Geoscience Reference
In-Depth Information
within an individual species or the population of a species,
whereas
macroevolution
is evolutionary change within a
larger taxon such as a genus or family. The major process
for evolution is natural selection, the differential repro-
duction among genotypes in a population as a result of
differing suitabilities to environmental conditions.
Adaptive radiation
is the evolutionary process in which
many species evolve and diversify from a common
ancestor to occupy a wide range of modes of life.
Allopatric
speciation
is the formation of a new species because of
geographical isolation. There are two common cases. First,
there is the case of a species whose continuous range is
subdivided into two isolated subpopulations by the
appearance of a new barrier such as mountain building
or continental drift. A second is where geographical
separation is brought about by dispersal across a pre-
existing ecological barrier such as the ocean or a mountain
range. Evolution requires reproductive isolation followed
by evolutionary divergence.
taxonomy, and in his book
Systema Naturae
developed the
well known biological classification, and the binomial
system of naming species. He used the number and
arrangement of plant stamens and pistils in his system of
ordering plants. Working without modern methods of
present-day plant geneticists like the electron microscope
and techniques of DNA analysis, it is surprising how
accurate Linnaeus was. Modern DNA work to unravel
evolutionary trees at the Royal Botanic Garden, Kew,
has supported the work of Linnaeus; all families remain
intact, and 87 per cent of Linnaeus's genera are the same.
Discrepancies between the Linnaean system and the
modern DNA groupings are unexpected; the lotus is not
a water lily but in the same family as the plane tree, and
the papaya is not a passion fruit but a cabbage!
The Shannon-Wiener information
theoretic index
Two major characteristics make up diversity - the variety
or number of species in the system (
species diversity
or
species richness
) and the evenness of the abundances
of species within the system (
equitability
of species
abundance). The data in
Table 22.2
show the relative
dominance of five tree species in three woodlands.
Woodland A has perfectly even equitability and the largest
number of species. Woodland B has fewer species but is
still relatively even. Woodland C (a pine plantation) has
few species and very uneven equitability.
One index of species diversity is the total number of
species, and so for the three woodlands the values of 5, 3
and 2 respectively would reflect the different diversities.
However, that would be a crude, unweighted measure,
taking no account of the relative proportions. A better
index of diversity would take into account both species
diversity and relative abundance. The most widely used
of several measures which do this is the
Shannon diversity
index
, which is calculated by:
Biological diversity has become shortened since the mid-
1980s into
biodiversity
, and has received wide currency
since the Rio de Janeiro summit in 1992. There are a few
differing definitions; one is that it is '
the totality of all genes,
species and ecosystems in one location
'. The UN Convention
on Biological Diversity of 1992 gives the definition:
the variability among living organisms from all
sources including,
inter alia
, terrestrial, marine and
other aquatic organisms and the ecological complexes
of which they are part; this includes diversity within
species, between species and of ecosystems.
Diversity of ecosystems is not an easy property to
define or measure. First it is necessary to define precisely
the limits of the community being described in time and
space, i.e. its temporal and spatial bounds. Thus one can
define the diversity of seabirds on an island in spring, the
diversity of plants in an oak woodland or the diversity
of insects in the whole of the arctic tundra biome. The
boundaries in space, time and community are normally
set by the logistics of the defined hypotheses and the
sampling programme of a particular biogeographical
investigation.
Although our knowledge of the number of species on
Earth is incomplete, it has been developing for many
centuries. The Swedish biogeographer Carolus Linnaeus
(1707-78) was the founder of
n
H
1
=
P
i
log P
i
i=1
woodlands (%)
Woodland
Oak
Ash
Birch
Alder
Pine
A
20
20
20
20
20
B
40
30
30
0
0
C
0
0
10
0
90
modern biological
