Splitting up the beetles: Come together on Banks Peninsula?

Megadromus guerinii specimens. Image from Adrian Paterson.

I’ve always been a fan of the beetles. They are some of the most obvious insects in our environments as we grow up and they don’t seem to have the ‘ick’ factor for people, like flies and spiders do. I’ve also always been a fan of the Beatles. My parents were not hugely into John, Paul, George and Ringo but they did have a copy of the ‘Hard Day’s Night’ record and I played that a lot as a kid. In my teens we got the Beatle’s Box from Reader’s Digest which had most of their music on a bunch of cassettes and that introduced me to the whole range of their output. Finally, when my eldest son was born, a trainee radio station for the local broadcasting school decided that their three month practical would be to play only Beatles music. Much time was spent listening while changing nappies and bottle feeding the babe. My interest in the Beatles has persisted through till today.

beatles & beetle
Temporal variation in Beatles, from clean cut to hippy.

One of the fascinating things about the Beatles is that the music that they wrote at the end of the 1960s was very different to what they wrote when they first became famous. It is hard to believe that the same people who wrote “She loves you, yeah yeah” and “Please, Please me” also wrote”I am the Walrus” and “Blackbird”.  Even the clean cut image of 1963 was radically different by 1967 with the messy and colourful Sgt Peppers. There is no particular reason why a person unfamiliar with the Beatles would link “Happiness is a warm gun” with “Twist and shout” after listening to them. Which brings me to taxonomy.

The science of taxonomy is all about finding and naming species. Historically, taxonomy was based on phenotypes (or what species look like). By and large this works fairly well and the vast majority of species have been named in this way. Sometimes this doesn’t work so well. Different species may look the same because they inhabit similar niches and convergent evolution occurs. The same species may have populations or individuals that look very different due to different responses to features in a habitat. TO solve this potential problem we can examine the genotypes of the individuals involved. Whether individuals differ genetically from one another should not be influenced by convergent evolution caused by the environment.

Lots of phenotypic variation in M. guerinii. Image from Adrian Paterson.

As biogeographers we are interested in the distributions of species – where we find them in the landscape. A crucial factor is that we are sure about the species we are working with. Taxonomy, systematics (how species are evolutionary related) and biogeography are a tightly linked package.

One local region that we spend a lot of time working on is Banks Peninsula. This is an area of about 1150 km2 which was built by volcanic activity from 11 to 6 million years ago. Peaks on the peninsula have eroded from 1500m to around 760m over the last few million years. For much of its history Banks Peninsula was an island until the Canterbury Plains, a large alluvial outwash from the Southern Alps, connected the peninsula to the mainland sometime in the last tens of thousands of years.

With this isolation Banks Peninsula has a high level of endemicity (species only found on the peninsula). As such we do a lot of biogeography on species found there. One such species is the big black forest beetle Megadromus guerinii. M. guerinii is found over the peninsula but is restricted to pockets of mostly native scrub and forest. From a conservation management point of view, it is useful to know if we are dealing with one large population or several small fragmented populations.

Rob Cruickshank and Mike Bowie, working with several students, initially found that beetles from Akaroa township were genetically very different from individuals a few hundred metres up the hill. After close inspection they were unable to find any visual differences between these beetles. They wondered whether this indicated that different populations were fragmented and very isolated. More worryingly there might be at least two different species here which might be confusing the analyses. They then decided to sample across the whole of Banks Peninsula.

Banks Peninsula
Banks Peninsula, the eastern mtDNA beetle populations live on the taller hills, the western mtDNA populations on the lower slopes. Image from Adrian Paterson.

In a study published in New Zealand Journal of Zoology, led by postgraduate student Ben Wiseman, we now have a better picture of what is happening with M. guerinii. Despite there being no consistent visual (or phenotype) differences, there are clear genotype differences in the mitochondrial genes which group M. guerinii into two groups or clades. There is an eastern group which lives on the hills above Akaroa and on the outermost slopes of Banks Peninsula. There is also a western group spread throughout the rest of the peninsula. Both groups have reasonable gene flow within them but not between them.
Complicating the analyses is the fact that the nuclear genes looked at in this study show no consistent differences between the east and west populations (and so agree with the phenotype and not the mtDNA genotype). Why would different genes give different results? Mitochondrial genes tend change at a faster rate than nuclear genes  because the are inherited maternally (making effective populations smaller) and have less stringent error correction mechanisms (increasing the mutation rate). This means that sudden range expansions or the appearance of barriers should be detected first in mitochondrial DNA.

Despite the large difference in mtDNA between east and west beetle populations, there is no overwhelming evidence for them being different (cryptic) species. What we are likely seeing is that M. guerinii has colonised the western Banks Peninsula in the last few thousand years. With fragmentation of habitat the nuclear genes may eventually catch up and speciation may occur. For now the taxonomy is sound and there is one species. Like the Beatles through the 1960s despite their large changes in music they were still fundamentally the same group, likewise Megadromus guerinii is a species that has changed a lot over the last few thousand years but is still fundamentally the same species. Also like the Beatles, given time, these beetles will split up.




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