1. Sexual Selection

2. Sex and the Marine Environment

3. What is sexual selection?
Natural Selection:
Differential reproductive success due to variation among individuals in survival and reproduction.

4. What is sexual selection?
Darwin (1871) defined sexual selection as:
Differential reproductive success due to variation among individuals in success at getting mates.

5. Sexual selection is non-random variance in reproductive success.
Two forms of sexual selection:
Intrasexual selection: direct competition for mates between members of the same sex, usually male-male competition.
Intersexual selection: differences in attractiveness to the opposite sex, usually non-random mate choice by females.
RANDOM MATING piece of Hardy-Weinberg : hinges on both natural & sexual selection.
Some debate – are NS and SS really different things?
Intraspecies’ selection versus interspecies / interenvironmental

6. Mating systems Monogamy One male mates exclusively with one female
Polygamy Individuals mate with more than one partner
Polygyny Some males mate with more than one female
Polyandry Some females mate with more than one male
Promiscuity Males mate with more than 1 female and vice versa
Monogamy
Polygyny
Promiscuity

7. Mating systems Mating systems influence sexual selection
Strength of sexual selection
Variance in mating success
All males have
same mating
success = 1 mate
Some males = 0 mates
Some males = 1 mate
Some males = 2 mates
Most males = 0 mates
One male = 8 mates
Monogamy
Moderate Polygyny
Strong Polygyny

8. Harem polygyny
Elephant seals (Mirounga) - breeding females cluster together on beaches
- allows males to defend a harem of many females at once
Male elephant seals weigh up to 3x more than females!
Male reproductive success is highly variable:
8 individual males inseminated 348 females in one study!
Bull male elephant seals engage
in violent, bloody fights over
females – large size confers an
advantage in male combat

9. Polyandry When is polyandry favored? What is the consequence?
When males become limiting resource for reproduction
This often occurs when low offspring survival requires
male parental care, so that males have the greater
Reproductive effort and lower reproductive rate
What is the consequence?
Sexual selection is stronger on females than males
Saddleback Tamarins,
Spotted Sandpipers,
and Red-necked
Phalaropes are
examples of polyandrous
species

10. The form of Sexual Selection is directly related to the relative investment in offspring production.
Bateman’s principle: The sex that invests more in offspring production has fewer reproductive opportunities.
Thus, the sex that has a higher investment
Should be more discriminating (choosier).
Become a limiting resource for the opposite sex.
This is usually the female sex

11. ANISOGAMY
FEMALES: Sex that produces relatively few, large well-provisioned gametes (eggs)
Theoretical work – proposes disruptive selection on gamete size from ancestral isogamous condition.
Initial favoring of more, smaller gametes to increase fertilization chances.
Subsequent disruptive favoring of larger (but fewer) egg gametes to increase fertilization success (viability, nutrition, etc.)
In humans – sperm and eggs are smallest / largest cells in the body respectively.
MALES: Sex that produces many, “cheap” gametes (sperm)

12. Females are more invested
Females also often invest more than males in other forms of care
Females bare the burden of pregnancy
Care for offspring
Only in fish is male care more common than female care
Males guard the eggs in their territory so they can also secure multiple mates

13. Why don’t males care for offspring more often?
Females can sometimes mate multiply (polyandry)
Therefore, not all offspring may be sired by the male

14. Why don’t males care for offspring more often?
Females can sometimes mate multiply (polyandry)
Therefore, not all offspring may be sired by the male (paternity sharing)
Opportunity costs
A male’s mating success is more driven by how many pairings he can achieve
Especially because females are choosy

15. Why do females invest more in offspring care?
If females invest more prior to mating, there is competition amongst males for female investment
This causes variability amongst male success
As long as females continue to provide care, it is less likely males will contribute (a positive feedback loop)
Dominant males with great mating success are not inclined to care after mating

16. The form of Sexual Selection is directly related to the relative investment in offspring production.
Bateman’s principle: The sex that invests more in offspring production has fewer reproductive opportunities.
Thus, the sex that has a higher investment
Should be more discriminating (choosier).
Become a limiting resource for the opposite sex.
Evolution of gender and sex resources
This is usually the female sex

17. The asymmetric nature of sexual selection often leads to dramatic sexual dimorphism in characters directly related to male-male competition and/or female choice.
Male-Male competition – Intrasexual selection
Female choice – Intersexual selection.
The peacock example is an example of…?
Peacock
Peahen

