Inherit the Warmer Wind

Some organisms' genes are changing in step with Earth's climate

While Christina Holzapfel and William Bradshaw were post-doctoral fellows at Harvard University, they discovered a love for each other—and for bogs. The pair used to spend entire days knee-deep in peat, admiring the soupy, muddy scenery. “It’s a peculiarity, I know,” says Holzapfel. “But when you see the pitcher plants in bloom, with these exquisite pink blossoms shining out over the generally mucky mess, it’s a stunning thing.”

SPRING FORWARD. Genetic variation within populations of the great tit (pictured) could keep hatch times in step with the springtime appearance of caterpillars, which the birds use to feed their hatchlings. iStockphoto
SQUIRRELY SCHEDULE. Birth dates for red squirrels like this one have advanced an average of 18 days over the last 10 years. This may be so they can take advantage of the spring boom of spruce seeds, which has grown in recent years presumably because of global warming. C. Kolacz
PITCHER UP. Northern populations of the pitcher plant mosquito (pictured) now respond to day length more like southern populations of the insect, presumably because of warmer temperatures in higher latitudes. CDC

Now, 30 years later, Holzapfel and Bradshaw are married and jointly running a lab at the University of Oregon in Eugene. The couple’s attention focuses on one of a bog’s typical residents—a tiny mosquito that makes its home inside carnivorous pitcher plants. This pitcher plant mosquito (Wyeomyia smithii) helps itself to insects captured by the plant, digesting parts of the bugs and leaving the rest behind for the plant.

Much of the two scientists’ work revolves around a phenomenon known as photoperiodicity, in which the mosquitoes rely on day length to determine when to go dormant in the fall. Variations in this trait are controlled by genes. Five years ago, the two scientists got their first clues that the W. smithii specimens they’d analyzed while at Harvard weren’t quite the same as the ones they continue to capture and study today.

Other researchers had already observed that global warming seemed to be changing the actions of some organisms (SN: 3/8/03, p. 152: Spring Forward)—animals from birds to butterflies were migrating out of their long-time habitats, and plants were flowering too early or going dormant too late. Some scientists had batted around the idea that not all these changes were superficial—that instead, populations might be responding to global warming by modifying their genes.

Holzapfel and Bradshaw remembered that idea one morning over coffee as they flipped through the decades of data they’d collected on photoperiodicity in W. smithii. “We were totally shocked by what we saw,” recalls Holzapfel. Photoperiodic time tables, hardwired in the mosquitoes during thousands of years of evolution, appeared to be gradually changing in many of the populations—a result, the two scientists say, of warmer temperatures in each population’s habitat.

Holzapfel and Bradshaw’s mosquitoes were one of the first organisms in which scientists observed genetic changes that might be attributed to global warming. Other scientists have more recently reported that the genetic makeup of organisms ranging from fruit flies to birds might also be responding to climate trends. Although these adaptations may enable some animals to keep pace with global warming, animals that don’t evolve quickly could be at risk.

A bug’s life

It makes sense that insects would be among the first animals to show signs of genetic change in response to global warming, says Bradshaw. Many insect species have survived climate swings and other environmental changes that have taken place in the past few hundred thousand years. This suggests that in some bug populations, genomes contain enough variety to adapt to changes.

Furthermore, insects are fast and prolific breeders. So, a few individuals with a gene variation that helps them survive an environmental change can quickly spread the novel trait throughout the population.

Bradshaw explains that in W. smithii, an appropriate photoperiodicity is pivotal for an individual insect’s survival. Go dormant too early in the fall, and the mosquitoes don’t have enough energy stored to survive the winter; wait too long to go dormant, and the insects could freeze to death.

Since day length is the same in a particular location year after year, but differs from place to place, mosquitoes in individual locales throughout the pitcher plants’ range—from north Florida up to Manitoba, and from Nova Scotia across to Minnesota—have evolved separate photoperiodic clocks to regulate their life cycles. These clocks are so location specific that Bradshaw and Holzapfel have relied on them to check where a bug came from.

“When we brought animals into the lab, we would measure their response to day length just to be sure we were working with what we thought we were working with,” says Holzapfel.

In 2001, the pair reported its evidence that global warming has warped some mosquito populations’ responses to day length. Holzapfel and Bradshaw had analyzed data that they’d collected from experiments between 1972 and 1996, in which they’d placed insects collected from different places in tiny compartments stacked in a big cabinet—”mosquito Hiltons,” says Holzapfel. Each compartment had an air-cooled light that turned on and off to simulate a different day length.

The researchers found that the mosquitoes’ responses to light—evident in their development and dormancy patterns—differed significantly among bugs collected in northern and southern locations in 1972. However, by 1996, many of the northern populations were acting more like their southern counterparts. The most probable explanation for this change, says Bradshaw, is that global warming has extended the growing season for northern mosquitoes. With warmer winters, the bugs have more time to grow without going dormant.

Other research teams have seen a similar northern-southern merge in the genes of fruit flies. George W. Gilchrist, who studies a fruit fly species known as Drosophila subobscura, notes that researchers have long been fascinated by a peculiar genetic quirk in this and some other fruit fly species. Small sections of these insects’ chromosomes are reversed in some individuals but not in others.

“It’s like pieces of a bar code that are flipped backward,” says Gilchrist.

Though it’s not clear what these chromosomal inversions do for the insects, researchers have noticed that the reversals follow a pattern: Wild fruit flies at the same latitude tend to have similar patterns of inversions, with the patterns shifting in a gradient extending northward from the equator. Some researchers have hypothesized that these inversions may permit fruit flies to survive in particular climates.

