Phylogenies and Clades

A phylogeny illustrates the evolution of species and their relationships to other groups.

A phylogeny is the evolutionary history of groups of organisms. A phylogenetic tree is a diagram that shows the history of evolutionary relationships among two or more groups of organisms. For example, all vertebrate animals are linked together on a phylogenetic tree because they all evolved to have spines. Sharks, ray-finned fish, amphibians, primates, rodents, and reptiles all appear on the same limb of a phylogenetic tree because they share the common trait of a spine.

Each branching in a phylogenetic tree is an example of macroevolution in action—when a new characteristic appears, that is an evolutionary change. If that new trait is inherited, this may ultimately result in the evolution of one or more species. For example, developing a hair- or fur-covered body was a macroevolutionary event. This character trait was passed on to felines, canines, rodents, and primates. However, the appearance of felines and canines on the same branch of a phylogenetic tree does not imply that one genus is more or less advanced than another. Their evolutionary stage is equal and would be presented as such on a phylogenetic tree.

A clade is a taxonomic group that includes an ancestor and all of its descendants. Members of a clade may or may not be from the same family or genera. Groups with a recent common ancestor may be close relatives like siblings or cousins. A common ancestor may refer to an earlier organism that, through evolution, gives rise to new species that are related. There are many species of birds, but all birds descended from a common ancestor.

Species with a more distant ancestor, such as a crocodile and an amphibian, may have evolved different characteristics or traits. Both species have four limbs inherited from their common ancestor, so they may appear on the same phylogenetic tree. However, amphibians lay strings of eggs that are not amniotic eggs (eggs that contain fluid-filled membranes), and they do not have the openings behind the eye that crocodiles do. These differences depend on genetic changes and that might be indicated by presenting crocodiles and amphibians on different branches on a phylogenetic tree.
A clade shows a grouping with a single common ancestor and all descendants of that original species. Primates and rodents have a common ancestor that produced descendants with the presence of body hair. Crocodiles, dinosaurs, and birds evolved from a common ancestor that passed on the trait of an opening in the skull above the eye socket, called the postorbital fenestrae. A more inclusive clade could show all species or genera that produce offspring with an egg that does not dry out, called an amniotic egg, thus including primates, rodents, crocodiles, and birds.

Homologous versus Analogous Structures

Physical structures may be homologous or analogous in form and function.

Similar or shared physical structures in organisms can arise as a result of homology or analogy. Homologous structures are similar in organisms as a result of inheritance from a common ancestor, an earlier organism that, through evolution, gave rise to new species that are related. For example, the forelimbs of a toad, a fox, and a crocodile have evolved to meet the specific needs of the species, but they are homologous because the structures and forelimbs were found in a shared common ancestor. The same is true of body parts of insects; for example, antennae and mouthparts. A grasshopper chews its food, but a butterfly sucks pollen. Antennae may be used to touch, smell, and even hear. These structures are also homologous and evolved from common ancestry.

Other physical structures may be analogous, having independent evolutionary paths but appearing similar in function. An analogous structure is a structure in one organism that is similar in structure or function to a structure in another organism because of convergent evolution, not common ancestry. Convergent evolution is a process by which two species acquire similar features due to similar selection pressures and not shared ancestry. Astrophyta live in North American deserts and are relatives of cacti. Euphorbia are succulent plants that come from Africa and evolved independently from Astrophyta. Both share similar spherical shapes, have eight symmetrical sectors, and can take in and retain large amounts of water. They are from different evolutionary paths and have developed in different locations, but they look so much alike that people might think they are in the same plant genera. Likewise, the fins on swordfish and the flippers on whales are analogous; they perform the same function in assisting both organisms in swimming, but fish (swordfish) evolved independently from mammals (whales).

Physical structures among both plants and animals may be quite similar in construction and function. The reproductive organs of mammals, from humans to elephants to squirrels, have a similar form. Mammals also have four limbs, whether these are arms and legs as in primates or four legs as in bovines and equines. All vertebrates have spines, whether they are amphibians, reptiles, or birds. Pines, elms, spruces, and apple trees have bark-covered trunks, vascular systems for water transport, and a means of producing seeds.
Thigh bones of mammals are homologous in shape and function, meaning they are derived from a common ancestor. Analogous structures appear across many organisms; these are similar structures that result from convergent evolution, not common ancestry.

Convergent Evolution

Species evolve similar structures independently of each other in a process called convergent evolution.

Organisms that evolve independently of each other but bear similar analogous structures are examples of convergent evolution, which is a process by which two species acquire similar features due to similar selection pressures and not shared ancestry. Evolution of similar species may happen at the same time or at different times, within similar ecosystems or in totally different ecosystems. The evolution of these traits is not the result of genetic relationships. Convergent evolution occurs in organisms from different lineages that inhabit very similar environments.

Some organisms develop similar internal traits, such as the proteins in fish that protect them from freezing in Arctic waters. Although fish in the Arctic may have all developed from a common ancestor (an earlier organism that gives rise to new species), fish in Antarctic waters evolved separately. However, they too evolved to have the same protein-based, anti-freezing chemistry. As another example, coconut crabs and several insect species have nearly identical scent organs.

Adaptations may result in externally obvious traits, including prehensile tails or the shape of spherical succulents. New World monkeys, tree pangolins, and opossums have prehensile tails, allowing tree dwellers to grasp branches. Spherical succulents, such as Euphorbia and Astrophyta, developed separately but have nearly identical shapes. Other plants have protective thorns, spines, or prickles, although there is no direct relationship between acacias, cacti, or roses.

Another example of convergent evolution is the development of wings in birds and insects. Convergent evolution produces physical structures that are analogous: they have similar functions but do not have identical structures. A comparison between a bird's wing and an insect's wing shows that both wings enable the organisms to fly. The wings both flap back and forth from the torso, but the structure is different. Bird wings have structures homologous with human arms, with a humerus, ulna, radius, and carpals, while insect wings, such as those of a butterfly, have no bones. The evolution of eyes is another example. Eyes evolved independently at least three times. The compound eyes of arthropods are composed of units called ommatidia, which have a cluster of photoreceptor cells surrounded by pigment cells and support cells. Mollusk eyes and vertebrate eyes both have retinas and hard lenses to focus light.
Convergent evolution produces analogous structures: the parts have similar functions without having derived from a common ancestor. Bird wings and butterfly wings evolved separately. Although the structures of these wings differ and evolved distinctly, they function similarly. Birds and butterflies lack a common ancestor.