MOUSE – SPRING 2011
Biogeography is the study of the geographic distributions of organisms, trying to
answer questions such as: “Why are these species where they are? Why is the composition of the
biota thus and so here, but otherwise over there? Why is the biota so rich here, but depauperate
there?” There are really two sorts of answers, ecological and evolutionary.
An ecologist notes that there are broad climatic zones and physiographic zones that are quite
similar, though on opposite sides of the planet, each containing a similar type of biota, a
similar biome, a ‘desert community’ or an ‘alpine community’ or a ‘rainforest community’,
etc. In other cases, one finds related species, widely distributed, tied to a specific habitat type.
The answer is at least partly that the ecology is driving the system.
An evolutionary biologist is far more likely to worry about the geological and migrational
history of the organisms, and the question of “How did this species or these species get here?”
Darwin and Wallace, who didn’t know about continental drift, were inclined to concentrate on
routes of migration and ability to spread, but as we shall see, it is important to think about not
only when ancestors were but where they were at that time. The geological history matters.
- Darwin got it mostly right (barring ignorance of continental
drift), and he pointed out three salient observations or facts, in support of his lineage theory:
Each species has a definite site or region of origin. The species spreads by colonization. How
far it spreads depends on two things – whether the intervening habitat is favorable, and how
mobile the organism is. The original species becomes modified as it spreads out, and new
varieties (yielding species) arise, but they are collectively geographically restricted.
Any barrier to movement (by which Darwin meant migration & colonization) would stop the
spread. A bird is not normally deterred by a serious expanse of water, in between, but an
elephant is. A very tall mountain range may stop a lowland snake, but not necessarily a deer.
The point is that you have to be able to get there from here, or isn’t going to happen. The
codicil to that is that sometimes the critters can surprise you; they really do get around.
Within a region, say one of Wallace’s
, the organisms are related to
each other, even though they occupy very different ecological situations. That is a statement
about ancestry, not a statement about ecological similarity. There are exceptions, but they
occur in situations where migration was easy or for organisms that are extremely mobile.
Extremely isolated oceanic islands do not have a native mammalian fauna, other than bats.