Biology Reference
In-Depth Information
Fig. 3.1
you might think. For those organisms that are visible to the naked eye (often called
macroscopic organisms), the basic unit of classification is the species. There are sev-
eral different definitions of a species, but the most widely used is that a species “is
a group of actually or potentially interbreeding natural populations that is reproduc-
tively isolated from other such groups”. Thus all humans are members of the same
species, but chimpanzees are a different species. Horses and donkeys are differ-
ent species because, although they occasionally mate and produce offspring called
mules, these offspring are not fertile.
Estimates as to the total numbers of living species that have been described so
far range from 1.5 to 1.75 million, and Fig. 3.2 gives an approximate breakdown
into the major groups. The reason for this range of values is the varying definitions
of species that are used by different biologists. However there is no disagreement
that these numbers are much too small, and that many more species remain to be
discovered. Estimates of howmany more living species remain to be described range
from 5 million up to 100 million.
These numbers are large, but the fossil record shows that the number of extinct
species is much larger than the number of living species. About half of all the known
animal species alive today are insects, and it is thought that many more species
of insects exist than have been described, especially in tropical rain forests. The
most common insects are beetles. The biologist John Haldane was once asked by
a theologian what he had deduced about the nature of the Creator from his study
of biology; to which Haldane replied “He must have an inordinate fondness for
beetles”.
