How can race be seen as both a biological and social construct?
Race is biological/genetic in those races that are the result of dozens of generations worth of purpose-breeding. You know a german shepherd when you see one. Humans haven't been purposebred much. Skincolour says NOTHING about your other thousands of genetic traits
Race is biological. There are obvious physical characteristics that are directly linked to racial types. A pathologist can easily determine the likely race and gender of a cadaver based on it's proportions and other physical features.
Since all humans are the same species, it is a social construct since people like to divide groups by ethnicity and race. It is biological because the difference in races reflects the adaptations isolated populations made to their specific environments.