Question

1. Although we teach natural selection using a single allele, we recognize that natural selection works upon the entire phenotype of an organism, not an individual loci.

About 85% of the human population has a muscle in the forearm called the palmaris longus.

a) What is this feature, and what benefit does it provide?

b) Respond to this common criticism of evolution, providing evidence to back your statement:   “If natural selection is truly working, we should be perfectly adapted to our environment. Thus, if evolution were true, this muscle would not be present. Thus, evolution is not true and we should teach scientific creationism (intelligent design) instead.”

c) Respond to this common fallacy of evolution, providing evidence to back your statement, “If most humans have this muscle, then obviously there is a benefit for the individuals. We would not waste energy in making useless structures. Evolution would remove the useless structure from the population.”

d) Explain the advantages of NOT having this particular feature.

Answer 1

a] Palmaris longus is a superficial muscle in the forearm lying on the medial side of the flexor carpi radialis. It arises especially from the medial epicondyle of the humerus and inserts into the palmar aponeurosis. It acts to flex the hand.

It is not present in everyone. There is a great variation in the percentage of population having or not having this muscle among the Africans, Asians and Native Americans.

The palmaris longus is one of the five muscles that act a the wrist joint. It runs to the palm and activates flexibility at the wrist.

Research has found that the presence of this muscle in the forearms does not give any discernible advantage over the people who do not have this muscle.

b] Evolution has helped organisms to adapt to their surroundings. This is a fact because we know that those organisms which could not adapt to their changing environment, became extinct. Dinosaurs, mammoths etc are examples of this. Monkeys and other related arboreal mammals who need a strong grip to hold on to the branches of trees have a strong palmaris longus. In the course of evolution, as chimpanzees and gorillas began spending time on the ground and bipedal movement took over, the use of this muscle was reduced and its importance was lost

As for the presence of palmaris longus is concerned, it can also be considered as an example of evolution in which due to the reduction in its usage, it has become vestigial like the appendix. What is puzzling is that unlike other vestigial organs, palmaris longus might be present in some populations and absent in some. We can attribute this to genetic inheritance i.e  this feature is controlled by the presence or absence of some gene.

Another example could be sickle cell anemia. This is a genetic disorder and is advantageous in that people suffering from this disease cannot be infected by malaria germ. We can not attribute this to evolution or 'intelligent design'. It is a genetic disorder resulting in the abnormal shape of the red blood cells with an advantage that it protects such people from malaria.

c] Palmaris longus dos have some clinical significance. It is commonly used for tendon grafts due to its length and diameter. Added to this is the fact that it can be used without producing any functional deformities. When a tendon becomes ruptured at the wrist, the palmaris longus tendon may be removed from the flexor retinaculum and grafted to take the place of the ruptured tendon.

d] The palmaris longus muscle itself is a weak flexor muscle and provides no substantial flexing force. So, even if it is removed, there is no hindrance to the movement of the wrist. So, obviously, there is no disadvantage in not having this muscle.

The advantage of not having this feature could be that it c be attributed to 'intelligent design'. The absence of this muscle cannot be of any specific disadvantage.