Homology (biology) in the context of Testis

Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last common ancestor of those groups. The cladistic term for the same phenomenon is homoplasy. The recurrent evolution of flight is a classic example, as flying insects, birds, pterosaurs, and bats have independently evolved the useful capacity of flight. Functionally similar features that have arisen through convergent evolution are analogous, whereas homologous structures or traits have a common origin but can have dissimilar functions. Bird, bat, and pterosaur wings are analogous structures, but their forelimbs are homologous, sharing an ancestral state despite serving different functions.

The opposite of convergent evolution is divergent evolution, where related species evolve different traits. Convergent evolution is similar to parallel evolution, which occurs when two independent species evolve in the same direction and thus independently acquire similar characteristics; for instance, gliding frogs have evolved in parallel from multiple types of tree frog.

A limb (from Old English lim, meaning "body part") is a jointed, muscled appendage of a tetrapod vertebrate animal used for weight-bearing, terrestrial locomotion and physical interaction with other objects. The distalmost portion of a limb is known as its extremity. The limbs' bony endoskeleton, known as the appendicular skeleton, is homologous among all tetrapods, who use their limbs for walking, running and jumping, swimming, climbing, grasping, touching and striking.

All tetrapods have four limbs that are organized into two bilaterally symmetrical pairs, with one pair at each end of the torso, which phylogenetically correspond to the four paired fins (pectoral and pelvic fins) of their fish (sarcopterygian) ancestors. The cranial pair (i.e. closer to the head) of limbs are known as the forelimbs or front legs, and the caudal pair (i.e. closer to the tail or coccyx) are the hindlimbs or back legs. In animals with a more erect bipedal posture (mainly hominid primates, particularly humans), the forelimbs and hindlimbs are often called upper and lower limbs, respectively. The fore-/upper limbs are connected to the thoracic cage via the pectoral/shoulder girdles, and the hind-/lower limbs are connected to the pelvis via the hip joints. Many animals, especially the arboreal species, have prehensile forelimbs adapted for grasping and climbing, while some (mostly primates) can also use hindlimbs for grasping. Some animals (birds and bats) have expanded forelimbs (and sometimes hindlimbs as well) with specialized feathers or membranes to achieve lift and fly. Aquatic and semiaquatic tetrapods usually have limb features (such as webbings) adapted to better provide propulsion in water, while marine mammals and sea turtles have convergently evolved flattened, paddle-like limbs known as flippers.

A hand is a prehensile, multi-fingered appendage located at the end of the forearm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs. A few other vertebrates such as the koala (which has two opposable thumbs on each "hand" and fingerprints extremely similar to human fingerprints) are often described as having "hands" instead of paws on their front limbs. The raccoon is usually described as having "hands" though opposable thumbs are lacking.

Some evolutionary anatomists use the term hand to refer to the appendage of digits on the forelimb more generally—for example, in the context of whether the three digits of the bird hand involved the same homologous loss of two digits as in the dinosaur hand.

A forelimb or front limb is one of the paired articulated appendages (limbs) attached on the cranial (anterior) end of a terrestrial tetrapod vertebrate's torso. With reference to quadrupeds, the term foreleg or front leg is often used instead. In bipedal animals with an upright posture (e.g. humans and some other primates), the term upper limb is often used.

A forelimb is not to be confused with a forearm, which is a distal portion of the human upper limb between the elbow and the wrist.

Evolutionary developmental biology, informally known as evo-devo, is a field of biological research that compares the developmental processes of different organisms to infer how developmental processes evolved.

The field grew from 19th-century beginnings, where embryology faced a mystery: zoologists did not know how embryonic development was controlled at the molecular level. Charles Darwin noted that having similar embryos implied common ancestry, but little progress was made until the 1970s. Then, recombinant DNA technology at last brought embryology together with molecular genetics. A key early discovery was that of homeotic genes that regulate development in a wide range of eukaryotes.

William Tecumseh Sherman Fitch III (born 1963) is an American evolutionary biologist and cognitive scientist at the University of Vienna (Vienna, Austria) where he is co-founder of the Department of Cognitive Biology. He was elected a Member of the National Academy of Sciences in 2025.

Fitch studies the biology and evolution of cognition and communication in humans and other animals, and in particular the evolution of speech, language and music. His work concentrates on comparative approaches as advocated by Charles Darwin (i.e., the study of homologous and analogous structures and processes in a wide range of species).

Divergent evolution or divergent selection is the accumulation of differences between closely related populations within a species, sometimes leading to speciation. Divergent evolution is typically exhibited when two populations become separated by a geographic barrier (such as in allopatric or peripatric speciation) and experience different selective pressures that cause adaptations. After many generations and continual evolution, the populations become less able to interbreed with one another. The American naturalist J. T. Gulick (1832–1923) was the first to use the term "divergent evolution", with its use becoming widespread in modern evolutionary literature. Examples of divergence in nature are the adaptive radiation of the finches of the Galápagos, changes in mobbing behavior of the kittiwake, and the evolution of the modern-day dog from the wolf.

The term can also be applied in molecular evolution, such as to proteins that derive from homologous genes. Both orthologous genes (resulting from a speciation event) and paralogous genes (resulting from gene duplication) can illustrate divergent evolution. Through gene duplication, it is possible for divergent evolution to occur between two genes within a species. Similarities between species that have diverged are due to their common origin, so such similarities are homologies.

Comparative anatomy is a study of similarities and differences in the anatomy of different species. It is closely related to evolutionary biology and phylogeny (the evolution of species).

The science began in the classical era, continuing in the early modern period with work by Pierre Belon who noted the similarities of the skeletons of birds and humans.

Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and lift, which help the fish swim. Apart from the tail or caudal fin, fish fins have no direct articulations with the axial skeleton and are attached to the core only via muscles and ligaments.

