A tiny brown bird sits on a stick as if nothing had changed, barely perceptible against Southern California’s parched riverbed brush. However, something has. For over a century, a subtle change has been taking place deep within that bird, within the very code that defines it. Only now are scientists learning to read it.
Researchers’ long-held suspicion that environmental stress affects more than just an animal’s behavior or movement has been confirmed by two recent studies on bird populations in California. At the molecular level, it alters their identity. It turns out that traits are not the only things that the environment selects for. The instructions can be rewritten by it.
Without a genetics lab to unravel its story, the southwestern willow flycatcher is one of those birds whose tale is largely unseen. The flycatcher is endangered and has been declining for decades in places like San Diego because it depends on riparian habitats that have been shrinking long before most people started paying attention. However, there has also been a shift in the San Diego population’s genome, and it’s not exactly in the direction you would anticipate from a troubled bird.
Sheela Turbek, a postdoctoral researcher at Colorado State University, oversaw a study that compared blood samples from live birds throughout the species’ breeding range with DNA from flycatchers collected in San Diego around the turn of the 20th century—taxidermied museum specimens, birds that have been sitting in drawers for more than a century. This kind of comparison was impossible until very recently. Older specimens typically contain fragmented, deteriorated DNA, and it took technological advancements that have only recently surfaced in the last ten or so years to extract anything useful from them. Turbek says it’s really difficult even now.
The team discovered something unexpected. Over time, the San Diego population’s genetic diversity has grown. Surprisingly, the increased diversity seems to be concentrated in regions of the genome associated with heat tolerance and the capacity to regulate heat in humid conditions. Gene flow is the most likely explanation; birds from other Western populations moved to San Diego and bred with local birds after habitat disruption elsewhere. The newly mixed gene pool was then subject to natural selection, which pushed the population in the direction of variations more adapted to a shifting local environment. To the best of Turbek’s knowledge, this is the first instance of genetic climate adaptation in a population of wild birds.
That sounds more optimistic than it probably is, so it’s worth pondering for a while. Here, Turbek herself exercises caution. San Diego’s population continues to drop. Even if genetic adaptation is occurring, it is most likely not keeping up with the rate of environmental change. Finding evidence of evolution in a population that is still declining is somewhat depressing.
It is more difficult to describe the savannah sparrow’s version of the same tale as encouraging. Phred Benham, a postdoctoral researcher at UC Berkeley, spent years researching two savannah sparrow subspecies that have adapted to live in coastal saltmarshes in California.
He claims that while just driving around California and observing what was happening to those marshes, he developed an interest in the question. Looking at a century of genomic change, he discovered that increased gene flow has diluted the very adaptations that allowed saltmarsh life by introducing genetic material from inland relatives into the coastal specialists. The diversity of genes increased. Local adaptations began to become hazy. A genome may become less adapted to the particular circumstances that a population faces while also becoming more diverse.
The science becomes truly complex at this point. By bringing in new variation, gene flow can save a population that is in decline. The specialized adaptations that have developed over many generations in a given environment may also be undermined. It is possible for both to be true simultaneously. Birds in California seem to be experiencing both of these things at the moment.
Beyond the DNA sequence itself, epigenetics is another aspect of all of this that scientists are starting to take seriously. Without altering the underlying code, mechanisms such as DNA methylation can change how genes are expressed. Stress, temperature, diet, and habitat conditions are examples of environmental cues that can turn on or off gene expression patterns. In certain situations, these changes can last for generations. Research on wild birds in their natural habitats is still in early stages, but the evidence is accumulating that what a bird experiences during its lifetime, or even before it hatches, can shape the phenotype it actually expresses. The genome is not as fixed as it once seemed.
It’s still unclear exactly how much these epigenetic changes matter at the population level over evolutionary time, or how reliably they pass from one generation to the next in wild birds. These are open questions. What seems increasingly hard to dispute is that the boundary between environment and inheritance is more porous than the standard textbook picture suggests. Environmental shifts don’t just select for who survives. They participate, in ways scientists are still working out, in shaping what survives.
There’s a feeling, watching this research accumulate, that we are just beginning to understand what environmental disruption actually costs — not just in lost populations or behavioral changes, but in the molecular architecture of species. Birds, with their well-studied genomes, accessible blood samples, and century-long museum records, are giving scientists a rare opportunity to read that cost in something close to real time. What the evidence suggests, so far, is that evolution is happening. Simply put, it’s not happening quickly enough.
