Posts Tagged ‘ selection ’

Intrasexual Selection

Introduction

Intrasexual selection is when members of the same sex (within a species) compete with each other in order to gain opportunities to mate with others, e.g. the male against male competition for females. Because intrasexual selection often involves fighting, species or individuals well adapt for intrasexual selection will have developed better armourments (weapons) than their competition.

On the other hand, there is intersexual selection. Often known as female choice, it is the process where the female choses the male based on certain ornaments e.g. a peacock’s tail. The ornament is not usually beneficial to the male (e.g. bright colours make it an attractive target for predators) but the female prefers the larger ornaments as it signals the male’s is able  to cope with the hindrance – and therefore a better genetic make-up which will be passed on to her offspring. The reason the females choose is to prevent wasting invested time and energy on offspring which are of poor genetic merit.

Competition

There are two main types of competition over females, scramble and contest competition.

  • Scramble: Typically whoever gets to the female first. An example with dung flies; brightly coloured male dung flies are attracted to a dung pat. Shortly after females will arrive at the dung pat. These females are quickly grabbed by the males. Very shortly after female arrival rate decreases and the number of both males and females around the pat decreases

In a similar scenario, male damselflies also grab females as soon as they arrive. However male body size also contributes to reproductive success. Larger males live for longer and hence have more possible days or reproductive ability but smaller males have a higher daily mating rate as they are more agile and able to grab the females faster. It is therefore beneficial for reproduction to be of intermediate size.

  • Contest: Contest competition is a more typical form of competition where the male with the best fighting technique, largest body size or the largest weapons will win the female. Although not always guaranteed to win, they have a much higher chance than inferior males. This has however; inevitably lead to the production of larger male offspring by reproductive selection – as the larger males are more likely to reproduce and pass on their genes.
  • Alternative Mating Techniques: Smaller males would therefore seem to be at a disadvantage during contest competitions. Fortunately there are species where the smaller males have developed alternative mating tactics to ensure reproductive success. For example:
  • Red deer – Smaller males with small antlers are much less likely to win in a contest competition. Instead they wait near a female deer and when the large male intending to copulate with her engages in a contest with a competitor, the smaller deer sneaks in and copulates with the female.
  • Sunfish – Males defend their territory and wait for females to come and lay their eggs. When a female arrives at the nest she will lay her eggs as the male fertilises them. However subordinate males may quickly dart in-between the male and female. The subordinate male mimics the female as not to alarm the dominant male and both males deposit sperm, this gives the subordinate male a chance to fertilise the eggs of the female.
  • Coho Salmon – There are two forms of male Coho salmon, the larger males known as Hooknoses and the smaller males known as Jacks. Females and Hooknoses spend 3 years at sea before returning to reproduce; Jacks spend only 2 years, meaning a larger proportion return – a lower mortality rate. As is typical with other species, the larger males compete for females by fighting, whilst the smaller males sneak to mate with the females. When comparing Jacks to Hooknoses, both have the same level of reproductive fitness (resulting in a mixed evolutionarily stable strategy).

Sperm Competition

Once a male has mated with a female, it is still possible for the sperm of another male to fertilise the female. Some species have therefore developed methods to prevent this. The basic methods are pre/post copulation guarding. Prior to copulation the male will guard the female until she is sexually receptive and after copulation the male will guard the female until she has laid her eggs.

There is also the basic sperm competition, where the sperm ‘compete’ against the sperm of other males within the female reproductive tract. Two examples of more dedicated sperm competition are:

  • Scrapers – Males who compete by this method use bodily structures to remove the sperm of other males from the female reproductive tract
  • Mating Plugs – Males which use the mating plug method, copulate with a female and when they disengage a ‘plug’ is left within the female. This plug prevents further males from mating with the female.
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Intersexual Selection

Introduction

Intersexual selection, often known as female choice, is the process where the female choses the male based on certain ornaments e.g. a peacock’s tail. The ornament is not usually beneficial to the male (e.g. bright colours make it an attractive target for predators) but the female prefers the larger ornaments as it signals the male’s is able  to cope with the hindrance – and therefore a better genetic make-up which will be passed on to her offspring. The reason the females choose is to prevent wasting invested time and energy on offspring which are of poor genetic merit. A study which monitored female choice in peacocks, found that 19 out of 22 times the female mated with the male which had the largest tail.

