Re: VRD - evolution in finches

Peter and Rosemary Grant's  research  on finches in the 
Galapagos Islands clearly demonstrate that natural selection is acting on 
these birds. 

Their findings are actually more subtle than your question suggests: for 
the past twenty years, the Grants have measured practically every 
individual of the medium ground finch (Geospiza fortis) on the island of 
Daphne Major and recorded their births, breeding success, and deaths. 
Individual fortis finches vary in size and appearance, just as people do. 
Most importantly, their beak size varies from one individual to another.

Birds need their beaks to eat, and their beak size dictates the size of the 
seeds they're capable of cracking open. But different sizes of seeds are 
more available from one year to the next. 

During extremely dry years the only available seeds are big and tough, so 
only the fortis with the biggest beaks can get anything to eat. Finches 
with beaks too small to crack the big seeds starve to death. When their 
breeding season starts, female fortis compound the selection of big beaks 
by mating with the largest birds, so bigger beaks are passed preferentially 
to the next generation of fortis.  In extremely wet years, selection on 
beak size is reversed -- many, many, small seeds are available for the 
birds to eat, and individuals with smaller beaks are selected 
preferentially. The preferential survival and reproduction of individuals 
with a particular set of physical features is natural selection. 

So, the Grant's research project has demonstrated that natural selection 
can effect a population of animals or plants in a very short period of 
time. But natural selection is not evolution, it is a mechanism that can 
lead to evolution. What has not been observed is the process of speciation 
-- the splitting of one species into two species. While natural selection 
can take place very quickly, no one knows how fast speciation can occur. 
But it's likely that it can happen very quickly -- for example, the 
approximately 405 unique cichlid fish species found in  Lake Malawi  must have evolved since the lake's 
formation 2 million years ago. 


References:
Grant, P.R. 1999. Ecology and Evolution of Darwin's Finches. Princeton 
University Press, Princeton, NJ
Weiner, J. 1994. The Beak of the Finch. Vintage House: New York.