I read this paper last week, and since then I have been trying to make sense of some of the conclusions expressed. I am hoping someone can explain to me what I am missing.
By way of background, the Tasmanian population of Wedge-tailed Eagle is estimated at about 420 breeding pairs and 1000 - 1500 individuals. It has been described as a separate subspecies, Aquila audax fleayi, though there has always been a level of disagreement about whether it deserves subspecies status.
The question the paper set out to answer was whether the degree of genetic difference in the Tasmanian population was consistent with the Tasmanian birds having been separated from mainland Australian eagles since the landbridge to Tasmania was last in existence around 13 000 years ago. The clear answer from their research is no. Or as they express it, “Coalescent analyses indicate that population divergence appreciably postdates the severance of terrestrial habitat continuity and occurred without any subsequent gene flow”.
So far so good. But when I delve into the paper I find the following:
The Tasmanian population of A. audax became isolated very recently from the mainland population, contrary to the paradigm of marine transgression-induced vicariance of continental island populations. Coalescent estimates employing ‘conventional' avian mitochondrial substitution rates suggest divergence 3–95 years ago. While these divergence dates must be considered an underestimate because the first European sighting of A. audax in Tasmania was documented in 1836, this likely reflects the lack of unique molecular variation in the Tasmanian population, which confidently refutes divergence times compatible with divergence accompanying the flooding of Bass Strait greater than 13 kya. Furthermore, under time-dependent molecular rates, such an ancient divergence is even less likely. Despite the inferred recency of founding, there was no evidence for subsequent or ongoing gene flow, from coalescent analysis, Bayesian clustering or tests of allele frequency homogeneity. These results suggest that the Tasmanian population was founded appreciably later than the marine transgression that flooded the Bassian plain and severed terrestrial connection to mainland Australia approximately 13 kya.
This is where I have trouble reconciling things with the Wedge tailed Eagle population in Tasmania being a fairly large population of a species that has a long lifespan and doesn’t reproduce rapidly. Questions that come to mind include:
- How do sight records from 1836 rebut the suggestion that divergence occurred 3-95 years ago? Just because genetic divergence happened at a particular date doesn’t mean the species would not have been present in Tasmania earlier than that (a minor point I admit, but I am trying to understand the logic behind the authors’ position generally).
- Given the significant size of the Tasmanian population, if the level of genetic difference is so slight as to suggest divergence occurred that recently, wouldn’t incomplete lineage sorting render the degree of genetic difference essentially meaningless?
- If we take one extreme of the range of dates, genetic divergence may have occurred 3 years ago. How can one confidently say there’s no evidence of subsequent gene flow since then – after all, Wedge-tailed Eagles don’t breed until they are five or six years old so any gene flow to another individual is logically impossible in that time frame.
To explain where I am coming from another way, with 450 pairs in Tasmania, the plausible population histories seem to be:
- the species has been in Tasmania a very long time (say perhaps since Tasmania was linked to the mainland 13 000 years ago)
- a small number of eagles crossed Bass Strait a long time ago – long enough ago to establish a population that’s now over 1000 birds. That would have had to be many, many generations ago.
- a big influx of eagles occurred relatively recently. Table 2 seems to be estimating the ancestral size of the population, but I am not sure I understand it. It seems to suggest 70 birds in the 3-95 year ago range, but how could that lead to 1000 birds today, in a population that is thought to be declining, not thriving?
To me a logical hypothesis would be that Wedge-tailed Eagles have probably been in Tasmania for a long time, and there has been continual gene flow between Tasmania and mainland Australia for most of that time. If there really is strong evidence to suggest gene flow has stopped recently, then human induced habitat change or population reduction from persecution might explain it. Maybe that's what the paper is suggesting and I just don't understand it well enough.
Any offers of enlightenment would be appreciated.
