Lumpers and splitters

So I mentioned the whole Rufous-sided/Eastern/Spotted Towhee thing and promised I’d get back to it.  Here’s me getting back to it.   Much of the reasoning for the splitting of Rufous-sided Towhees into Eastern and Spotted Towhees comes from how we think about what it means to be a species.  We’ll begin this story with a flashback (cue the smokey/wavy visual and the ‘back in time’ music):

Back in the day, people looked at two birds and said that they were the same species if they looked the same.  Now this particular group of Towhees do look fairly similar.  The “rufous-sided” part of the name comes from the color pattern of the bird:  a nice white belly, a black back and thick reddish-brown patches (rufous-colored) on the sides.   Spotted towhees, though, have white spots on their wings and back, whereas the Eastern Towhees do not.  Originally, people didn’t think that the color pattern difference and the song differences were enough to make them a distinct species and so they were given the moniker of sub-species. These people are the ‘lumpers’.  “If things look the same, we’ll put them in the same category,” they say.

As time went on, people began thinking about these differences and about what it means to be a species and they realized that these physical differences may be small compared to other kinds of genetic differences.  “If these birds are genetically distinct, then maybe they should be treated as different species as well,” went the thinking.  These people are the ‘splitters.’  As a result, various people began to look at the genetic differences.  A popular way to examine these differences is to look at mitochondrial DNA sequences.  Mitochondria are organelles inside cells that have their own DNA and get passed down in the egg cell from the mother, only.  As a result, they can be very useful to show similarities and differences between populations.  When people studied the towhees, they found that birds out west (the Spotted subspecies) were all more similar to each other and very different from the birds in the east (the Eastern subspecies).

Ball, R.M., Jr. and J.C. Avise.  1992.  Mitochondrial DNA phylogeographic differentiation among avian populations and the evolutionary significance of subspecies. Auk.  109:  626-636.

So splitting the species into two distinct species is really a way of showing that these two groups really are more different from each other than you’d guess just by looking at them.


About thomasbiology

I'm an Associate Professor of Biology at Queens University of Charlotte with a background in animal behavior with an emphasis in bird song. I've got two secret goals with this blog (well, since I'm sharing them, they're not so secret): 1. To encourage people to look at the natural world around them- not just as a hiking destination, but to notice all the little things moving around them all the time; and 2. To show some of the science that relates to these little things moving around. There's some really fascinating research out there that so few people get to see.
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