BIOL 4160 Evolution Phil Ganter 301 Harned Hall 963-5782
California Sea Lions (actually in Oregon), Zalophus californianus - Why is the male so big?

Sex and Evolution

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Link to a list of Specific Objectives for lectures

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• Evolution of Mutation Rate
• Sex and No Sex
• Sex Ratio and Sex Allocation
• Inbreeding and Outbreeding
• Sexual Selection and Conflict
• Alternative Mating Strategies

This lecture addresses the questions of:  Why sex?  Hermaphrodite or separate sexes?  How many of each sex?  With whom to mate?

Evolution of Mutation Rate

Mutations are not directed toward solving a problem or attaining a goal, individual mutations are random (within their domain of possible changes)

• Given this, populations have mutation rates related to the probability of mutation
  • The rate of point mutations have been most closely studied
• Changes in the apparati that copy and repair DNA affect the rate of mutation so we know that the mutation rate is open to the possibility of natural selection
• Each lineage will find its own minimum (perhaps a theoretical minimum exists but natural selection will not necessarily find it)

Asexual organisms

• If mutations are always harmful, then:
  • Natural selection should minimize the rate of mutation by selecting for the most efficient replication mechanism and repair systems
  • Note: I disagree with the textbook, which asserts that selection for the minimum (or, presumably, the maximum) does not qualify as an adaptation.  I feel that the min or max can be adaptations.
• We will not consider the possibility that all mutations are beneficial, as we know this is not so.
• If mutations are sometimes beneficial and sometimes harmful (neutral mutations do not count here), then:
  • The rate of mutations will be above the minimum by some degree related to the probability of positive and negative consequences from the mutation, which is related to long-term variation in the environment (both biotic and physical)
  • When a positive mutation occurs, it will probably be swept (selective sweep) to fixation and so will the rest of the genome (including those alleles responsible for the mutation) through the phenomenon of hitchhiking
    • Some models show that fixation is not necessary, only increased representation
  • In variable environments, the lineages with greater mutation rates would be more genetically variable and more likely to have variation that is most fit to the new conditions

Sexual Organisms

• In asexual organisms, the benefits and costs of mutations accrue to the replication/repair systems that caused them
• In sexual organisms, recombination means that positive mutation's value will not only accrue to the particular lineage in which they arose but can be transferred to other lineages
  • Since the data suggests that harmful mutations are more common that beneficial, higher rates of mutation mean increased chances of association with harmful mutations and we would expect that selection would minimize this probability by minimizing mutation rates
  • The textbook illustrates this outcome by postulating "mutator loci" but these not be the cause of mutation for the argument to work (although mutator loci have been discovered in prokaryotes)

Sex and No Sex

Some terminology to describe the variations in sexual systems:

• asexual (amictic, apomictic), sexual (mictic), parasexual (transformation, conjugation, transduction)
• recombination, suppression of recombination (crossover), legitimate and illegitimate crossover
• isogamy, aniosgamy, mating type, sex, dioecious (gonochorism, bisexual), monecious, (unisexual, cosexual, hermaphrodite [simultaneous and sequential])
• parthenogenesis, vegetative reproduction

Sexuality is more common than asexuality in most eukaryotic lineages (we do not know the relative frequency of parasexuality in prokaryotes but it appears to be common and the more we find out, the more common it becomes)

• The predominance of sexuality is even more pronounced if one considers that there are few ancient asexual lineages, indicating that asexuals are at greater risk of extinction

Disadvantages of sex

• Cost of sex - the potential growth rate of sexual population is as little as 50% of the potential of asexual populations
  • females determine the maximum size of the next generation
  • allocating some reproductive effort to males (or male reproductive effort in hermaphrodites) reduces the number of females (or female effort) in the next generation, lowering the growth rate
• Recombination cost - if a particular set of alleles at several loci maximize fitness in an organism, recombination will break up these combinations in an organism's offspring (recombination at the level of crossover and meiotic segregation)
  • There is another consideration pertinent to recombination
    • The rate of evolution is proportional to the genetic variation in the population
    • Recombination increases genetic variation
    • The increased rate of evolution means that recombining populations adapt more quickly
  • This make recombination both favor and disadvantage the evolution of sex, so it is hard to determine its overall effect
• Other types of cost
  • failure to find a mate
  • danger of mating - exposure to predators, disease

