Last month, a California resident was infected with dengue fever after a bite from a local mosquito. This case made the news because it was the first, but it may not be last. Dengue and another mosquito-transmitted disease, malaria, are on the rise around the world. Several factors are behind this trend. Climate change allows mosquitoes to live in broader geographic regions. A city-dwelling mosquito species has been transported across continents and established itself in dense urban areas. And, importantly, mosquitoes have changed.
Where's the evolution?
We’ll review two of those recent changes in mosquitoes to see if they represent rapid evolution in action, where a genetic variant (also called an allele) has spread through the population, or phenotypic plasticity, where differences among individuals are caused by their environments and not by which gene versions they carry.
Mosquitoes are increasingly resistant to our insecticides. The insecticide DDT was one of the first broad weapons we had against mosquito-borne disease. Wiping out the mosquitoes prevented them from passing on disease. DDT was first used against mosquitoes in the 1940s; however, after just a few years, resistance emerged. For the last 40 years, control efforts have relied on pyrethroid insecticides, which are deadly to mosquitoes, relatively safe for humans, and can be applied to bed nets to protect sleepers. However, pyrethroid-resistant mosquitoes have been on the rise since the 1990s, and now threaten to make pyrethroids ineffective. We know that this shift from non-resistant to resistant mosquitoes is caused by the evolutionary spread of resistant gene versions, in part, because we’ve been able to identify the specific genetic changes that cause resistance. Such gene versions spread through the action of natural selection. It works like this:
- There is genetic variation in the mosquito population. One individual differs from the next because of mutations that have accumulated in the population in the past. Some of those randomly occurring mutations might cause an insect to be able to survive exposure to a particular insecticide.
- There is selection. We apply the insecticide to kill the mosquitoes. If any insects happen to have a mutation that allows them to survive the treatment, they can pass that gene version on to the next generation of mosquitoes. Other mosquitoes, without the advantageous gene version, die and do not pass their genes on to the next generation.
- Over time, the resistant gene version becomes more common in the mosquito population. Because resistant mosquitoes have a better chance of surviving to reproduce, each generation, more and more mosquitos with the resistant gene version hatch.
Insecticide resistance is a mosquito adaptation to an environment in which they are frequently exposed to insecticides. Indeed, resistance may only be the start. Resistant mosquitoes might be using the smell of insecticide to help locate food sources. In one experiment, resistant mosquitoes were more attracted to a potential blood meal behind an insecticide-treated net than one behind an untreated net!
Mosquitoes have shifted where and when they bite. Some disease-carrying mosquito species do most of their feeding indoors at night. Hence, control efforts often focus on spraying around homes and on the use of insecticide-treated bed nets. However, in at least some of these cases, mosquitoes have changed their behavior, biting outdoors and in the evening or morning. Have these new, advantageous behaviors evolved, that is, are they encoded by gene versions that have become more frequent because mosquitoes carrying them were able to get food and avoid deadly insecticides? Or are they more common now because of phenotypic plasticity – that is, are the behaviors a response to something in the environment? For example, are the mosquitoes repelled by the pyrethroids and so just seek food where and when they can get it? The answers to these questions are not yet clear. A recent experiment showed that some differences in preferred biting time was caused by genetic differences between mosquitoes – that is, there is genetic variation in the population related to this trait. This suggests that natural selection could act on biting time in ways that shape the behavior of the population, but it’s not yet clear if it actually has done this.
One thing the research does make clear is that mosquitoes, with their large population sizes and multiple generations each year, can evolve quickly – much more quickly that we are developing new insecticides and prevention strategies. Biologists and public health workers must take this evolution into account in any long-term strategy to reduce the enormous human suffering caused by mosquito-borne diseases.
- Carrasco, D., Lefèvre, T., Moiroux, N., Pennetier, C., Chandre, F., and Cohuet, A. (2019). Behavioural adaptations of mosquito vectors to insecticide control. Current Opinion in Insect Science. 34: 48-54. Read it »
- Liu, N. (2015). Insecticide resistance in mosquitoes: Impact, mechanisms, and research directions. Annual Review of Entomology. 60: 537-559. Read it »
- An article on the recent dengue fever case in California from the University of Minnesota
- An in-depth review of the resurgence of mosquito-borne diseases from the New York Times
Understanding Evolution resources:
- Consider the example of two types of moth belonging to the same species: one has dark wings and lives in an area where the trees it rests on are darkened by soot, and the other has light colored wings and lives in an area where trees have light colored bark. The difference in the coloration of two moth types could be caused by natural selection or by phenotypic plasticity.
