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This is Science Friday from NPR News. I'm Ira Flatow. When you go hunting and fishing, what do you keep, and what do you throw back? We always keep the biggest fish, right? And we throw back the smaller ones. In fact, the game warden will fine you, give you a big fine, if you don't do that. And when you hunt for a game, you always go after the biggest bighorn sheep or the strongest, biggest caribou with the most impressive set of antlers. Well, these trophies may look good on your mantelpiece or cook up into a nice steak or stew, but hunting the biggest and the strongest fish in wildlife may not be helping the herd.

According to a paper out this week in the journal Proceedings of the National Academy of Sciences, human selection, as opposed to natural selection, tends to speed up the evolution of the hunted, whether it's bighorn sheep, fish or flowers, and that results in smaller species being left to develop behind them. Joining me now to talk about that is my guest, Chris Darimont. He is a postdoctoral fellow in the environmental studies department at the University of California in Santa Cruz. He joins us from a studio there. Welcome to Science Friday.

Dr. CHRIS DARIMONT (Postdoctoral Fellow, National Science & Engineering Research Council, University of California, Santa Cruz): Thanks very much, Ira.

FLATOW: So, we should be throwing the big ones back?

Dr. DARIMONT: I think there's some good rationale to, indeed, do that, throw back the large ones and allow them the opportunity to breed, so the traits we value, like size in the fish that we target, are here to stay.

FLATOW: Tell us about the spiral that we create by keeping the big ones and not the little ones.

Dr. DARIMONT: Well, I think everyone has a story to which they can relate, whether it be their grandpa telling them of these massive trout that he used to catch and that his grandkids can no longer catch. And it's a very simple process. We see it on our farms, for example, where we allow, say, the best producing cow to breed. Well, we do the same sort of thing in nature, but the consequences are far different; that is, we give a selective advantage to certain individuals, in this case, the smallest, because we've captured the largest, and the consequences are very different. Whereas on the farm, we get increasingly desirable traits in future generations, in the wild what scientists are reporting on more and more is increasingly what we term undesirable traits in prey like fishes or game or even plants.

FLATOW: Mm-hmm. So, we are changing the evolution of these fish and plants?

Dr. DARIMONT: We sure are, and you know, our study was the first to put together many studies that have reported on single case-by-case studies on specific species. And many of these studies reported seemingly very, very rapid changes. A good example comes from Canada, a population of bighorn sheep that hunters have targeted, large ones, for about 30 years, those with, you know, the big trophy horns. And in only three decades, they've seen a decline of about 20 percent in the average mass and the average horn size of this population, and that seems stunningly fast. That really raises eyebrows. But what we did is put all these studies together and ask a very simple question: How fast are those changes occurring? And we had a showdown, so to speak, within a database with other systems from nature. And what we've found was quite stunning.

FLATOW: And is the process reversible?

Dr. DARIMONT: Good question. It depends. It should be, because evolution has a characteristic whereby traits can be pushed in one direction and then in another direction. So, in theory, absolutely, things are reversible. But to create conditions that, say, horns will be allowed to grow bigger or fishes will be allowed to become bigger in the near future, we need to sort of radically change the way we target these wild organisms in our harvests. And what I mean by that is, as we mentioned, maybe forgoing our preference for the largest and, probably just as importantly, radically restricting the proportions of individuals within populations we take. So, instead of taking 50, 60, 70 percent of a fish population a year, we might think about taking far less per year to start the sort of reversals about which you speak.

FLATOW: You're going to reeducate all those fish wardens, game wardens, aren't you?

Dr. DARIMONT: Well, it's - there are big barriers. As you mentioned, cultural and management barriers; a good example of some of the cultural barriers is think about fishing derbies or the Boone and Crockett Club. Their very notion is that we should be targeting the very largest, and this is really deeply ingrained in our culture as sportsmen or sportswomen out there, and indeed, in socioeconomics systems, too. So, for example, fish boats; they're geared up to take the largest. So, to change their gear to target smaller fishes presents enormous...

FLATOW: Yeah.

Dr. DARIMONT: Economic barriers, too.

FLATOW: So, you're not just saying, if we take the biggest fish, we're leaving the smallest behind. You're saying something a little bit different. You're saying by leaving the smallest behind, the - we're allowing for the evolution of smaller fish. Is that correct?

Dr. DARIMONT: That's absolutely correct. What we're reporting on is the - what we call phenotypic evolution to smaller sizes. And all that means is that we are able, as scientists, to measure these traits in real time, over small chunks of human lifetimes, to measure with our rulers or our scales how the very essence of these organisms are changing. And they're changing very, very rapidly, much more quickly than the pace of evolution that we've observed in natural conditions or under other human-caused influences.

