Homologous and Analogous Traits in Anthropology

Homologous Traits in Whales and Horses: 

Humpback Whale 

"Homologous traits are those shared between two different species because their common ancestor possessed the trait and passed it on, even though they appear to the eye to be different structures. This means that this shared trait indicates a genetic relationship exists between the species. Differences in those traits are due to differences in the environment, resulting in different functions and evolutionary selection pressures. The process that produces these different but genetically-related traits is called divergent evolution."

Two species with a homologous trait are whales ande horses. Horses, from the family Equidae, are four-legged land mammals in a variety of sizes depending upon the breed. Whales, or porpoises, in the family Cetecea, are large (generally speaking) ocean-dwelling mammals. Both the horse and the whale have similar forelimbs, containing the ulna, radius, humerus, and phalanges. 

 In the horse, the humerus, radius, and ulna differ in size from that of the whale. In addition to size, the carpals, metacarpals, and phalanges are developed into one long toe (ending in a hard hoof) with the rest of their phalanges being vestigial or non-existent. The humerus and radius are elongated to accomodate long distance coverage of land. 

In whales, the ulna, radius, and humerus are more compact with five separate phalanges. These bones are not visible from the outside and each flipper appears as a broad, flat limb used for navigating through water. 

Though the general "make-up" of the forelimbs of whales and horses is the same, their limbs differ tdue to their very different environments. Because whales live in large bodies of water, they need limbs that will propel them through water efficiently for great distances. And because horses live on land and were once considered prey, they needed to develop long, lean, sturdy legs to cover long distances quickly. 

The common ancestor of these two vastly different species were early vertebrates. In recent years, fossils and research have shown tha tthe forelimb development in vertebrates is controlled by only a few genes. According to this research, mutations in the genes of these early vertebrates led to the limb development of all other vertebrates that came after. Because of this, all vertebrates have nearly the same number and structure of bones, with small genetic differences leading to developmental variety (demonstrated by the differences and similarites of horses and whales). 

Wild horse taking a mud bath.

Analogous Traits in Monarch Butterflies and Geese:

"Analogous traits, on the other hand, are not the result of a common genetic history, but are due to a common environmental pressure that caused both species to develop a similar adaptation to that stress. The mechanism that produces this type of matching trait is called convergent evolution. It indicates a simlar evolutionary environmental stress, not an evolutionary genetic relationship."

Monarch Butterfly

Two species with an analogous relationship are monarch butterflies and Canadian geese. The monarch butterfly is a milkweed butterfly found in the Nymphalidae family and its orange and black pattered wings are distint and familar throughout North America. The monarch has a wing span of approximately 9-10cm, making it incredibly small in comparison to the Canadian goose. The goose can have a wing span any where from 127cm-185cm and is found in the Anatidae family.  

While they are very different, they have one thing in common: wings. The wings of both the goose and butterfly are pairs of specialized appendages that enable them to maneuver through the world airborne. Both the butterfly and the goose can fall prey to many predators in the animal kingdom. Because of this, it was important (for survival) to develop a trait for escape. For both the goose and the butterfly, this evolutionary development came in the form of wings. This commonality creates an analogous trait between the two entirely different species.

Canadian Goose

This analogous trait was not present in the common ancestor of the Canadian goose and monarch butterfly. We know this because butterflies evolved from an arthropod (insect) without wings and the goose (and other birds) evolved from lizards that were also wingless. In addition to this, the appendages of these separate species are made entirely differently. The wing of a bird is really a forelimb made of bone and muscle. The bones of the forelimb are structured the same as the whale. The butterfly's wings are made of a scaffolding of veins and tiny scales. Also, the goose (like all other vertebrates) has a skeletal system, developed brains, and a central nervous system--insects do not. Because of these differences, scientists know that these similar structures, the wings, are products of convergent evoloution rather than something inherited from a common ancestor. The link between these two species is entirely superficial. 

Sources: 

Introduction to Physical Anthropology, 2013-2014 edition

http://www.hwdsb.on.ca/hillpark/Departments/Science/Watts/SBI3U/Assigned_Work/Evolution/Homologous_and_Analogous_Vestigial_and_Competition.pdf

http://evolution.berkeley.edu/evolibrary/article/0_0_0/eyes_04

http://bioweb.cs.earlham.edu/9-12/evolution/HTML/live.html