When history is evolution: the seals of Banks Peninsula

Recently, I caught up with an old school friend. It seems a long time since we played aspects of the same character in the musical “I was a teenage Jekyll and Hyde” at South Otago High School. Brent has a successful career as a history teacher, publishing text books and generally inspiring a generation of Auckland high school kids. Both of us are passionate about history, Brent in the social sense (New Zealanders during World War 2, and social change of the last 50 years) and me in the biological sense (evolution of the New Zealand biota). We’re both interested in how the past influences the present, just at different timescales. Usually.

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The harbor of Banks Peninsula. Seal colonies are at the heads (right hand end) and on the far sides of the hills.

 

Sometimes, when you are an evolution researcher, it is easy to envy the historian who has many different sources of information to reconstruct the past. Often there are even eyewitness accounts of what happened. The historian spends a lot of time sorting out the various accounts but, hey, at  least they have them! With evolutionary questions we are reconstructing events that usually happened before humans were a species. In evolutionary studies we have to make do with a fossil or two, perhaps a morphological trait and DNA. If we are lucky. No eyewitnesses there! Usually.

Over the last few years Rob Cruickshank and I have had a PhD student, Arsalan Khoyi-Emami, working on the New Zealand fur seal. Seals, like most pinnipeds, have been through harrowing times over the last few centuries as hunting by humans have crashed their populations almost to extinction. Peak hunting in New Zealand occurred in the early 19th Century and populations have spent the last couple of hundred years recovering. Here we have evolutionary responses in historical time – human history shaping the evolution of another species. We even have eyewitness accounts in the form of harvesting returns and observations from people like Captain Cook about where seals were found in the 1770s. Luxury!

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Arsalan about to take a sample from a NZ fur seal juvenile. Image from Adrian Paterson.

One question that we had was a biological one about what the seals forage on.  Another question is taking the historical point of view and asking what impact the severe reduction in numbers from hunting has had on the seal population. Given the growth in population over the last two hundred years we were also interested in how individuals had recolonised different areas of the mainland. Not only would this information tell us about the recent history of the New Zealand fur seal but it might help us to look into the future of this species to see how it may continue to recolonise and grow.

We decided to intensely sample one area of New Zealand, Banks Peninsula. An old remnant of several huge volcanoes, Banks Peninsula is currently home to 15 or so fur seal colonies. As far as we can tell, from eyewitness data like surveys, there may have been as few as 1 or 2 colonies prior to 1970. So the increase has been rapid and recent. Arsalan collected small tissue samples from over 100 individuals spread around the peninsula. Obtaining samples from large, aggressive seals on slippery, precarious rocks accessible only by boat is no small feat. Arsalan reports the findings of these labours in a new New Zealand Journal of Zoology paper.

Arsalan used the control region of mitochondrial DNA (a very fast evolving gene region) to assess the genetic groupings of the Banks Peninsula population. First up the DNA suggested that genetic diversity around Banks Peninsula was similar to that of the wider population elsewhere in New Zealand and that individuals often change colonies around the peninsula itself. There was a little geographical signal in that seals of neighbouring colonies shared more gene variants than those further away. This supports the idea that the seals simply spill over from one colony to form a new colony in the next bay (rather than moving out from large central colonies).

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Jekyll and Hyde – only their mothers can tell them apart! Both interested in history though. Image from Julie Paterson.

The bulk of the genetic signal suggested that fur seals had recolonised Banks Peninsula from the general New Zealand population although one colony around Horseshoe Bay was a little different, perhaps because it was a population that survived in a Banks Peninsula refuge through peak hunting. The current breeding population was estimated at around 2500 individuals which has increased from around 350 founders. The growth rate is still typical of an expanding population.

 

Often when a population moves through a very small size (or bottleneck) there is a loss of a lot of genetic differences from the population (surviving individuals become very similar genetically). However, Arsalan’s results indicate that there is a lot of genetic variation remaining in this population. This diversity may be because the hunting bottleneck, although extreme, only lasted for a very short time (2 or 3 seal generations). Most genetic variants were only found in one or two individuals, however, and are still at risk.

So human history shaped the evolution of the New Zealand fur seal. Human hunting, first by Maori from the 15th Century on, and then by Europeans in the late 18th and early 19th Centuries have left evolutionary scars on these populations that are still in recovery mode. Sometimes it is nice to work at the point where evolution and history overlap.

 

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