You're kidding right? There's pretty clear biological evidence of multiple human races. Don't want to call them races? Fine, come up with another word. The fact remains that there are biological differences between different groups of people though.
As Flilix said, there are biological differences between individual humans too, even siblings. So, do siblings count as different races?
It's possible. A few generations back in my family there was a caucasian brother and a black sister. Technically they were only half siblings, but they were still siblings nonetheless.
And of course there are biological differences between individual people, we're not all clones! But there are still clear differences in the genes related to skin pigmentation and skeletal structure between different groups of people. There's other differences too, but those ones are the most recognisable.