What specific observational evidence are you talking about? Just a couple of examples, please.
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Some links to help you:
http://www.talkorigins.org/faqs/faq-speciation.html
http://www.talkorigins.org/faqs/speciation.html
http://www.origins.tv/darwin/transitionals.htm
http://www.gnxp.com/MT2/archives/002551.html