Vestigial Structures

Vestigial Structures Definition

Vestigial structures are various cells, tissues, and organs in a body which no longer function in the same way the ancestral form of the trait functioned. A vestigial structure can arise due to a mutation in the genome. This mutation will cause a change in the proteins that are required for the formation of the structure. Although the structure no longer functions, the prevalence of the vestigial structure may increase in the population if it is advantageous. In cave-dwelling fish, for example, the development and upkeep of eyes are an unnecessary energetic expense when there is no light. Therefore, vestigial eyes may be selected for over function eyes.

Since the earliest days of studying the anatomy of various animals, vestigial structures have been encountered and observed in almost every species. The process of evolution is an imperfect one. While evolution constantly drives to adapt organisms perfectly to the conditions present, it can only work with what it is given. Therefore any time a population moves environments or the environment changes, resulting adaptations must be made. In many organisms, vestigial structures are the result of a large evolutionary change that resulted in a previously functional structure to become burdensome and useless.

Vestigial alone means lacking function or use, and can apply to behaviors, chemical pathways, and other aspects of an organism’s existence that are not directly physical. However, these useless aspects are also controlled by the genome, and have become vestigial because of a mutation. The mutation, although advantageous to the population, has not completely left the population. That is why vestigial structures, behaviors and pathways are still present.

Examples of Vestigial Structures

Vestigial Structure in Fruit Flies

The wild fruit fly and common laboratory organism Drosophila melanogaster was one of the first to have its small genome mapped. During the mapping of the genome, scientists found many genes that if inactivated would cause vestigial mutations in the fruit flies. Hundreds of mutations where found that could produce vestigial structures. The wings, eyes, feet, and many organs could become vestigial through the deactivation of different genes. Using these flies as a model, scientist were able to accurately and clearly show how vestigial structures can arise through simple sexual reproduction, and how these vestigial structures could become frequent in a population.

Populations of fruit flies have been developed to have different vestigial structures for different purposes. Flies with vestigial wings are bred and used as feeder insects for pet frogs. Because humans supply an environment with plenty of food and no predators, the flies can still grow and reproduce. When it is time to feed the pet frogs, the flies can be easily tapped out of their culture tube. With no wings, the flies cannot fly away or otherwise escape the frog’s enclosure. In other cases, scientist may want to test the sensory organs of flies. By producing flies with vestigial eyes, for instance, the other senses can be tested without the variable of sight being added in.

Snake Limbs

Before the days of fossil records, x-rays, and DNA analysis, it was long assumed that snakes gave rise to lizards, not the other way around. When scientist started really observing the anatomy of snakes, they began to realize that many snakes still have vestigial structures where a lizard’s limbs would have been. Other vestigial structures in snakes, such a vestigial lung, were also evidence that snakes evolved from an ancestor that used two lungs and walked with 4 limbs. This, coupled with a fossil record that showed a decline in limb size leading to snakes and mounting DNA evidence revealed that the opposite was true; snakes came from lizards, not the other way around.

Loss of limbs is also seen in whales. The ancestors of whales were organisms somewhat like hippos, which slowly moved into the water. In the water, limbs create drag and making swimming less efficient. Slowly, the front limbs were changed to fins, and the back limbs were lost entirely. However, the skeleton of a whale will reveal a set of bones, not attached to the main skeleton, where the hind-limbs used to be. The bones to not leave the body, and seem to only provide minor support to the muscles. These vestigial structures are a clue that like snakes, whales came from a 4-legged ancestor.

Vestigial Structures in Humans

Humans have a wide range of traits that are considered vestigial structures. One of the most obvious is the tailbone, or coccyx. The coccyx is a small series of fused vertebrae that exist at the base of the pelvis. In our ancestors, it probably formed a large prehensile tail, capable of grabbing branches. As we evolved into bipeds, less time was spent in the trees and more time spent walking and sitting on the ground. As seen in the transition from monkeys to great apes, the loss of a tail represents a less arboreal, or tree-based lifestyle.

If you’ve ever had your wisdom teeth removed, you know that vestigial organs can be more than useless. Sometimes, they can even be painful or harmful to an organism. In the case of wisdom teeth, the human skull has been shrinking as we evolve. Part of the reason is that our diet has become much softer and easier to chew, because we cook or otherwise process our food. While our jaw has become smaller, the last tooth in the jaw has not been lost. In most people, this tooth will cause pain as it comes in, and may deform the other teeth in the jaw.

Have you ever gotten goose-bumps when you get cold? When this happens, small vestigial muscles at the base of your hair follicles pull the hair so it stands upward. In our ancestors this created a much fluffier and thicker coat, which could hold more air. An animal’s coat functions by trapping air and heating it up. Humans have lost the coat, but retained the muscles that make hairs stand up. The pathways that cause the hair to stand up can also be considered vestigial. While they do help us know we’re cold, they certainly don’t help warm us up.

Related Biology Terms

  • Adaptation – A change in a population that helps them survive or reproduce in the given environment.
  • Evolution – The process through which adaptations are developed, which involves the inheritance of genes through common decent.

Quiz

1. The human appendix is often described as a vestigial organ, as the human appendix is much smaller than in many of our mammalian relatives. However, recent evidence has shown that the appendix may harbor bacteria and fungi that help repopulate your intestines after a sickness. Is the appendix a vestigial organ?
A. Yes
B. No
C. Maybe…

Answer to Question #1

2. In the corner of the human eye is a small bit of pink tissue. Some of the tissue contains tear ducts, but much of it does not appear to have a function. This vestigial structure is similar to the nictitating membranes of certain aquatic vertebrates, which helps them see underwater. What does this suggest about human ancestors?
A. At one point, our ancestors lived in the water.
B. Our closest relatives are frogs.
C. We used to be mermaids.

Answer to Question #2

3. A population of fish are decorated with small spines. When they expand into a new territory, the spines are no longer needed to defend against predators, and become vestigial. Millions of years later, the spines are adapted into small lures, which attract small prey fish towards the mouth of the spiny fish. Are these still vestigial structures?
A. No
B. Yes
C. They are vestigial spines, but functional lures.

Answer to Question #3
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