Biology Reference
In-Depth Information
water presents a barrier to the passage of most land animals and plants. This distri-
bution pattern is explained by evolution if you assume that each species appears at a
particular location, and then gradually disperses away from its point of origin until
it meets a barrier that it is unable to cross.
Darwin devoted two chapters in his famous topic to the geographical distribution
of species. The first sentence reads:
In considering the distribution of organic beings over the face of the globe, the first great
fact that strikes us is, neither the similarity nor the dissimilarity of the inhabitants of various
regions can be wholly accounted for by climatal and other physical conditions.
As an example, Darwin points out that both the North and South American conti-
nents share the same range of humid forests, arid mountains, grassy plains, marshes,
lakes and rivers as does the European continent, but that there are almost no species
of animals and plants that occur in both locations. A further example is the similarity
in climate between parts of Australia, South Africa and western Southern America.
But this similarity is not accompanied by any similarity in the animals and plants
that occur in these regions. Kangaroos were not found in Europe and rabbits were
not found in Australia, until humans transported them there.
Similar observations have been made for marine environments. The shelled ani-
mals such as crustaceans and sea urchins, are quite different between the eastern
and western shores of South America. On the other hand, many of the fish species
are the same on the opposite sides of the isthmus of Panama, suggesting that in
the recent geological past, there was a free flow of water between the Pacific and
Atlantic oceans. Since Darwin's time, geologists have established that the isthmus
of Panama formed about three million years ago, an event that had dramatic effects
on the world's climate. The blockage of exchange between the two oceans resulted
in a rerouting of ocean currents, the most important being the formation of the Gulf
Stream that today keeps Britain and northwestern Europe at a habitable temperature
in the winter. The formation of the Panama land bridge allowed animals and plants
to migrate between North and South America, an event called by fossil experts
the “Great American Interchange”. For example, the ancestors of the opossums,
armadillos and porcupine found in North America today came across the newly-
formed land bridge from South America, while the ancestors of animals such as
cats, bears and racoons went in the opposite direction.
Another observation that impressed Darwin is that, although species at differ-
ent locations on a continent are distinct, nevertheless they are more similar to one
another than to those on other continents. He pointed out that if you travel from north
to south on the same land mass, successive groups of species replace one another but
are also closely related. The same conclusion applies to islands. Their inhabitants
are distinct from those on the nearby mainland, but are more closely related to them
than to those on different continents. During his stay on the Galapagos Islands,
Darwin collected a number of different mockingbirds as shot specimens. On his
journey home he realised that all the mockingbirds caught on each island were of
the same species, but different from those caught on nearby islands, while all the
