Overview of Lecture: Sex & Reproduction. see the schedule for reading and watching assignments

Transcription

1 Overview of Lecture: Sex & Reproduction. see the schedule for reading and watching assignments Bullet Points: Copulation vs genetic recombination Demographic & genetic costs of sex What are the ultimate benefits? Parthenogenseis & seasonal sex Environmental change & sex Sexual selection is a game Inter- & Intra-sexual conflict Human mating systems Evolutionary Psychology The science of beauty Beauty and student engagement, learning and evaluation of teaching

2 Learning Goals: 1. Be able to explain the demographic and genetic costs of sexual recombination. Describe the hypothetical benefits of recombination and use an example to help explain why sex is more common in changing or unpredictable environments. 2. Be able to explain how a game- theoretic perspective can help explain how/why sexual selection can select for different traits in males and females. Briefly, what is the essence of Batemen s Principle? Why is it clearly a caricature of human mating systems. Why can t one sex or the other win the battle of the sexes? 3. Be able to describe and explain how/why the removal of sexual competition in Pitnick s experimental evolution study with fruitflies results in the rapid evolution of different morphologies, compared to controls with typical sexual competition. What does human morphology relative to related primates suggest about the ancestral human mating system? 4. Be able to describe what the field of evolutionary psychology seems to be showing about the evolution of human perceptions of attractiveness. 5. Be able to describe what the study by Wolbringa & Riordan suggests about the impacts of college teacher appearance on student engagement, learning and Student Evaluation of Teaching (SET).

3 Few subjects pervade our everyday thinking more than sex, and few urges are more insistent. By sex the author means the proximate pleasure of copulation, not the ultimate functions of recombination & reproduction. Fitness depends on lifetime reproductive success. Many proximate aspects of animal form and function (and thought) can be viewed as adaptations contributing to ultimate reproductive success. There are two principal modes of animal reproduction. Asexual reproduction is the cloning of one parent. In most cases, just mitotic cell division w/o meiosis & syngamy. Sexual reproduction is the creation of offspring by the fusion of haploid gametes to form a diploid zygote. Haploid gametes are formed by meiosis. The female gamete, the ovum (unfertilized egg), is usually a relatively large and nonmotile cell. The male gamete, the spermatozoon, is a small, motile cell. Sexual Reproduction and the Evolution of Sex Otto, S. (2008) Sexual reproduction and the evolution of sex. Nature Education 1(1) Sexual reproduction increases genetic variability among a mother s offspring

4 Why sex and recombination? Barton NH, Charlesworth B SCIENCE 281 (5385): SEP Most [animals] reproduce sexually, despite the automatic reproductive advantage experienced by asexual variants. In a population of parthenogenetic females cloning themselves: the pop. can produce babies faster everyone s a mom;; & each baby reproduces 100% of a mom s genes, not 50%;; But babies, families & the pop genetically more homogeneous;; less variable ecology, more vulnerable to coevolving parasites This implies the operation of selective forces that confer an advantage to sexuality and genetic recombination, at either the population or individual level. The effect of sex and recombination {combining isolated beneficial mutations at} different genetic loci, which increases the efficiency of natural selection, is likely to be a major factor favoring their evolution and maintenance. With sex, isolated mutations at diff loci in diff individuals can be combined in one individual: beneficial mutations can interact synergistically & deleterious mutations can be purged in batches both processes can speed adaptation and select for sexual recombination. Sex releases the speed limit on evolution. Colegrave N. NATURE 420 (6916): DEC I show that sex can increase the rate of adaptation {to novel nutrient conditions} in the facultatively sexual single-celled chlorophyte Chlamydomonas reinhardtii...

