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Rapid Evolution

We normally tend to think of evolution as something that happens slowly.  In a reasonably stable ecosystem this would normally be true. When something changes there is pressure on organisms to adapt to the change.

The fossil record seems to indicate that many of the important changes happened when something changed in the environment.  These changes stimulated very rapid evolution and the emergence of new forms.  These eras of rapid evolution would then lead to fairly stable conditions when small improvements of the forms would occur.

Change in the Environment

One effect was observed a long time ago in the Midlands of England.  This was the area I was born in.  There are a lot of heavy industries, and the trunks of the trees were turning black from the soot in the air.

The Peppered Moth, Biston betularia, had been well camouflaged against the tree’s bark with lichens on it, but when the trunks turned black the light coloured moths stood out and the birds could find them more easily.

There was also a rare black coloured form of this moth.  The birds found it easily, so it was not likely to build up in numbers, but once the trees turned black, the black moths survived and their light coloured relatives got eaten.

The proportions of the colours of the moths reversed themselves quite quickly and the black moths became the common ones.

Introduced Animals

Although factors can change in the physical environment, the change can be a new animal.

This has happened in several cases in Australia.

Introduction of the Rabbit

This could have been a new food source for our native predators, but the Wedge-tailed Eagle was already being killed in huge numbers, and the Foxes had already been introduced and were killing off the native ground Rabbit predators.  Dingos had also been killed in large numbers.  Quolls had been killed off both by Humans and by Foxes, and Foxes were killing the young Goannas in large numbers.

The introduction of the Rabbit could have been a major driving force for evolution in Australia’s predatory animals, but many of these had already been killed off.

The Cane Toad

This poisonous animal has been a major driving force for some very rapid evolution.  Some of this has been measured.  For example two species of snake that eat Cane Toads are now evolving small heads.  This means that they can only eat small Cane Toads which have a much lower percentage of poison than the big ones.

Quolls are being killed off in large numbers as Cane Toads invade their areas.  But in some areas that have had Cane Toads for a long time there are populations of Quolls still surviving.

We do not know how they are doing this.  There are several possibilities.  One is that these have some resistance to Cane Toad poison.  There is no evidence for this.  Another possibility is that they have a genetic aversion to eating Cane Toads.  A third possibility is that that have a learned aversion to the Toads, and that the mothers pass this aversion onto their babies.  In this case, it would be cultural evolution rather than genetic.

However they are coping with the Cane Toads, the surviving populations can now be evolving other mechanisms to cope with the Toads.

Rapid Evolution in Cane Toads

Apart from the rapid evolution that the Cane Toads are causing, extremely rapid evolution is occurring in the Cane Toads themselves.

When Cane Toads first were released they were spreading at about 10 Kilometres a year.  Now they are spreading at more like 55 Kilometres a year. 

The Cane toads that are faster, and have a genetic tendency to move in a westerly direction are the ones that are ahead.  These Toads are bigger, and have longer legs.

Perhaps when they reach the Indian Ocean or some other barrier that they cannot cross, these changes will not be advantageous, and there will be a tendency for the Toads to become more like the ones first introduced.

Sources

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/Evolution.html

http://ukmoths.org.uk/show.php?bf=1931

http://www.canetoadsinoz.com/evolutioncausedbycanetoads.html

http://www.canetoadsinoz.com/toadevolution.html

   
   
2 Different Colour variations of Biston betularia

 
Photo by Martinowksy [GFDL (www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (www.creativecommons.org/licenses/by-sa/3.0/)], from Wikimedia Commons