1. Sexual Selection in the Sea

2. Darwin’s postulates & evolution IF –Variation: phenotypic variation among individuals within population –Inheritance: some variation passed genetically to offspring –Reprod. Excess: not all offspring survive –Selection = differential fitness! Survival & reprod. NOT random—some phenotypes have greater fitness than others THEN EVOLUTION = composition of population changes generation to generation

3. Sexual Selection Sexual selection = selection for traits concerned solely with reproduction –Traits selected for via: 1)Intrasexual selection = competition for access to mates or resources mates attracted to 2) Intersexual selection = mate choice by one sex for members of the other sex (i.e., males advertise, females choose best displays)

4. Sexual selection vs. natural selection: why the distinction? Although the two forms of selection operate the same way, they can conflict with each other!!! Traits increasing reproductive success may be detrimental to survival

5. Asymmetry of the Sexes Anisogamy = “gametes of size” Females: few large gametes (eggs = expensive) Males: many small gametes (sperm = cheap) Conflict between sexes! Reproductive Fitness: Females: low variation Males: high variation

6. Bateman’s Principle RS depends on access to mates RS depends on ability to produce eggs & rear young = choosy= competitive If this pattern holds…

7. What determines strength of sexual selection? 1.Operational Sex Ratio (OSR) = the ratio of receptive females to receptive males 2.Parental Care Greater the difference in PI between sexes = strong SS 3.Mating system

8. OSR = receptive females/receptive males OSR skewed toward one sex or the other (biased sex ratio)  strong SS Males >> Females  males compete Females >> Males  females compete

9. Parental Investment PI = costly parental activities that increase the chance of survival for offspring, but decrease the chances of producing additional offspring Male PI = Female PI  SS minimized Greater asymmetries in PI between parents/sexes  stronger SS Males >> Females Females >> Males

10. Mating Systems Monogamy = males pair with single female for breeding season Polygamy: more than one mate –Polygyny = males mate with multiple females –Polyandry = females mate with multiple males –Polygynandry = males and females mate multiply

11. Sexual Selection in Elephant Seals

12. Sexual Dimorphism Body Size: males = HUGE!!! Snouts

13. Elephant Seals: Mating System Polygynous mating system Males battle for territories Largest males win battle scars

14. Elephant Seals: Mating System Females choose largest males Males defend harems Alternative strategy: sneaker males http://www.biosbcc.net/ocean.htm

17. Inter-sexual selection: which sex chooses? Sex with greater parental investmentSex with greater parental investment –Individuals with more invested have more to lose –Choose mates that will “maximize return” on that investment Females generally invest more into offspring so they are most often choosierFemales generally invest more into offspring so they are most often choosier Dr. Pfennig, UNC Biol 63 lecture

18. Why choose? Direct benefits –Nuptial gifts –Parental care –Fertilization success –Avoidance of parasites Indirect benefits –“Good genes” (Zahavi 1975; Hamilton & Zuk 1981) –“Sexy sons” (Fisher)

19. Sex Role Reversal Sex role reversal –Females compete –Males choose Ubiquitous in the Family Syngnathidae) –Seahorses –Pipefishes –Sea dragons Sea dragon Pipefish Seahorses www.seaworld.org/.../ fish/bony/ sea-horse.htm www.abc.net.au/.../ img/ water/sealeafy.jpg Adam Jones

20. Male Seahorses www.mbayaq.org/.../ content/images/ssh_sea_02.jpg Adam Jones brood pouch eggs When would you expect sex role reversal to occur?