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Owen codified 3 main criteria for determining if features were homologous: position, development, and composition. In 1859, Charles Darwin explained homologous structures as meaning that the organisms concerned shared a body plan from a common ancestor, and that taxa were branches of a single tree of life. [2] [7] [3]
Homologous structures - structures (body parts/anatomy) which are similar in different species because the species have common descent and have evolved, usually divergently, from a shared ancestor. They may or may not perform the same function. An example is the forelimb structure shared by cats and whales.
Diagrams that show the development of male and female organs from a common precursor. Sperm ducts and fallopian tubes are not homologous, as the sperm ducts originate from the Wolffian ducts, whereas the fallopian tubes originate from the Müllerian ducts. Homologous structures in the external genitalia
This can be done in terms of the chemical elements present, or by molecular structure e.g., water, protein, fats (or lipids), hydroxyapatite (in bones), carbohydrates (such as glycogen and glucose) and DNA. In terms of tissue type, the body may be analyzed into water, fat, connective tissue, muscle, bone, etc.
Such structures are described as homologous and suggest a common origin. In cases where the similar structures serve different functions in adults, it may be necessary to trace their origin and embryonic development. A similar developmental origin suggests they are the same structure, and thus likely derived from a common ancestor.
Homologous structures indicate common ancestry with those organisms that have a functional version of the structure. [15] Douglas Futuyma has stated that vestigial structures make no sense without evolution, just as spelling and usage of many modern English words can only be explained by their Latin or Old Norse antecedents. [16]
In arthropods, an appendage refers to any of the homologous body parts that may extend from a body segment, including antennae, mouthparts (including mandibles, maxillae and maxillipeds), gills, locomotor legs (pereiopods for walking, and pleopods for swimming), sexual organs , and parts of the tail . Typically, each body segment carries one ...
Homologous recombination, genetic recombination in which nucleotide sequences are exchanged between molecules of DNA; Homologous desensitization, a receptor decreases its response to a signalling molecule when that agonist is in high concentration; Homology modeling, a method of protein structure prediction