Saturday, January 29, 2011

Sexual Selection

Sexual selection, for those who are not familiar with the term, is the mechanism used by the females to select the most genetically dominant males within a particular species for sexual reproduction. This selection is a means to promote favorable traits of survival on to the future generations. The males are in a constant and often aggressive struggle with one another to prove to the females that they are the most able and dominant among all the other males, i.e. the more genetically fit for survival.  Sexual selection is considered to be a part of natural selection, which can be understood as nature’s test to select the fittest. The term fittest in natural selection is an organism that has to ability to adapt quickly to nature’s harsh conditions and hence is more likely to survive and have offspring.

Examples of Sexual Selection in Nature and other Related Hypotheses
There is no denying that there are plenty of examples of sexual selection in the natural world. The mesmerizing beautiful feathers of male peacocks that are used to seduce the females are a great visual example of this mechanism in action. In other organisms, the females require the males to engage in more aggressive approach to show superiority in strength like with kangaroos, rams, or giraffes.
The Long-tailed Widowbird (Euplectes progne) © Isak Pretorius Photography
An emerging belief about sexual selection is that some females select according to “sensory exploitation” which means that females most often find exaggerated male traits attractive. For example, the larger the combs in the male goose, the more likely it is to be chosen as a mate by the females. Another example of this mechanism can be seen in the long-tailed widowbird of Africa, in which the females choose the dominant males according to their exaggerated tails. These long-tailed males are also more likely to be caught by the predators because of the long feathers. So this hypothesis was further expanded on by the handicap hypothesis, which stated that these females judge the superiority of the male according to its ability to survive despite these exaggerated traits that could potentially put it in harm’s way. 

Sexual and Natural Selection
Science today believes that this sexual selection is part of natural selection, because like natural selection, it encourages the passage of genetically dominant traits to the offspring. In other words, female sexual selection is directly related to the survival of the fittest. However, here lies the problem. This statement in detail means that female sexual selection is a selection of the males that are quick to adapt, as these are the ones that are the fittest and the more genetically favored to survive according to natural selection. So are these females actually choosing the ones that are quick to adapt as science states? Maybe we can test this theory by observing nature.

Eastern Bluebird
Let us look at the sexual behavior of the eastern bluebird, typically know for its monogamous relationships. A typical male fathers a certain percentage of chicks that are not related to it, either because it is unaware of female’s adulterous behavior or believes that at least one of the chicks might be related to it. There is no way for the male to figure out its offspring among other chicks, and so it is trapped taking care of all the female’s chicks. In a similar fashion, the male might be the biological father of some of the neighboring chicks. In this particular case of the bluebirds, the female chooses the mate that she wants to procreate with, and the male has the responsibility of making the nest for all the chicks.

The male bluebird (left) with a relatively dull-colored female (right). © FNAL (Fermi Lab)

 Now, let’s look at it in greater detail. First, the female chooses a male that it believes will be able to father its chicks. Then it goes around having affairs with other males that are more sexually appealing to it. These female bluebirds sometimes copulate with males that never even build a nest anywhere or fathered chicks. The original mate innocently provides for chicks believing that at least one of them is related to it. 
In this case, it is clear that the monogamous relationship husband can provide for the family by building a nest and is chosen to father all the chicks in the nest, but the female still goes around copulating with other males that are not good with building nests or as a father. The actual partner was clearly the more efficient in adapting quickly because it managed to build a nest from scraps and was able to compete with the other males by being faster in providing a home, when it was necessary. Since adapting quickly is key in surviving, the actual partner is the more genetically fit with the best traits. The male partner was also chosen by the female to father its chicks, which shows how the male has other genetically valuable traits that the other female’s lovers lack. Nevertheless, the females still mate with the unsuccessful mates that lack the fathering abilities and did not built nests, i.e. the ones that were not quick to adapt.
This is not in line with what is believed in natural selection. The female chooses males that might seem stronger, colorful, etc, but these have little to do with survival in nature. Nature is unpredictable, and so the best trait for survival is the ability to adapt quickly. By the term “survival of the fittest”, it does not mean that the strongest is the fittest, or the more colorful, but the one that adapts quickly because that is the one fit for survival.
Therefore, sexual selection cannot really be a part of natural selection because the female’s selection makes way for unfit male genes in the gene pool. These males are unfit because they are not good fathers and they cannot build a nest. The female is seduced by the color, tail, strength, etc of the male, but this is not as valuable for natural selection, which is all about the ability to adapt. The actual partner that fathers the chicks and is quick to build the nest, is the one that is more fit but gets the short end of the stick in this particular case.

Human Beings
The above mentioned case of the bluebirds can be found to be practiced among human beings and other organisms in a similar manner. However, there is another example of how sexual selection does not match with natural selection in human beings.

A human pedigree chart. © Prof Atsma 2005

            With human beings, an intelligent and educated family on average decides to limit itself to two or three kids. The educated family is more learned about available contraceptives and is more knowledgeable about the responsibility required in providing for a large family. On the other hand, an uneducated couple usually builds a much larger family. Either they are unaware about contraceptives or they do not think through about the possible consequences that arise from a larger family. In short, the educated woman gives birth to two or three kids, and the uneducated woman gives birth to much more. This shows how particular person’s sexual choices might have nothing to do with natural selection. The selection that occurs from one’s sexual choices in life is a limit that one puts on oneself and has almost no observable relation to promoting the fittest, which on the other hand is known to be the ultimate purpose of natural selection. Therefore in other words, natural selection and sexual selection are two unrelated systems of selection.

Related Supporting Articles:
"Women on "the Pill" Choose Better Dads as Mates, Study Finds." World Science - Science News. Web. 13 Oct. 2011. DOI: 10.1098/rspb.2011.1627


  1. Interesting. It got me thinking of survival of the fittest in society. Take Haiti for example. In nature, (weird example I know) Haiti would have been washed under and no resue attempts would have been made, and they would have died out as a society.

  2. how peculiar...this is much more intriguing than biology to me

  3. i liked the part of sexual and natural selection

  4. This is a lot more informative then some of my boring lecture classes, good work.

  5. Very interesting and informative. Great post!

  6. interesting stuff, cant wait for more

  7. interesting post, want to see more, keep it up!

  8. As someone mentioned earlier, this is alot more info then 10 hours of classes!

  9. If all that is true than I'm fucked... or not fucked... you get the picture. I'm not getting laid. :(

  10. that stuff look kinda AWESOME!!!