natural vs. sexual selection

[gator@mail.utexas.edu (c. brochu)]

>Sorry, but I can't buy this argument.  Evolution does not
>necessarily search out the best solutions, any solution (horn shape)
>that works well enough for it's bearer to breed gets carried on.
>
>If you believe that there is a single horn shape that would best facilitate
>animal combat, check out the changes in swords through the ages.
>
>Steve New


I think you're making an invalid comparison.  Sword shape was under
conscious control; ceratopsian horn shape was not.

I agree that evolution doesn't necessarily form the absolute best
morphology, but different selective conditions will yield different
evolutionary rates.

Studies of living populations show that structures used primarily for
defense or prey capture are under strong selective pressure to maintain
their functionality - any drastic change in that shape is more likely to be
harmful than helpful and would be selected against.  Carnivore canines and
claws have evolved, but not very rapidly.  The group I study - crocodylians
- have evolved very very slowly, largely (I think) because their skeleton
is designed for function more than  show.

On the other hand, if a structure is primarily used for mate recognition or
intraspecific intimidation, changes in shape or size that accumulate during
speciation might be passed on; this increases fitness by preventing
hybridization, and if female choice evolves in tandem with male
characteristic, evolution might proceed rapidly.

Horn and antler shape have evolved rapidly in grazing mammals largely
because they are under strong sexual selective pressure.  They primarily
use these things to signal other members of their species (intimidation:
"mine are bigger than yours!"  or mate attraction:  "Mine are bigger than
his!  Mate with me, not him!") or to tell other kinds of mammal, "hey - I'm
a different species. Don't mate with me."  Of course they will use them
defensively if necessary - I mean, if I had a big pointy thing sticking out
of my forehead, I would use it to defend myself too - but their size and
shape are first and foremost for display.

This may sound counterintuitive, but this is based on experimental and
observational evidence.  Had antelope - and, I think, ceratopsian - horns
evolved primarily for defense, there should be much less interspecific
variation.

regards, chris



__________________
Christopher Brochu
Department of Geological Sciences
University of Texas at Austin
Austin, TX 78712
512-471-6088

gator@mail.utexas.edu