Sexual selection: it is part of natural selection. Sexual selection acts on an organism's ability to obtain or successfully couple with a mate. Selection makes many organisms go to extreme lengths in order to find a mate such as: peacocks need to develop a beautiful tail, elephant seals fight over territories…. Sexual selection is often powerful enough to produce features that are harmful to the individual’s survival. For example, extravagant and colorful tail feathers or fins are likely to attract predators as well as interested members of the opposite sex.
In nature females always invest more energy into producing offspring than males invest, and as a result in most species females are a limiting resource that male have to compete for. In most cases it is the females who have a relatively larger investment in producing each offspring and in contrast sperm are cheaper than eggs. There is such a large production of sperm that a single male can easily fertilize all of female's eggs.
So clearly female will not produce more offspring by mating with more than one male since her eggs are the limiting factor where in contrast a male is capable of fathering more offspring if he mates with several females. Generally, a male's potential reproductive success is limited by the number of females he mates with, whereas a female's potential reproductive success is limited by how many eggs she can produce. This results in sexual selection, in which males compete with each other, and females become choosy in which males to mate with.
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As a result of being heterogamy, males are fundamentally less likely to stick to one female, and females are fundamentally selective when it comes down to selecting a mate. Sexual selection takes two major forms one is called intersexual selection and one is called intrasexual selection. intersexal selection is simply males competing with each other to be chosen by females. Intrasexual selection means that members of the less limited sex which are always males compete aggressively among themselves for access to the limiting sex the female.
The limiting sex is the sex which has the higher parental investment, which therefore faces the most pressure to make a good mate decision. In his essay Emlen comes up with few hypotheses as to why or how natural selection may have shaped patterns of horn evolution in such a way as to generate divergence in horn morphology, change from one type to another. First we need to understand the benefit of having a particular horn type and how they’re used. One thing he presents is that eetles live underground, in the same cylindrical tunnel no matter if it’s in the hot desserts or tropical area, but male have to keep the tunnels defended against other males trying to invade it therefore horns have defensive and weaponry use. Next important idea that is brought up in his essay is the cost of having certain horns. At this point we don’t care about the benefit or function of the horns, the only thing matters is its trade-offs. The main reason for that is the growth and size of beetle horn can stunt the relative size of other morphological structures such as antenna, wings, and eyes.
The first identified trade off was that males who possessed long horns also had small eyes. Based on his research he found a correlation between where the horn is located and the adjacent structure to it. After breeding beetles for multiple generations he realized that horns that grow on the center or front of the head affect the size of the antenna, horns that grow on the thorax affect the size of the wing, and finally horns that grow on the base of the head affect the eyes.
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