In the traditional approach to working with evolution, as exemplified by plant and animal breeding and the fields of genetic algorithms and genetic programming, humans guide the evolution of a population of replicators by deciding which members of the population are allowed to reproduce. The breeder provides a ``fitness function'' or a ``selection criteria''.
Breeding manages evolution within the species, producing variations in the forms of existing species. However, evolution is also capable of generating new species. Even more significantly, evolution is capable of causing an explosive increase in the complexity of replicators, through many orders of magnitude of complexity. The Cambrian explosion may have generated a complexity increase of eight orders of magnitude in a span of three million years. Harnessing these enormously more creative properties of evolution requires an approach completely different from traditional breeding.
We know how to apply artificial selection to convert poor quality wild corn into high-yield corn. However we do not know how to breed algae into corn. There are two bases to this inability: 1) if all we know is algae, we could not envision corn; 2) even if we know all about corn, we do not know how to guide the evolution of algae along the route to corn. Our experience with managing evolution consists of guiding the evolution of species through variations on existing themes. It does not consist of managing the generation of the themes themselves.
As a thought experiment, imagine being present in the moments before the Cambrian explosion on Earth, and that your only experience with life was familiarity with bacteria, algae, protozoa and viruses. If you had no prior knowledge, you could not envision the mahogany trees and giraffes that were to come. We can't even imagine what the possibilities are, much less know how to reach those possibilities if we could conceive of them.
Imagine that a team of Earth biologists had arrived at a planet at the moment of the initiation of its Cambrian explosion of diversity. Suppose that these biologists came with a list of useful organisms (rice, corn, pigs, etc.), and a complete description of each. Could those biologists intervene in the evolutionary process to hasten the production of any of those organisms from their single celled ancestors? Not only is that unlikely, but any attempts to intervene in the process are likely to inhibit the diversification and complexity increase itself.
If the silk moth never existed, but we somehow came up with the idea of silk, it would be futile to attempt to guide the evolution of any existing creature to produce silk. It is much more productive to survey the bounty of organisms already generated by evolution with an eye to spotting uses for existing organisms.