Biology Reference
In-Depth Information
that on average each new particle carries one mutation. A small minority of these
mutations cause the virus to become resistant to the antiviral drugs being used. Thus
the resistant variants increase with time in the presence of the drugs. The result is
that the drugs rapidly become less effective with time, making this disease difficult
to treat.
Evidence for evolution is also seen from the domestication of plants and animals
by humans in the last ten thousand years. In the first chapter of his famous topic,
Charles Darwin talked about the evolution of domestic varieties of animals such as
dogs and pigeons from wild ancestors. Darwin was especially interested in the way
that selective breeding by pigeon fanciers has produced all the varieties of pigeon
that they show at exhibitions. All the breeds favoured by pigeon fanciers have been
bred from the wild rock pigeon - Fig. 4.16 shows a few examples. Dogs have been
evolving from wolves for at least ten thousand years, and possibly for ten times
longer. Selection of favourable traits by humans has produced the 300 or so vari-
eties of dog that we see today. Figure 4.16 shows just two modern varieties of dog,
compared with their remote ancestor.
The domestic plants that we all rely upon for food have also been produced by
breeders selecting those variants that possess desired characteristics - desired that
is by humans, not by adapting to the natural environment. Figure 4.16 shows the
different crop plants that have been bred from the wild cabbage by plant breeders.
All these are examples of microevolution. This process produces new varieties, but
these varieties are still the same species. So for instance modern dogs can, and do,
interbreed with wolves.
Fig. 4.15