Why are there often many different types of males in an animal group? According to the theory of evolution, only the "best" should have prevailed over time. A team from Julius-Maximilians-Universität Würzburg (JMU) has now set out to find an answer to this question. Scientists from the German Cancer Research Centre (DKFZ) in Heidelberg and the University of Halle-Wittenberg were also involved in this research. The result: so-called "mate copying" - i.e. simply copying the mate choice of other specimens - plays a key role.

Scientists from the Chair for Computational and Theoretical Biology (CCTB) at the University of Würzburg were responsible for this study. Under the leadership of Chaitanya Gokhale, Professor of Theoretical Evolutionary Biology, Srishti Patil (DKFZ) developed new mathematical models that significantly expand the previous theoretical framework. Grounding the work in the empirical work of Dr. Sabine Nöbel from Halle, they have together published the results of their study in the scientific journal PNAS .

"In classical evolutionary biology, the dogma has long been that individuals have an innate preference in choosing their mates. According to this, every living being uses private information to find the biologically fittest partner. Examples of such information are one's own sensory impressions, experiences or genetic instincts," says Chaitanya Gokhale, explaining the background to the study.

However, modern research is undergoing a paradigm shift: according to this, many animals rely on social information instead. They observe the choices of their conspecifics and copy their decisions. This "mate copying" is a powerful mechanism of inheritance.

"You can compare this process to choosing a restaurant: Instead of examining the menu, i.e. private information, in detail, you prefer to sit where many guests are already seated. So you use social information," explains Gokhale. This behaviour is widespread in nature. It is found in vertebrates such as birds and fish, but also in invertebrates such as the fruit fly Drosophila melanogaster.

While independent mate choice usually aims to maximise the genetic quality of offspring, copying means that individual decisions depend on collective trends. This raises a crucial question: How does this social learning affect the long-term diversity of an entire population?

As evolutionary processes extend over periods of time that can hardly be observed experimentally, scientists use mathematical models to clarify such questions. While previous models are usually limited to a choice between two options, the new model developed by the Würzburg research group offers a wider choice. It can simulate complex systems with any number of different types, so-called morphs - a prerequisite for modelling reality in nature.

The researchers' analysis shows that in mate choice, the type of copying determines the fate of phenotypic polymorphism - i.e. the lasting diversity within a species. There are two opposing variants to choose from:

Conformity: Here, the majority follows the trend. The model shows that this can paradoxically lead to the fixation of traits that have a lower biological quality. A rarer, actually fitter type then has little chance of asserting itself against the established social trend.

Anti-conformity: If individuals deliberately copy the minority, diversity in the population remains stable.

In addition, the new model makes it possible to identify the "critical copying probability". This threshold value marks the point at which social information overrides natural selection. For example, the research team's empirical background and calculations show a threshold value of around 38 per cent for three different morphs. This means that if around 40 per cent of the population follows the example of other individuals when choosing a mate, a biologically inferior type can suddenly dominate the group.

The researchers also show that diversity remains particularly stable when the probability of copying adapts to the environment. This creates a feedback loop: "If the decision in favour of a common male type is associated with costs for females - for example, because this type is biologically inferior - the probability of copying should decrease," says Gokhale. This dynamic balance prevents social information from completely eliminating natural selection, thereby protecting polymorphism.

With their model, the Würzburg research team provides an important explanation for the so-called "lek paradox". In lek mating systems, in which males court females together at mating grounds, strong selection should theoretically lead to all males looking identical at some point. The fact that they nevertheless remain so diverse can now be explained mathematically by the interplay of private information and conformist or anti-conformist copying.

These findings are of great value for conservation biology. They allow more precise predictions about whether an endangered species will lose its phenotypic diversity due to social trends, or whether social learning can help favourable new traits spread quickly within a population.

The study also emphasises that evolution is not determined by genes alone. It is also shaped by the way information flows and is processed within a community. Mathematical modelling bridges the gap between theoretical biology and observable nature.