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It is not easy being small. Especially when nature seems to favor larger individuals. But why don’t all animals evolve to be bigger? Is it sometimes better to be small? Or are some incapable of evolving?

To answer these questions, we studied a wild population of snow voles (a small rodent species) in their alpine habitat. Genetic analysis indicated a hidden evolutionary change: voles evolved to become smaller but the average body size of the population stayed the same. To understand the underlying causes, we separated genetic and environmental influences on vole body size.

We found that young voles with genes for small bodies developed faster. This allowed them to survive better when environmental conditions changed (earlier arrival of winter). As a result, the population evolved towards a smaller body size. Our study shows that populations can evolve rapidly. But without a genetic perspective and understanding the underlying causes, we may not be able to detect these changes.

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