5 Animals may reproduce asexually or sexually exclusively, or they may alternate between the two modes. In aphids, rotifers, and the freshwater crustacean Daphnia, X each female can produce eggs of two types, depending on environmental conditions such as the time of year. Asexual reproduction occurs in good condition early in the season {summer}. Females produce clonal parthenogenetic female offspring. Sexual reproduction occurs during adverse environmental conditions: food shortages & end of season {fall }. Some males develop from parthenogenetic eggs and some females produce darker eggs that require fertilisation by the males. {males made only when times are tough & sex-recombination is apparently a good idea! } The fertilized eggs form dormant resting eggs : may remain viable in sediment for years - disperse through time & space. Differentiation in sex investment by clones and populations of Daphnia. Tessier AJ, Caceres CE ECOLOGY LETTERS 7 (8): AUG 2004 document an ecological gradient in dormancy that is manifest as a genetic gradient in clonal variation for the propensity to engage in sex. A pattern

6 Variety sparks sexual evolution A changeable environment encourages a move away from asexual reproduction. Joseph Milton Published online 13 October 2010 Sex is a costly business at least in evolutionary terms. The question of why it emerged in the first place is the subject of heated academic debate. After all, why not simply produce clones asexually, saving energy and passing on genes more efficiently? Research published online today in Nature shows that An experiment a species of rotifer that reproduces both sexually and asexually, opts for sexual reproduction more often in varied habitats than in homogeneous settings, suggesting that varied surroundings may have contributed to the evolution of sex. [in heterogeneous environments, offspring s environment different from mom s] "We could actually see how the investment in sex changed over time, so we could test predictions about the evolution of sex, " says Lutz Becks, The TA: Higher rates of sex evolve in spatially heterogeneous environments Lutz Becks & Aneil F. Agrawal Nature 468, (04 November 2010)

7 If evolution by natural selection adapts animals to their environments, why are males & females so different? Environment includes social env as well as ecological env. Males & females are in different social (mating) env s. Parasitic male anglerfish Sex is a game [speaking technically, of course: where the payoff to each player depends on what the other players do] w/ asymmetries among the players that lead to conflicts and different optimal tactics Mating systems begin with sex but differentiate around parental care.

8 Bateman s Principle: - the battle of the sexes (without parental care) Fruit flies: mean # offspring # mates There appears to be stronger selection on males - eager? than on females- choosy? to mate more than once - in fruitflies w/o pair bonds or paternal care This is a controversial caricature, see - Rethinking Bateman s Principles: Challenging Persistent Myths of Sexually Reluctant Females and Promiscuous Males Z Tang-Martínez 2016 J. Sex Research 53, In a diploid sexual species, in each generation, half of the chromosomes and genes come from males & half from females. Excepting the X & Y sex chromosomes (in mammals;; Z & W in birds: females are ZW), all of your genes spent ~ half their history in in males and ~ half in females. Neither sex wins the battle of the sexes. If one male has relatively more offspring, other males have relatively less, males don t have more offspring than females on average, just greater variance in reproductive success than females.

9 Males' evolutionary responses to experimental removal of sexual selection. Pitnick et al Proc Royal Society Of London B 268: We evaluated the influence of sexual selection upon male reproductive traits in a naturally promiscuous species, D. melanogaster. Sexual selection was removed in two replicate populations through enforced monogamous mating with random mate assignment or retained in polyandrous controls. {lots of males & females per bottle} Monogamous mating eliminates mate competition, mate discrimination, sperm competition, cryptic female choice and, hence, sexual conflict. Levels of divergence between lines in sperm production and male fitness traits were quantified after generations of selection. Monogamy males evolved a smaller body size and the size of their testes and the number of sperm within the testes were disproportionately further reduced. the fitness of monogamous males and their mates was greater when reproducing in a non-competitive context: females mated once with monogamous males produced offspring at a faster rate and produced a greater total number of surviving progeny than did females mated to control males. The results indicate that sexual selection [male-male competition for access to mating] favours the production of increased numbers of sperm in D. melanogaster and favours some male traits conferring a direct cost to the fecundity of females.

10 Intrasexual selection for dominance & access in one sex selects for sexual dimorphism within species Does size dimorphism in humans suggest anything about ancestral mating systems? Across primates, harem size is correlated w/ dimorphism Comparing human dimorphism (~1.1) to pattern in primates suggests humans evolved dominance and mating system with at least a little polygyny BBC. 1.5m Chinese 'descendants of one man' Over 1.5 million served. Not by McDonalds but Giocangga, the grandfather of the founder of the Qing dynasty, Recent genetic research suggests that over 1.5 million Chinese men are the direct descendants of this one single man.... the average Chinese man at the time of Giocangga would only have around 20 descendants living today.

