What is Sexual Selection?
Sexual selection is a form of evolution. It allows animals to develop characteristics that increase their chances of reproduction comparative to others, whilst having a neutral or negative effect on survival. There are two kinds of sexually selected characteristics: primary and secondary.Primary sexual characteristics are those directly involved in the mating process (e.g. the reproductive organs that produce sperm and eggs). Secondary sexual characteristics are features of an animal that are indirectly involved in the mating process, but can help gain the interest of a potential mate or fight off rivals.
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| Male elephant seal fight for mating rights to a harem of females. These fights are costly, with both males risking injury and even death to mate. Their huge size and larger teeth make them sexully dimorphic from the females Image credit: Floris van Breugel |
The female is usually the parent who invests the most. This may be because she has to produce many eggs; go through gestation; builds a nest, den or other safe place for young; lactate or simply provide food in order for her young to survive. As a consequence females are often 'choosy' about who they mate with as in some species they receive nothing but genes from the male. As the male has few costs to bear in rearing offspring, males tend to develop secondary sexual traits which many models suggest bear costs of their own.
Sexual dimorphism between the sexes is often high. In some species males may look completely different to females, with larger bodies, weapons, or bright colours; whilst females remain cryptically coloured and unadorned.
Sometimes though sexual selection can get carried away with exaggerating a trait well beyond its optimum for survival chances. This is where natural selection limits the extent to which sexual selection can exaggerate a trait. For example: the peacocks tail, whilst large, can only grow to a certain size before the cost of having such a large tail becomes greater than the benefit of attracting females.
aka. Fischer's model of runaway selection. This model states that arbitrary differences in male expression of a particular trait (let's say tail feather length) are preferred to varying levels by females. As females choose the male (perhaps with the longest tail feathers) that male trait is passed on to their offspring, along with the females preference for that trait. Greater expression leads to greater preferences and so on, until the trait becomes so exaggerated that it is no longer viable and natural selection limits it. The trait of the male and the preference of the female can become genetically linked.
Good genes model:
As the name states, this model assumes that male traits are reliable signals of male genetic quality (i.e. healthier males can produce better ornaments). Females that choose to mate with males that have these traits pass these 'good genes' on to their offspring. For traits to remain as reliable signals this model assumes that there are costs to producing or maintaining the trait, that unfit males will not be able to cope with (and so won't produce extravagant traits).
Sensory bias model:
Monogamy:-
Monogamy means that one male and one female mate together exclusively. Whilst the only true form of monogamy is a pair mating for life; seasonal monogamy is also present in some species where a pair may mate over one or more breeding seasons. Although monogamy is most evident in bird species it is present in some mammals, rodents (prairie voles) and even fish! (French Angel fish).
Monogamy is thought to have arisen in situations where having both parents present vastly improves the chance of offspring survival. Studies have shown that when food availability is increased, male birds are more likely to desert their mate because the female is able to adequately feed offspring by herself. This is known as faculative monogamy.
Polygamy:-
There are 3 types of polygamy:
Promiscuity:-
Individuals of either sex will mate with any member of the opposite sex, such as in Bonobos.
Sneaky Males Cartoon - It's not all about survival of the fittestModels of Sexual Selection
Run-away model:aka. Fischer's model of runaway selection. This model states that arbitrary differences in male expression of a particular trait (let's say tail feather length) are preferred to varying levels by females. As females choose the male (perhaps with the longest tail feathers) that male trait is passed on to their offspring, along with the females preference for that trait. Greater expression leads to greater preferences and so on, until the trait becomes so exaggerated that it is no longer viable and natural selection limits it. The trait of the male and the preference of the female can become genetically linked.
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| Run-away selection in process: peacocks are well renowned for their extravagant plumage, a trait presumably preferred by the females. A male with a large, brightly coloured tail may also demonstrate good genes (he may have fewer parasites and be good at escaping predators. Image credit: Roger Clark |
As the name states, this model assumes that male traits are reliable signals of male genetic quality (i.e. healthier males can produce better ornaments). Females that choose to mate with males that have these traits pass these 'good genes' on to their offspring. For traits to remain as reliable signals this model assumes that there are costs to producing or maintaining the trait, that unfit males will not be able to cope with (and so won't produce extravagant traits).
Sensory bias model:
This model states that females can gain direct fitness benefits from her evolved preference for a male trait (e.g. she will be less likely to mate with diseased males). Under the sensory bias model the female prefers the male traits because it exploits pre-existing biases in her sensory system (for foraging or predator avoidance) that are under direct selection.
Mating Systems
All species of animals have specific constructs for mating. The type of mating system present in a species can dictate the presence or absence of sexually selected traits and is dependent on how they can best maximise their reproductive fitnessMonogamy:-
Monogamy is thought to have arisen in situations where having both parents present vastly improves the chance of offspring survival. Studies have shown that when food availability is increased, male birds are more likely to desert their mate because the female is able to adequately feed offspring by herself. This is known as faculative monogamy.
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| Emperor penguins remain monogamous throughout a single breeding season. Without both parents providing care, the chick would not survive Image credit: Claus Possberg |
There are 3 types of polygamy:
- Polygyny - 1 male mates with 2 or more females
- Polyandry - 1 female mates with 2 or more males
- Polygynandry - 2 or more male mate with 2 or more females
Promiscuity:-
Individuals of either sex will mate with any member of the opposite sex, such as in Bonobos.
Find Out More:
Birds of Paradise Animation - Fun National Geographic animation explaining sexual selection in birds of paradise
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