- SPECIES PROBLEM
MOUSE – SPRING 2011
The Central Difficulty:
We have bandied around the label “species” for this course rather
freely, but we have yet to define what the term “species” actually means in evolutionary terms.
We’ve been concentrating on the nitty-gritty of how the gene pool changes over space and time,
that is to say with “evolution in the small” or
, i.e., evolution within a species.
– A population (or a collection of populations) changes because mutations are
pumped in, because genes are exchanged between adjacent populations, because genetic drift
provides some sampling variation, and because natural selection sorts out what works better.
– Populations diverge over evolutionary time, and eventually either go extinct or
become species, but that process is too slow for us to watch in real time. It can take hundreds
or thousands of generations. But, even that is too fast to see it happen in the fossil record.
– There is a suspicion that something important happens in the process of a single
species becoming more than one species. Even Darwin could not explain how that happened,
in other than divergence terms, and mostly we draw what inference we can from organisms
that we catch in the process, or species that have recently gotten through it, whatever “it” is.
– Species were with us long before Darwin. Linnaeus had even given them
names and divided them up into genera, families, and so on. We “knew a species when we saw
We didn’t need to speak of evolution to talk about species. They obviously existed.
– From a taxonomic point of view, all we needed were piles, into which we could
place specimens. The piles might have internal variation in morphology, anatomy, color, or
genotype, but with greater divergence among piles than variation within them.
– For asexual organisms, most of our species are piles. We could divide
into several species, but the current taxonomy is based on the idea that different strains
are similar, relative to their differences from
, two related genera.
Where to draw the line around
is a still matter of convenience, and based on utility.
– No matter how we define our species, there are cases where the edges between
species are a little fuzzy. There are even times when we have to draw a pretty arbitrary line
around our species. The variation within them is the same order of magnitude as the
differences between them, because nature “could care less” about our choice of piling rules.