The coexistence within species of two or more different forms has puzzled biologists for more than a century. According to evolutionary theory, natural selection should remove inferior individuals from the population, and thereby erode diversity and promote monomorphism. New research published in the journal Scientific Reports demonstrates that polymorphism as such can be beneficial for both individuals and populations by offering protection against predators.
That colour polymorphism may offer protection against predators was suggested more than a century ago. A few studies have confirmed that individuals that are members of polymorphic groups may benefit from reduced predation risk. However, there has been no evidence that polymorphism can increase survival of populations.
Researchers from Linnaeus University in Sweden and Åbo Akademi University in Finland photographed grasshoppers, displayed the images on computer screens in groups that represented four levels of polymorphism, and then asked humans to act as 'predators'.
"We aimed to investigate whether and how polymorphism of natural camouflaged colour patterns affects survival of populations. We also wanted to examine whether the protective effect is similar or different for alternative colour morphs," says lead-author Dr Einat Karpestam.
The study included 70 human subjects and more than 3300 grasshopper images. Overall, humans needed more time and detected fewer grasshoppers when the images were presented in mixed than in homogeneous groups.
"Our results are important because they offer the first experimental demonstration that the protective effect of polymorphism can extend to the level of populations. This finding helps resolve the old puzzle why colour polymorphism is so common in nature," says Dr Sami Merilaita.
But are these results reliable? Can computer-based detection experiments really inform about natural processes? According to senior author Professor Anders Forsman the answer is yes.
"Our previous research shows that results obtained using this approach mirror natural selection imposed by real predators, such as birds, when they hunt for live and free-moving prey in the wild."
These results have some interesting implications for us as humans. For instance, the protective effect of polymorphism depended on individual characteristics of the prey. Some colour pattern phenotypes benefitted more from being members of mixed groups than others. This implicates a conflict of interest among individuals with regard to what is the optimal group composition. This is likely to influence individual decision making and can negatively impact on interactions and group dynamics.
Another lesson from the study is that dividing ones attention can be costly.
"It seems that we perform better if we focus one thing at a time. This realization is important to bear in mind in an era when multi-tasking is considered something to strive for," says Professor Anders Forsman.
Explore further: Bird color variations speed up evolution: research
Einat Karpestam et al. Colour Polymorphism Protects Prey Individuals and Populations Against Predation, Scientific Reports (2016). DOI: 10.1038/srep22122