Advantages of Sex

• Breaking up Muller's Ratchet
  • First, a ratchet mechanism is one that will allow motion in one direction but prevents motion in other directions.  In a way, the one-way valves in our veins are a ratchet mechanism
  • In an asexual lineage:
    • Given that mutations will occur, an asexual lineage will build  up the total number of mutations in the genome (this is the ratchet)
    • Most mutations are deleterious, so the fitness of the individuals decreases with increasing number of mutations
    • As fitness declines, so does the population size of the clone, and the risk of extinction increases
  • Here, recombination will break the ratchet by reconstituting the lineages with fewer mutations
• Adapting to fluctuating environments
  • If the environment changes from generation to generation, then preserving successful combinations of genes from one generation for the next might not guarantee success
  • Recombination means individual's offspring are not identical and the parent has a greater chance that some of its reproductive output will be successful in the future
    • Problem for this model was that greater variation in physical factors seems to favor asexual lineages (asexual fish in temporary ponds, sexual relatives in permanent lakes) and the actual level of variation is small (the average temperature, year to year, in TN does not vary by much)
  • Solution comes from the importance of parasitism and disease in fitness - here, the agents of disease are very variable and there is a real short-term advantage for variation
    • We see the disadvantage of genetic homogeneity when entire fields of genetically identical crops are wiped out by a single disease agent
  • Variation in both parasite and host leads to an Arms Race, where changes by one opponent are matched by changes in the other

Sex Ratio and Sex Allocation

Sex ratio is almost a self-defining term but we must make the distinction between the sex ratio of a population and the sex ratio of individuals (individual sex ratio. brood sex ratio)

What is the optimal sex ratio?

• Rare Sex Advantage
  • Every generation, half of the genes come from male parents and half from female parents and this equality of contribution is constant
  • If one sex is more common than the other in a population, any gene that favors production of more of the rare sex by a female will have a selective advantage and the magnitude of the advantage is inversely correlated with the frequency of the rare sex (lower frequency leads to greater advantage)
• There is no rare sex advantage when there is no rare sex, i. e. when the sex ratio is 1:1 and that is the expected population sex ratio

When do we expect biased sex ratios?

• When there is Local Mate Competition
  • If the local males are all brothers of the available females (i. e., they are all offspring of the same female parent), then there is no rare sex advantage as all grand-offspring will carry the female's alleles from both parents as both parents are offspring of the original female
  • Without any rare sex advantage, ecological considerations play a role and the question of why produce males becomes relevant
  • Here, the female should produce as few males as possible to maximize the number of female offspring, who control the growth rate of the lineage
• When the rare sex is predictable
  • The textbook's example is excellent - female parasitic wasps normally expect local mate competition because they lay their eggs in a larvae with no other females eggs and so, when they emerge from the pupae and mate, they mate with siblings
    • When a female wasp detects that another female has laid eggs (called superparasitism), then she can expect that the first female's offspring will be mostly female and can gain a reproductive advantage by laying mostly male eggs (note that the brood sex ratio must be adjustable but this is not difficult for haplodiploid species)
  • The textbook presents data to show that, not only does the female wasp detect the presence of a prior brood but can judge the size of that brood and the relative size of the brood she will lay
    • The male advantage is only there when the second female's brood is much smaller than the first female's brood
    • When the reverse is true, the first females's daughters would be the rare sex if the second female produced all sons (remember that the second brood is larger)
      • In this instance, the second female should also produce more daughters than sons

Sex Allocation

Sex Allocation is the proportion of effort and resources given to each sex by an individual

• In animals, dioecy is most common but plants are mostly monecious
• In evolution, dioecy and monecy are seen as point along a spectrum (or gradient) of possible sex allocations with the end points (all allocation to one sex or the other) being the dioecious condition and everything in between being monecious (hermaphroditic)
• Two questions arise:
  • What conditions favor allocating all effort to one sex?
    • This depends on the relationship between reproductive success (number of viable offspring) and resource allocation to a particular sex
    • If it is accelerating (upward bending curve), dioecy is favored
    • If it is decelerating (a downward bending curve) then monecy is favored
    • A linear relationship favors neither
  • If one allocates effort to both sexes, then what effort should be put into each (what is the ratio of sex allocation, akin to asking what is the optimal sex ratio for dioecious species)?
    • The argument here is similar to that for standard sex ratio:  under most conditions, a 1-to-1 ratio of male to female effort is predicted
    • If one sex receives less effort in a population, any species that puts more effort into functioning as that sex will get a larger portion of the 50% contribution of the low-effort sex makes to the next generation (the 50% contribution of both sexes does not change for hermaphrodites)