11 More on Primate sexual dimorphism & mating system (from The Third Chimpanzee, Diamond 1992) Chimps are not size dimorphic;; males do not control access to females Inverse correlation between size dimorphism vs testicle size;; hyp: the more males compete for & control access to copulation, the less sperm competition Increasing size dimorphism correlated with increasing male-male competition for access to copulation Gorillas are very dimorphic;; males control access to a harem of females Human mating system seems to have evolved w/ some competition for access to copulation;; what about sperm competition? Human sperm competition: testis size, sperm production and rates of extrapair copulations. Simmons et al Anim Behav. 68: rates of extrapair paternity are in the region of 2% suggest that the risk of sperm competition in humans is relatively low...

12 Allometry of female breasts Human female breasts are padded w/ fat, much larger than necessary to nourish babies. Hyp: Large and/or shapely breasts advertise youth, health & high future reproductive potential. Angier (1999) argues that male obsession w/ breasts is maladaptive (ie, just stupid)

13 Men's preferences for women's breast size and shape in four cultures JanHavlíček et al The morphology of human female breasts appears to be unique among primates due to their permanent fat deposits. It has been previously suggested that female breast morphology arose as a result of sexual selection. breasts become less firm with age and parity [# previous births], and breast shape could thus serve as a marker of residual fertility. [potential future reproduction aka residual reproductive value ] Therefore, cross-culturally, males are hypothesized to prefer breast morphology that indicates high residual fertility. [why cross-culturally?] To test this, we performed a survey on men's preferences for breast morphology in four different cultures (Brazil, Cameroon, the Czech Republic, Namibia). As stimuli, we used two sets of images varying in breast size (marker of potential fertility) and level of breast firmness (marker of residual fertility). [see for images] Individual preferences for breast size were variable, but the majority of raters preferred medium sized breasts. [stabilizing selection] We found directional preferences for firm breasts across all four samples. This pattern supports [is consistent with] the idea that breast morphology may serve as a residual fertility indicator, [Can you think of any plausible alternative hypothetical interpretations?]

14 The evolutionary psychology of physical attractiveness: Sexual selection and human morphology. Barber N 1995 Ethol & Sociobiol 16: Abstract: Psychological evidence suggests that sex differences in morphology have been modified by sexual selection so as to attract mates (intersexual) or intimidate rivals (intrasexual selection): Women compete with each other for high quality husbands by advertising reproductive value by exaggerating morphological indicators of youthfulness such as a small nose and small feet and pale, hairless skin. Low waist-hip ratio is sexually attractive in women... Men's physical appearance tends to communicate social dominance, which has the combined effects of intimidating reproductive rivals and attracting mates. The male beard is not obviously related to phenotypic quality... Darwinian aesthetics: sexual selection and the biology of beauty. Grammer et al. Biological Reviews 78 (3): AUG 2003

15 Studies of human physique and sexual attractiveness: Sexual preferences of men and women in China. Dixson BJ, et al Am J. Human Biol. 19: Waist-hip ratio in women is often considered a key feature in physical attractiveness. Measured directly from this image, the frontal WHR of the model in this image is approximately Waist-to-Hip Ratio across Cultures: Trade-Offs between Androgen- and Estrogen-Dependent Traits E Cashdan 2008 Current Anthropology 49: women with a WHR of about 0.7 are more fecund, are less prone to chronic disease, and (in most cultures) are considered more attractive. [and are relatively young] Optimal Waist-to-Hip Ratios in Women Activate Neural Reward Centers in Men. Platek SM, Singh D (2010) PLoS ONE 5(2): e9042.

