Genes important in the embryonic development of animals, such as homeobox-containing genes, have been relatively well conserved during evolution; that is, they are more similar among different species than are many other genes. explain why?

Homeotic genes are master regulator genes that direct the development of particular body segments or structures.
When homeotic genes are overactivated or inactivated by mutations, body structures may develop in the wrong place—sometimes dramatically so!
Most animal homeotic genes encode transcription factor proteins that contain a region called the homeodomain and are called Hox genes.
Hox genes are turned on by a cascade of regulatory genes; the proteins encoded by early genes regulate the expression of later genes.
Hox genes are found in many animals, including fruit flies, mice, and humans. Mutations in human Hox genes can cause genetic disorders.
Introduction

How many legs does a fruit fly have? Even if you're not particularly into fruit flies, you may know that insects tend to have six legs total—as compared to, say, the eight legs of spiders. Also, you may have noticed that a fly's legs usually grow out of the middle part of its body—its thorax—and not, say, out of its head.

Image credit: modified from Drosophila melanogaster, by Madboy74 (CC0/public domain)
What's responsible for this orderly organization of body parts in something as tiny as a fly? As it turns out, a set of master regulator genes are expressed in different regions of a fly's body during development. These genes turn on the right genetic "program" for development of each section of the body. They make sure, for example, that the fly's thorax carries legs while its head does not.
In this article, we'll take a closer look at these and other homeotic genes, also called selector genes. By definition, these are genes that "select" the identity of entire segments or structures in the bodies of developing organisms.