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Teaching climate change and rapid evolution: the case of the comatose fruit flies

Anusha Krishnan

Chill coma recovery test is used as a tool to study the genetics of fruit flies
Chill coma recovery test is used as a tool to study the genetics of fruit flies   (Photo: Drowsy Drosophila. Modified to current form by Manoj Rangan)

European great tits (Parus major) which need caterpillars to feed their young, time their egg-laying schedules to coincide with maximum availability of this creepy-crawly food. However, earlier spring onset due to global warming, is causing caterpillars to mature earlier than the birds’ eggs hatching, leaving parent birds with scarce food resources for chicks.

Are these birds able to cope with this change? The answer to this is as yet undecided, since the battle between climate change and the birds’ adaptation is ongoing. A study on a Dutch population of these birds has found that a genetic subpopulation can vary the timing of their egg-laying. This population can lay eggs earlier, and so, feed their chicks with the early-emerging caterpillars. This has led to an intense selection for birds capable of varying their egg-laying timings, and the Dutch great tit population has shown distinct genetic changes over a span of just 32 years.

This case of the European great tit highlights two important processes – one, global warming can create immense selection pressure on living organisms; and two, certain populations can evolve rapidly to adapt to this pressure.

As evidenced by the example above, climate change is a key driver of evolution, and understanding this is of immense importance — yet most schools and colleges provide no linking study matter between these two processes. “Climate change and evolution are treated as separate topics in the biology pacing guides, scope and sequence, and Florida science standards”, says Julie Bokor, from the outreach centre at the University of Florida. “Often, climate change is dealt with in environmental science classes, while evolution is in biology courses,” she adds.

How then, can students in a classroom be taught about climate change, its effects on genetic variation, and consequent changes in populations and species survival?

Enter the fruit fly (Drosophila melanogaster), which can be used in classrooms to explore interactions between climate change and rapid evolution, thanks to a three-lesson module designed by a group of teachers and a scientist. The module (which includes Bokor as an author) includes an experiment on chilled comatose fruit flies, and is aimed at linking climate change to lessons on evolution; a detailed version of the module is freely available to instructors as a teaching resource.

At temperatures between 4–7°C, fruit flies go into an inactive state known as a “chill coma”. For fruit flies, the chill coma recovery time (CCRT) is a known heritable trait that is dependent on several genes. Fruit flies from temperate regions are known to have shorter CCRTs than those from tropical regions. As flies in a chill coma cannot find food, mates, or avoid predators, CCRT is likely to be adaptive in seasonal temperate climates, where a sudden cool period may be followed by rapid warming.

First Lesson

A basic understanding of adaptation is required for students to be able to interpret data on how quickly comatose flies recover. Thus, the first lesson in the module introduces a fundamental question — are all species affected equally by climate change? To stimulate their minds, students are assigned two articles as homework. One, that future climate change not only includes global warming, but also leads to extreme weather events such as heat waves and cold snaps; and two, an article that introduces the concept of phenotypic plasticity, where a single genotype can produce multiple phenotypes depending on the environment.

Following a discussion of these concepts, students must work in small groups to analyse eight climate-affected species to predict which species would be more populous (“winners”) or less populous (“losers”) in response to expected changes in climate. This analysis activity uses ‘species cards’ based on real data from a study that analysed species vulnerability to climate change, and aids in correcting two common misconceptions about evolution and climate change — (a) that evolution only occurs over very long periods of time, and (b) that climate change negatively impacts all species.

Second Lesson

The second lesson aims to help students explore the role of natural selection on the long-term survival of a species using an active laboratory setup. Students use a modified form of a widely used protocol to address the question — is there potential for natural selection to act on the fruit fly?

To assess the impact of temperature on the rapid evolution of fruit flies, small groups of students observe six vials containing ten flies each; the flies in each vial are from genetically distinct lines. Flies are chilled on ice for three hours to induce a chill coma, and the time taken for flies to recover (CCRT, defined as the fly’s ability to walk) is noted by the students. Based on the pooled data collected, students must create and compare graphs of mean CCRTs for the six fruit fly lines.

In an instructor-mediated class discussion, students must identify CCRT as a genetic trait on which natural selection can occur. Following self-study sessions about classic mechanisms of evolution (mutation, gene flow, genetic drift, non-random mating, and natural selection), a post-lab question set is used to help students connect the lab-activity with the study material.

Third Lesson

Lesson three aims to help students synthesise their knowledge of climate change and evolution to tackle the question, “What patterns of natural selection might occur as a result of climate change?”. In this one-day lesson, students learn about different types of selection (directional, disruptive, and stabilising), following which, they must complete an assessment in the form of a “natural selection in the face of climate change” activity. Based on a fact sheet with a species description and a problem that the it faces due to climate change, students will again work in small groups to identify how a population might respond to climate change.

This lesson must conclude with a discussion on the limits of evolution — namely,

1. Species do not evolve by choice;

2. Evolution is limited by existing genetic variation; and

3. The pace of evolution may sometimes not be able to keep up with environmental changes due to climate change.

The teaching module, which was implemented on high school students, has been reported by the authors to engage students at a higher level than previously used methods. “This curriculum unit provides opportunities for students to make their own connections between real world occurrences,” says Jessica Mahoney, an author in the publication, and a classroom teacher.

“Although this is an interesting setup, procuring six strains of Drosophila would be difficult for most Indian undergraduate classes”, points out Helen Roselene, Head, Department of Environmental Sciences, Mount Carmel college, Bengaluru. However, if experiments with live fruit flies are not possible in a class setting, the authors have provided a data set on CCRTs for use by teachers who can conduct the lesson as a purely analytical exercise.

In all, the module encourages cross-curriculum-based inquiry and may help students engage with climate change policies worldwide. “India is one of the countries likely to be highly affected by climate change,” says Nirmala Raghunandan, Head, Department of Biology, St. Joseph’s pre-university college, Bengaluru. “Introducing this module in Indian schools could be really useful as it can help sensitise students to the effects of climate change, and educate the next generation of leaders and decision makers on how climate change can affect evolution,” she adds.

Written By

Anusha is a part-time science writer and editor and a full-time mum. She loves writing about new scientific discoveries, and believes that the art of storytelling is crucial for successful science communication.