Are there any tools for organising literature with tags? So when writing a manuscript I usually go about as follows:
When reading papers on my topic I copy/paste or summarise those parts, which may be relevant for me and collect all these bits in a Word Document. It'll look like this:

Species A grew taller than Species B (Smith et al. 2013)

Growth rate depends on genetics (Miller et al. 2012)

For Species A growth rate did not differ between experiments (Jones et al. 2013)


So basically a list of statements that I then regroup (manually) by topic. However, that last part is quite tedious (and I have to decide on how to group the bits, e.g. by "Species A" or by "growth rate"). 

So I am looking for a tool (online or downloadable) with which it's possible to collect those text bits, assign tags to each and then select which tag to group by and get the selection of statements. 

Are you aware of such a tool?
I use Qiqqa to manage my PDFs when writing research papers. It allows you to highlight text or add notes to the PDFs. You can then tag the notes, and run "reports" which essentially will pull all of the tagged sections from all of the PDFs in your library into a single paper (similar to the Word document you mentioned) so you don't have to do that manually. It will show the original snippet from the paper, any notes you've made regarding that snippet, and the citation for the paper. There's also a neat brainstorm function that will let you visualize papers, tags, or notes and move them around/link them together; I use that to organize papers sometimes. 

There is a free version which will let you try out some features, and an affordable pay version that has more enhanced options. There are some helpful how to videos on Youtube that provide a nice sense of functionality and may help you decide if it's the tool you're looking for. I've been using it since I started grad school and it's been tremendously helpful in organizing my academic writing.

http://www.qiqqa.com/67642