Answer:
<em>Hox </em>Gene
Explanation:
First, you're question is very vital, there are many ways in classifying along with identifying all living organisms that includes; morphological analysis, molecular systematics (studying the similarities and differences of the genetic data such in the sequences of DNA, RNA, and rRNA ), homology, cladistics, etc. based on phylogenetic tree, which the study of the evolutionary among various species.
But through it said that all living organisms shared one common ancestor. However, what makes them different from one to another is the homeotic genes that called <em>Hox </em>Genes; which specify the fate of a particular segment or region of the body, meaning the number and arrangements of the<em> Hox</em> genes varies considerably among different types of animals.
For instance, Sponges have at least one homologous to<em> Hox</em> genes, also insects have nine or more <em>Hox </em>genes resulting in multiple <em>Hox </em>genes occur in a cluster in which the genes are close to each other along a chromosome. Therefore, increases in the number of<em> Hox</em> genes have been instrumental in the evolution of many animals species with greater complexity in body structure.
Overall, more <em>Hox</em> genes, more complexity in body structure resulting in the differences of their morphological structure.
Hope that answered your question!
In my opinion
Mass extinctions happen because of climate change, asteroid impacts, massive volcanic eruptions or a combination of these causes. One famous mass extinction event is the one that lead to the extinction of dinosaurs, 65 million years ago.
I hope it’s help
An established population that is accustomed to change.