16 MACHINE MEASURES BEAUTY OF FACE (Feb, 1933) Facial attractiveness: evolutionary based research. AC. Little et al Philos Trans R Soc Lond B Biol Sci. 366: We review the facial characteristics that influence attractiveness judgements of faces The influence of shape and colour cue classes on facial health perception A L Jones 2017 Facial cues to health can be divided into two broad classes facial shape, which is linked to previous health and is relatively fixed;; and facial coloration, changes over the short-term, reflecting current health. both make contributions to perceived health, but that averageness explains a larger proportion of variance.

17 How beauty works. Theoretical mechanisms and two empirical applications on students' evaluation of teaching. [SET] T Wolbringa & P Riordan 2016 Social Science Research 57: Plenty of studies show that the physical appearance of a person affects a variety of outcomes in everyday life. [salary, trust, election results, etc.] We want to test five hypotheses H1. There is a high degree of inter-individual agreement on the physical attractiveness of instructors among students. (Beauty Consensus) H2. Students skip classes less when instructors are physically more attractive even if course quality is held constant. (Attractiveness Attention Boost) H3. Physically attractive instructors receive more favorable evaluations of teaching. (Attractiveness Treatment Advantage) [Hot or not: do professors perceived as physically attractive receive higher student evaluations? Riniolo et al J. General Psychol. 133: 19 35] H4. The effect of physical attractiveness on course ratings and absenteeism is stronger, if the sexes of rater and ratee differ. The effect is more pronounced if a male student rates a female instructor. (Intersexual Attraction Effect) H5. Greater physical attractiveness of an instructor leads to worse ratings in the case of her failing to meet the expectations of students. (Beauty Penalty) Study 1: a descriptive analysis of actual UG classes in Germany. the results are consistent with the experiment below, but possible confounds?

18 How beauty works. Theoretical mechanisms and two empirical applications on students' evaluation of teaching. [SET] T Wolbringa & P Riordan 2016 Social Science Research 57: Study 2 uses data gathered in a series of laboratory experiments conducted at the University of Munich. After receiving the [fake identical] CV [and morphed photograph] all participants listened to an eleven-minute audio recording of a lecture. [on] the analysis of social structure: the discussion on classes, strata, and milieus the voice was systematically adjusted for male or female voice characteristics using the software MorphVoxPro. [same lecture, different voice, photo & CV] After listening to this lecture, one half of the participants completed a SET questionnaire Then either an easy or hard test on the content of the lecture followed. The other half of the participants completed both documents in the opposite order. the outcome of interest is students' ratings of the overall quality of the lecture

19 How beauty works. Theoretical mechanisms and two empirical applications on students' evaluation of teaching. [SET] T Wolbringa & P Riordan 2016 Social Science Research 57: Results On average, attractive instructors received SET ratings 0.24 grade points higher. The beauty premium is markedly stronger for female instructors (0.40) than for male instructors (0.10) [on a 5 point scale] We find no evidence that attractiveness effects become stronger if rater and ratee are of the opposite sex. Attractiveness is significantly related to [student] performance in the hard test Students listening to an attractive instructor answered 0.44 more questions correctly There are no differences between treatment groups in the performance in the easy test. When we look at ratings given after the test, the positive effect of attractiveness disappears for female instructors and becomes negative for male instructors. Students who completed a hard test rate the teaching significantly worse. If attractive instructors demand too much from their students [give a hard test] and the test takes place before the SET the [beauty] premium turns into a penalty. Attractive instructors who gave students a hard test were punished more severely. The statistical model explains only 6.6% of total variation in the data

20 How beauty works. Theoretical mechanisms and two empirical applications on students' evaluation of teaching. [SET] T Wolbringa & P Riordan 2016 Social Science Research 57: Conclusions Raters revealed a high degree of consensus when evaluating instructors' appearances and these beauty evaluations in turn resulted in differential treatments of instructors. differences in instructors' productivity could not solely explain the latter effect. The results from study 1 [a descriptive study of actual college classes] showed that courses of attractive instructors were attended more regularly Analyses of the experimental data in study 2 [described above] revealed differences in the SET ratings of attractive and unattractive lecturers although students listened to exactly the same audio recording Students listening to an attractive instructor scored better on the harder test. Attractive instructors who gave students a hard test were punished more severely with lower SET scores. [see Vote for Ugly: The New Yorker ]