Inbreeding and Outbreeding

Inbreeding means mating with close relatives (including oneself, so hermaphrodites are included), outbreeding with distant relatives

• Inbreeding decreases genetic variation in a lineage or population over time, which means more fixed alleles (mostly deleterious) over time
  • Inbreeding can produce Inbreeding Depression - a loss in viability, fecundity or growth experienced by offspring produced by inbreeding
• Inbreeding also reduces the chance of unexpected matings

Plants and animals have many mechanisms to avoid inbreeding

• Plants - incompatibility loci, sequential hermaphroditism, asynchrony
• Animals - mostly behaviors that lessen the chance of incest

Is inbreeding always detrimental?

• Can reduce chance of not finding a mate (increases reproductive assurance)
• Can prevent recombination for the most fit combination of genes (prevention of Outbreeding Depression)
  • This situation usually requires a chance linkage between a locus affecting the probability of inbreeding and the loci influencing the increased fitness
  • Locally adapted allele combinations would be preserved and inbreeding could sweep those combinations through local populations

Sexual Selection and Sexual Conflict

Sexual Selection

• Selection based on increased mating success (or increased gamete success) as the advantage to be gained is Sexual Selection
  • Can take many forms (contests among members of a sex for mates, attempts to overcome resistance to mating by members of the opposite sex, sperm competition)
• Sexual selection is an outcome of Anisogamy
  • Large egg means females make fewer gametes than males
  • Sperm outnumber eggs in population at 1:1 sex ratios
  • Causes mating strategy differences between the sexes
  • Female may only mate once to fully fertilize all of her gametes and so the male is an important choice
  • Males, no matter how often they mate, will not fertilize all gametes and each mating choice is not critical for overall success
  • Leads to greater variation in male mating success than female mating success (this difference is one measure of the intensity of sexual selection)