In the Sept. 22 Science, Gilchrist and his colleagues published evidence that these inversion patterns in fruit flies on three continents—Europe, North America, and South America—have changed in response to climate. When the researchers compared chromosomal data taken in the late 1970s and the early 1980s with recent data, they found that fruit flies living at latitudes farther from the equator have gradually changed to resemble those living near the equator.

“Almost every site sampled is warmer now than it was before—the chromosomes now look like the chromosome patterns from a slightly warmer place,” says Gilchrist.

Early birds

Researchers aren’t seeing genetic changes just in fast breeders such as insects. Some studies suggest that populations of animals that take years to breed are also beginning to show genetic responses to climate change.

Stan Boutin of the University of Alberta in Edmonton and his colleagues have kept track of individuals in a red squirrel colony in the southern Yukon for 15 years. “They’re a pretty rare mammal, in that you can follow them right from birth through their entire lives,” he says. That’s because the squirrels are territorial, so Boutin and other researchers can easily track individuals year after year. The squirrels eat only one food, spruce seeds, so Boutin’s team can document how much each animal consumes. Also, these animals reproduce in grass nests that are easy to spot in trees. Researchers can see when females give birth, count the number of offspring, and tag them.

“It’s like living in a town where no one ever leaves, and you have birth certificates for all of them,” he says. Indeed, Boutin and his colleagues have developed an extensive pedigree for the animals.

Combining this information with the characteristics of individuals in the colony, Boutin and his colleagues in 2002 spotted some traits that seem to be controlled genetically. Most notably, mothers who give birth early pass on that tendency to their daughters.

Because of today’s warmer spring temperatures, the trees produce more cones than they did a decade ago. Over the past 10 years, Boutin says, the animals’ birth dates have advanced, on average, about 18 days—enabling them to take advantage of the larger spring boom of spruce cones.

Boutin notes that since the size of the cone crop varies significantly from year to year, even without global warming’s effects, colonies of red squirrels probably harbor gene variants that prompt some moms each year to give birth at the most advantageous time.

“The population may be genetically preadapted to cope with the rapid climate change that we’re seeing,” he says.

Some bird populations also seem to harbor genes that may similarly help them cope with global warming. Although great tits may have the genetic potential to adjust to conditions that come with warmer temperatures, they haven’t made that adjustment yet, notes Daniel Nussey of the University of Cambridge in England.

The birds rely on a food chain that’s been skewed in recent years by global warming, Nussey says. Trees are budding earlier, causing an earlier springtime boom in caterpillars that use the buds for food. Great tits, in turn, harvest these spring caterpillars to feed their babies. However, the great tits haven’t altered their laying schedule to keep up with the caterpillars.

“What you have here is an emerging mismatch between levels in the food chain that’s driven by climate change,” says Nussey. “There’s no evidence of negative consequences yet, but you can see that there could be a major problem if it goes on.”

If the birds stick to their age-old laying schedule while the caterpillar boom shifts earlier, then they won’t have enough food for their babies, he explains.

Much like Boutin’s red squirrels, individual great tits are easy to track, says Nussey. In one great tit population that researchers have studied for 50 years, the birds nest in a series of human-made boxes in a forest in the Netherlands. Scientists can peek inside a box to see when each female has laid the first egg of her clutch, check when the eggs have hatched, and eventually catch and tag the offspring. With these methods, researchers have compiled a pedigree for the colony that stretches back several decades.

Using this pedigree, Nussey and his colleagues reported in the Oct. 14, 2005 Science that the birds’ laying habits appear to have a genetic basis. Mothers who lay early tend to pass on that trait to their daughters. The researchers also found that females that lay early have more offspring that reenter the population as adults to breed than do females that lay later.

If this trend continues, says Nussey, birds that lay early might replace those that lay later—correcting the mismatch between the caterpillar boom and the hatching of the birds’ offspring.


As global warming has progressed and led to further ripples of environmental change, says Nussey, some researchers predict widespread die-offs of many species. However, he notes that his work and that of others is showing that at least some species may adjust to these changes.

“The message from our study is actually quite positive about climate change. If animals have the evolutionary potential and can change their responses, then this could potentially rectify some of the problems” associated with global warming, Nussey says.

However, it’s unlikely that every species possesses this intrinsic capability to adapt, says Camille Parmesan of the University of Texas in Austin. Parmesan, who studies adaptation to climate change in a variety of butterfly species, adds that a population must have both the right genetic variations and a breeding period that’s not too long to keep pace with global warming.

The species that are already being negatively affected by climate change probably don’t have these traits, she explains.

“The [species] we thought would be most sensitive—the mountaintop species and corals—they haven’t evolved. They just died,” Parmesan says. She adds that there’s no evidence so far that long-lived creatures, such as polar bears and penguins, in climates drastically affected by global warming are adapting to the changing conditions.

It’s impossible to predict how the presence or absence of such evolution could ultimately change ecological communities, says Bradshaw. “We won’t recognize communities—they’ll be different,” he says. “Is different bad or good? Different is different. Whether it’s bad or good depends on your point of view.”

As global warming continues, some organisms will probably die out and some will stay—but life will go on, adds Holzapfel.

“We are hopeful people, hopeful that adjustments will be made so the world will keep going,” she says. “I’m hoping that W. smithii is one of the ones that stick around.”