These views imply that natural selection is not, in fact, free to act randomly. Some structures are either protected from change, or conservative or inert. In other words there are constraints as to what may vary. Lets look at some of the possible constraints that may exist. Structural constraints .
Structural constraints are more likely to affect unicellular than multicellular organisms.
The maximum rate of mutation and the numbers of recombinations formed will both constitute genetic constraints on evolutionary change. We shall see later that some genes are phylogenetically highly conserved, since we find them in many species and higher taxonomic units. These are concerned with fundamental aspects of development and so are somehow prevented from mutating or have a very high priority service contract. We can imagine a priority on any gene product, varying from absolute invariability for the most vital, through partially restrained to free for all. The suite of genes governing Aunt Maud's nose shape are less conserved.