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Chapter 19
Adaptive Radiation
An organism’s ecological niche is the role that it plays within a community including
factor’s such as habitat, food, competitors, enemies and the use it makes of resources
in the environment.
Darwin’s finches
When Darwin visited the Galapagos
Islands in 1835, he found many
different species of finches (birds)
which were different from each other
in terms of their beak size and beak
shape which enabled them to make
use of different sources of food. The
nearest mainland is 600 miles away
and has only one species of finch
which feeds on seeds on the ground.
On the mainland, the finch cannot
use other forms of food because
competition is too strong from other
species of birds. When the first
finches arrived in the Galapagos
(probably blown there by freak
storms), there were no competitors.
The birds increased in number and occupied the many available ecological niches.
Sub-populations became isolated by barriers (as described in Ch 18 notes) and
speciation occurred. Each group was subject to natural selection so that subpopulations diversified (became different) and adapted (as a result of natural selection
on variation present within the population) to an available ecological niche.
Adaptive radiation
Adaptive radiation is the evolution of a group of related organisms (sharing a common
ancestor) along several different lines by adapting over a long period of time to a wide
variety of environments.
See p 139-140 Torrance ‘New Higher Biology’ for details of adaptive radiation in
marsupials in Australia and in British buttercups.
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Chapter 19
Homologous structures and divergent evolution
A group of related organisms which have undergone adaptive radiation are found to
possess structures which have the same evolutionary origin (common ancestor) and
are structurally alike although they may perform different functions. These are said to
be homologous structures.
The pentadactyl (five digit) limb
in mammals has become
adapted to suit different
functions – the basic structure
remains the same but there is
divergence of structure to fulfil
different functions.
This
process
is
divergent evolution.
called
Convergent evolution
Sometimes animals from very different evolutionary origins show similar structural
features because they have become adapted over a long period of time to occupy
similar ecological niches in different parts of the world. Moles in Australia, Africa and
Europe all need powerful digging claws and other characteristics which suit them to life
under ground. This process is called convergent evolution and occurs when animals
whose origins are very different, have coincidentally evolved very similar features.
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