Fisher’s Runaway Process

Fisher’s runaway process is a method which explains the reasoning for selection and development of male ornaments.

  • Males have a gene which determines the ornament trait e.g. tail length
  • Females have a gene which makes them find the male ornament appealing
  • Initially females will base their choice on what is best for their offspring. For example the utilitarian optimum is the optimum tail length for flight in birds; females will therefore select males who have a tail length closest to this.
  • Continuing with the example, the female will produce male offspring that have a longer tail length and closer to the optimum for flight.
  • The male offspring are more likely to be selected and reproduce, meaning even more offspring produced with longer tails.
  • Natural variation and selection for the largest tail length will eventually lead to males that have tail lengths exceeding the optimum, yet the females continue to prefer the males with the longer tails.
  • A trade-off is produced; a longer than optimum tail length leads to decreased flight ability but increased reproduction success.
  • The two traits eventually reach equilibrium, as males with too long a tail are unable to survive.

Zahavi’s Handicap Principle

Zahavi’s Handicap Principle says that although exaggerated ornaments are selected for by females, it does not actually benefit the female directly (however, you could argue that it helps to spread her genes because her offspring have the exaggerated feature, which in turn leads them to greater reproductive success). The exaggerated features are not beneficial for the male; they act as a disability e.g. the large tail that prevents flight, bright colours which alert predators or larger ornaments which make escape difficult.

The reason the female still chooses the male with the disabling trait, is because it shows that despite the disability, the male is still able to survive. This in turn must mean that the male has good genes, which is why the female choses him.

A distinction is often made between indicator genes, which indicate that beneficial genes will be passed to the offspring (‘good genes’) for example a colourful plumage, and genes which will simply make the male offspring sexually favoured when searching for a mate (‘pure Fisherian’).

The Hamilton & Zuk Hypothesis

This hypothesis states that female swill be more likely to choose healthy male, i.e. those with a resistance to parasites.

This is often seen in birds, a way to determine whether the male is healthy or not is to observe the colour of its plumage. The more colourful the male, the better its resistance against parasites and the more likely it is to be sexually selected by the female (as she will want to pass on those parasitic resistant genes to her offspring). This is because a bird burdened with a parasite will be unable to meet the metabolic rate required to produce a colourful plumage whilst trying to remove the parasite.

However, this is not apparent in all birds only those where parasitic burden is likely to occur. Birds where incidence of parasitism is low do not tend to display bright colours as there is no need for the female to base her mate choice on parasitic burden. If parasite presence is high amongst a species, then that species is more likely to display bright colours as a way to show they are not burdened with the parasite – thus increasing their reproductive success.

An example of this has been shown with sticklebacks. The males display a bright red colour on their stomach; females choose males with the brightest stomach. To prove this was the case, scientists bathed the experiment tank with green light to remove the red colouring from the stomach. The result was random female choice. Then some of the males were infected with a parasite, they proceeded to lose colour an when females were given the choice of which mate they chose the more colourful, non-infected males.

Intersexual Role Reversal

Intersexual selection is not always a female’s choice however. There are examples where males are the ones who invest more time in the upbringing of offspring. There are some species of bird where the females lay their eggs in many nests leaving the males to raise the offspring.

A more specific example is that of bush crickets. Bush crickets only feed on the pollen of one specific plant, early in the season this plant is numerous and pollen is high. However later in the season, the plant number decreases and therefore so does the pollen. The mating process of bush crickets sees the males transfer a sack of sperm during mating, when the female is ready to lay her eggs she is able to eat a protein rich sack from her own body to give her enough energy.

Early in the season when pollen is high males outnumber females so intersexual selection acts as normal. However later in the season when pollen begins to run low, the females are able to consume their protein rich sack to ensure they have enough energy. Males do not have a structure similar to this and so decline in number. This leads to females outnumbering males, and the occurrence of a sexual role reversal – therefore males are now the ones able to choose which females they mate with.