In the interest of contributing where I can, I am attempting to get word out before someone without specialized knowledge is surprised by the newest argument for irreducible complexity.  The link to the article in question is here: (http://www.evolutionnews.org/2010/07/messenger_rna_regulated_in_mor036201.html#more)- there is a link to a paper in Molecular Cell, a highly respected journal in the field of biological sciences.  I am currently a Ph.D. candidate in the molecular genetics program at The Ohio State University, working in a lab that looks at microRNA during vertebrate development.  The author makes an argument from ignorance, simply asking “how exactly can these things be explained by traditional Darwinian selective pressure?”   (emphasis in the original document).  I didn’t actually see anything about irreducible complexity anywhere except in the title of the post.  The article was posted on July 2, 2010, making a good case for the author being either too lazy or unwilling to investigate this question on their own. 

                In fact, this is a question that has intrigued scientists since the unexpected discovery of this gene regulatory mechanism; as you would expect, there is a correspondingly large amount of work on this question.  Here is a (very) short summary of the evidence of the evolution of microRNAs :

                -The microRNA pathway is composed of proteins and RNA, both of which are produced from sequences of DNA, which has previously been shown to be under ‘traditional Darwinian selective pressure’.

                -All microRNAs must form the same structure in order to function (referred to as a ‘hairpin-loop’- think, literally, of a hairpin) which is dependent on the sequence of the microRNA; if this sequence is maintained, we would expect a functional microRNA; if this sequence is not maintained, we would expect a non-functional microRNA.  This is, indeed, just what we see across lineages and species, from plants to animals.

                -piwi-interacting RNAs (piRNA) are a class of microRNAs that function in germ cells (gonadal tissue) in animals, from flies to humans.  However, the proteins responsible for the function of this pathway (referred to as piwi proteins) are found in ciliates (single-celled organisms) and slime molds (organisms that alternate between a single-celled state and a multicellular state).  Neither of these organisms has germ cells; however, it is apparent that the proteins function is a similar pathway to that of the piRNAs.

                To address the (conspicuous in its absence) argument for irreducible complexity:

                -microRNAs have been shown to function in a combinatorial fashion: you need more than a single microRNA in order to achieve the desired effect.  Further, microRNAs are often produced as ‘families’ that function in the same or a similar fashion.  This means that of you remove one of the microRNAs from the organism, the pathway will still function, though not necessarily at the same level.

                -The microRNA pathway is known to function as a ‘fine-tuning’ mechanism as opposed to a ‘molecular switch’ (on versus off).  All microRNAs reduce the level of a given gene product in a cell, to an extent.  Turning a gene off completely effectively removes that gene product from the cell while attenuating the gene with microRNAs ‘turns down’ the gene product.  You can delete microRNAs in an organism, and the expected cells will grow, the correct tissues will still develop, and the organism will be alive (although it may have cancer, or not be able to reproduce).