Biology Reference
In-Depth Information
Chapter 3
Darwin's Theory of Evolution
Evolution is a vast subject, so what I shall do in the space available is to provide
in Chapter 3 a summary of the evolutionary theory proposed by Charles Darwin,
and in Chapter 4 some of the principal evidence in support of this theory. For more
detailed information on both these topics, you should look at the topics on evolution
in the Further Reading at the end of each Chapter.
The aim of evolutionary theory is to explain the two most striking features of life
on Earth - its astonishing diversity, the fact that there are so many different sorts of
living organism, and the exquisite adaptation of each organism to its environment.
Let us now look at each of these features in turn.
Biodiversity
Organisms occur in many different environments, from polar seas to tropical
forests. Each distinctive environment is called an ecosystem. The biodiversity of
a given ecosystem is defined as the total number of different organisms it contains.
Figure 3.1 illustrates the diversity of organisms found on a tropical reef at the Great
Barrier Reef of Australia.
A major aim of biologists is to characterise and name every different type of
organism living in all the ecosystems on the planet today. One reason for this is the
belief that the stability of the Earth's ecosystems depends upon their diversity, but
that this diversity is being rapidly reduced by human activities that are changing nat-
ural habitats at an increasing rate. The loss of tropical rain forests to agriculture and
logging is causing particular concern. Many biologists think that the rate of species
loss is greater now that at any time in human history, and fear that the global ecosys-
tems on which we depend for our food production may collapse if the diversity is
reduced further. One estimate is that 25% of existing species will be lost by the end
of the twenty-first century. An additional argument for preserving biodiversity is an
aesthetic one - the world would be a less interesting place if there were no tigers,
whales or frogs. Several international organisations are dedicated to fighting this
reduction in biodiversity.
How many different types of organism are thought to exist? How do we distin-
guish between different types? These questions are more difficult to answer than
