Adaptation of species

The principle of evolution: every living thing has its own set of hereditary traits, recorded in its genes. The variation in these between individuals of the same species makes them respond differently to a changed or new environment. Those who are more successful under new conditions will have more offspring, and thus contribute more strongly to the hereditary traits of the next generation. In this way, the hereditary traits of an entire population - a group of animals, plants or micro-organisms - change. The population adapts.

Extraverted great tits

Animal ecologist Kees van Oers specialises in how personalities of individuals within a population determine how they deal behaviourally with changing conditions. "Personalities are consistent differences in behaviour between individuals of the same species. Animals have their own strategies by personality to solve the problems posed by a changing environment.” A mix of these strategies influences the survival of a population or even species. “For example”, Van Oers says, “we found that extroverted great tits with a high problem-solving ability were the first to settle in the far north of Scandinavia - as a result of climate change."

Evolutionary adaptation is actually selection on genetic variation. The selection brought about by a new environment can create, on the one hand, generalists who are flexible and can cope well with change, but also animals that are instead specialists. Van Oers: "These two groups alternate, because generalists are good at discovering and colonising new areas, while specialists take over from them later, in more stable situations."

Urban evolution

Plant ecologist Koen Verhoeven studies the adaptation of plants to the urban environment. "The field of urban evolution is fairly new, but until now it was mainly focused on animals. An important effect of urbanisation is that it is warmer within than outside a city. Here in the Netherlands, the average difference is about two degrees, and on sunny days even much more. That affects species. Our research on dandelions between the centre of Amsterdam to outside the city shows that urban specimens grow relatively well at higher temperatures. Moreover, they need a shorter cold period to flower in spring. These are two forms of genetic adaptation to higher temperatures and milder winters resulting from changing landscape use by humans."

At the same time, Verhoeven sees urban evolution as a predictor of the effects of climate change on plant species over the next century. "That two degrees Celsius is exactly the difference soon expected for global temperature rise. So if you look on a slightly longer timescale, these adaptations could start happening throughout the Netherlands."

Adaptation makes resilient

Aquatic ecologist Steven Declerck is investigating how rotifers, microscopic aquatic organisms, can quickly adapt evolutionarily to climate change and various forms of pollution. It turns out that the creatures have a particularly high ability to adapt quickly to one or more of these factors. Nature's resilience is sometimes greater than expected. However, that adaptation does not appear to happen equally well in any combination or sequence of stressors.

Populations of species do not live in isolation, but in ecological communities within which they influence each other in all kinds of ways. Declerck: "Many community ecologists base their interpretation of changes in nature on the idea that species have immutable characteristics. However, populations can adapt micro-evolutionarily quickly and they do so in interaction with each other. Under the influence of a stressor, this can lead to unexpected changes in species composition, with consequences for ecosystem functioning."

Population size and the evolution of microorganisms

Microbial ecologist Mark Zwart studies the effects of population size on changing traits in microorganisms. Zwart: "Viruses and bacteria reproduce asexually. That means they make copies of themselves. Evolution therefore takes place only through random errors in copies. The larger the population, the greater the chance that the same mutation occurs and that a trait changes permanently. We call that the repeatability of evolution." But sometimes a combination of several such mutations are needed to lead to one new trait. Zwart: "The interaction between mutations is often very complex. That makes such a trait unlikely to emerge, even in large populations." 

Yet microorganisms are known to be able to adapt quickly to new conditions. Zwart explains how this is possible. "There are also mutations that do not change the gene of the trait, for example a protein, but the number of times that gene is in the DNA and therefore how much of the protein is produced. That can make a difference in different environments."

Through experiments in the lab, Zwart is testing the success of this copy number variation for rapid adaptation by plant viruses. "Some of these viruses divide their hereditary material, the genome, into segments that you can compare to chromosomes in higher organisms. Instead of making each segment once for each new cell, the ratio often varies tremendously. We think this must have an evolutionary advantage, especially when it comes to rapid adaptation to new environments. In that case, a virus population does not have to 'wait' for an advantageous mutation, but takes advantage of the ubiquitous variation in the number of gene copies."

Phenotypic plasticity

Adaptation to rapid environmental changes may also come from so-called phenotypic plasticity, in which characteristics of one individual change under the influence of the environment in such a way that they contribute to the organism's success in a changed environment. 

Phenotypic plasticity also plays a role in the development of behaviour. "In environments heavily modified by humans, such as cities, great tits develop different behavioural traits," explains animal ecologist Van Oers. "They have to guard against unfamiliar predators, new types of food and a greater variety of habitats. Urban specimens are therefore more aggressive and bold than great tits from the forest, and they are less afraid of people. Moreover, they are better at learning and solving problems. If we raise urban and forest tits in the same environment, all these behavioural differences turn out to be plastic traits. Except, incidentally, for one characteristic: the better problem-solving ability of urban animals is genetically fixed. That is an example of rapid evolutionary adaptation over the past decades."

On, not in the DNA

In recent years, it has been discovered in many different species that temporary modifications on the DNA play a role in this. By putting a molecular layer on the DNA, as it were, it is possible to temporarily switch a gene on or off. Because these adjustments are not in but on the DNA, it is known as 'epigenetic modification'. In the example of the great tits that moved to northern Scandinavia, this probably plays a role. Van Oers: "That epigenetic variation gives individuals more opportunities to temporarily adapt to different environments."

Plant ecologist Verhoeven studies the mechanism in plants. "Epigenetic changes can also create phenotypic plasticity in plants. There, some of the epigenetic changes even appear to be transmitted from parents to offspring. The question has arisen how important epigenetic changes in plants are for the rapid adaptation of traits. Then, for example, even the trait to cope better with heat could in principle pass from generation to generation without genetic evolution. That would be a kind of phenotypic plasticity over several generations."