Does founder-effect speciation occur in the Silvereye?

Testing this controversial speciation model with genomic data.

Speciation typically takes thousands to millions of years as geographically isolated populations slowly accumulate genetic differences. However, theoretical work suggests that speciation can occur more rapidly. In combinatorial speciation models, for example, the rearrangement of existing genetic variation might give rise to new species in the evolutionary blink of an eye. Another possibility for rapid speciation concerns founder effects, an idea first articulated by Ernst Mayr in 1954. He envisioned how a small number of individuals became geographically isolated from the mainland population. Building on the theoretical work of Sewall Wright (1942), he described a scenario in which the reduced genetic variation and the effects of random genetic drift contribute to the genetic differentiation between the island and mainland population. In addition, extensive inbreeding between the few founders might result in more homozygous loci which are exposed to natural selection. The synergy between genetic drift and stronger selection could speed up the speciation process. Or as Mayr put it in his paper: “It may have the character of a veritable ‘genetic revolution’.” Theoretically, this reasoning makes sense, but what does the actual data say? A recent study in the journal Molecular Ecology tested this founder-effect speciation model for the Silvereye (Zosterops lateralis), a species that has colonized numerous islands in the Pacific Ocean.

Countless Colonization Events

Over the last 200 years, Silvereyes have spread across several islands from their source population on Tasmania. They sequentially reached South Island (New Zealand), Chatham Island, North Island (New Zealand), Norfolk Island and Tahiti. In addition, there have been much older colonization events from the Australian mainland to Heron Island and Lord Howe Island that occurred thousands of years ago. This study system allowed researchers to compare the genetic consequences of founder effects between recent and ancient colonization events. If Mayr’s proposal of “genetic revolutions” is correct, recently founded populations would quickly approach the level of genetic divergence observed in the older island populations. To test this idea, Ashley Sendell-Price and his colleagues sequenced the genomes (using RADseq) of Silvereyes from nine different island populations in the Pacific Ocean.

An overview of the colonization events from Tasmania (in black) to several Pacific Islands, and from Australia (pink) to Heron Island (blue) and Lord Howe Island (purple). The graph on the right shows differences in morphology between the island populations. From: Sendell-Price et al. (2021).

Accumulating Divergence

The genomic analyses showed that genetic divergence accumulates with the sequential colonization events. A more detailed look at these genetic differences revealed that distinct genomic regions diverged in each of the island populations, reflecting the stochastic nature of the founder effect. Although genetic divergence increases with each founder event, the recent island populations do not reach the level of genetic divergence observed in the older populations. This finding indicates that founder-effect speciation is not as rapid as Mayr envisioned it to be. Over time, the recently founded populations might reach the divergence level of the older populations, but it is not a “genetic revolution.” Hence, the authors conclude that “founder-event speciation may be rare in nature.”

The level of genetic divergence (measured as Fst) increases across sequential colonization events, but does not reach the level of genetic divergence at the older populations. From: Sendell-Price et al. (2021).

References

Sendell‐Price, A. T., Ruegg, K. C., Robertson, B. C., & Clegg, S. M. (2021). An island‐hopping bird reveals how founder events shape genome‐wide divergence. Molecular Ecology30(11), 2495-2510.

Featured image: Silvereye (Zosterops lateralis) © Bernard Spragg | Wikimedia Commons

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