Topic 2 - The Evolution Of Sex

Question 1

What is pathogenesis?

Answer 1

Also known as ‘virgin birth’; the development of offspring from an unfertilised egg.

Question 2

Explain the costs of sexual reproduction.

Answer 2

1) two-fold cost of sex, only half the individuals are female and able to produce offspring.
2) sexually reproducing individuals pass on only half their alleles.
3) potentially unfavourable allele recombination can result from poor mate selection.

Question 3

What are the benefits of sexual reproduction?

Answer 3

sex is maintained because sexual populations can purge themselves of harmful mutations & it provides sexual organisms with genetic variability. Sexually reproducing organisms exchange genes, thereby producing offspring with greater genetic variation than asexual organisms.

Question 4

What is meant by genetic load?

Answer 4

The burden imposed by the accumulation of deleterious mutations. The genetic load equals the chance that an average individual will die before reproducing because of the deleterious mutations it possesses.

Question 5

What is a possible consequence of high genetic load in a population?

Answer 5

Genetic load could become so great that the population fails to survive and reproduce, and goes extinct.

Question 6

Explain the red queen hypothesis.

Answer 6

Under the Red Queen hypothesis, interactions between organisms, such as between parasites or pathogens and their hosts, generate ever-changing environmental conditions that favour the maintenance of sexual reproduction relative to asexual reproduction or self-fertilisation. ‘It takes all the running you can do, to keep in the same place’.

Question 7

Define coevolution.

Answer 7

The process of reciprocal evolutionary change in two or more interacting species (genes, sexes) by natural selection. Each party in a coevolutionary relationship exerts selective pressures on the other so that they affect each other’s evolution.

Question 8

What is anisogamy?

Answer 8

Having gametes of different sizes. Individuals producing the larger gamete (egg) are defined as female, individuals producing the smaller gamete (sperm) as male.

Question 9

Explain fecundity.

Answer 9

The reproductive rate of an organism measured in the number of eggs, seeds or propagules (in the case of asexually reproducing organisms) that it can produce.

Question 10

How might a female improve the survival prospects of her offspring?

Answer 10

She could care for them by providing food, warmth and protection.

Question 11

What is meant by potential reproductive rate?

Answer 11

The offspring production per unit time that each sex would achieve if unconstrained by mate availability. Because females generally produce large eggs and care for offspring, their potential reproductive rate is lower than that for males.

Question 12

Explain operational sex ratio (OSR).

Answer 12

The ratio of sexually active males (those ready to mate) to sexually fertilisable females (those ready to mate). Differs from the sex ratio which takes into account inactive or non-competitive individuals.

Question 13

How does the definition of sexual selection differ from that of natural selection?

Answer 13

Natural selection favours traits that give individuals an advantage over others with respect to survival and hence reproduction. For example, an animal that runs faster than others in the population will be favoured by natural selection if, by running faster, it escapes predation and survives to reproduce. Sexual selection, however, favours only those traits that afford an individual a mating advantage over other individuals in the population. Such traits may or may not be advantageous in terms of survival.

Question 14

How is fitness defined?

Answer 14

Fitness is a measure of the relative success of an individual in surviving and reproducing and therefore contributing offspring to future generations.

Question 15

Why does reproductive success generally vary more among males than females?

Answer 15

It varies more among males than females because of the limited number of female gametes available for males to fertilise (a consequence of the differences in the size of male and female gametes). This means that males must compete with each other for access to eggs by mating with as many females as possible. Some males will be better than others at finding and fertilising females, so variation in reproductive success among males will be high. Because of the competition among males for females, it is likely that all females will mate. Variation in reproductive success among females will therefore be low.

Question 16

Explain intrasexual selection.

Answer 16

Members of the same sex (usually males) within a species compete with one another for access to members of the opposite sex. Usually results in the evolution of physical or behavioural features that determine the outcome of aggressive encounters, e.g. weapons, large body size.

Question 17

Explain intersexual selection (female mate choice)

Answer 17

males compete among themselves to be chosen by females. Females are selective about the male they choose to mate with.

Question 18

What benefit or benefits could females gain by mating with more than one male?

Answer 18

If a single male transfers insufficient sperm to fertilise all her eggs, it would benefit a female to mate with more than one male and thereby increase fecundity. Females may also benefit indirectly by producing genetically variable offspring. Furthermore, if females are able to fertilise their eggs selectively using sperm of preferred males, they may provide better genes for their offspring.

Question 19

What are the indirect benefits of choice in female mate selection?

Answer 19

Factors that increase the genetic fitness of offspring. Indirect benefits include high quality genes for offspring (adaptive indirect benefits) or genes that make offspring more attractive (arbitrary indirect benefits).

Question 20

Explain Fisher’s runaway theory of sexual selection.

Answer 20

A mechanism of sexual selection proposed by R.A. Fisher to explain evolution of exaggerated male ornamentation. According to Fisher’s theory, the initial female preference for the male trait is an arbitrary preference; there is no necessity for there to be a relationship between the trait expression and the male’s inherent genetic quality.

Question 21

Explain the ‘good genes’ hypothesis.

Answer 21

‘Good genes’ hypotheses of female choice hold that females gain indirect benefits from their choice through the superior genetic quality of offspring. According to these hypotheses, female preferences for a trait are not arbitrary. Instead, they choose males with traits that are honest, reliable signals of underlying male genetic quality.

Question 22

Explain sexual conflict in terms of sexual selection.

Answer 22

Conflict that arises between the sexes because strategies that maximise the fitness in one sex can reduce the fitness of the other sex. Can lead to antagonistic coevolution and sexual ‘arms races’.

Question 23

What is antagonistic coevolution in the context of sexual selection?

Answer 23

In a sexual context, antagonistic coevolution is driven by conflict between the sexes in maximising reproductive success. Adaptations that benefit males at the females’ expense select for counter-adaptations in females that mitigate male-imposed harm. Often referred to as an arms race between the sexes. Antagonistic coevolution can also occur between predators and prey, between parasites and their hosts and between parents and offspring.

Question 24

What effect would male parental care have on potential reproductive rate of males and females?

Answer 24

Because males care for the young, their potential reproductive rate decreases. Females, on the other hand, are free to mate again as soon as they are physiologically able to do so.

Question 25

What is cryptic female choice?

Answer 25

Female behaviour, physiology or morphology that selectively biases paternity in favour of some conspecific males and against others after copulation has already begun. The female’s choice is cryptic in that she may accept a male for copulation but reject him as a father.