- How would you explain the coloration difference if it were caused by natural selection (i.e., evolution)? Use your own words.
- How would you explain the coloration difference if it were caused by phenotypic plasticity? Use your own words.
- Review the process of natural selection. Explain why genetic variation is so important to this process.
- Review some background information on natural selection. Explain how a mutation causing resistance to DDT would spread through a population of mosquitoes that are exposed the pesticide. Make sure to include the concepts of variation, selection, and inheritance in your explanation.
- Imagine that you read an article about mosquito-borne diseases that states, “Mosquitoes developed resistance in order to avoid being killed by insecticides.” Review some common misconceptions about natural selection and then rewrite that sentence in a more accurate way.
- To examine whether changes in mosquito biting time were caused by natural selection (i.e., evolution) or by phenotypic plasticity, researchers performed an experiment to see if the grandchildren of evening-, night-, and morning-biting mosquitoes tended to bite at the same times as their grandmothers did.
- Review the process of natural selection. Which component(s) of natural selection (variation, selection, and/or inheritance) was the experiment designed to assess?
- Why was this experiment important in distinguishing between natural selection and phenotypic plasticity as explanations for the difference in biting time?
- Teach about natural selection: In this lesson for middle school through college, students participate in demonstrating how natural selection works. They play the roles of predators with different feeding appendages (spoons, forks, chopsticks, or straws) and compete to gather beans as prey.
- Teach about natural selection and phenotypic plasticity in humans: In this set of sequenced lessons for the high school and college levels, students learn how to devise an experiment to test the difference between acclimation and adaptation; investigate how scientific arguments show support for natural selection in Tibetans; design an investigation using a simulation based on the Hardy-Weinberg principle to explore mechanisms of evolution; and devise a test for whether other groups of people have adapted to living at high altitudes.
- Teach about the evolution of resistance: In this lesson for the high school and college levels, students learn why evolution is at the heart of a world health threat by investigating the increasing problem of antibiotic resistance in such menacing diseases as tuberculosis.
- Carrasco, D., Lefèvre, T., Moiroux, N., Pennetier, C., Chandre, F., and Cohuet, A. (2019). Behavioural adaptations of mosquito vectors to insecticide control. Current Opinion in Insect Science. 34: 48-54.
- Govella, N. J., Johnson, P. C. D., Killeen, G. F., and Ferguson, H. M. (2023). Heritability of biting time behaviours in the major African malaria vector Anopheles arabiensis. Malaria Journal. 22: 238. https://doi.org/10.1186/s12936-023-04671-7
- Kupferschmidt, K. (2016). After 40 years, the most important weapon against mosquitoes may be failing. Science. doi: 10.1126/science.aal0254
- Liu, N. (2015). Insecticide resistance in mosquitoes: Impact, mechanisms, and research directions. Annual Review of Entomology. 60: 537-559.
- Naddaf, M. (2023). Dengue is spreading in Europe: how worried should we be? Retrieved November 2, 2023 from Nature (https://www.nature.com/articles/d41586-023-03407-6)
- Nolan, S., (2023). What to know about dengue fever as cases spread to new places. The New York Times. Retrieved November 2, 2023 from The New York Times (https://www.nytimes.com/2023/10/24/health/what-is-dengue-fever.html)
- Nolan, S., (2023). Mosquitoes are a growing public health threat, reversing years of progress. The New York Times. Retrieved November 2, 2023 from The New York Times (https://www.nytimes.com/2023/09/29/health/mosquitoes-malaria-disease-climate-change.html)
- Porciani, A., Diop, M., Moiroux, N., Kadoke-Lambi, T., Cohuet, A., Chandre, F., … and Pennetier, C., (2017). Influence of pyrethroïd-treated bed net on host seeking behavior of Anopheles gambiae s.s. carrying the kdr PLoS ONE. 12: e0164518.