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Evolution on an Ecological Scale

Drought on the Galapagos Islands wiped out 85% of finch populations and shifted their evolution in a single generation. Credit: schame87/Adobe

Drought on the Galapagos Islands wiped out 85% of finch populations and shifted their evolution in a single generation. Credit: schame87/Adobe

By Martino E. Malerba

Darwin’s theory of natural selection may be simple and intuitive, but some of its key assumptions are now being called into question.

At the end of the 19th century, Charles Darwin revolutionised our way of thinking about nature: in every species, the individuals with the highest “fitness” for their specific environment are more likely to survive and pass on their successful genes to the next generation. This mechanism of “natural selection” implies that all species continuously evolve, generation after generation, by accumulating beneficial mutations that can gradually improve the fitness of the species. Darwin’s theory of evolution is the most popular concept of how life reached its current state.

Following Darwin’s natural selection theory on how species evolve, scientists have studied natural communities by taking the “evolution as stage, ecology as play” approach. Simply put, interactions between species and their environment allow natural selection to mould individuals, thus imposing a gradual change on the performance of a species. Generation after generation, species will either evolve to become increasingly more adapted to their environment, or they will become extinct.

Many types of species–environment or species–species interactions make up the ecological drivers for evolution. Some of the most important include competition for resources, predation, parasitism, mutualism and facilitation. We can now use Darwin’s theory of natural selection to infer how the interactions of a species and the environment shapes its evolution.

This way of explaining what we see in nature is appealing because may be simple and intuitive, but some key assumptions of Darwin’s revolutionary intuition are now being called into question.

Darwin’s theory of evolution implies a one-way link from ecology to evolution: the success of a species in its environment defines how individuals perform in the present and evolve in the future. This assumption is so prevailing that most scientists think they can ignore evolutionary processes when investigating natural biodiversity. Instead, tremendous advances in the past decade have revealed that evolution can occur on timescales much shorter than the “long lapse of ages” emphasised by Darwin. In fact, evolutionary changes are occurring all around us, all the time! This has led to the game-changing realisation of “rapid evolution”.

Many external characteristics of a species are highly heritable, which can lead to rapid and extraordinary changes in the way a species interacts with its surroundings. A famous example of rapid evolution is the case of Darwin’s finches on the small Galapagos island of Daphne Major. After a severe drought caused a shortage of seeds between 1976 and 1977, 85% of the local populations died of starvation. The remaining 15% had a special adaptation for drought conditions – a short beak that could break into the harder seeds of drought-resistant plants. When the rain returned, the surviving finches mated with each other and the new generation of birds rapidly evolved a shorter and stronger beak size that could help the species tolerate future droughts.

This example is important because it was among the first evidence of evolution and ecology both acting on similar time scales to influence interactions between individuals. This is because the new offspring of drought-adapted finches will behave differently to their parents. While the accepted paradigm to explain natural communities had previously been that “ecology drives evolution,” we now realise that evolution can also shape future ecological processes, suddenly forming a reciprocal two-way “ecology–evolution” interaction. This is often referred to as “eco-evolutionary dynamics”.

When I first read about the idea of rapid evolution I found it extremely fascinating. Could it be that rapid evolution has been the missing link for understanding how such incredible bio­diversity can persist in nature? If so, how can I find out?

I came up with an approach to better answer this question, and embarked in the longest experiment of my life. I took a species and evolved it to different body sizes to test how its performance could change in response to my evolutionary treatment.

I evolved both smaller and larger individuals of a common species of microalga that differed in cell size by as much as 2500% (Fig. 1). This is the same difference in size between a dog (50 kg) and a wild buffalo (1200 kg). I then assessed the physiological and ecological consequences of this size shift. My plan was to use my evolved species to show the importance of evolution in shaping assemblages of species in nature.

What I found was exciting. Evolving a species to different sizes massively altered the way the organisms behaved and performed in their environment. For example, larger organisms could work harder than an equal mass of smaller individuals, but they also consumed more resources to survive.

In other words, larger individuals have potentially much greater performances, but they also need more maintenance. Instead, smaller individuals don’t work as hard but are more efficient at using resources in the environment. It is a bit like in cars: Ferraris have great driving performances but consume more petrol than smaller city cars.

The body size of a species has profound influences on their internal organisation. By selecting for size, we are simultaneously selecting for a range of internal mechanisms that are advantageous for that size. This means that increasing body size doesn’t correspond with an equal increase of the internal organs within the body.

For example, I found that larger cells don’t have as much sunscreen pigments compared with smaller cells. This is because larger bodies are more protected than smaller ones from dangerous UV rays, as light tends to dissipate as it penetrates a cell.

Finally, my experiment also showed that larger cells can store more resources than smaller ones (Fig. 2). This makes them more likely to cope with fluctuating resources than smaller cells.

I then turned my attention to multiple interacting species. By influencing demographic parameters, eco-evolutionary dynamics are likely to have repercussions on the broader community. For example, the evolution of a prey species can induce a change in the ecology and evolution of its predator.

Scientists explored these predator–prey eco-evolution feedbacks decades ago, but the supporting evidence remains scarce. My experimental system represented a great opportunity to test these mechanisms.

I wanted to see if the evolution of my algae could generate an evolutionary response further up in the food chain and affect the ecology and evolution of an algal predator. This could reveal why there seem to be so many interacting species in nature: each species constantly evolves in response to its surrounding community. Nothing stays still and every species is in a constant “evolutionary” motion.

The next stage in my experiments is to feed aquatic grazers (such as copepods) with an equal biomass of my evolved algae of different sizes and look for a change. These experiments are still on their way, but preliminary results show that copepods feeding on larger algae grow faster, reproduce more and evolve new morphologies. So far the evidence suggests that predators can evolve in response to the evolution of their prey.

If these results are confirmed, my algae could tell us about how species interact in nature. Perhaps species coexist in nature thanks to a complicated mix of direct and indirect ecological and evolutionary effects, acting together at different levels across a food chain.

These are exciting times to work in biology. There is so much to discover in nature, but also an increasing urgency to come up with ways to mitigate our catastrophic impact on the planet. Our planet is now in the midst of its sixth mass extinction of plants and animals, with humans driving more species to extinction. Dozens are disappearing every day.

There is also little doubt that we should do more to reduce our impact on the planet. One way to do this is to understand the mechanisms promoting biodiversity in nature and use them to prevent further loss of species. This is no easy task.

For now, the only thing we can do is keep running our experiments and hope to find a breakthrough that can help maintain a healthy planet for the sake of generations to come.

Martino E. Malerba is a postdoctoral researcher in the Marine Evolutionary Ecology Group at Monash University.