18. Sexual dimorphism

19. Selection for combat in males
Natural selection selects for male morphologies that make them more competitive
Larger body size
Greater strength
Weaponry

20. Selection in Males: Body Size
Larger male copepods can carry larger spermatophores
Mating with large males allows females to fertilize more eggs and reduces the need for frequent matings

21. Weaponry
Fiddlers!
h4ales compete for the possession of burrows that are more vlslted
females, thus taking over burrows is an alternative strategy to burrow digging Male waving activity, mudball
distance and claw size can beconsidered as multiple visual signals available to females for mate choice. Females' first choice cn- tenon would seem to be
burrow quallty

22. Weaponry
Narwhals!
Biology traditionally considers the tusk a secondary sexual characteristic, similar to the mane of a lion or the tail feathers of a peacock. It may help determine social rank, maintain dominance hierarchies, or help young males develop skills necessary for performance in adult sexual roles. Narwhals have rarely been observed using their tusk for fighting, other aggressive behaviors, or for breaking sea ice in their Arctic habitat. The tusk is an innervated sensory organ with millions of patent nerve endings connecting the external ocean environment with the brain.[19][20][21][22] There is at least one recorded case of a tusk being used against another species. A broken tusk was found embedded in the melon of a beluga, suggesting a figh
Females sometimes possess tusks but it is rare

23. And in terrestrial birds
Some really awesome courtship behaviors
Marine birds

24. (Megaloceros giganteus)
Sexual selection can be very strong and often opposes natural selection.
This can lead to exaggerated and sometimes maladaptive development of male traits.
Irish Elk! Most recent carbon dating of fossils – extinct ~7700 years ago.
Lot of debate as to why extinct. Combination of human hunting, climate change.
Antlers – nutritive stress during unstable food periods wrought by climate change. Other deer w/ less ridiculous ornamentation seemed to do better.
Irish Elk
(Megaloceros giganteus)

25. Sexual vs natural selection
Guppies (Poecilia) - sexual selection can favor traits that reduce survival
- laboratory selection studies by John Endler
Natural selection (predation) favors
spot patterns that match background
Sexual selection (mate attraction) favors
male patterns that contrast background

26. Intra- and inter-sexual selection
Intrasexual selection
All of the examples thus far have involved interactions within a sex -
Darwin’s theory of intra-sexual selection was readily accepted, even in his time
Intersexual selection
Intersexual selection has always been controversial
We know mate choice occurs because we can observe
it directly
However, it is debated if and how some sexually
dimorphic traits evolve in response to mate choice

27. Are females really choosy?
Females can prefer elaborate ornaments
Long tailed widowbirds
Elaborate ornaments are often costly

28. Why are females choosy? Two broad benefits: Good resources Good genes
Competitive males often secure the best territories
Good genes
Body size and other competitive characteristics will be inherited by the females offspring
Humback whale size and mating calls
But not all mating systems involve competition for mate territories

29. But what are the genetic benefits?
1) Fisher’s hypothesis: gaining attractive sons
Females want sexy sons to attract more mates in the future
Often produces runaway sexual selection
Usually begins with a trait that confers some adaptive benefit and then is further exaggerated because it is selected for by females

30. Fisher’s runaway model
Fitness
Tail length
Total male fitness
(survival + mating)
Fitness due to survival
Survival Selection
Sexual Selection
Female choice
adaptive for survival

31. But what are the genetic benefits?
1) Fisher’s hypothesis: gaining attractive sons
Females want sexy sons to attract more mates in the future
Often produces runaway sexual selection
Usually begins with a trait that confers some adaptive benefit and then is further exaggerated because it is selected for by females
2) Handicap hypothesis

32. GOOD GENES MODEL The Handicap Principle (Zahavi 1975)
ELABORATED MALE TRAITS MAY BE INDICATORS OF HERITABLE GENETIC QUALITY (I.E. FITNESS).
The Handicap Principle (Zahavi 1975)
Some males may have a heritable trait that reduces viability.
Only males with “Good Genes” can survive despite the handicap.
Females that mate with these males will have offspring with higher fitness.
Female offspring particularly fit because they inherit good genes without the heritable handicap

