Homology & Analogy

Jacklyn van der Colff
Anthropology 101

Homology and Analogy

Homology:

Homologous traits can be found in species that look nothing alike, sound nothing alike, and seem to have no close relation; however, many species that seem to be day and night have a common ancestor. 

An example of homology can be seen in these two species:

Panthera uncia: Better known as the endangered Snow leopard, this species of cat is found in central and south Asia. They live in cooler climates and are usually found at higher elevations. Snow leopards are members of the Panthera lineage, which is a branch of the Felidae. They are genetically similar to many other big cats, such as tigers, lions, and jaguars.

 

Common dolphin: The Common Dolphin technically includes two types: the short-beaked and the long-beaked, but for this example, we are focused on the flipper of the dolphin. They are found in the ocean and are from the Delphinidae family.

These two seemingly different species, actually have a homologous trait- their forelimbs.

The leopard and the dolphin, as can be seen above, both have a type of forelimb, in this case, the paw of the cat and the flipper of the dolphin. Both of these forelimbs contain the same bones, the radius, and the ulna. 

However, in these two species, the homologous trait is used for different things. The snow leopard uses its paws aid in balance and reception during hunting, groom themselves, and being able to keep balanced when climbing. The dolphin, on the other hand, uses their flippers for steering and communicating with other dolphins. The actual bones, although still similar to the ones in other land mammals, are slightly shorter in the dolphins' forearm.
The reason that these traits have different uses, is because they are needed for different things. A cat living on land in Asia is going to have very different needs than the dolphin living in the Atlantic ocean; whereas the cat uses the forepaw to walk, the dolphin uses the forelimb (flipper) to swim. They do have a common ancestor- even shared with humans and almost all other land mammals. 

During the time period called "Paleozoic", the first varieties of boned fishes and the first four-limbed amphibians appeared. It is believed based on the fossil record and DNA testing that, these reptiles were the ancestors to modern day mammals. The main reason for the seemingly large difference in species is the split of Pangea. This split caused great isolation between species that could at the time have even been a member of the same family. As time passes, natural selection and evolution occur which allows ecological niches to develop. Another way to explain this difference in use is the concept of generalized and specialized traits; most mammals have feet and hands, but there are small changes that make them specialized to each species; such as, varying lengths, weights, and sizes to accommodate the species. These two species are descendants of the first vertebrate or more specifically it could have been the first land living whale, the Pakiceuts. 

For better visualization- here are photos of bone structure to compare:

By looking at these images, we can clearly see that the forepaw of a cat and the flipper of a dolphin have very similar structures. 

Analogy:

Analogous traits are traits that look similar but are actually not genetically related between different species. Heres an example:

Dragonfly: Dragonflies are insects that have two large multifaceted eyes, thin transparent wings, and an elongated body. They are found all around the world but particularly in wetlands or marshes  

Pigeon: Pigeons are birds with small and "fatter" bodies, they have short necks, slender bills, and their diet usually consists of fruits, plants, and seeds. They are commonly found all around the world and even in cities. 

The analogous trait that these species share is their wings. Dragonflies have these transparent wings that often look like they are many different colors. Pigeons also have wings, but these wings are covered in feathers. Both sets of wings are used for the same thing: flying. Both sets of wings are placed on similar parts of the bodies.  A pigeon wing contains multiple bones, such as an ulna and a radius. Whereas dragonfly wings don't contain any bones at all. They do have s similar roundish shape to them, despite the difference in makeup. It would not be an uncommon mistake to think that a structure with the same usage and general structures are homologous, but in this case, they are definitely not.  Despite both being wings, they do not share a common ancestor. 

Even though all organisms are the descent of one single individual, not all traits are from this common descendant. It is believed that insects came to life over 400 million years, about 250 million years later, the first bug to fly evolved. It was recently discovered that birds actually evolved from dinosaurs. One of the first theropod dinosaurs had hands with several digits, however, it can be seen through their lineage that birds would eventually lose some of them but this loss allowed for flight, a which means the evolution from the dinosaur is what actually gave birds their wings. Because of fossil records and scientific studies, we know that these creatures got their analogous trait from different ancestors.

Note: Some of my font appears to change color in the actual post which is not intentional and I think it is a glitch in blogger- sorry if a section is hard to read!