2 the e volution of sex - university of british columbiabio336/bio336/lectures04/lec30_2.pdf · 5...

1

Recombination is almost always expected to decrease fitness

among offspring

Unless the allele combinations built up by selection in one

generation are detrimental in the next.

The evolution of sexRapidly changing environments

2

If the direction of selection changes periodically and rapidly, recombination might increase fitness by decreasing the frequency of previously selected

genotypes.

This only works if the combinations of alleles that are beneficial change over the period of 2–5 generations.

For example: cold & wet turns to cold & dry turns to hot & dry turns to hot & wet turns to cold & wet…

The evolution of sexRapidly changing environments

3

Parasites evolve to infect the most common host genotype, which then decreases in fitness and drops in frequency, causing another host genotype to become

most common…

The evolution of sexRapidly changing environments – the Red Queen

Generation

Gen

oty

pe

freq

uen

cy

Host

Parasite

4

This has been called a Red Queen process.

“Now here, you see, it takes all the running you can do to keep in the same place.” – The Red Queen to Alice in Through the

Looking Glass, by Lewis Carroll (1872)


Generation

Gen

oty

pe

freq

uen

cy

Host

Parasite

5

Recombination allows hosts with common genotypes to produce offspring with rare combinations of alleles.


Generation

Gen

oty

pe

freq

uen

cy

Host

Parasite

6

Some models of sex propose that recombination increases fitness variation, and speeds up the response

to selection. This requires:

• An association between beneficial & deleterious alleles, which recombination breaks down.

• Associations may arise via drift (or other stochastic processes) or selection.

• A reliable source of directional selection over long periods of time.

• Changing environments or recurrent mutation.

The evolution of sexSummary of models

7

Other models suggest that recombination directly increases the fitness of offspring. This requires:

• A rapidly changing environment, such that different allele associations are selected every 2–5 generations.

• The only likely candidate for such change is host-parasite coevolution.


8


Recombination increases genetic varianceLinkage disequilibria form by:

Directional selection comes from:

Selection Stochastic processes

Changing environments ? ?

Mutation-selection balance ? ?

Recombination increases offspring fitness due to host-parasite coevolution

?

9

Most lab evidence suggests that deleterious mutations don’t exhibit negative epistasis.

• Selection is unlikely to cause associations between beneficial & deleterious alleles.

But we don’t know for sure about beneficial mutations.

The evolution of sexSo, which model is supported by data?

10

The evolution of sexSo, which model is supported by the data?




Changing environments ?? ?

Mutation-selection balance X ?


?

11

Lab experiments show that recombination can speed up the response to selection when associations

between alleles are allowed to form stochastically.

But we don’t know whether directional selection (changing environments) is common enough in the

wild to make this an important effect.

Also, we don’t know whether this would work under mutation-selection balance.


12






Mutation-selection balance X ??


?

13

Field studies show a correlation between parasite loads and sex in several species.

In one case, genotypes have been shown to fluctuate as predicted by the Red Queen model.

But we don’t know the genetic details of infection, and parasites can’t be expected to be equally important in

all species.


14






Mutation-selection balance X ??


?

15

We still don’t have enough data to know which model (or models) explains sex.

It’s entirely possible that different processes are important in different populations, or that more than

one process might act at a time.


16

Individuals rarely mate at random for a number of reasons:

• Dispersal may be limited

• Individuals may or may not be able to self

• Individuals may reproduce asexually

• Individuals may compete for mates

• Individuals may choose particular mates

Sexual selectionIntroduction

17

Non-random mating has a number of important evolutionary consequences.

In this lecture, we will focus on the evolution of mate choice and sexual selection.

In particular, we will concentrate on how these processes may explain differences between the sexes

(sexual dimorphism).


18

"Sexual selection depends on the success of certain individuals over others of the same sex, in relation to the propagation of the species; while natural selection depends on the success of both

sexes, at all ages, in relation to the general conditions of life.

