Sex adds a whole other level of complexity to the lives of organisms. On top of the struggle to survive fueling natural selection, they must also compete to find a mate and reproduce. In this episode, we discuss one of the more bizarre mechanisms driving evolution: Sexual Selection.
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Sexual selection is the selective pressure on an organism to reproduce successfully, in contrast to natural selection, which focuses on an organism’s survival. These two evolutionary mechanisms can work together but also against each other.
Darwin first described sexual selection in On the Origin of Species. He recognized two main types of sexual selection:
Intrasexual selection occurs when members of the same sex compete for access to mating rights, such as through competitive fights and displays.
Intersexual selection occurs when one sex controls mating rights for the opposite sex, usually involving elaborate displays by one sex trying to impress their potential mates.
These can lead to some truly bizarre behaviors
One of the results of intense sexual selection is that males and females can often end up evolving different body types or behaviors. This is know as sexual dimorphism.
Sexual dimorphism is often strongly correlated with parental care. In species where one sex provides most of the care for eggs or babies, sexual dimorphism tends to be more extreme, with one more drab sex dedicated to caring for young, and the other sex highly decorated for attracting mates (most birds are like this). In species where both sexes care for the young together, they tend to have less extreme dimorphism, or hardly any at all (albatrosses are a good example).
But strict sexual selection is not the only cause for sexual dimorphism. In many animals, females are larger simply because they have to carry the eggs or babies. Other cases can arise for environmental reasons or to avoid cannibalism.
And it gets even more complicated! In some animals, males come in two forms: the classic big competitive males and “sneaky males” who resemble females to avoid competing with the other males. Sneaky males are famous in elephant seals and cuttlefish.
Exaggerated features in the fossil record often bring sexual selection to mind, since such traits often evolve for display or competition – think moose antlers, peacock feathers, etc.
But fossils can be tricky. It can be difficult to tell if an exaggerated feature in an ancient species is sexually-related or if it evolved for another reason. Paleontologists generally look for a few traits that sexually-selected features tend to exhibit: their growth rate changes during ontogeny; the show positive allometric growth; they’re generally costly to maintain without a survival purpose; and they’re often sexually dimorphic.
None of these criteria are guaranteed to identify sexual selection in an organism but they are good sources of support. Using these techniques, sexually selected traits have been identified in certain extinct animals such as Protoceratops and some pterosaurs.
More Sexual Selection
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