Sexual Selection Sexual Selection Sexual Selection

1 2 Sexual selection Sexual selection Introduction Introduction

Individuals rarely mate at random for a number of Non-random mating has a number of important reasons: evolutionary consequences. • Dispersal may be limited In this lecture, we will focus on the evolution of mate • Individuals may or may not be able to self choice and sexual selection. • Individuals may reproduce asexually In particular, we will concentrate on how these • Individuals may compete for mates processes may explain differences between the sexes (sexual dimorphism). • Individuals may choose particular mates

3 4 Sexual selection Sexual selection Introduction Classification

"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 Following Darwin, two main forms of sexual selection sexes, at all ages, in relation to the general conditions of life. are recognized: The sexual struggle is of two kinds: in the one it is between the • Intrasexual selection (competition within a sex for individuals of the same sex … in order to drive away or kill their rivals …; while in the other, the struggle is likewise between the the opportunity to mate) individuals of the same sex, in order to excite or charm those of • Intersexual selection (choosiness on the part of the opposite sex, … which … select the more agreeable one sex for mates) partners." Darwin (1871) – The Descent of Man and Selection in Relation to Sex 5 6 Sexual selection Sexual selection Classification Intrasexual selection (male-male competition)

Although female-female competition and male Horns, antlers, tusks, spurs choosiness are known, we'll focus on the more common and other weapons provide patterns of male-male competition and female choice. some of the most extreme examples of sexual Why might these be the more common patterns? dimorphism.

7 8 Sexual selection Sexual selection Intrasexual selection (male-male competition) Intrasexual selection – Examples

Possible explanations: • Weapons against predators Barrette and Vandal (1990) • Weapons against other males studied sparring in caribou. Of • Indicators of male strength 713 matches between males of and fighting ability (male-male different antler size, males with competition) smaller antlers withdrew 90% of the time. • Indicators of sexual vigor and quality (female choice) 9 10 Sexual selection Sexual selection Intrasexual selection – Examples Intrasexual selection – Examples

Eberhard (1979,1980) studied Brown and Bartalon (1985) the use of horns in seventeen placed forked fungus beetles species of beetles, finding that together in a terrarium. they tend to be used either Males with the longest horns to pry a rival off his site or to had higher access to females. lift and drop the rival to the Female preference was not ground. observed.

11 12 Sexual selection Sexual selection Intrasexual selection Intersexual selection Some of the more profoundly beautiful traits seen in nature have evolved in response to female choice.

"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) 13 14 Sexual selection Sexual selection Intersexual selection Intersexual selection But why should female preferences exist in the first place? 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)

15 16 Sexual selection Sexual selection Intersexual selection – Fisher’s runaway process Intersexual selection – Fisher’s runaway process

Phase 1: Female preferences initially evolve because they favor a trait in males that is also favored by natural Phase 1: “Whenever appreciable differences exist in a selection. The offspring of choosy females are then species, which are in fact correlated with selective more likely to carry the advantageous trait. advantage, there will be a tendency to select also those individuals of the opposite sex which most clearly Females preferring discriminate the difference to be observed, and which Red males red males produce Daughters with survive more red sons preference most decidedly prefer the more advantageous type.” better and daughters Red sons that prefer red males Fisher (1930) 17 18 Sexual selection Sexual selection Intersexual selection – Fisher’s runaway process Intersexual selection – Fisher’s runaway process

Phase 2: Once female preferences exist, they can favor even more extreme traits in males. This can in turn Phase 2: “...the further development of the plumage favor the evolution of stronger female preferences, character will still proceed, by reason of the advantage leading to a runaway process. gained in sexual selection, even after it has passed the point in development at which its advantage in Natural Females preferring Higher fitness despite Selection has ceased.” Daughters with carrying a more and red males produce Red males more red sons preference get more more extreme trait “There is thus...a potentiality of a runaway process.” mates that is no longer and daughters Red sons that prefer red males favored by natural Fisher (1930) selection, Increase in frequency

19 20 Sexual selection Sexual selection Intersexual selection – Fisher’s runaway process Intersexual selection – Fisher’s runaway process

The runaway process will halt when genetic variation is In a breeding experiment exhausted or when the trait becomes so costly that with the threespine natural selection balances sexual selection. stickleback, Bakker (1993) observed a genetic correlation between red Females preferring Daughters with red males produce Red males get coloration among sons and more red sons preference more mates but die preferences for red more often and daughters Red sons coloration among daughters, that prefer red males as expected under the Increase inX frequency Fisherian process. Sexual and natural selection balance 21 22 Sexual selection Sexual selection Intersexual selection – direct benefits Intersexual selection – direct benefits

The Fisherian model of sexual selection is ineffective in the face of costs to female choosiness, such as: Occasionally, however, female preferences may be • Time and energy in evaluating mates directly beneficial, such that natural selection favors the • Risk of remaining unmated evolution of preferences, which in turn lead to the elaboration of the preferred trait in males. Example: Engelhardt et al (1982) found that choosy female seaweed flies had reduced fertility.

