• Biology Evolution Sexual Selection
  • Sexual Selection

    A Form of Natural Selection

    Have you ever wondered why a peacock’s feathers are so big and gaudy? Or why a Lyrebird’s song is so complex? Maybe you’ve simply been outraged by sexual dimorphism in general. Well the answer to your questions and outrage is Sexual Selection, a more specific form of Natural Selection.

    Natural Selection is a process in which favorable traits are maintained in a population, and subsequently passed down through generations, while unfavorable traits are eliminated from a population over time. Organisms that exhibit traits that help them to survive, live longer and are able to pass those good genes to the next generation. Organisms that exhibit traits that are to their detriment generally do not live as long and do not pass on their negative genes.

    A Competitive Nature

    All organisms are in a never-ending competition for survival and reproduction. Often times, the competition is based on exploiting food resources in a given habitat. For example, a squirrel’s memory helps him to remember where he hid his nut, and a deer’s strong, flat teeth allow her to munch continuously on vegetation. The squirrel is in competition with other squirrels for access to nuts, and the deer is in competition with other deer for access to veggies. This is called intraspecific competition, where one species competes with other members of their own species for resources. If the squirrel and the deer directly competed with one another, it would be called interspecific competition.

    Sexual Selection

    The competition for limited resources is also called “The Survival of the Fittest”. “The Fittest” do not always have to be those organisms that could gather the most resources, though. Remember, characteristics are preserved in a population by passing them down through the generations. If an organism is unable to reproduce, it doesn’t matter how well they can gather food resources, their traits are not going to stay in the gene pool for long.

    Sometimes, favorable traits are those characteristics that help the organism attract attention from the opposite sex, thus ensuring their genes stay in the pool. Sexual selection is a form of intraspecific competition that favors traits for reproduction and raising offspring. The most drastic examples occur in polygamous species, those species that have more than one mate in their life. Characteristics that have been sexually selected for in a population generally come in two different flavors: Weapons and Ornaments.

    Weapons and Ornaments

    Weapons are traits that individual males possess to help them defend their territory against rival males. For example, male deer have large antlers on top of their head to both mark their territory and to fight rival males. Those individuals that have the largest rack will generally keep their territory and have their choice of female mates. If he can keep his territory defended, the genes that code for large antlers will be preserved and his offspring will hopefully be as reproductively successful.

    Weapons can sometimes get a little out of hand. The Irish Elk is example of an animal that was sexually selected into extinction. Their antlers were size-selected for so long that the rack got too big for the body and they were unable to support themselves. Many paleontologists believe this contributed to their extinction. How like a man… We just keep pushing it don’t we?

    Ornaments are features on an organism’s body that stand out (Juicy Couture) and help potential mates decide if the suitor is worthy of mating with. Ornaments can be a number of different decorations or behaviors. Bright colors, like those on the face of a male Mandrill, can often give insight into how healthy that particular male it. Birds of Paradise often have long colorful feathers, like Peacocks, to impress and attract females.

    Some behaviors can be thought of as “ornamental”, like the song of a lyrebird, the agility of a swallow, or the beautiful dancing grebe. Mammals may not have the most obviously sexually selected traits, but remember that vision is not your only sense. Mammals are very sensitive to smells and many of the traits that we select for are based on the release of pheremones. Think about that the next time your attention is diverted towards someone with a sweet smelling perfume, or cologne, they may just be trying to cheat selection.

    Written by Rob Nelson

    Rob is an ecologist from the University of Hawaii. He is also an award winning filmmaker. As principle director of the Untamed Science productions his goal is to create videos and content that are both entertaining and educational. When he's not making science content, he races slalom kayaks and skydives.

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