I'm a postdoc at a US university, and my principal investigator (PI) asked me to cover some of his lectures next semester. I do not want to do any lecturing as I want to focus solely on my research. I am involved in two funded projects, and I therefore have a lot to do research wise.
Can I say no to covering lectures or would that be considered a mark against me?
I don't think a flat "no" would go over very well. Covering classes is something that colleagues are expected to do for one another, within reason. Yes, everyone has their own projects that this takes time away from. But it's really the only way that faculty are able to travel during the school term, which most find essential to keeping an active research program. Saying "I don't want to do any lecturing" will come across as unreasonable and possibly selfish. Keep in mind that "focusing solely on research" is a luxury that basically nobody ever has.
However, you can certainly negotiate, and maybe reduce the amount, while still showing that you're willing to help. Keep in mind that the PI presumably has an interest in your research success, so if the teaching would impact this in specific negative ways, beyond a generic "this is an hour of my time that I could be spending on research", you can point this out and suggest a compromise.
"I'm happy to help out, but seven classes over the semester is kind of a lot. I am going to need to [achieve specific research goals] next semester and I think this might start to get in the way. Maybe you can find another person who can cover some of them, and we'll split them up?"
"I can do January 18, but on March 7 there is a conference that I was really hoping to attend. Perhaps someone else could cover that date?"
"February 10 would be fine. April 3 is the week before [important deadline] and it might be kind of a crunch, so maybe someone else could teach on that day."
You can also ask for materials or other help that will lessen the amount of work for you.
With such assistance, covering a lecture should not end up taking an excessive amount of time away from your research. You really should only need to prepare and deliver the lecture - you likely won't have to deal with creating assignments, grading exams, supervising TAs, holding office hours, answering emails from students, or the other things that end up taking the majority of time when teaching.
First, I agree with everything Nate Eldridge wrote in his excellent answer.
To add to that: you may think your job is only to do research, but the funding agency that provides the funds that pay your salary may not see things that way. Your PI pays you out of (I assume) a grant he got from the National Science Foundation or other similar government agency, as part of the grant’s “training” component, that is, the PI promised the agency to help train the future generation of researchers and educators (and the NSF cares about such things, including the teaching part, see here). One could make a very plausible argument that giving you some teaching experience is an essential part of that training. So, even aside from the fact Nate and others already explained that not being willing to do any lecturing will indeed come across as a black mark against you on an informal basis, you may also have a hard time arguing even at a purely formal level that lecturing is “not in your job description”.
Another thing to consider is the mindset of your PI, who as a regular faculty member likely teaches around 100 hours a year and also has many administrative duties, and yet manages to do productive research. From his point of view you have an incredible luxury to be able to focus essentially all your time on research. Depending on his personality and the level of stress he experiences from having such a workload, he may even be a bit resentful of you for having such unbridled freedom in comparison to him. From such a vantage point, a refusal on your part to take on just a few hours’ teaching load - a trivial amount, relatively speaking - is almost certain to appear selfish and immature.
Finally, to balance my answer a bit, I will go in the other direction and mention the possibility that a normally innocent request by a PI to his postdoc to teach a few lectures can in fact cross a line into potentially abusive behavior if the extent of the teaching grows above a certain point, especially if this is done without the knowledge and approval of the PI’s department. I have heard of cases of faculty who got their postdocs to do essentially all their teaching for them, and got in trouble for it. It would certainly be reasonable of you to be on your guard about a potential abuse of authority of this type, and to set a reasonable limit on the amount of teaching you are willing to do, and also ask some questions about whether this has been cleared with the department.
Your personal relationship with your advisor is probably a very important consideration. This might depend on your assessment of how he/she would react. But in general, it is probably a good idea to get the experience of lecturing in a way that involves only a short term commitment. While I think you can refuse, and most legal systems would back you up, it may not be wise to refuse.
But you might want to try to structure it so that you benefit maximally from the experience. For example, asking now for the opportunity to discuss things with your advisor after the first such lecture would be an advantage. Also, it puts an obligation on the PI.
Your future may depend on your relationship, but also on the breadth of experiences that you have as a student. You could think of it as an opportunity, rather than an obligation. The PI seems to trust you. That is a valuable thing in itself.
The short answer is: it depends.
The objective of a postdoc is to obtain sufficient research experience so that you are ready to be a 'grown-up' academic and university professor. This is achieved mostly by doing amazing research, but your PI's assessment of your work is a significant part of this. So I would say that even if you feel that this is taking up a significant chunk of your time, it's better to keep a good relationship with your PI, even it means a few hours lost to teaching.
That said, almost any academic position would require some teaching, and perhaps this is the PI's way of assessing your teaching capabilities so that they can make an educated statement in a future reference letter.
If it is just a few lecture hours, I personally would not make a big deal out of it. If they are asking you to grade exams/consult students or something more time consuming, perhaps it would be worthwhile to diplomatically discuss it ('are you sure that this is the best use of my time given that I am involved in two projects?').
If you feel like this is part of a growing pattern of assigning you a bunch of tasks not related to your research, it may be good to raise this issue.
You can.
But how do you see your career progressing? Perhaps a combination of research and a small amount of teaching could be good for you and, for the future students.
Your choice, so good luck anyway.
I had the Professors coming to me and saying, will you do X lectures, we have the pay rates already agreed and signed... And the material, examples, handouts all prepared, printed and ready to go...
Yes you can. How you do it is the key point you should worry about.
I was in the exact situation once before. As a 1st timer postdoc my supervisor came to me informally to ask whether I could lecture in his place in case he'd go for "trips". Immediately my internal alarms went off. Why did he drag me into a separate room, and why was he smiling so much? Clearly he was pushing beyond limits.
Still, I was quite friendly and open. I asked how did that contribute for my record with the department and project? I wanted certificates per each class given, and beforehand some rough estimation of how many classes to expect and more or less when. I emphasised on how time-consuming the project and writing of papers were sure to be. And I asked whether officially my postdoctoral funding rewarded or at least demanded giving out classes (e.g. how many hours).
He immediately realised the resistance. He said "Well, in case you don't want to help with classes, it's OK", to which I said, "of course I am available, but I must understand the official details before making plans."
In the end, I lectured about 6-8h in total, out of 3 years as a postdoctoral fellow in that lab. Surely it could have been more. My impression is that he was wary of "leaving for trips" and relying on me to cover him up. I bet he wasn't really communicating absences, where talking about official procedures gets a bit unsettling.
Thus, my advice is: don't let others abuse you, but make concessions where you see some clean opportunity for experience or "points" with the department. You do not depend on your supervisor for jobs later -- this is usually a myth and they won't move a finger in any direction.
Most post-doctoral positions involve research work under the supervision of a more senior researcher. They do not include teaching or institutional-administrative duties. This is related to their fixed- and short-term nature - and to their significantly lower pay.
With due respect to your PI - making precarious researchers serve as a backup teaching workforce is a further degradation of their (or should I say our) position in academia. So while - as @NateEldridge suggests - accepting a backup request is the collegiate thing to do, it is also collegiate for your PI to make sure you are then recognized as having been employed as a teacher, even if for a short period of time, and paid for it (with consideration of the extra relative overhead per class hour when you are teaching just once or twice).
With that said,