FLATOW: And what makes this a bad thing?

Dr. DARIMONT: Well, there are number of concerns that immediately result from our study, and probably the most important one relates to the reproductive ability of populations. So, one of the patterns we see in addition to a decrease in size, especially among the commercially-fished organisms, is we see a shift to breeding at earlier ages and at smaller sizes. Now, those younger, smaller breeders have far fewer babies, they produce far fewer eggs, and as a result, the population is much less able to recover from very high harvest pressure. So, this is not only a threat to many populations, this evolutionary process that we impose as humans, but it's also a threat to the very harvesting industries that depend on these fishes.

FLATOW: Now, you also talk about not just big-game fish, trophy game and fish, but about plants. How...

Dr. DARIMONT: Yeah, it's a...

FLATOW: How is it working with plants?

Dr. DARIMONT: Yeah, isn't that neat? Yeah.

FLATOW: Yeah.

Dr. DARIMONT: A good example is in the Himalayas. There's an incredibly beautiful plant called the Himalayan snow lotus. And a few years ago, researchers that had looked at specimens of the snow lotus over 100 or so years, about 105 years, noticed a vast decrease in the height at flowering. And they related this to size-selective plant harvesting by people in the Himalayas. The larger plants simply fetched more money at the markets; they were more powerful in the eyes of people that valued them for their spiritual powers. And as a result, if you kill these plants before they set seed and so those seeds that might code for plants that grow extra fast and extra big, those seeds don't enter future generations. So, we can see how this predation of plants, if you will, in a size-selective manner can lead to these changes in plants and, indeed, wide variety of organisms.

FLATOW: We have a Twitter from EmptySandwich(ph), a Twitter saying: Aren't the largest the oldest? Wouldn't they have already procreated the most?

Dr. DARIMONT: Good question, that's an excellent question. Well, it depends on breeding structures. But I'll give you an example, an Atlantic cod that's really instructive. So, we target the large ones, which are also, on average, reproductively mature individuals. So, as a consequence of that, those that reproduce earlier and at smaller sizes they have their babies before they enter that size class that become attractive to us as fishermen or easily caught in our nets. So, in fact, the absolute opposite is happening, that those who wait to reproduce and get older, they face such a high risk of being killed before they reproduce that we see this evolution towards reproduction at earlier ages.

FLATOW: Mm-hmm. So, where do you go from here? What else would you like to know?

Dr. DARIMONT: Well, we'd like to know a little more specifically how the speed of evolution works over the time series of exploitation. So, for many of the data in our database, we have data as to how fast they change from year to year, be they bighorn sheep or Atlantic cod or this Himalayan snow lotus. So, what we would like to ask is, are there periods over that time series, that duration of time we hunted them, where they underwent particularly rapid changes? And do those periods relate to, say, when our technology greatly evolved or our exploitation levels went through the roof? So, these are the sort of questions we're going to be asking now with the data.

FLATOW: Mm-hmm. And so, you're just going to analyze more of the data, or do you need to go out and collect more information?

Dr. DARIMONT: Well, luckily, many other people, such as a colleague, Stephanie Carlson from Berkeley, have done the work and published these really excellent papers on single species evolution, those that were hunted. So, we've got a tremendous database across a huge variety of organisms, and we've taken a first cut here and established that, yeah, the changes we're seeing in these harvested organisms are happening very, very quickly, even superseding this speed at which organisms are changing from other human influences, just like pollution.

FLATOW: Wow. But you think you could reverse that?

Dr. DARIMONT: Well, I think if society and management chooses to consider these evolutionary processes carefully - and I think we're starting to see this now - that potential solutions are there. If we decrease the amount of individuals we take every year and we, perhaps, forego our preference for the largest, then we could see some shifts back to things the way they used to be, the trope, the size, that Grandpa used to see, so to speak.

FLATOW: Mm-hmm. Chris, I want to thank you for taking time to be with us today.

Dr. DARIMONT: Oh, it's been my pleasure, Ira. Thank you.

FLATOW: You're welcome. Chris Darimont is an NSERC postdoctoral fellow in the Wilmers Lab of the environmental studies department at UC Santa Cruz. We're going to take a break and talk about something really mysterious: methane on Mars spewing out from someplace, from some unknown origin. Is it because of the rocks, geology, or is it because there might be unseen life below the surface there? Mysteries we'll talk about after this break. Stay with us.

(Soundbite of music)

FLATOW: I'm Ira Flatow. This is Science Friday on NPR News.

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