Biology: Comparative Morphology: Studies of Structure and Function

Introduction

Morphology, one of the life sciences, studies an organism's outward characteristics: its anatomy, shape, and appearance. One of the first steps in identifying an organism is examining these prominent features; this helps distinguish one species from one another and identify new species or subspecies. Morphology can also be studied on a much smaller scale, investigating specific organs, tissues, or cell types.

The ability to compare the morphology of two organisms is an important basic skill for life scientists. Simple, careful observation and comparison have led, for example, to most of the discoveries in the field of paleontology as well as the discovery that whales are mammals.

Historical Background and Scientific Foundations

From antiquity through the Renaissance, laws and cultural taboos prevented physicians from dissecting human bodies. Their anatomical knowledge came from surgery, treatment of serious wounds and injuries, and from animal dissections. In a certain sense, most early investigations of human anatomy were exercises in comparative morphology. Since direct investigation was not possible, physicians filled the gaps in their knowledge with comparison to animal models.

One of these was the Roman physician Galen of Pergamum (AD 129–216), who was greatly respected for his writings on medicine and anatomy. Despite many errors created by his inability to conduct human dissections, Galen was the best source of anatomical information until the Renaissance. He encouraged physicians to be curious and to investigate for themselves. Despite this urging, however, his work remained unchallenged for more than 1,000 years.

Andreas van Wesel (1514–1564), known more commonly by his Latinized name Vesalius, was among the first physicians to question Galen's authority. He was born in the early sixteenth century into a prominent Dutch medical family that had long served the Holy Roman Emperors. Early in his career Vesalius began to perform human dissections, challenging Galen's dominance with the results. Since Galen had never dissected a human corpse, Vesalius published corrections to his writings, showing many flaws born of blind comparison between humans and animals.

IN CONTEXT: ANIMAL STUDIES INFLUENCE EARLY MEDICAL THOUGHT

Medieval understanding of the nervous system was basically limited to observations of animal anatomy, tempered by philosophies prevailing since antiquity. The influence of Greek physician Galen of Pergamum (AD 129–216) on medical theory and practice was dominant in Europe throughout the Middle Ages and into the Renaissance. Galen considered the best physicians were also philosophers, and that philosophy promoted medicine. Galenic tradition held that illness was a result of an imbalance of body fluids, or humors. While dissecting calves, Galen noticed a network of nerves and vessels at the base of the calf brain that he mistakenly assumed also existed in humans. Galen labeled this area the rete mirabile, and stated that this was the site where vital life spirits were transformed into man's animal spirits. After the advent of Christianity, these spirits were unified into the concept of a Christian soul, and physicians debated the soul's base in the human body, presumably in either the heart or the brain.

Influence on Other Fields of Study

One of the first discoveries in paleontology was made possible by the use of comparative morphology. In 1666 Italian fishermen caught a large shark. It was sent to Niels Steensen (1648–1686), better known by his Italian name, Nicolaus Steno, who was a Danish anatomist working in Florence. Upon examining the shark's teeth, Steno noticed that they were very similar to so-called “tongue stones,” small triangular stones that had long been found in the earth. Steno realized that these stones were fossilized sharks' teeth, and that over time, the living material had been replaced by stone.

Comparative morphology also played a large role in the early classification of plant and animal species. Swedish naturalist Carl Linnaeus (1707–1778; also known as Carolus Linnaeus or Carl Linné) developed the first consistent system for classifying organisms, specifically plants. His system was based on the characteristics of plants' male and female sex organs. He grouped them by type then compiled broader groups based on shared characteristics, relying heavily on comparative morphology. Linnaeus's work allowed a vast number of plants and animals to be classified systematically, based not on artificial categories (such as domestic animals) but on common traits. His work formed the foundation of modern taxonomy; his method of comparative morphology is still the starting point for classification.

Modern Cultural Connections

Comparing the features of one organism to another helps scientists learn about both. While logic decrees that those with the most similar morphology are most closely related, natural selection sometimes gives unrelated organisms similar forms.

Structures that develop in similar ways because they share a common origin are called homologous. The front appendages of most mammals are a very broad type of homologous structure. Those that do not arise from a similar origin are analogous structures. These develop when a similar environment exerts similar evolutionary pressures on different organisms; examples are the similar shape of dolphins and fish, or the wings of birds and bats. A striking example of evolutionary homology (convergent adaptation) is the similarity between the eyes of animals in different zoological phyla, such as squid and octopuses, which are mollusks, and those of vertebrates, including animals and humans.