The sexual struggle is of two kinds: in the one it is between the individuals of the same sex … in order to drive away or kill their rivals …; while in the other, the struggle is likewise between the individuals of the same sex, in order to excite or charm those of

the opposite sex, … which … select the more agreeable partners."

Darwin (1871) – The Descent of Man and Selection in Relation to Sex


19

Following Darwin, two main forms of sexual selection are recognized:

• Intrasexual selection (competition within a sex for the opportunity to mate)

• Intersexual selection (choosiness on the part of one sex for mates)

Sexual selectionClassification

20

Although female-female competition and male choosiness are known, we'll focus on the more common patterns of male-male competition and female choice.

Why might these be the more common patterns?

Sexual selectionClassification

21

Horns, antlers, tusks, spurs and other weapons provide some of the most extreme

examples of sexual dimorphism.

Sexual selectionIntrasexual selection (male-male competition)

22

Possible explanations:

• Weapons against predators

• Weapons against other males

• Indicators of male strength and fighting ability (male-male competition)

• Indicators of sexual vigor and quality (female choice)

Sexual selectionIntrasexual selection (male-male competition)

23

Barrette and Vandal (1990) studied sparring in caribou. Of 713 matches between males of different antler size, males with smaller antlers withdrew 90%

of the time.

Sexual selectionIntrasexual selection – Examples

24

Eberhard (1979,1980) studied the use of horns in seventeen species of beetles, finding that they tend to be used either

to pry a rival off his site or to lift and drop the rival to the

ground.


25

Brown and Bartalon (1985) placed forked fungus beetles

together in a terrarium. Males with the longest horns had higher access to females. Female preference was not

observed.


26

"Among the explanations for sexually dimorphic horns, antlers, tusks, and spurs, the empirical support is

strongest for the idea that they have evolved and are favored in males as weapons in contests over females."

Andersson (1994) Sexual Selection (p. 314)

Sexual selectionIntrasexual selection

27

Some of the more profoundly beautiful traits seen in nature have evolved in response to female choice.

Sexual selectionIntersexual selection

28

But why should female preferences exist in the first place?


29

We’ll discuss three theories for the existence of female preferences:

• Female choice and male traits co-evolve (Fisher's Runaway Process).

• Choosy females gain direct benefits from their mates.

• Females are choosy because of a sensory bias (Historical constraint)


30

Phase 1: Female preferences initially evolve because they favor a trait in males that is also favored by natural selection. The offspring of choosy females are then

more likely to carry the advantageous trait.

Sexual selectionIntersexual selection – Fisher’s runaway process

Red malessurvivebetter

Females preferringred males produce

more red sons– and daughters

that prefer red males

Daughters withpreference

Red sons

31

Phase 2: Once female preferences exist, they can favor even more extreme traits in males. This can in turn favor the evolution of stronger female preferences,

leading to a runaway process.







Red sons

Red malesget more

mates

Increase in frequency

32

The runaway process will halt when genetic variation is exhausted or when the trait becomes so costly that

natural selection balances sexual selection.







Red sons

Red malesget more

mates

Increase in frequency

33

In a breeding experiment with the threespine

stickleback, Bakker (1993) observed a genetic

correlation between red coloration among sons and

preferences for red coloration among daughters,

as expected under the Fisherian process.


34

The Fisherian model of sexual selection is ineffective in the face of costs to female choosiness, such as:

• Time and energy in evaluating mates

• Risk of remaining unmated

Example: Engelhardt et al (1982) found that choosy female seaweed flies had reduced fertility.

Sexual selectionIntersexual selection – direct benefits

35

Occasionally, however, female preferences may be directly beneficial, such that natural selection favors the

evolution of preferences, which in turn lead to the elaboration of the preferred trait in males.


36

Possible direct benefits of being choosy:

• Lower risk of mating with the wrong species (species recognition)

• Mate may provide paternal care

• Mate may provide food (e.g., a nuptial package)

• Mate may be more fecund

• Mate may be less likely to pass diseases/parasites to female

• Offspring may be more fit (Good genes hypothesis)


2 the e volution of sex - university of british columbiabio336/bio336/lectures04/lec30_2.pdf · 5...

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