The Loom

Getting Sexier All The Time

swallow.gifI have a short piece in today’s New York Times about how male swallows are evolving longer tails, which female swallows find sexy. Here’s the original paper in press at The Journal of Evolutionary Biology. Measuring the effects of natural selection is tough work, the details of which are impossible to squeeze into a brief news article. Scientists have to document a change in a population of animals–the length of feathers, for example–but then they have to determine that the change is a product of genetic change. We are much taller than people 200 years ago, but it’s clear that most, if not all, of this change is simply a response of our bodies to better food and medicine. The authors of the swallow paper carried out a number of studies that suggest that the length of swallow tails is genetically based, and that those genes are changing. If they’re right–and other experts I contacted think they are–it’s a striking example of how quickly the sex lives of wild animals can evolve.

Things get a little fuzzier when the researchers propose what’s driving the evolution. They think desertification in the springtime range of the swallows in Algeria is to blame. But it’s very hard to eliminate other possibilities, since these swallows have complicated lives, migrating from Europe to South Africa and back every year. It’s much easier to make a case for the forces driving the evolution of Darwin’s finches, which generally sit obediently on the island on which they were born and are subject to cycles of droughts and heavy rains.

But it’s a question very much worth investigating. Global warming may well produce ecological changes that could produce just these sorts of rapid evolutionary changes in animals and plants. In some cases, species may be able to adapt quickly enough to their new environment. In other cases, they may lose the race.


  1. #1 Joseph Poliakon
    November 30, 2004

    AChTunG Bird Lovers! It’s not about Global Warming affecting genetics, but rather continuous improvement of barn swallow A-C-T-G sequencing to build better bug catchers. Longer barn swallow tail feathers mean better and snapper aerodynamic control pursuing bugs in flight, compared to shorter feathers. The attendant increase in bug catching volume of the improved long-feather bio-design, results in lower long-feathered baby bird mortality rates with more long-feathered birds surviving to breeding age.

    Trying to tie longer barn swallow feather evolution to Global Warming is a bit of counter-intuitive conjecture. Today’s barn swallow is not the 21st Century’s coal miner canary of Global Warming. Warmer climes would mean more bugs not less. More bug food would mean that even swallows built to aerodynamically inferior, short feathered barn swallow A-C-T-G DNA bio-specs would be fit enough to survive, thrive and proliferate too.

    Warmer climates caused by Global Warming would mean rising ocean levels. Only if barn swallows start evolving into webbed footed water birds might one attempt to surmise that Global Warming is an evolutionary driving force affecting their genetics.

  2. #2 Carl Zimmer
    November 30, 2004

    Dear Joseph: Consider the fact that swallow tails have increased half an inch in twenty years. If this change is simply the ongoing adaptation of swallows to better flight, then over the past 5,000 years swallow tails should have become 10 feet long. Obviously an explanation must be found elsewhere.

  3. #3 coturnix
    December 1, 2004

    Have the females’ tales grown, too?

    What would affect the rate of evolution by SEXUAL selection (as opposed to natural selection)?

    Have the females suddenly become pickier (and why)? Or has the sex-ratio skewed toward the male? Or is it natural selection anyway – everyone pairs up, but a smaller percentage of broods survive than 20 years ago, with long tails aiding in survival (better flight or better parenting?), thus raising the “cut off” point on the continuum of tail lengths?

  4. #4 Carl Zimmer
    December 1, 2004

    As I mention in the article, female tails have *not* gotten longer, which strongly suggests that sexual selection is at work. The scientists propose that weaker males are not able to survive in the harsher conditions they face in the spring time. These are also the swallows with shorter tails. As a result, a higher portion of the males mating each year have longer tails, and they pass those genes on to their offspring.

  5. #5 William Gruzenski
    December 1, 2004

    If our understanding of emerging infectious diseases is considered, we are now confronted with illnesses because a particular virus or bacterium has ‘jumped’ a species, often times with fatal results. We simply do not have time for Darwinian evolution to occur in its traditional manner. Many of the answers come from our ability to mutate at the genetic level. This is constantly occurring and at a rate that would require the burden of everyday measurement. More antibiotics is not the answer. So we try vaccines. And there is some promise here whether we are dealing with viruses or cancers or simply the minute misfoldings of a protein (prions). There must be a way to live together!

  6. #6 Jason Malloy
    December 1, 2004

    Evidence for evolution and global warming – I should post this over at Free Republic and watch everyone’s head explode.

  7. #7 Carl Zimmer
    December 1, 2004

    Dear Jason: This particular study is not evidence for global warming. The spread of the Sahara has not been linked to man-made climate change. I was just pointing out that predictions of man-made climate change include the spread of arid regions. If these scientists are correct, then man-made climate change could produce similar evolutionary changes in the future.

    Given the high ratio of heat to light in the debate over global warming, I just want to make these issues clear.

  8. #8 Jason Malloy
    December 1, 2004

    The spread of the Sahara has not been linked to man-made climate change.

    Thanks. My assumption.

  9. #9 Sarah Dempsey
    December 2, 2004

    Have we learned nothing yet? Simply, warmer climates caused by Global Warming would mean the hole is getting bigger. Thus, it may be evolutionary driving forces affecting their genetic make up, but solutions to this occurrence are in the distant and more research is necessitated. There is no easy answer to explain this phenomenon. We should let the swallows have fun and we will see what comes of it.

  10. #10 Pericles
    December 4, 2004


    It is so simple. The air over the Sahara has changed so some male swallows who happened to have longer tail feathers, flew faster and got the good looking girls. This obviously selected for higher and higher speeds for male swallows.

    Either that or the girls have (as girls will) decided that this century’s look is longer tails for boys.

    Naturally, this means that in 5000 years, swallows with tails 10 feet long will regularily exceed the sound barrier, leading to female swallows selecting their mates by the loudness of the sonic booms.


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