evolutionary biology has changed since 1859, the year Darwin first published “On the Origin of Species.” But the subject of evolution isn’t the only thing that’s changed since then. There’s been plenty of actual evolution, too. For although we tend to think of evolutionary change as being something that only takes place over the course of millions of years, it isn’t. It’s going on here, now, all around us. So, this week, I thought I’d round up some examples of recent evolutionary change in nature. (What do I mean by recent? Within the last 40 years.)
I’m not intending to be comprehensive — that would take a book or two. Instead, I want to sketch a few examples of natural selection that have caught my fancy, and through them consider different aspects of evolutionary change, and what it takes to show it.
Galápagos finches. No discussion of evolution in nature would be complete without mention of the evolution of beak size in finches in the Galápagos archipelago.
Every year since 1973, large numbers of medium ground finches (Geospiza fortis) living on the island of Daphne Major have been marked, weighed and measured, and so have their chicks. In these finches, survival largely depends on the ability to open seeds; this depends on beak size. Bigger beaks allow the opening of larger seeds. How many seeds there are depends on the weather; some years seeds of all sizes are abundant, and the finches thrive. In other years, most seeds are scarce, and many birds die. Large-scale death affects the genetic make-up of the population, because both beak size and body size has a large genetic component. If all the birds with smaller than average beaks die in a given year, they take their genes with them.
Over the course of 30 years, annual measurement of finches shows that both body size and beak size evolved significantly. But they didn’t do so in a smooth, consistent fashion. Instead, natural selection jittered about, often changing direction from one season to the next.
As the abundance of different seeds fluctuated, so too did the beak sizes. One year, larger beaks were more successful; then it was smaller beaks. Over time, the average shape of the beak kept shifting, but it did so in an unpredictable, erratic sort of way, like a drunk man staggering about. Thus, some of the most dramatic changes were later reversed, and if beaks had only been measured at the beginning and at the end of the thirty years, the total amount of evolutionary change would have been underestimated. (Beak size has continued to evolve: the arrival on the island of a competitor for large seeds has subsequently favored small beak sizes in Geospiza fortis. Many individuals with larger beaks starved to death.)
Field mustard. Between 2000 and 2004, southern California had a severe drought. For many plants, including field mustard (a scrawny annual plant with little yellow flowers), a drought means a shorter growing season. A shorter growing season means that plants that flower earlier are more likely to leave seeds than plants that flower later — which are in danger of dying before they’ve finished reproducing. Since flowering time has a large genetic component, a drought — by favoring plants that flower earlier — could cause an evolutionary shift towards early flowering.
Has it?
Yes. The beauty of plants is that they make seeds — small packets of genes that can be stored for a period. This means that the genes of the past can, in principle, be compared directly with the genes of today. And an experiment in which field mustard plants grown from seeds collected in 1997 and in 2004 were planted together, under controlled conditions, showed clear differences in flowering times: the plants from 2004 flowered significantly earlier.
Moreover, in both years, seeds were collected from two sites, one where the soil is sandy and doesn’t hold water well, and the other where the soil stays wet for longer. As you’d expect, plants from the dry site showed a more dramatic shift than plants from the wet site. In the course of just 7 years, then, natural selection caused the plants to evolve an earlier flowering time.
Croatian lizards. In 1971, five pairs of adult wall lizards (Podarcis sicula) were brought to the tiny Croatian island of Pod Mrčaru from the nearby island of Pod Kopište. These five pairs have since given rise to a thriving lizard population — and one that has developed some interesting differences from the lizards that live on Kopište.
Lizards on Mrčaru now have larger heads and stronger bites than those living on Kopište, and they eat far more in the way of leaves and other plant material. Whereas the diet of native Kopište lizards is only about 7 percent plant matter, Mrčaru lizards are much more prone to a vegetarian habit. In spring, their diet is about 34 percent from plants; in summer that almost doubles, to 61 percent.
Plants are hard for animals to digest, and most plant-eaters rely on micro-organisms to help them. They also, typically, have complicated stomachs — think of the fermentation chambers in a cow, or the enlarged crop of that strange leaf-eating bird, the hoatzin. Intriguingly, the Mrčaru lizards appear to have evolved something similar. Their stomachs now have cecal valves, which divide the stomach into compartments, allowing for slower digestion and fermentation. Cecal valves are rare among lizards and snakes: fewer than 1 percent of species have them. At the same time, the Mrčaru lizards have acquired some novel micro-organisms in their guts (but whether these are helping break down plant fibers, or are some sort of sinister parasite, remains to be seen).
