The power of chance

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’There is no long-term plan as to where the journey is going,’ says

’There is no long-term plan as to where the journey is going,’ says evolutionary geneticist Jochen Wolf about the development of species. Biodiversity wall at Natural History Museum in Berlin. Photos: Stephan Höck, I. Schulz/ Picture Alliance/ ImageBROKER; Collage: chm

Life is constantly evolving, and yet "progress" is not the right word for this process of organismal change, says evolutionary geneticist Jochen Wolf.

A chasm of more than three-and-a-half billion years separates the beginnings of life from today’s abundant biological diversity. Virtually all environments on Earth - even the supposedly most inhospitable ones such as Antarctica or the ocean depths - are inhabited by living organisms. Evolution has thrown up a bewildering variety of forms, variants, and survival strategies. Not just temporally, but also in evolutionary terms, we have come a long way from the first single-celled creatures in primordial times to the organizational complexity of the animals and plants that surround us today. Intuitively, therefore, we could see evolution as a story of progress: a continual, directed progression toward more biodiversity, better adaptation to the environment, higher complexity, and ever more finely spun ecological webs.

From a scientific standpoint, however, the very term "progress" is problematic when applied to evolution: "After all, progress implies change toward something defined by humans as ’better’ and this raises the question as to what ’better’ actually means in this context," says Jochen Wolf, Professor of Evolutionary Biology at LMU. Such an anthropocentric viewpoint can lead to misunderstandings, he explains. First and foremost, evolution is a principle of life operating by means of random, heritable mutations and selection.

In the view of the evolutionary biologist, success can first and foremost be quantified between individuals and not between species. The basis of evolution is individual variability: small changes in DNA building blocks which individual creatures pass on to their offspring. An individual is successful when it manages to reproduce and hand down this version of its genes. The currency for success, so to speak, is fitness: In evolutionary biology, this term refers to how well an individual with a certain genotype is adapted to its environment. Through sexual reproduction, various gene variants are repeatedly mixed by means of recombination. Selection acts like a filter which lets through the combination of mutations produced by random variation that are advantageous in the current environment. "However, the selection filter is completely blind with respect to the future. There is no long-term plan as to where the journey is going," emphasizes Wolf.

Experiment: evolution in fast forward

This finely calibrated balance of mutations, their mixture through sexual reproduction, and their screening by means of selection is difficult to capture in experiments, as evolution is usually a long-drawn-out process in which a wide variety of genes interact. And so Jochen Wolf decided to recreate the whole process in the laboratory using a simplified system. For more than seven years, he has been letting mutations take their course in fission yeast cultures. All the cultures originate from a single yeast cell, i.e the cells were genetically identical at the beginning of the experiment. Since then, the populations have been evolving separately in their respective mini-incubators. The generation times are short, and random mutations can produce new variants.

Distinction between top and bottom

In this setting, Wolf studies adaptation to two habitats that differ in a very straightforward way: "bottom" and "top." Analyzing the cells in a suspension, he tracked how quickly they settled to the bottom and selected for further breeding those that remained aloft the longest and those that reached the bottom the fastest. He observed that adaptation can happen very quickly: "Within a few dozen generations, we found unbelievable fitness differences between the ancestors and the populations adapted to the new habitats," says Wolf. The crucial difference was made by random new mutations, which led to "highly idiosyncratic and divergent" adaptations. Some cells started to develop thread-like hyphae; others got thicker; while yet others grew longer - none of which was predictable.

In their special habitats, the adapted populations clearly outperform their ancestors. In this sense, we could indeed speak of progress, says Wolf. Back in the original habitat, however, the ancestors would presumably have the edge again and not the supposedly higher evolved specialists, as the new mutations would then be irrelevant or most likely in fact a disadvantage. "Evolution is very context-dependent and always only works within a specific environment," emphasizes the biologist. Meanwhile, the starting point - that is to say, the information already encoded in the respective genome - also plays a decisive role in constraining which path can be taken. Nature has developed many potentially successful survival strategies, and which of them prove successful and which do not, depends on the circumstances, the starting point - and is sometimes also the result of coincidences.