Fish fins are distinctive anatomical features with varying internal structures among different clades: in ray-finned fish (Actinopterygii), fins are mainly composed of spreading bony spines or "rays" covered by a thin stretch of scaleless skin, resembling a folding fan; in lobe-finned fish (Sarcopterygii) such as coelacanths and lungfish, fins are short rays based around a muscular central bud internally supported by a jointed appendicular skeleton; in cartilaginous fish (Chondrichthyes) and jawless fish (Agnatha), fins are fleshy "flippers" supported by a cartilaginous skeleton. The limbs of tetrapods, a mostly terrestrial clade evolved from freshwater lobe-finned fish, are homologous to the pectoral and pelvic fins of all jawed fish.

Placoderms (from Ancient Greek πλάξ [plax, plakos] 'plate' and δέρμα [derma] 'skin') are vertebrate animals of the class Placodermi, an extinct group of prehistoric fish known from Paleozoic fossils during the Silurian and the Devonian periods. While their endoskeletons are mainly cartilaginous, their head and thorax were covered by articulated armoured plates (hence the name), and the rest of the body was scaled or naked depending on the species.

Placoderms were among the first jawed fish (their jaws likely evolved from the first pair of gill arches), as well as the first vertebrates to have true teeth. They were also the first fish clade to develop pelvic fins, the second set of paired fins and the homologous precursor to hindlimbs in tetrapods. 380-million-year-old fossils of three other genera, Incisoscutum, Materpiscis and Austroptyctodus, represent the oldest known examples of live birth.

A cuticle (/ˈkjuːtɪkəl/), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non-homologous, differing in their origin, structure, function, and chemical composition.

A testicle, also called testis (pl. testes) is the male gonad in all gonochoric animals, including humans, and is homologous to the ovary, which is the female gonad. Its primary functions are the production of sperm and the secretion of androgens, primarily testosterone.

The release of testosterone is regulated by luteinizing hormone (LH) from the anterior pituitary gland. Sperm production is controlled by follicle-stimulating hormone (FSH) from the anterior pituitary gland and by testosterone produced within the gonads.

The spinal column, also known as the vertebral column, spine or backbone, is the core part of the axial skeleton in vertebrates. The vertebral column is the defining and eponymous characteristic of the vertebrate. The spinal column is a segmented column of vertebrae that surrounds and protects the spinal cord. The vertebrae are separated by intervertebral discs in a series of cartilaginous joints. The dorsal portion of the spinal column houses the spinal canal, an elongated cavity formed by the alignment of the vertebral neural arches that encloses and protects the spinal cord, with spinal nerves exiting via the intervertebral foramina to innervate each body segment.

There are around 50,000 species of animals that have a vertebral column. The human spine is one of the most-studied examples, as the general structure of human vertebrae is fairly typical of that found in other mammals, reptiles, and birds. The shape of the vertebral body does, however, vary somewhat between different groups of living species.

Alternatives to Darwinian evolution have been proposed by scholars investigating biology to explain signs of evolution and the relatedness of different groups of living things. The alternatives in question do not deny that evolutionary changes over time are the origin of the diversity of life, nor that the organisms alive today share a common ancestor from the distant past (or ancestors, in some proposals); rather, they propose alternative mechanisms of evolutionary change over time, arguing against mutations acted on by natural selection as the most important driver of evolutionary change.

This distinguishes them from certain other kinds of arguments that deny that large-scale evolution of any sort has taken place, as in some forms of creationism, which do not propose alternative mechanisms of evolutionary change but instead deny that evolutionary change has taken place at all. Not all forms of creationism deny that evolutionary change takes place; notably, proponents of theistic evolution, such as the biologist Asa Gray, assert that evolutionary change does occur and is responsible for the history of life on Earth, with the proviso that this process has been influenced by a god or gods in some meaningful sense.

In many animals, a penis (/ˈpiːnɪs/; pl.: penises or penes) is the main male sexual organ used to inseminate females (or hermaphrodites) during copulation. Such organs occur in both vertebrates and invertebrates, but males do not bear a penis in every animal species. Furthermore, penises are not necessarily homologous.

The term penis applies to many intromittent organs, but not to all. As an example, the intromittent organ of most Cephalopoda is the hectocotylus, a specialized arm, and male spiders use their pedipalps. Even within the Vertebrata, there are morphological variants with specific terminology, such as hemipenes.

Evidence of common descent of living organisms has been discovered by scientists researching in a variety of disciplines over many decades, demonstrating that all life on Earth comes from a single ancestor. This forms an important part of the evidence on which evolutionary theory rests, demonstrates that evolution does occur, and illustrates the processes that created Earth's biodiversity. It supports the modern evolutionary synthesis—the current scientific theory that explains how and why life changes over time. Evolutionary biologists document evidence of common descent, all the way back to the last universal common ancestor, by developing testable predictions, testing hypotheses, and constructing theories that illustrate and describe its causes.

Comparison of the DNA genetic sequences of organisms has revealed that organisms that are phylogenetically close have a higher degree of DNA sequence similarity than organisms that are phylogenetically distant. Genetic fragments such as pseudogenes, regions of DNA that are orthologous to a gene in a related organism, but are no longer active and appear to be undergoing a steady process of degeneration from cumulative mutations support common descent alongside the universal biochemical organization and molecular variance patterns found in all organisms. Additional genetic information conclusively supports the relatedness of life and has allowed scientists (since the discovery of DNA) to develop phylogenetic trees: a construction of organisms' evolutionary relatedness. It has also led to the development of molecular clock techniques to date taxon divergence times and to calibrate these with the fossil record.

In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have evolved in their most recent common ancestor.

In cladistics, synapomorphy implies homology.