How can evolution be measured?
Evolution can often be traced in a single genus, species or related species where there is a complete fossil record.
The effects of natural selection can be measures and studied wishing a single genus or species where there is a very complete fossil record and absolute age dates are present (see Pic.). The horse family is a good example of this wherein there is an excellent record of fossil horses over time and the dates are also known.
Biologists can then study how things like their teeth, hooves, and overall size have changed over time. More recently, humans have become a natural (or unnatural!) selection agent for horses as breeders select different breeds for speed, ability to prance around or whatever!