When dealing with specific questions relating to areas of currently active research, questions regarding concepts which are easy to assign to a research area with frequent publications, it is of course very difficult to defend refusing to provide references. When someone asks, "do mutations of the gene ABC1 lead to cancer?", the answer is, in the procedural sense, trivial: You look for research examining ABC1 vs. cancer, either the research says yes, no, or maybe, or there is no research on ABC1 and the answer is we don't know.
However that still leaves classes of questions pertaining to biology, which are interesting and not common knowledge, but would not be answered directly by literature:
- Answering a question may require taking several different given research findings, putting them together and making deductions from there. In this case you will have references for the premises, but your conclusion (and the correctness of your reasoning), which is the answer to your question, will not necessarily be confirmed by any publication (it may be that the question happens to be something few scientists would bother to publish on, because answering it takes so much effort compared to the career benefits).
- The question may be too broadly scoped and interdisciplinary for a publication to deal with it. It may ask for synthesis of disciplines or fields which are just not combined very often by actual scientists.
While it is nice to have sources and it is a bad habit not to give sources for your claims, I think it wouldn't be too shocking to ask you to accept that the scientific community is not exactly objective, fair and impartial with regard to what science it prefers to do. I mean this in the sense that, out of all the questions and unexplained phenomena to which the scientific method is applicable (and for studying which experimental methods exist or can be feasibly devised), individual scientists as well as the community as a whole, will be more likely to research some and not others. One reason is that any scientific research project is done by human beings with careers to worry about (and it seems increasingly so as time passes and costs of research as well as the dependence on cooperation from the rest of the community and governments/funding agencies are growing), and they will at some point ask the question, "is doing this research worth the effort, time and money? Is that expenditure of effort, time and money something that I can afford, considering how many papers and cites I'll get out of it?"
Human curiosity, on the other hand, is at least relatively neutral - it doesn't really matter how complicated, useful, or career-friendly a question is for us to become curious about its answer.
This site (I assume) is meant to address the latter, rather than the former, and as such there will be times when a question is asked and finding sources is challenging. Therefore, naively expecting every answer to have sources would be quixotic. I think it would be more realistic to treat it as a guideline rather than a rule, with the caveat that "answers without sources are allowed, but you better have good reasons for not giving sources (and explaining those reason in the answer doesn't hurt)". The latter part, requiring an explanation for why sources are eschewed when they are, might resolve the whole problem entirely, in fact.