33. GOOD GENES MODEL Hamilton and Zuk added to this hypothesis:
ELABORATED MALE TRAITS MAY BE INDICATORS OF HERITABLE GENETIC QUALITY (I.E. FITNESS).
Hamilton and Zuk added to this hypothesis:
Sexual displays are reliable indicators for genetic resistance to disease or parasites
Advertisements of healthy animals

34. Evidence of females choosing for sexy sons
Females prefer brighter red colors
And the intensity of red in sons is correlated with the red in his father
But is the only benefit of this trait increased matings?
Males with bright red color should not show increased viability, just increased mating success

35. Evidence of females choosing for good genes
Reddish color is derived from carotenoids in a fish diet
Better color, better foraging,
Even subordinate males express this color early in the mating season, but only more fit males are able to maintain it though the breeding season
They guessed that males in poor condition would go all out to produce the reddest signal they could early in the mating season, because they might not have the chance to mate for long before expiring. Healthier males, on the other hand, would keep some of their strength in reserve, because there would be more opportunities for them to mate in the future.
This assumption was borne out when the researchers tested their hypothesis. At the start of the breeding season, all of the males had a good red throat, and females paid little attention to the degree of redness when choosing a mate. They didn't trust it as a signal of reproductive fitness. As the summer wore on, however, only the fittest males could maintain the colouration, and only at this late stage did females develop a strong preference for redder throats.

36. Healthier male fish are more red
Red coloration declines in males infected with parasites
Healthy red males were also likely to have a strong MHC profile

37. Genetic Benefits 1) Sexy sons and daughters
Only the sexy are selected
2) Good genes as fitness indicators
Only good quality males that can afford elaborate ornaments can survive
But then how is variation maintained in a population?
Mutations
Traits are quantitative (many genes are responsible)
Variation in choice over time

38. Female behavior can also help to maintain genetic variation
Female copying- females imitate the choices of other females
3 morphed isopod
Female isopods prefer to mate with other females and can copy matings of other females
This may account for the maintenance of the 3 male morphotypes
Female isopods prefer to breed with other females in a spongocoel and while this increases the harem size for alpha males it also gives beta and gamma males more of a chance for mating success

39. Sex Role Reversal
When males make large contributions to parental investment, males may become the choosy ones
Seahorse mating
In pipefish and seahorses males have a brood pouch in which heholds and protects the fertilized eggs
Females can produce several clutches of eggs, so the limiting resource becomes males

40. Sex Role Reversal
When males make large contributions to parental investment, males may become the choosy ones
This produces a similar sexual dimorphism in females that have larger body sizes to compete for males

41. Sexual Conflict Over Mating
If males need to maximize matings and females need to choose the best partner
Then females need to resist mating and males need to be aggressive
Results in a sexual selection arms race
Males competing for a female sea turtle

42. Sexual Conflict Over Mating
Male Traits
Female Traits
Enforced Copulation
Resistance
Intromittant organs which enhance mating success
Elaborate reproductive tracts that are obstacle courses for sperm
Mate guarding, frequent copulation
Seek extra pair copulation
Remove or displace sperm of rival males
Sperm Ejection
Copulatory plugs and ant—aphrodisiacs
Sperm choice
Accessory glands to manipulate females
Chemical defense

43. Sexual Conflict Over Mating
The male guppy grows claws on its gonopodium (anal fin) to counteract sexually-selective females.

44. Chase Away Sexual Selection
This sexual conflict can drive antagonistic co-evolution between the sexes
Females will evolve resistance to male ploys because acquiescence is costly
Female swordtail prefer mlaes wit longer swords, and so do swordless platyfish, suggests a pre-existing female preference/bias for tails
But the preference and the trait should evolve together according to fisher, but female preference is stronger is species that don’t have swords, suggesting female attraction to swords has declined as swords evolveds

45. Sexual Conflict
Selection has favored Littorina saxatilis female snails that do not signal their gender in their slime trails
The females of most species of snail excrete a substance in their mucous trails that enables males to find them more easily, since they can distinguish between trails from females and those from other males. The males follow the mucous trails laid down by females in order to find a partner for mating. However, the females of one of the species studied (Littorina saxatilis) have stopped labelling their mucous trails. The consequence of this for the females is that they copulate less frequently, since the males often follow the trails of other males and must therefore spend twice as long looking for a female. This may appear strange at first sight, since we expect it to be in the females' interests to mate. But we show that copulation is costly for the females and that they already achieve more copulation than is required to fertilise all of their eggs," says Kerstin Johannesson, professor of marine ecology.