How can I identify research opportunities I am qualified for, and position myself as a competitive candidate, in a different field from my degree? I currently have a BSc. in Environmental Science and would like to go back for a biology focused PhD in the next few years. I keep seeing the recommendation that having a good research fit will be important to potentially being admitted. 

My question is, how do I go about getting research experience in biology when my degree is in something unrelated?

Answers to the question on How to gain research experience after master program? suggest various possible research opportunities. I've looked at jobs and volunteer opportunities like those, but I'm not sure I'm qualified for them. Even when I find a job that says ‘no experience necessary’, they are still asking for a biology degree. Presumably anyone with one would have had some experience in a biology lab, whereas most of my lab experience is in geology labs.

I don't understand why they would take someone without a relevant degree or experience (even if it's in-class experience) over interns, under/grad students, various types of volunteers with relevant degrees. Without understanding that, I'm not sure how to approach people about these potential opportunities.

How can I identify research opportunities I am qualified for, and position myself as a competitive candidate for these positions, when I don't have a relevant degree or experience in the field?
Frankly, most undergraduate students (and new graduates) are under-qualified to do research in any field, including their own. For various altruistic and non-altruistic reasons, we're often willing to take them on anyways. 

When I hire an undergraduate or M.S. student to work in my research group, I look for qualities that tell me they'll be train-able, and also a good investment for me. Things like:


motivation (as demonstrated by showing that they actually know what we are currently working on, have looked at our publications before asking for a job, etc.)
interest in pursuing graduate studies related to my field of research 
soft skills (oral and written communication, organization, note-taking, etc.)
ability to learn new concepts and skills quickly and somewhat independently
basic quantitative and technical ability


I don't expect anybody I hire at this level to have relevant experience or experimental skills specific to my lab, whether they have a degree in the field or not. 

So, keep pursuing opportunities like these, be honest about your lack of a biology degree, but highlight your eagerness to learn and your "transferable" abilities.

You might also look for research opportunities in cross-disciplinary labs that combine your new interest (biology) with your previous degree (environmental science). For these opportunities, your degree might actually give you a competitive advantage over biology students.