Geoscience Reference
In-Depth Information
ecosystems
A natural ecosystem is a self-regulating community of
organisms which are in equilibrium with their physical
environment. Species adapt through processes of natural
selection to the conditions - biological and non-biological
- which exist in the ecosystem. Processes of
natural
selection
have worked through evolution to give us a
world which can support a rich variety of species. Precisely
how many species has been estimated by Wilson (1992)
to be 1ยท4 M, including all plants, animals and micro-
organisms. However, some biologists estimate that this is
probably less than a tenth of the number that actually
inhabit Earth! In fact the true number probably lies
somewhere between 10 M and 100 M species. For
example, the biodiversity of micro-organisms in the
world's soils is scarcely known.
Table 22.1
gives an
indication of the total number of species on Earth.
of organisms. An
evolutionary species
has a single lineage
of ancestor-descendant populations distinct from other
such lineages, whilst a
morphological species
is the smallest
population permanently separated by a clear discontinuity
in heritable characteristics such as morphology.
Speciation
Speciation
is the process whereby two or more genetically
distinct species emerge from a single ancestor. Evolution
is a directional change over time in the frequency of genes
in a population. Genetic variation is the raw material for
evolution, and the mechanism for evolution, especially
in small populations, is
genetic drift
, the differential
reproduction among genotypes in a population as a result
of chance.
Microevolution
refers to evolutionary change
Insects
751,000
Evolution
Other animals
281,000
Higher plants
248,400
Sir Charles Darwin and Alfred Russel Wallace in the mid-
nineteenth century introduced theories of
evolution
which have since been refined and extended. As with any
sub-discipline, the part of biogeography that deals with
evolution and biodiversity uses a number of technical
terms that need defining and understanding. A
taxon
is
any discrete and recognizable biological group such as a
species, a genus or a family of animals or plants. A
species
is a group of organisms of the same kind which can
reproduce among themselves but not with other groups
Fungi
69,000
Protozoa
30,800
Algae
26,900
Mosses
12,000
Ferns
12,000
Bacteria
4,800
Viruses
1,000
Total
1,447,900
Source: Modified After Wilson (1992).
