Posts Tagged ‘ hamilton ’

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.

Introduction to Kin Selection

Introduction

Some organisms tend to exhibit strategies that favour the reproductive success of their relatives, even at a cost to their own survival and/or reproduction. The classic example is a eusocial (highly social) insect colony, with sterile females acting as workers to assist their mother in the production of additional offspring. Many evolutionary biologists explain this by the theory of kin selection. Natural selection should eliminate such behaviours; however, there are many cases, such as alarm calling in squirrels, helpers at the nest in scrub jays, and sterile worker castes in honey bees, in which these animals cooperate despite an obvious disadvantage to the donor.

This sacrifice of individual success for the aid of other individuals is known as altruism.

There are thought to be four possible ‘routes’ to altruism – why it might arise, these are:

  • Kin selection – Keeping altruism in the family, possibly shared in the genes. Altruism within a family helps it to proliferate well.
  • Reciprocal altruism – ‘One good turn, deserves another.’ Altruism expressed by an individual is at some point returned. E.g. social grooming in primates, the individual doing the grooming is eventually groomed back.
  • Selfish mutualism – ‘What’s in it for me?’ Altruism which is expressed only because an individual also gains from it. E.g. feeding in house sparrows, they will call for help to break up large pieces of food which they are unable to carry alone thus losing some of the resource but gained more than they would have alone.
  • Group selection – ‘For the good of the group.’ Groups within a population – not necessarily family – which benefit by co-operation.

Kin Selection

John Maynard Smith described Kin Selection in 1964 as “…The evolution of characteristics which favour the survival of close relatives of the affected individual, by processes which do not require any discontinuities in the population breeding structure.”

It goes on the idea that because similar genes are more prevalent within a family (either by kind [species] or by descent [ancestral]), any altruistic genes expressed within the family are more likely to become more prevalent within the entire species.

Kin selection refers to changes in gene frequency across generations that are driven at least in part by interactions between related individuals. Under natural selection, a gene encoding a trait that enhances the fitness of each individual carrying it should increase in frequency within the population; and conversely, a gene that lowers the individual fitness of its carriers should be eliminated. However, a gene that prompts behaviour which enhances the fitness of relatives but lowers that of the individual displaying the behaviour (altruistic genes), may nonetheless increase in frequency, because relatives often carry the same gene; this is the fundamental principle behind the theory of kin selection. According to the theory, the enhanced fitness of relatives can at times more than compensate for the fitness loss incurred by the individuals displaying the behaviour.

Hamilton’s Rule

Whether or not altruism is favoured within a family or species depends on whether or not Hamilton’s rule is met:

Hamilton’s Rule: rB-C>0 or rearranged rB>C

Altruism is favoured when rB>C

  • C – The cost of displaying altruism, any disadvantages to the individuals.
  • B – Benefit to the individual(s) who receive aid.
  • r – The coefficient of relatedness. The probability that 2 individuals contain a gene identical by descent at the same locus. It has a value of 0-1.

Possible r values:

Relationship Coefficient of Relatedness
Identical Twins 1.0
Parent to an offspring 0.5
Siblings 0.5
Half Siblings 0.25
Unrelated individuals 0.0

Hamilton’s rule therefore predicts that we expect closer related individuals to express greater amounts of altruism. For example:

Would a mother warn her child of a predator, thus exposing herself as a target? If doing so has an arbitrary value of 2, but the benefit of saving the child of 5 then using Hamilton’s Rule the following must be true:

r(0.5) x B(5) – C(2) > 0

0.5 x 5 – 2 = 0.5

Thus as the value is greater than zero, altruism in this situation is favoured, would the same be true between half siblings? (r=0.25)

r(0.25) x B(5) – C(2) > 0

0.25 x 5 – 2 = -0.75

Because the result of Hamilton’s rule is less than 0, altruism in the same situation but a half-sibling attempting to warn a half-sibling, is not favoured.