Common Ancestry

Common descent is a term within evolutionary biology that refers to the common ancestry of a particular group of organisms. The process of common descent involves the formation of new species from a particular group of organisms. population ancestral . When a recent common ancestor is shared between two organisms, they are said to be closely related. Conversely, common descent is also traced back to a universal common ancestor of all living organisms using molecular genetic methods. Such evolution from a universal common ancestor is thought to have involved several events from speciation as a result of natural selection and other processes, such as geographical separation.

Common descent theory

The theory of common ancestry states that all living organisms are descended from a single ancestor. Thus, Common Ancestry Theory helps explain why species living in different geographic regions exhibit different traits, some traits are highly conserved among broad classifications of animals (e.g., Vertebrates or tetrapods), apparently different species (e.g., Birds and reptiles) share inherited traits. Physical and genetic traits and successfully adapted organisms tend to produce more offspring.

While the theory of common descent is mainly derived from physical observation of various phenotypes (e.g., size, colour, beak shape, embryological development, etc.), modern advances in genetics and associated molecular techniques have been able to demonstrate that the process by which the DNA ultimately translates into proteins is maintained among all life forms. Small changes in DNA between organisms have revealed shared ancestry, as well as insight into major changes that resulted in various speciation events. Phylogenetic trees and cladograms are often used to hypothesise about the evolution of various organisms and shared common ancestry.

Examples of common ancestry

Chromosome human 2

Compelling evidence for the shared common ancestry of humans with the great apes is the fusion event that occurred when two common chromosomes in apes fused to form chromosome 2 in humans (as illustrated below). This resulted in humans having only 23 pairs of chromosomes, whereas all other hominids have 24 pairs. The great apes (e.g., chimpanzees, gorillas and orangutans) have two chromosomes with DNA sequences almost identical to those found on human chromosome 2. Other evidence for such a fusion event is the residual presence of telomeres and a centromere indicating that the genetic information was historically found on two separate chromosomes.

Endogenous retroviruses

Endogenous retroviruses are residual DNA sequences found in the genomes of virtually all living organisms as a result of ancient viral infections. Since the retroviral sequences are incorporated into the DNA of the organism host, such sequences are inherited in offspring. Since such infections are random events, as is the location at which the viral genome is inserted into the genome, identification of the same retroviral sequences in multiple species is indicative of shared ancestry. Such analyses of endogenous retroviruses often reveal speciation events (e.g., feline endogenous retroviruses reveal separation between large and small cat species) and how closely related two species can be, as seen in endogenous retroviruses shared between humans and other primate species.

The presence of atavisms

Atavisms are the occurrence of a missing trait observed in an species ancestral ancestry that is not observed in more recent ancestors. Atavisms are an example of common ancestry, as they provide evidence of phenotypic or vestigial characteristics that are often conserved throughout evolution. Examples include the appearance of hind limbs in whales as evidence of a terrestrial ancestor, teeth exhibited by chickens, extra toes observed in modern horse species and hind flippers of bottlenose dolphins. Atavisms tend to arise because ancestral genes are not deleted from the genome, but silenced and then reactivated in subsequent offspring.

Vestigial structures

Like atavisms, vestigial structures are structures that are homologous to those found in ancestral species; but which have become underdeveloped, non-functional or degenerate in more recently evolved organisms. Such structures provide evidence of adaptations to a new environment, in which the organ ancestralor member is no longer needed, or has been modified to better suit a new purpose. There are a number of examples of vestigial structures observed in nature. Examples include the hind limbs and pelvic girdle observed in whales (as shown below) and snakes; non-functional wings exhibited by some insect species, non-functional wings of flightless bird species (e.g., ostriches), abdominal segments of barnacles; and embryonic limb buds exhibited by various species lacking hind limbs (e.g., dolphins).

Pentadactyl limbs

The presence of pentadactyl limbs is an example of a trait homologue exhibited by all tetrapods and is highly conserved throughout evolution, despite some modifications. Such limbs are first observed in the evolution of fish to amphibians and consist of a single proximal bone; two distal, carpal, five metacarpals, and phalanx. Although the general structure of pentadactyl limbs is similar, several modifications have been made throughout evolution as adaptations to specific environments or lifestyles. Examples include the modified pentadactyl wings of bats; the elongated forearms of primate species; the flippers of dolphin and whale species; and the modified toes of horses to form a hoof.

Fossil evidence

The fossilised remains of various organisms combined with modern dating methods provide some of the most compelling evidence of common descent and evolutionary history. Fossilisation occurs when the bones of a decaying animal become porous and mineral salts from the surrounding soil infiltrate the bones, turning them to stone. Other methods include preservation in ice, imprinted remains (e.g. plants or footprints), tree resin and peat. Since the fossils are found in sedimentary rock; which is made up of layers of silt and mud, each layer corresponding to a specific geological period that can be dated. Therefore, the extinction The evolution and emergence of various species can be observed throughout history by using the fossil record.. The fossil record also shows many extinct species, such as dinosaurs.

Biogeography

The biogeography presents compelling evidence of common ancestry by showing speciation and novel traits through adaptations to environmental pressures. One of the most famous examples is that of island biogeography and Charles Darwin’s observations of the beaks of finches residing in the Galapagos Islands. In these finches, the beaks had adapted for the specific vegetation found on the island, resulting in a departure from the ancestral finches found on the mainland. The long-term effects of geographical separation are also seen with the evolution of new species found nowhere else in the world. An example of this is the presence of marsupial species on the continent of Australia; and the emergence of polar bears as a result of geographic isolation in the Arctic.