Machine tags (MachineTag) are just standard tag with extra information to extend the information in the tag. They are called machine tags because there are often use by program to get extra information from the tagged works or urls.
Scientific classification (like for plants and animals) can be done in different ways like the complete hierarchical classification used in the Scientific Notation or like the Binomial nomenclature/classification. This can be easily expressed as a machine tag.
In case of binomial classification (we'll take the example of the Phacelia, a plant in the above picture), the predicate is binomial and the value is composed of the binomial name.
As a binomial classification is composed of the genus and the species, you can deduce a part of the scientific notation from the binomial classification like :
Here is a simple approximative search to the binomial name of species. The binomial names are based on the ITIS 2009 catalogue of life. The search is fuzzy because it's pretty easy to make mistake in binomial name and easier to search the proper terminology. The search output the full binomial machine tag (useful for a cut-and-paste in flickr).
Following the example above, you can create the following machine tag for "Phacelia Tanacetifolia" for the family classification :