Article: Readiness of America's biology teachers questioned

Data spanning 1987 to 2007 show changing demographics among public high school biology teachers. The workforce has become less experienced and has been destabilized by turnover, and biology teachers are more likely than other science teachers to work outside of their discipline.

I do not have enough personal knowledge about this. However, I do know that of the 6 people who teach biology at my high school, none teach all biology all the time. Four of the 6 teach biology related courses, while 2 teach other sciences. 

Bold-faced is mine. 

The authors support alternatives to the typical calls for more stringent certification and targeted professional development. In their view, it would be better to match curricula to existing expertise. They propose a model in which "instead of offering a static, predetermined slate of science courses at each school, district, or county, the curriculum is chosen largely as a function of the expertise of those teachers who they employ." 

Currently, curriculum is "top down" and controlled at the state and federal levels.

Actually having biology teachers teach their areas of expertise is a throw back to a previous, and long gone era.