There's a great detailed explanation of this and many other evolutionary processes such as the eyes, in the book "Your Inner Fish"

https://www.amazon.com/Your-Inner-Fish-Journey-3-5-Billion-Y...

I have previously read the site kindly submitted for discussion today and have recommended it (here on Hacker News, too, as I recall) before. This site is well worth a read. A comment asks for a recommendation of a book about macroevolution, transformation to new species among descendant organisms, and I can not only recommend two books, but also a free website. The website is 29+ Evidences for Macroevolution,[1] which I have recommended often to readers here on Hacker News. The books are Why Evolution Is True by Jerry Coyne[2] and The Greatest Show on Earth by Richard Dawkins.[3] Both books are very readable and interesting and well deserve your attention. There are other good recent books about evolution that help fill in the research findings that have occurred since you or I finished our formal schooling.[4]

[1] http://www.talkorigins.org/faqs/comdesc/

[2] http://www.amazon.com/Why-Evolution-True-Jerry-Coyne/dp/0143...

[3] http://www.amazon.com/The-Greatest-Show-Earth-Evolution/dp/1...

[4] http://www.amazon.com/The-Ancestors-Tale-Pilgrimage-Evolutio...

http://www.amazon.com/Your-Inner-Fish-Journey-3-5-Billion-Ye...

http://www.amazon.com/Endless-Forms-Most-Beautiful-Science/d...

http://www.amazon.com/Making-Fittest-Ultimate-Forensic-Evolu...