I just read this great blog post about how complicated evolution is. I am including an excerpt and you can read the whole post by clicking the link.
" Evolution isn't linear. There are always many species in the mix, and they are co-evolving, competing, innovating, reproducing, dying, sometimes even going extinct. You have variations within species. There is simultaneous divergence and convergence. There are contrary winds. There are genetic drifts and back-currents. There is hybridization. There are invasive species and a changing competitive landscape. "