Sex and Sexual Selection Flashcards
The costs and benefits of sexual reproduction
what is sex?
we really mean here sexual reproduction
the formation of a new organism containing the genetic material of two parents
requires mechanisms for the amalgamation of genetic material and controls on processes coded for by genes. these in the meiosis, recombination crossing over.
but many species do not have sex:
asexual reproduction doubles the genetic advantage over sexual reproduction in producing genetically identical descendants. two benefits to this,
1) individuals don’t have to half the genetic material that they pass onto offspring as to sexually reproducing species who have to.
2)they can produce genetically identical copies of themselves, thus avoiding the risk of having to mix their genes with those of another individual who might have poorer quality genes.
One example of asexual reporduction is vegetative reproduction:
an asexual stand of aspen trees in Colorado, all of which are genetically identical and thousands of years old. can be seen in grass as well as weeds.
another form of asexual reproduction is Parthenogenesis:
Development of an unfertilised egg into an independent organism –requires only one individual’s genetic material. one that can do this is Aphids: parthenogenetic, but also viviparous and with telescoping generations.
Why would an individual raise an offspring carrying half its genes when it could produce an offspring carrying all its genes?
The costs of sex:
- Sex halves each individual’s genetic contribution to the next generation. it does this through the process of Meiosis, it relates to the formation of both male and female games (sperm and eggs). the process starts with the Diploid cells which have the full compliment of an individuals genes, then undergo two cell divisions to produce the sperm and the egg which are Hapliod (have half of individuals genes). if fertilisation occurs with a mates games then the resulting offspring will be back to having dipliod cells.
- The cost of recombination; It causes the break-up of successful genotypes and the recombination of their elements (elimination of gene linkages with selective advantages). Matched pairs of chromosomes exchange parts during meiosis (only one member of the pair exchanges parts). Crossing over has the effect of increasing the number of possible combinations of genes among offspring.
3) comes after fertilisation - Costs of homozygosity; Sex throws up disadvantageous homozygous pairs (e.g. sickle cell anaemia, haemophilia). - Costs of Courtship and Mating;
- Time and energy is invested in securing mates,
- Investment in behaviour and ornamentation associated with sexual reproduction
- Courtship may attract predators
- Possibility of failure to mate; few matings
- Possibility of infanticide
- Sexually transmitted diseases
Benefits of sex:
1. Sex has evolutionary potential.
Because of recombination, different combinations of genes arise in each new generation –although some may be unfavourable (see above), some others may be very favourable.
As a result, sex can speed up evolution: fast responses to Darwinian selection (Fisher, 1958; Muller, 1932). Favourable mutants appear together in the same genotype sooner than they would if they had to occur sequentially in a single lineage.
Therefore sexual species should suffer lower rates of extinction.
2.Sex is an adaptation to an uncertain future (Williams, 1975).
Species which can alternate between asexual and sexual reproduction use sexual reproduction when the environment is unstable or unpredictable.
Aphids:
- Wingless, parthenogenetic clones through summer
- Winged, sexual (male and female) generations in autumn
like the Lottery Principle, rather like not putting all your chickens eggs in one basket, sexual reproduction is like buying a bunch of different tickets giving you more of a chance of winning. in contrast, asexual reproduction allows you to buy many more tickets but with no additional chance of winning.
3. specific type of lottery in which the winning tickets buy you the ability to fight against various pathogens including parasites and diseases.
The pathogen theory: Antagonistic pairs of species, such as parasite and host, react to each other’s ongoing evolution by evolving counter-ploys.
Red Queen Hypothesis “Now, here, you see, it takes all the running you can do, to keep in the same place”.
Sex enables immune systems to keep finding new ways to beat the same enemy (let alone new ones).
Support for the pathogen theory:
•Parasites live within individuals of other species and biotic interactions are expected to be relatively intense. - Therefore, asexuality is rare in parasites.
•Species that feed on dead organisms lack a major source of biotic interaction. Their food does not evolve counter-strategies.
- As expected,asexual forms are common in such species.
What is sexual selection
it relates to the obsessive individuals against other members of the same sex in terms of producing offspring in the next generation. it leads to varies products such as ornamentation, weapons, sexual dimporhism, and behaviour (psychology).
Sex determination:
The female is, by definition, the sex that produces the larger immobile gamete. The gamete may be provided with resources. Males have small, motile or dispersing gametes. In mammals, eggs carry the X chromosome and sperm carry either X or Y.
Sex roles:
Depending on the embryo sex… there are subsequent differences between males and females in:
-Metabolic rates, starting from a few cell divisions after fertilisation
-Hormonal profiles and systemic effects of hormones
-Controls on reproductive development and function–endocrinological and neuroendocrinological(brain & body differences)
-Rearing environments, as experiences and reinforcement may be gender-specific
Together, these produce the phenotypic features of sexes:
•Reproductive organs
•Secondary sexual characteristics (develop at reproductive maturity)
•Behavioural characteristics
These differences shape the various ways that males and females can maximise their reproductive success (fitness) –sexual selection
Darwin proposed that one important factor leading to differences in reproductive success is differential access to mating opportunities. Charles Darwin suggests the sexual struggle is of two kinds;
- one that is between the individuals of the same sex, generally the males, in order to drive away or kill their rivals, the females remaining passive (Intrasexual selection)
- whilst in the other, the struggle is likewise between the individuals of the same sex, generally the females, which no longer remain passive but select the more agreeable partners.” (Intersexual selection).