If everything was set back to the beginning

Of the one-off chance events in the long history of the Earth, the absorption of prokaryotic cells - in essence, bacteria - into host cells is surely one of the most consequential. After all, it gave rise to today’s mitochondria and chloroplasts, which are responsible for the supply of energy and photosynthesis in eukaryotic cells. This so-called endosymbiosis was an early evolutionary quantum leap and was quite likely a necessary condition for the subsequent explosive radiation of more complex organizational structures, such as we encounter today in animals and plants. If the evolutionary clock was set back to the beginning, according to Wolf, this jump might not be repeatable in the same way. Other universal principles, however, would probably reemerge in a similar fashion. These include things like polymers capable of replication and tiny compartments separated by membranes. The evolutionary biologist assumes that multicellularity would also likely emerge again, as there are advantages to cooperation, as would a type of nervous system with which living systems can react to environmental stimuli.

An engineer would be more likely to build a bacterium than a person.
Jochen Wolf

Is growing complexity, then, a sign of evolutionary progress? Well, it is certainly no clear indicator of the ability of an organism to thrive: Microbes are among the least complex of organisms in human definition, and yet bacterial pathogens get around the world at a rapid pace. And the coronavirus SARS-CoV-2, a biological particle with an even simpler structure, which some biologists do not even consider to be a microorganism, swiftly covered the globe despite all the measures undertaken to prevent it. Due to their rapid succession of generations and the high number of individuals, microbes have advantages when it comes to adapting quickly to changing conditions.

If anything, Wolf sees a connection between ineffective selection and more complexity: If deleterious mutations are preserved, they may have to be balanced out by further mutations. In other words, the complexity grows. Wolf compares this with a crooked hut that needs to be repeatedly propped up with a board here and a board there. Accordingly, the LMU researcher does not see complexity as necessarily being a sign of success: "An engineer would be more likely to build a bacterium than a person."

It’s always a question of strategy: Some species invest a lot in the survival of their few individuals; others invest in having a lot of offspring.
Jochen Wolf

In general, the criteria for evolutionary progress are framed too much according to human categories for Wolf’s liking. People tend to think teleologically and crave to find causal, meaningful relationships between events along a timeline. But crucially, evolution is simply not a linear progression toward a defined goal or according to a set plan. Wolf is keen to stress this point: "If everything was geared toward the goal of a super-organism that outcompetes all other beings, we would not have a tree of life, but a highway of life." Evolution can follow winding paths and diversions and go down blind alleys and endow characteristics with brand new functions. Feathers, for example, were probably originally "invented" by nature to display colors and impress the opposite sex, possibly also for heat insulation - flying came long after the first feathered dinosaurs.

Most mutations are harmful to organisms

The most important driver of evolution is changing environmental conditions, which can increase selection pressure and benefit new variants. Most mutations are harmful, however, and therefore it is important for organisms that their number remains low. How successful this is, in terms of whole populations, depends in part on a population genetics principle: the interplay between genetic drift and selection. By genetic drift, evolutionary biologists mean the random change in the frequency of certain gene variants. The smaller the population, the more significant is the role played by genetic drift, as the statistical probability for both disappearance and fixation of individual variants increases. This reduces genetic variability, with the result that selection is less able to remove deleterious mutations - the classic inbreeding effect.

An important variant of genetic drift is the so-called bottleneck effect, whereby population size plummets because of events like natural disasters and plagues. Jochen Wolf has demonstrated, using genetic analyses, that the genetic variability of numerous seal species was significantly influenced by historical fluctuations in population size. Such comparative population genome analyses can be an important tool to identify endangered populations and potentially initiate conservation measures. The absolute size of a population, however, is not a reliable indicator of the vitality of a species. "It’s always a question of strategy: Some species are successful with few individuals and invest a lot in their survival; others invest in having a lot of offspring," says Wolf. "Both strategies can be successful."

At the same time, it is clear that when the challenges posed by environmental changes become too great, many organisms reach the limits of adaptation and become extinct. Climate change could reveal just such limits to adaptability. Although the far-reaching changes could also be a spur to evolution, Wolf fears that much of today’s variation will be destroyed. Preventing this from happening would undoubtedly be a human success story.

Monika Gödde


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