Drift drives phenotypic evolution in a rapid island radiation

Background

As populations accumulate phenotypic differences over time, they eventually evolve into different species. The phenotypic diversity and lineage diversification are central to understanding how new species arise. Faster species formation is often linked to faster trait evolution.

Sexual signalling traits, such as plumage coloration, are not merely ornamental but can influence mate choice. Thus, it has been used to study the phenomenon of lineage diversification. Several hypotheses have been proposed to explain why rapidly evolving feather colors may accompany the formation of different lineages (Figure 1).

Under sexual selection, certain phenotypes are favoured over others, where birds choose their partners based on color. Species recognition offers a different explanation, where varying color patterns are advantageous in the identification of the mates of the same species. In genetic drift, a color variant is fixed due to sheer sampling luck. Random chance events, such as drift, are known to shape the smaller populations.

In the preprint highlighted here, the authors hypothesised that if sexual selection is the driving force, then color change should occur irrespective of the population size. But if genetic drift is the dominant force, populations with lower genomic diversity and smaller population sizes should exhibit faster rates of color evolution.

The preprint’s main aim was to study the kingfisher birds that diversified across islands, where selection- and genetic drift-predictions could be compared ¹. Such diversification of an ancestral species into multiple species, termed as island radiation, provides an ideal setting because an isolated population often experiences reduced gene flow and a smaller population size.

The authors combined the genetic data and feather color measurements of 96 taxa across the Indo-Pacific kingfishers to assess how color evolution correlates with the patterns of species diversification.

Key findings

Diversification and phenotypic evolutionary rates are positively correlated

The authors observed and verified a positive correlation where species exhibiting a higher lineage diversification rate also had a higher rate of plumage color evolution. This relation was consistent across sexes and the two methods for calculating lineage diversification rate. These results also highlighted within-species heterogeneity, challenging the longstanding belief of a species being a monolith.

Tempo of phenotypic diversification increased with decreasing genomic diversity

Next, the authors tested whether patterns of plumage evolution were better predicted by selection or genetic drift. Differences in color between males and females, often allied with sexual selection, showed no association with the rate of color evolution. Similarly, species sharing the same geographical space did not exhibit faster color evolution and were instead weakly associated with lower evolutionary rates.

In contrast, genome-wide heterozygosity emerged as the strongest predictor of color evolution. It was found that lineages with lower genomic diversity displayed a higher rate of chromatic (hue and saturation) and achromatic (brightness) color evolution. Range-limited kingfishers, such as the Pacific, exhibited faster rates of color evolution than the widely distributed taxa, such as the Red-backed or Sacred.

Genetic drift drives rapid phenotypic evolution in an island radiation

The rate of color evolution was best described by genomic diversity: birds with lower genetic diversity demonstrated a high rate of evolution. Adaptive processes were unable to account for these observations and did not emerge as the primary driver of the signalling trait. These results, hence, suggest that random evolutionary mechanisms, such as genetic drift, may be more important as a determinant of plumage coloration.

Conclusion and future directions

Earlier studies have implicated genetic drift in microevolutionary processes such as bat vocalisations ² , but this study provides rare evidence that drift may drive the evolution of sexual signalling traits in bird radiations. These findings highlight drift as an often-overlooked process shaping the phenotypic diversity in isolated populations.

The authors further suggest that structurally produced colors may be more susceptible to drift, as opposed to biochemically driven colors. Future studies across bird radiations would clarify the different mechanisms of color production and their evolutionary trajectories.

More broadly, these results raise an exciting possibility that genetic drift may play a larger role in phenotypic diversification than previously accounted for. Investigating other radiations and non-avian taxa will determine whether drift-driven divergence is a general principle of speciation or largely restricted to island systems.

What I like about the preprint

For the last few decades, biologists have presumed that most sexual signalling traits have an adaptive advantage. I find this study interesting because it challenges the deeply ingrained assumption that striking phenotypes, such as plumage colouration, arise solely through strong selection.

I also found the preprint authors’ approach to the problem interesting. Most studies analyse species that are already established and measure the color differences. As speciation begins before species exist, the authors here studied populations diverging in real time, rather than looking only at already formed species.

Questions for the authors

1. Can drift-generated color divergence become a driver of reproductive isolation?
2. Would this pattern persist outside island radiations?
3. Is there other signalling traits expected to show a similar role of genetic drift in lineage diversification?

References

1. McCullough J, Eliason C, Shultz A, et al. Drift drives phenotypic evolution in a rapid island radiation. bioRxiv. Published online June 11, 2026:2026.06.09.731170. doi:10.64898/2026.06.09.731170
2. Pratt HD, Etpison MT. The Birds and Bats of Palau.; 2008. Accessed June 14, 2026. https://library.sprep.org/content/birds-and-bats-palau

 

doi: https://doi.org/10.1242/prelights.44127

Read preprint (No Ratings Yet)