Analogous traits are traits that are shared by two or more taxa but are NOT inherited from the common ancestor of these two taxa. Thus, their dorsal fin is not a homologous trait, but rather is called an analogous trait (Fig. In other words, these two animals have a common trait (an enlarged dorsal fin) that was NOT inherited from their common ancestor. However, the common ancestor of a shark and a killer whale did NOT have a similarly enlarged dorsal fin. This fin helps to stabilize them while they are moving fast in water hunting prey. For example, the external structure and function of the enlarged dorsal fin of a killer whale (mammal) is similar to that of the enlarged dorsal fin of a shark (cartilaginous fish). Sometimes taxa from very different branches of the tree of life can independently evolve similar traits due to similar environmental pressures. Secondly, not all similarities among taxa qualify as homologies! In fact, taxa can share similar traits that they DO NOT inherit from a shared common ancestor. First of all, finding similar traits in the first place can be quite difficult. Unfortunately, identifying homologous characters is not an easy task. Ultimately, taxonomy should reflect the true evolutionary history of an organism: its phylogeny (and vice versa). Although the Linnaean system of classification was developed before Darwin published his theory of evolution by natural selection, modern systematists strive to incorporate evolutionary history into taxonomic classification schemes and vice versa. A named taxonomic group at any level of the hierarchical system is called a taxon (plural = taxa). The Linnaean taxonomic system is hierarchical in that it groups species and genera into more inclusive categories: family, order, class, phylum, kingdom, domain (Fig. Both parts of the binomial are either underlined or italicized (e.g. The genus name is always capitalized, while the specific epithet is not capitalized. The first word in each scientific name identifies the genus (plural = genera) to which the organism belongs, while the second word is the specific epithet. Taxonomists use the Linnaean system of classification, which gives each species a scientific name composed of two parts (binomial nomenclature). Because of this genetic isolation, individual species typically don’t pass on heritable traits to each other, and are, therefore, thought to be independent evolutionary units. Humans, for example, are a separate species from chimpanzees, because although humans can reproduce with each other (and thereby exchange genetic information), they cannot reproduce with chimps. A species is typically defined as a group of organisms in which genetic exchange through reproduction is possible. Taxonomy is the branch of biology that describes, names, and classifies species and groups of species. These different types of data don’t always tell the same phylogenetic story, so any phylogenetic tree is a hypothesis based on the type of data that was used and so is always subject to further scientific analysis and revision in light of new data. Many types of data can be used to generate phylogenetic trees, including morphological characteristics such as shape of body parts and molecular data such as nucleotide sequences of DNA. A phylogenetic tree is a diagram of the hypothesized evolutionary history of a group of organisms (Figure 1). Hypothesized lineages of organisms can be represented as branching trees, called phylogenetic trees. Systematic analyses help create classification systems that convey information about the evolutionary relationships of organisms. Biologists who work to reconstruct phylogeny are called systematists. The proposed evolutionary relationships that arise from analyzing heritable traits among organisms are referred to as phylogeny. Heritable features are those that can be passed down from one generation to the next. In an attempt to reconstruct the evolutionary history of life, biologists analyze patterns of change in the heritable features of organisms. The diversity of today’s biological world is the result of nearly 4 billion years of evolution.
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