Zoology comes from the Greek zoon, meaning animal and logos meaning study. Zoology is the science that studies animals as well as relations between them and their environment. Zoology studies also the functioning of the animals, which are their tissues, cell functions and properties.
We can define zoology as “a branch of biology that encompasses all aspects of animal biology, including relationships between animals and the environment.” With this definition so broad, we can study not only the morphology, systematics and ecology, but also the functioning of animals, the chemical constituents of tissues, training and development, properties and cellular functions. Experimental Zoology, for example, includes the subdivision on changes in experimental animals as patterns of genetic and experimental embryology morphology.
The various fields of research with animals have an inter-relationship with other disciplines with classical zoology. To the extent that specialization increases, branches of study become more and more restricted: celenterology – study of cnidarians; entomology – study of insects, which is divided into systematic and economic entomology, mammalogy – study of mammals, herpetology – the study of reptiles and amphibians, and so on.
Approximately 2.000 species are known comprising various groups of animals, but experts estimate that in certain groups, is known only a small fraction of species existing today, notably nematodes, insects and certain quelicerados, particularly mites. In some intensive surveys of fauna in tropical forest, it was estimated that the diversity of arthropods in one area may be a few hundred thousand or even a million species.
Some calculations indicate that the current diversity represents only about 1% of the biological diversity produced in history, with millions of extinct species over a few billion years of biological evolution. This brings the number of species in nearly two million today to a number perhaps above one hundred million.
The understanding of animal diversity requires an understanding of what biodiversity is. A term, which synonym is more frequent “variety of life” which includes a variety of structures and functions in the levels of genetic, species, population, community and ecosystem, a definition so broad as biology itself. There are several attempts to reveal the main features of biodiversity as a concept and then the three divisions arise most frequently mentioned in the news: genetic diversity, and taxonomic diversity of species and ecosystem diversity.
The study of biodiversity is thus equivalent to studying the history of life that began about 4.5 billion years. For a better understanding of animal life, we need to go back about 650 million years, during the emergence of multicellularity. Concerning the great plans for the organization of metazoan, they’ve been definitively established 430 million years ago. The Earth is initially captured by plants, then by invertebrates and finally by vertebrates. The conquest of the air is accomplished by insects 350 million years ago and birds about 130 million year ago. From the most primitive animals to millions of species present there are many changes, which we call evolution.
Thus, the inventories of biodiversity should look for patterns that repeat themselves, understand the processes that generated several species through adaptations and evolutionary history. In this context the reconstruction of phylogenies is essential for us to detect the origins of multicellularity and many other aspects that distinguish biodiversity in modern times.