Crossbills show there is more to evolution than natural selection

What explains the fit between beak morphology and pine seeds in a Spanish population?

A common misunderstanding about evolutionary theory is equating evolution to natural selection. However, evolution is much more than natural selection. Evolution refers to the change of populations over time (often phrased in terms of allele frequencies), while natural selection is just one mechanism underlying these changes. The idea behind natural selection is simple, but very powerful. It is the logical conclusion of these three premises:

  1. Individuals vary in certain traits
  2. These traits are heritable (they are passed on from parent to offspring)
  3. Variation in these traits leads to differences in survival and reproduction

And that’s it. A quick hypothetical example illustrates the workings of natural selection. Imagine a population of birds with different beak sizes. These birds feed on a tree species that produces seeds hidden inside small cones. Birds with a certain beak size can open these cones and access the seeds inside. These birds will be most successful in surviving (they have plenty of food) and producing offspring (they can feed their young). Because beak morphology is heritable, the young birds inherit beak size from their parents. Each generation, the variation in beak size will be determined by the surviving birds and their offspring from the previous generation. Over time, the average beak size of the population will converge upon the optimal size for cracking the cones.

Loxia curvirostra

A Common Crossbill in Austria © Frank Vassen | Wikimedia Commons



A recent study in the Journal of Avian Biology studied this scenario in common crossbills (Loxia curvirostra) from Spain. These birds show variation in beak size and forage on mountain pines (Pinus uncinata). Using a capture–recapture dataset spanning 27 years, the researchers investigated whether natural selection is driving beak morphology to an optimum size to feed on mountain pines. Between 1988 and 2014, birds were ringed and recaptured. How did beak morphology change over this period?

Statistical models indicated that the population optimum beak width is 11.43 mm. Apparent survival decreased when beak width deviated from this value. I write “apparent survival” because the capture-recapture data only suggest that a bird did not survive. When a bird is not caught again, it could be dead or it could have moved to another location.


For different age classes (juveniles, yearling and adults), apparent survival shows a peak around a beak width of ca. 11 mm. From: Gómez‐Blanco et al. (2019) Journal of Avian Biology


Selecting Environments

This results suggests that natural selection is keeping the population stable around a beak width of 11.43 mm. As I explained above, individual birds with smaller or bigger beaks don’t survive and are thus removed from the population. The reality, however, is not that straightforward. An alternative explanation is that maladapted birds do survive but fly to other locations where they have better access to food. Instead of the environment selection for particular individuals (i.e. natural selection), the individuals are selecting a certain environment.

This phenomenon could partly explain the result in this study. The authors write that “our estimate would be a rather unusually strong measure of stabilizing natural selection. Said otherwise, an unusual number of selective deaths would have to occur because of a phenotypic trait. Therefore, it appears probable that selective dispersal of locally maladapted individuals out of the study area has also contributed in order to produce this high value.” More widespread sampling of neighboring populations is needed to confirm this idea.


The estimated selection gradients from the present study (blue) versus a literature review (grey) indicates from strong selection on crossbills. Perhaps too strong? Gómez‐Blanco et al. (2019) Journal of Avian Biology


Alternative Explanations

This study illustrates that a match between a certain trait and the environment is not always the outcome of only natural selection. There are several alternative explanations that need to tested, such as individuals selecting certain environments. Other possibilities are phenotypic plasticity and adjustment of the environment. Phenotypic plasticity concerns the situation where a trait can vary in different environments (think of a tree growing higher in fertile soil compared to bare soil). Adjustment of the environment is self-explanatory: a population changes the environment to fit its needs, such as beavers building a dam to flood an area.

These alternative explanations highlight that we should be careful in attributing adaptations solely to natural selection and telling unfounded just-so stories (such as my hypothetical example above). Explore all possibilities before your draw conclusions.



Gómez‐Blanco, D., Santoro, S., Borrás, A., Cabrera, J., Senar, J. C., & Edelaar, P. (2019). Beak morphology predicts apparent survival of crossbills: due to selective survival or selective dispersal?. Journal of Avian Biology50(12).

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