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Module 1: Evolución

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Selección natural en acción

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XSIQ
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Biology - Natural selection in action

Natural selection in action

The current 13 species of Galapagos finches all descended from a common
ancestor

There are examples of natural selection in action. One of the classic
examples is the change in phenotype of the peppered moth _Biston betularia
_(see any text for details). During the industrial revolution in Britain,
the buildings and woods became coated in black soot. The light coloured
moths became easy prey but any darker [1] mutants were better camouflaged
and survived to pass on their genes for dark colour (melanin production) to
their offspring. The gene for dark colour increased in frequency over
successive generations. When controls were exercised over emission of
pollutants in the 1950s, the dark coloured moths were less well camouflaged
in the cleaner surroundings and they were preyed on (selected against). The
frequency of the lighter coloured moths increased.

Other recent examples supporting the concept of natural selection include:

* antibiotic resistance in bacteria;

* pesticide resistance in insects;

* heavy metal tolerance in plants.

Isolated islands, such as the Galapagos off the coast of S. America and
the Hawaiian Islands in the Pacific, have been referred to as 'evolutionary
laboratories' (Kinnear and Martin, 1993, _Nature of Biology,_ Jacaranda
Press p377). Initially the Hawaiian Islands had no inhabitants as they were
volcanic in origin. Colonisation occurred and a range of flora and fauna
became established in the process of succession. At some stage a few
honeyeaters arrived. Without predation and competition their numbers
increased and they colonised different islands. As populations increased,
competition and therefore selective pressures operated. Differences in bill
sizes and shapes became evident, specialised or adapted to different diets.
All the present honeyeater species are descended from the ancestral
colonisers of about 5 million years ago.

A similar process occurred to give rise to the different species of
finches on the Galapagos Islands.

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