How did natural selection impact the size and foot structure of horses?

[Sace Ver]

Explain how natural selection might have caused the changes in each of the following:
-Size of horses
-Foot structure of horses

I'm quite sure as to why the size of horses needed, and I'm not quite sure why the structure of the foot needed to change.

 

[.Scott]

Sounds like a homework problem.
What new problems would the horse have if its foot was much too small or much too large?

 

[epenguin]

Environmental change can have been a factor you could bear in mind in the study you are going to do.
For this is not a question you can answer by thinking about it.
But I am ready to tell you everything I know.
The first 'horses' were only the size of a dog.
Their evolution was studied by Darwin.
As a genus they are not by now successful, their only flourishing wild species is the plains zebra.
Otherwise, like their cousins the rhinoceroses they are on the brink of extinction as wild species.
So, for a change, they owe their survival and diversification to man.
Er.
That's it.
Except I could suggest an idea for an interested student might be to look into whether the following 'natural experiment' has told us anything - the wild mustangs of N. America are feral descendants of domestic ones and have only been there for a few centuries. Is it known if and how they have changed? But results are not guaranteed.