Intrasexual Selection:
where individuals of one sex (usually males) compete for mating opportunities. Consequence is selection for weaponry, fighting skills, body size.
Intersexual selection:
- where individuals of one sex (usually females) choose mating partners (inter-sexual selection)
- Females can usually maximise their reproductive success by selecting the best mates, rather than more mates
- Accounts for ornaments, colouration and other features in males –not necessarily visual (e.g. song of birds [sedge warblers], smells of mammals).
Bateman’s Principle:
He studied sexual selection in fruit flies
(1) females should be the choosier sex because eggs are expensive to produce and because a female’s potential reproductive success is limited compared with that of a male, and
(2) females’ greater choosiness in mate selection should translate into greater variance in the reproductive success of males
Different influences on reproductive success
- Males: depends on his number of mates
- Females: depends on her rate of reproduction
The outcome of this is that females benefit more from being particularly choosy over their mate( we will return to this in the next 2 lectures)
Variance in reproductive success:
The sex which invests more in the young becomes a resource for which other members of the less parental sex compete
As soon as 1 male fertilises > 1 female, there must be 1 male who gets 0.
Sexual selection should therefore produce different degrees of variation in the reproductive success of males and females.
Evaluation of evidence for the strength of sexual selection in our own species
Variance in reproductive success:
Moulay Ismail Ibn Sharif (1634? –1727)
Second ruler of the Moroccan Alaouite dynasty
Fathered 867 children - 525 sons, 342 daughters
Winston Blackmore (Mormon leader) Had 25 wives and at least 121 children
Feodor Vassilyev’s wife (~1707-1782)
Russian peasant
She had 69 children – (32twins, 21 triplets, 16 quads)
Livia Ionce (1944 -)
Romanian emigrant to Canada
She had 18 children –(8sons, 10 daughters)
Sexual strategies:
Relative to women, men evolved to:
•pursue short-term sexual relationships
•with as many women as possible (promiscuity)
•only have long-term relationships if they must
•court females
•compete over women with other men
Women evolved to:
•choose between men: pick “the best” male
•favour long-term relationships (childcare)
This places stronger selection on men and their displays.
Equivalents of the peacock’s tail: Facial symmetry, masculinity, height,status-seeking, risk-taking, displays of wealth, even male sense of humour, creative intelligence (music, art etc.)
therefore, it has been argued that humans are similar to peacocks.
However, other information suggests otherwise:
- we are relatively monomorphic (look relatively similar)
- costly offspring (altricial birth and extended childhood, likely due to bipedality (pelvic canal) and large brains. 10-13 million calories to rear offspring from birth to nutritional independence; not until they are 18 do they start acquiring more calories than they consume. Short human IBI (compared to apes) also means that mothers may have more than 1 dependent offspring.) All of this shows mothers require paternal care.
- Paternal care increases offspring survival in many foraging societies; Sex differences in reproductive investment are actually modest when seen across dependency.
- Pair-bonding is therefore a central feature of human mating
- Therefore men also choose, and women compete for, the most desirable partners; Both sexes have attractive traits that may reveal fitness and fertility
When it comes to sexual selection, humans are more like gibbons than peacocks.
There are large sex differences in sexual behaviour!
Buss & Schmitt (1993): 75 men, 73 women college students: Currently seeking a short-term mate (i.e. 1-night stand, brief affair). evidence showed no difference between male and female participants in their level of seeking for long term relationships, both sexes are interested in that. However, males reported a much higher level of seeking short term mate than women. Males also wanted more sexual partners in the future than females.
Clark & Hatfield (1989)
Approach by attractive man or womanI have been noticing you around campus. I find you very attractive. Would you….
-go out with me tonight? (M:50%, F:50%)
-come over to my apartment tonight? (M:69%, F:6%)
-go to bed with me tonight? (M: 75%, F:0%)
Sex differences are probably exaggerated:
Lippa (2009): 200,000 BBC viewers, 53 countries with >90 participants
Measured difference in Sociosexual Orientation (measure of willingness to engage in sex outside a committed relationship), but also height
The sex difference in SO (d = 0.74) was ~half the sex difference in height (d= 1.63). d stands for effect size.
Using similar logic:
1/3 men have SO score lower than average woman
If we randomly select 1 man, 1 woman: M>W only ~2/3 of the time.
Massive difference in our willingness to have uncommitted sex.
Clark & Hatfield (1989)
Study really asks about casual sex with a total stranger
- this is much more risky for women!
- zero women agreed, but interest in casual sex is not zero!
Also sex differences in how people view the proposal
- women more likely to attribute –ve personality traits
- more concerned about reputation (these are sex diffs, but not sex diffs in interest in casual sex!)
Other studies report smaller differences
- Denmark: 2% women and 38% men consented to “bed” invite, 8% women and 22% men to the “apartment” invited = 1.11 and 0.41 respectively
Summary
Classic Evolutionary Psychology vs “human evolutionary psychology”
- Humans are peacocks No!
- But humans ARE polygynous…. Sometimes, not usually
- Sex differences in sexual behaviour!…. Probably exaggerated