23 24 Sexual selection Sexual selection Intersexual selection – direct benefits Intersexual selection – direct benefits Possible direct benefits of being choosy: Example: Thornhill (1983) showed that female • Lower risk of mating with the wrong species hangingflies lay more eggs with males that provide larger (species recognition) nuptial food gifts. • 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) 25 26 Sexual selection Sexual selection Intersexual selection – direct benefits Intersexual selection – sensory bias

Example: Pleszczynska The runaway and direct benefit models predict that (1978) showed that male lark female choice evolves initially in response to buntings with more nest (a) selection for exaggerated male traits (runaway) cover in their territory or (b) selection for increased fitness of the female attracted more females, and or her offspring (direct benefit) these females had higher breeding success. That is, female choice is (at first) an adaptation.

27 28 Sexual selection Sexual selection Intersexual selection – sensory bias Intersexual selection – sensory bias

A possible non-adaptive hypothesis is that female preferences are simply a side-effect (a "pleiotropic" Example: Searcy (1992) found effect) of how sensory systems have evolved. that female common grackles In other words, the nature of the sensory system preferred males singing an may be a constraint on what males a female can artificial repertoire with four song see, hear, or otherwise detect. types even though males in this Male ornaments may then evolve to take advantage species sing only one song type. of these constraints. 29 30 Sexual selection Sexual selection Intersexual selection – sensory bias Intersexual selection – sensory bias Example: Smith et al (2004) showed that juvenile Example: Burley and Symanski sticklebacks preferred artificial food items that were red. (1998) artificially added crests to 30 s

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white crests (55%) than with no n crests (20%), red crests (12.5%), or Kerry Clayman 0 green crests (12.5%). Male long-tailed & zebra finch e e e e k k n d w t l c u e e g n i o l p i r a e n l h r l b r p l a u b e w r g p y o

colour

31 32 Sexual selection Sexual selection Intersexual selection – sensory bias Implications This experiment was performed on a species that does not exhibit red throated nuptial colors (a male mating trait)

Spinachia Apeltes Pungitius Culaea G. wheatlandi G. aculeatus (absent) (absent) (black) (black) (black and gold) (red) Regardless of how mating preferences have evolved,

origin of red coloration http://www.icapb.ed.ac.uk/research/evolecol.html their presence in a population has profound influences on the evolution of morphology, behavior, and communication. origin of nuptial coloration

Preferences for red evolved before the sexually selected trait appeared. 33 34 Sexual selection Sexual selection Implications Implications

Sexual selection has undoubtedly contributed to the In addition, sexual selection can lead to divergence in evolution of some of the more spectacular traits seen in traits and preferences causing reproductive isolation of the natural world (e.g. the radiant feathers of peacocks, populations, thereby contributing to speciation. the flashing lights of fireflies, the nightly song of crickets).

35 36 Sexual selection Sexual selection Implications Implications Males are very red Males have 6 lake little red Sexual selection has played a crucial role in the populations of Females explosive radiation of Drosophila species on Hawaii sticklebacks detect red (800-900 species) and of cichlids in the African Rift Lakes (>800 species). Females poorly detect red well

Water color Red Boughman 2001 37 38 Sexual selection Sexual selection References Intersexual selection – sensory bias Example: Basolo (1990) showed that female platyfish Malte Andersson's (1994) book entitled Sexual Selection is a preferred males with swords artificially attached, fantastic source of information about the theoretical and arguing that evolution in the sister taxa (swordtails) empirical support for various hypotheses about mate choice. was shaped by this pre-existing bias for swords. Additional pictures come from Sexual Selection (1989) by Here, the female preference evolved before the male Gould and Gould. trait – and in one species, the trait never evolved! Swordtail Platyfish Further reading: (males have swords) (no swords) • Burley & Symanksi (1998) Am Nat 152: 792-802 • Smith et al. (2004) Proc. R. Soc. Lond. B 271:949-955 • Boughman (2001) Nature 411:944-948 Ancestor prefers swords