Re: Does evolution create ever bigger animals, and if not, why?

The influence of evolution on animal size appears to depend on the type of 
animal, diet, availability of food, habitat, etc.  For example, forest 
antelope are much smaller than savannah species, predators of rabbits are 
much smaller than predators of zebras, etc..
  
One interesting trend may be related to the heat-conserving trend of bigness 
(which is related to the critter's surface-area/volume ratio); a tiny shrew 
radiates so much heat it must eat its own weight in high quality food each 
day to prevent starvation, while a giant elephant only needs to eat a small 
fraction of its own weight in poor-quality leaves each day.  

However, certain environments force a scaling down of size, as shown by the 
pygmy hippopotomus of Crete, the pygmy elephant of Sicily, and the pygmy 
mammoth that was found on some Alaskan Islands (all now extinct).  Something 
about islands, with their restricted habitats, have a profound shrinking 
effect on these biggest of animals.  When food is limited, the individuals 
with the smaller appetites tend to survive.  This was seen on the Galapogos 
Islands; when small seeds are scarce, the biggest finches did best during a 
drought that killed 80% of the population that were unable to crack the big, 
tough seeds which were all that remained in the nearly empty "pantry", while 
in a rainy El Nino year, when grasses with small soft seeds overgrew and 
replaced the large-seeded cacti, it was the smaller finches that survived 
best, since a large bird with a big beak wasnt able to collect enough 
calories from the tiny seeds.

Notice the important role that severe drought, famine and flooding have on 
animal populations.  Many think that evolution shifts into high gear during 
these periods of extreme climatic conditions.

Pretty complicated, eh?  I also think it is interesting to consider the 
implications of these evolutionary patterns to the human economy... how will 
the big houses, big budgets, big gas guzzling cars and big military of the 
US affect our ability to adapt to future changes?

Thanks for the great question!

Cheers,
Dean Jacobson