According to fossil records, the horses that lived 50 million years ago were much smaller, weaker and slower than modern horses. which process is most likely responsible for the changes that have led to the increased size, strength, and speed in horses? migration inbreeding evolution by natural selection commensalism

Evolution by natural selection

Explanation:

Most probably, the trait of being small, weak and slower did not convey advantage (or was even a disadvantage) in survival in their environment. The bigger, stronger and faster horses had an advantage and were able to survive long enough to pass down their genes to the next generation, Over succeeding generations more and more horses, therefore, became strong, bigger and faster. This is the process of natural selection at work.  

the father in the first generation probably had the recessive colorblind trait in his x chromosome. if he and his wife both had the colorblind trait, even if she had 1 recessive and 1 dominant, then that trait would be passed on to their kids.

(since you didn't attach the family tree this is just an assumption)