10 SE XU A L SE LECT IO N A ND B EH AV IO R AL E VO LUTIO N In this course you have already discussed how natural selection favors alleles that increase survival and reproduction However one can observe traits in some organisms that seem clearly to reduce some aspects of fitness For example in social insects many individuals are sterile workers and they feed and rear the offspring of other individuals Another example occurs in many animals in which individuals of one sex have conspicuous traits or behaviors that make them vulnerable to predators are energetically expensive to produce and don t contribute to the well being of offspring In this lecture we will concentrate on traits in this second category the evolution of conspicuous sexually dimorphic traits Darwin was at first puzzled by conspicuous traits such as bright colors horns and displays of males of many species seemingly lower survival of the bearers predation energy His solution Se xual S ele cti on he devoted most of a book The Descent of Man and Selection in Relation to Sex to this topic De fi niti o n Selection that arises from differences in mating success of mates that bear progeny over a defined time interval Arnold and Wade 1984 It is a kind of natural selection vs ecological selection Sexual Selection exists because one gender is a limiting resource for the other In many species females are a limiting resource for males b c females produce few large gametes that are energetically expensive while males produce many small gametes that are not expensive In some species females provide more parental care than males which also makes females a limiting resource However males can be a limiting resource for females in species in which males provide parental care many insects fish birds and mammals including primates Two examples 1 Female red p hal ar op e s lay more eggs than a male can tend they are more brightly colored than males and compete with other females for mates Oring 1986 2 Pi pe fi s h eggs deposited in the male s brood pouch or attached to his ventral surface Males fertilize eggs developing embryos are protected aerated osmoregulated and nourished by the male brooding structure males are choosy sex they prefer females with blue coloration Berglund et al 1986 Berglund Rosenqvist 1993 1994 Berglund 1991 Rosenqvist 1990 3 Mid wi fe T oa d 4 Gia nt W ate r bug Also in many species it takes two parents to successfully raise offspring so males and females are equally limiting many primates many birds 10 1 K INDS OF S E XUAL S E LECTIO N Darwin proposed two kinds of Sexual Selection 1 10 1 1 Int ra sex ua l Se lecti on contests between male females for access to females males a Male bl ac kbir d s defend territories using song and display epaulets Anderson showed that obscuring epaulets led to more trespasses by other males Andersson 1994 10 1 2 Int erse x ua l Sel ecti on Sexual selection via mate choice Females of many species mate preferentially with males with particular phenotypes size color pattern behavior Jumping spider courtship http tolweb org tree eukaryotes animals arthropoda arachnida araneae salticidae salticidae movies tarsalis mov Courtship behavior in Japanese crane see http www3 famille ne jp ochi eng kushiro2 html Striking how numerous and seemingly arbitrary these traits are In some species females prefer males with bizarre artificial traits WHY would females prefer these strange traits Several Hypotheses 1 Direct Benefits 2 Indirect Benefits 3 Sensory Bias 10 1 2 1 Di re ct Be nefits Male provides a res o urce that directly benefits the female food territory parental care Male house finches vary from orange to bright red Redder males bring food to the nest at a higher rate Hill 1991 and are preferred by females Not controversial but does not explain evolution of ornaments in species in which males provide nothing to females or offspring except gametes 10 1 2 2 Indi rect Be ne fit s Most difficult to explain are those species in which male provides no resources to the female and does not participate in parental care Male provides only genes What could be the cause of female preference in such species a G oo d Ge ne s M od el s Females choose mates of high genetic quality so that their offspring inherit this quality Females choose based on some trait that indicates high fitness both trait and fitness must be heritable VA 0 Mutation changing environments or parasite and pathogen co evolution could contribute to long term maintenance of heritable variation for fitness a R una wa y Se x ual S el ecti o n 2 First model described by R A Fisher 1930 developed mathematically by O Donald 1962 in O Donald 1980 Lande 1980 Kirkpatrick 1982 Male trait and female preference once initiated become self reinforcing Can lead to continual co evolution and elaboration of both trait and preference Kirkpatrick s genetic model of Fisher s Runaway process Females Males Genotypes P1 no preference P2 preference T2 no trait T2 trait P2 females tend to mate with T2 males P1 females mate randomly P1 females mate randomly Offspring of P2 females inherit both the T2 trait and the P2 preference generates a genetic correlation between preference and trait Since T2 males have higher mating success on average trait increases in frequency in population Preference increases in population b c of genetic correlation Other genes affecting trait and preference behave similarly Can lead to elaboration of trait and preference until the mating advantage of trait is outweighed by survival cost of the trait c S e ns o ry B ia s M o del s Female preference for a trait may be a by product of the female sensory system that has adapted to be sensitive to certain kinds of signals for reasons that are unrelated to mating Also some species display preferences for novel stimuli for reasons that are not understood e g grass finches prefer artificial crests Platyfish and swordtails are related species of Poecillid fish both in genus Xiphophorus Swordtails obviously have sword like extensions on their caudal fins Basolo 1990 1995 attached artificial swords to the tails of platyfish and showed that female platyfish preferred males with the artificial swords over those that had normal platyfish tails So the preference for swords exits in female platyfish even though the sword trait does not exist in this species 10 2 S EXUAL AND E C OLOGI CAL S ELECTIO N Sexual selection is a powerful concept that can explain traits that would otherwise seem