Comparative morphology also provides support for the theory of evolution. By studying both living organisms and the fossils of their extinct ancestors, zoologists and paleontologists can draw conclusions about their origins. By studying fossils, scientists can see how elephants evolved from small, trunkless animals, or how whales evolved from quadrupedal land mammals, losing their back legs as they adapted to life in the sea.

Some of the strongest evidence of evolution comes from comparative studies—comparing structural similarities of organisms to determine their evolutionary relationships. Organisms with similar anatomical features are assumed to be relatively closely related evolutionarily, and they are assumed to share a common ancestor. As a result of the study of evolutionary relationships, anatomical similarities and differences are important factors in determining and establishing classification of organisms.

Some organisms have anatomical structures that are very similar in embryological development and form, but very different in function. These are called homologous structures. Since these structures are so similar, they indicate an evolutionary relationship and a common ancestor of the species that possess them. A clear example of homologous structures is the forelimb of mammals. When examined closely, the forelimbs of humans, whales, dogs, and bats all are very similar in structure. Each possesses the same number of bones, arranged in almost the same way. While they have different external features and they function in different ways, the embryological development and anatomical similarities in form are striking. By comparing the anatomy of these organisms, scientists have determined that they share a common evolutionary ancestor, and, in an evolutionary sense, they are relatively closely related.

Other organisms have anatomical structures that function in very similar ways but are very different morphologically and developmentally. These are called analogous structures. Since these structures are so different, even though they have the same function, they do not indicate that there is an evolutionary relationship, nor that the two species share a common ancestor. For example, the wings of a bird and dragonfly both serve the same function; they help the organism to fly. However, when comparing the anatomy of these wings, they are very different. The bird wing has bones inside and is covered with feathers, while the dragonfly wing is missing both of these structures. They are analogous structures. Thus, by comparing the anatomy of these organisms, scientists have determined that birds and dragonflies do not share a common evolutionary ancestor, or that, in an evolutionary sense, they are closely related. Analogous structures are evidence that these organisms evolved along separate lines.

Vestigial structures are anatomical features that are still present in an organism (although often reduced in size) even though they no longer serve a function. When comparing the anatomy of two organisms, the presence of a structure in one and a related, although vestigial, structure in the other is evidence that the organisms share a common evolutionary ancestor, and that, in an evolutionary sense, they are relatively closely related. Whales, which evolved from land mammals, have vestigial hind leg bones in their bodies. While they no longer use these bones in their marine habitat, they do indicate that whales share an evolutionary relationship with land mammals. Humans have more than 100 vestigial structures in their bodies.

Comparative morphology is an important tool that helps determine evolutionary relationships between organisms and whether or not they share common ancestors. However, it is also important evidence for evolution. Anatomical similarities between organisms support the idea that these organisms evolved from a common ancestor. Thus, the fact that all vertebrates have four limbs and gill pouches at some part of their development indicates that evolutionary changes have occurred over time, resulting in the diversity observed today.

See Also Biology: Botany; Biology: Classification Systems; Biology: Comparative Morphology: Studies of Structure and Function; Biology: Concepts of Heredity and Change Prior to the Rise of Evolutionary Theory; Biology: Evolutionary Theory; Biology: Paleontology; Biology: Zoology.

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Web Sites

California State University, Stanislaus. Biology Department. “Introduction to Evolution: Comparative Anatomy.” http://arnica.csustan.edu/biol3020/anatomy/anatomy.htm (accessed January 26, 2008).

University of California, Berkley. “Comparative Anatomy: Andreas Vesalius.” Understanding Evolution for Teachers.http://evolution.berkeley.edu/evosite/history/compar_anat.shtml (accessed January 26, 2008).

University of California Museum of Paleontology. “Carl Linnaeus.” July 21, 2000. http://www.ucmp.berkeley.edu/history/linnaeus.html (accessed January 26, 2008).

Wilson, Bronwen. “Andreas Vesalius.” Boundaries of the Body and Scientific Illustration in Early Modern Europe.http://www.bronwenwilson.ca/physiognomy/pages/biographies.html# vesalius (accessed January 26, 2008).

Kenneth T. LaPensee