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Unit Chapters
Genomics
Proteins & Proteomics
Evolution & Phylogenetics
Introduction
A Brief History of Classification
Cladistics and Classification
Applications of Molecular Phylogenetics
HIV and Forensic Uses of Phylogenetics
The Origin of Bats and Flight
Challenges
Coda: The Renaissance of Comparative Biology
Microbial Diversity
Emerging Infectious Diseases
HIV & AIDS
Genetics of Development
Cell Biology & Cancer
Human Evolution
Neurobiology
Biology of Sex & Gender
Biodiversity
Genetically Modified Organisms
"Systems of classification are not hat racks, objectively presented to us by nature. They are dynamic theories developed by us to express particular views about the history of organisms. Evolution has provided a set of unique species ordered by differing degrees of genealogical relationship. Taxonomy, the search for this natural order, is the fundamental science of history."
- Stephen J Gould1


Introduction

Perhaps the most striking feature of life is its enormous diversity. There are more than one million described species of animals and plants, with many millions still left undescribed. (See the Biodiversity unit.) Aside from its sheer numerical diversity, organisms differ widely and along numerous dimensions - including morphological appearance, feeding habits, mating behaviors, and physiologies. In recent decades, scientists have also added molecular genetic differences to this list. Some groups of organisms are clearly more similar to some groups than to others. For instance, mallard ducks are more similar to black ducks than either is to herons. At the same time, some groups are very similar along one dimension, yet strikingly different in other respects. Based solely on flying ability, one would group bats and birds together; however, in most other respects, bats and birds are very dissimilar. How do biologists organize and classify biodiversity?

In recent decades, methodological and technological advances have radically altered how biologists classify organisms and how they view the diversity of life. In addition, biologists are better able now to use classification schemes for diverse purposes, from examining how traits evolve to solving crimes. These advances have strengthened evolutionary biology as a theory: a theory in the scientific sense, meaning a "mature coherent body of interconnected statements, based on reasoning and evidence, that explains a variety of observations."2 Molecular biology, genetics, development, behavior, epidemiology, ecology, conservation biology, and forensics are just a few of the many fields conceptually united by evolutionary theory.
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