How Climate Change Changes Evolution – One Bird's Story
As average global temperatures increase, biologists anticipate more negative impacts from this change on a great many species of plants and animals. In most cases, scientists expect climate change to damage ecosystem integrity and ultimately accelerate the extinction of a great many species (as appears to be already happening in many quarters of the globe).
However, in some cases, increasing seasonal temperatures (such as in warmer Springs) may actually help some species adapt to climate change more quickly, and perhaps providing them with an evolutionary advantage. A recent study of a song bird (Parus major, or great tit) population in the Netherlands, by Husby, Visser, and Kruuk, revealed a positive selection mechanism linking Springtime temperature increases, early egg-laying, and greater genetic variance.
This mechanism may help speed up microevolution and may play a vital role in how fast species can adapt to climate changes.
The ‘rate of evolutionary change’ is generally determined by two key variables:
1] The strength of natural selection (a mechanism in Nature that favors the survival of gene-based traits),
2] The amount of variation in genes (and thus the traits they encode) within a given species’ gene pool, that such selection may act upon.
In general, strong, natural selection of a trait results in a greater difference in the expression of that trait showing up between “successful” and “unsuccessful” parents. The greater the heritability of that trait (for example, earlier egg-laying) the greater the evolutionary change across successive generations, that is, the greater occurrence of a trait in a given population.
The researchers examined egg-laying dates, derived from more than 3,800 seasonal breeding records for nearly 2,400 females, and compared these to average daily temperatures derived from national weather service data.
Consistent with previous research, the authors found strong, positive selection on laying date, that is, the earliest breeders showed the highest genetic fitness. As a general rule in evolutionary biology, if an environmental effect (such as warmer spring temps.) strengthens natural selection (of a trait) and/or increases variation of genes (in that breeding population), then the rate of evolutionary change should be accelerated. This all translates, in theory, to a more adaptive breeding population — one that may better survive global climate change.
Indeed, the researchers’ statistical model (gauging genetic variance change versus temperature change) revealed an increase in genetic variance with the warmest temperatures. Also, where selection appeared to be strongest, this was associated with greater genetic variance (in egg-laying dates). All of which should portend for successful adaptation.
However, there’s one major problem: the population of this songbird species has declined over the past three and a half decades.
This reality perhaps underlines the sensitive ecological coupling between a species and its food source; it is believed that earlier egg-laying — selected for by warming temperatures — threw the birds out of sinc with the peak, seasonal hatching of caterpillars which are its principal food source. Early egg-layers didn’t get the higher caloric/nutritional intake that later ones did.
It would seem, then, that Nature may have played a cruel trick on the great tit. On the other hand, by increasing genetic variance, this mechanistic coupling may be providing the songbird with a genetic “toolkit”, or reservoir, in preparation for some future evolutionary shift. Alternately, it may be actually slowing the rate of the bird’s decline, as its current state of decline has only been observed since record keeping on them began. Their decline may have been pre-existent and on-going.
Thus, it is unclear if the mechanism revealed in this study can serve the long-term survival needs of birds and other animal species. Evolution does not act equally on all species occupying a niche (it didn’t here change the caterpillars’ behavior in tandem with the birds’). Adaptation is not simply about taking one beneficial trait and spreading it around, but rather, it seems to involve a “mosaic” of mutually supportive traits working within, interacting with, a favorable and diverse environment.
Some content for this article came from Climate Change Could Change Rates of Evolution by Liza Gross (published on PLoS Biology, Feb. 1, 2011). This article was an overview of the research paper Speeding Up Microevolution: The Effects of Increasing Temperature on Selection and Genetic Variance in a Wild Bird Population
by Arild Husby, Marcel E. Visser, Loeske E. B. Kruuk, also published on PLoS Biology, Feb. 1, 2011.
Top image credit: (Parus major, or great tit)) Sławomir Staszczuk.
19th Century Drawing: (Darwin’s finches) John Gould