This study is one of the most intriguing I’ve come across. It suggests that arrival in a new environment can result in dramatic changes to an organism within fewer than 40 lifetimes. But so far, the basis of these various changes remains unknown: there’s an outside possibility that they are induced by leaf eating, and are thus due to the environment rather than genetics. (This seems unlikely — even lizards that are just hatched, and haven’t had a chance to do much eating, have the valves. But without doing the genetics, we can’t be sure; until that has been looked at, the changes cannot definitely be attributed to natural selection.) For now, natural selection for efficient plant-eating is the main suspect for this whole suite of changes, but the case is not yet closed.
Other examples. I don’t have space to go into other examples in detail, but to give a sense of what else is out there, here’s a partial list.
The fruit fly Drosophila subobscura has been evolving bigger wings in higher latitudes in North and South America; mosquitoes that live in pitcher plants hunker down for the winter later in the year than they used to; in a forest in southern England, great tits have been shrinking (great tits are songbirds).
Double the time frame to the past 80 years, and I’d have to add many more; of these, my favorite is the decline in head size of Australian frog-eating snakes in response to the arrival of poisonous toads in 1935 (a smaller head makes it harder to eat a deadly toad). And I haven’t even begun to mention the countless examples of pests that have evolved resistance to pesticides and bacteria that have evolved resistance to antibiotics, nor the thousands of laboratory experiments showing evolution in the simple environments of test tubes and petri dishes. Also omitted: several examples of new species that are in the process of forming (I want to look at these in a future column).
In short, evolution never takes a vacation: it’s going on all the time.
Yet we tend not to notice it. Why? The finches can help us here. That study tells us two things. First, from one year to the next, even the most dramatic changes are, to our eyes, small — which is to say, you have to measure them to detect them. The reason is that although birds differ from one another in their abilities to handle the various seeds, the differences are subtle. It’s not as if one bird has a beak 100 times mightier than another’s. When you add to this the tendency of natural selection to jerk around, it’s no surprise that we often don’t notice evolution as it happens. It also sheds light on why changes in the fossil record often appear to be slow: these studies show that change can be continual without really getting far from the starting point. Second, getting data as good as that is hard work. Most datasets are not so complete or robust.
At least one other lesson can be drawn from all these studies. Natural selection has its most dramatic effects when an organism’s environment is perturbed in some sustained way — prolonged droughts, the arrival of species that compete for food, warmer winters, the use of pesticides. If we humans continue to increase our impact on the globe, we’re likely to see lots more evolution. And soon.
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NOTES:
For beak size in Galápagos finches, see Grant, P. R. and Grant, B. R. 2002. “Unpredictable evolution in a 30-year study of Darwin’s finches.” Science 296: 707-711 and Grant, P. R. and Grant, B. R. 2006. “Evolution of character displacement in Darwin’s finches.” Science 313: 224-226. For evolution of flowering time in field mustard, and for its genetic basis, see Franks, S. J., Sim, S. and Weis, A. E. 2007. “Rapid evolution of flowering time by an annual plant in response to a climate fluctuation.” Proceedings of the National Academy of Sciences USA 104: 1278-1282. For the evolution of cecal valves in Croatian lizards, see Herrel, A. et al 2008. “Rapid large-scale evolutionary divergence in morphology and performance associated with exploitation of a different dietary resource.” Proceedings of the National Academy of Sciences USA 105: 4792-4795.
For wing size in fruit flies, see Huey, R. B. et al 2000. “Rapid evolution of a geographic cline in size in an introduced fly.” Science 287: 308-309 and Gilchrist, G. W. et al 2004. “A time series of evolution in action: a latitudinal cline in wing size in South American Drosophila subobscura.” Evolution 58: 768-780. For hunkering down time in mosquitoes, see Bradshaw, W. E. and Holzapfel, C. M. 2001. “Genetic shift in photoperiodic response correlated with global warming.” Proceedings of the National Academy of Sciences USA 98: 14509-14511. For body size in great tits, see Garant, D. et al 2005. “Evolution driven by differential dispersal within a wild bird population.” Nature 433: 60-65. For head size in Australian snakes, see Phillips, B. L. and Shine, R. 2004. “Adapting to an invasive species: toxic cane toads induce morphological change in Australian snakes.” Proceedings of the National Academy of Sciences USA 101: 17150-17155.
Many thanks to Dan Haydon, Gideon Lichfield and Jonathan Swire for insights, comments and suggestions.