Forms of Sexual Conflict

• Contests among males for matings (or to prevent other males from mating)
  • Direct contests - when males confront rivals can lead to conflict (may be injurious to both winner and loser)
    • Often dominance and access to females is done through displays of some particular feature (color, song) and direct conflict is averted (lose can try elsewhere if no injury)
    • Mate Guarding - some females store sperm and can potentially have the sperm of more than one male or become receptive only briefly and here, mate guarding (either before or after copulation) requires direct contests
    • Some insects form sperm plugs that prevent subsequent mating
  • Indirect contests (like direct and indirect competition)
    • Out-last rivals
    • Find mates first
    • Kill offspring sired by rival
    • Make female less attractive post mating
• Sperm competition occurs when sperm of more than one male are present at fertilization
  • This can lead to some of the contests listed above
• Mate Choice
  • Sex with fewer gametes chooses mates from among available, competing mates
  • Choice often depends on possession of extreme phenotypes in chosen sex
  • Antagonistic selection may occur if exaggerated phenotype is beneficial for mating but carries ecological costs (is not favored by natural selection)
  • Reasons for mate choice
    • Sensory Bias
      • Some trait or traits that possess the ability to provoke positive mating responses from females may explain the origin of choice in some cases
      • This is a kind of "preadaptation" in which the preference develops before the male trait appears
    • Direct Benefits
      • If both parents contribute to the post-fertilization welfare of the offspring, choice of a good provider provides a direct benefit
      • Since the chooser cannot perceive the ability or willingness to provide directly, choice must be made on a character that is correlated with the desired traits
    • Indirect Benefits
      • If one mate only contributes its gametes to the next generation, then any benefits of choice will be through increased fitness of the offspring, not benefits directly experienced by the other mate
      • Good Genes Hypothesis: Mate doing the choosing needs "good genes" from its choice and will depend on a condition-dependent indicator of the presence of good genes
      • Such an indicator reflects the overall good condition of the  mate - correlated with the possession of good genes
• Runaway Sexual Selection
  • This model begins with two alleles for a male trait that can be used as a means of choice by females
    • T₁ is the normal allele and T₂ is a slightly exaggerated version (the exact details of the model will differ depending on the dominance relationship between T₁ and T)
    • The frequency of T₁ is t₁ and the frequency of T₂ is t₂
  • Originally, there is no genetic basis for choice by females but a mutation occurs at locus P that encourages P₂P₂ and (if the system works quickly) P₁P₂ females to prefer males with T₂ phenotype
    • The frequency of P₁ is p₁ and the frequency of P₂ is p₂
  • The appearance of mating preference has two consequences:
    • t₂ will start to increase as males with T₂ phenotype will get more matings because P₁P₁ females will not distinguish between T₁ and T₂ males and will mate with them with a frequency equal to their gene frequencies (t₁ and t₂)
      • P₂ females will choose T₂ males (in the simplest models the choice is exclusive but that is not necessary)
        • So T₁ males mate with only P₁P₁ females and T₂ males mate with P₁P₁, P₁P₂, and P₂P₂ females, an advantage that increases t₂ over time
  • P₂ will also start to increase, not through direct selection, but through hitchhiking
    • P₂ females choose T₂ males, which violates random mating and, as a result, causes a linkage disequilibrium between the P and T alleles (in other words, assortative mating links the P and T alleles, not a physical linkage but a behavioral linkage)
    • As the frequency of T₂ (t₂) increases, so will the frequency of any alleles linked to it - in this case P₂ is linked and, so, p₂ will increase
  • Further mutation for more extreme phenotypes is favored
  • If a new mutation occurs (T₃) that is even more exaggerated than T₂, it will most likely be preferred over T₁ and T₂ by P₂ females, which will give it the same advantage over T₂ that T₂ had over T₁
  • If a new mutation occurs (P₃) that causes a stronger preference for T₂ males, it will be more closely linked to T₂ than P₂ and p will increase as the result of hitchhiking
  • This is a "run away" system because the male trait gets more and more exaggerated and female choice gets more and more exclusive over time
    • Run away can be prevented by costs associated with either more exaggerated T traits and costs associated with strict choice (bypass a mating today and you are risking finding no mate tomorrow or, for strong choice, never finding a suitable mate)
  • Indirect Effects and the Runaway Hypothesis:  The exaggeration of the the preferred phenotype and overall fitness of the population may both increase if the expression of the T phenotype is enhanced by the presence of a "good gene" (Say allele B₂) because B₂ enhances the overall condition of the organism (that's why it's a good gene) over B₁
    • Then, the combination T₂B₂ is the most preferred phenotype is an indicator of "good genes" so that, in choosing T₂B₂, the mate is securing enhanced fitness for its offspring.
    • This assortative mating behavior links T₂, B₂ and P₂, and all increase both through sexual selection and natural selection
  • Sexy Sons and the Runaway Hypothesis:  Fisher proposed a model for mate choice that lead to a runaway situation without any indirect effects
    • In this situation, sons of attractive males are also attractive  (reason u nknown)
• Antagonistic Coevolution and Sex (more Sexual Conflict, but between sexes, not within sexes)
  • Antagonistic coevolution results when one group (a species, or, in the case of sex, one of the sexes) benefits at the expense of the other
    • The exploited group is then pressured by selection to change so as to reduce the exploitation
    • The exploiting group then is pressured to change to maximize its ability to exploit
    • The resulting sequential changes in exploiter and victim is known as an Arms Race (taken from Cold War jargon)
  • Sperm competition may lead to Antagonistic coevolution between sperm and egg
    • Sperm compete to quickly penetrate the egg, which may increase chance of polyspermy (more than one sperm nucleus entering the egg), which may disrupt normal development
    • Egg responds by making its outer covering resistant to entry, which promotes selection for more efficient ability to penetrate on the part of the sperm, which puts pressure on the egg...etc.
  • In competition among males for mating opportunities, males may harm females, which sets up a sexual conflict
    • Male Drosophila have toxins in their seminal fluid which harm females
    • Many arthropod males force copulation on their mates, often harming the female in the process of mating
  • Sexual conflict and Mate Choice
    • if females become more and more reluctant to mate, then male traits that encourage mating may become more and more exaggerated to overcome the reluctance (Chase-Away rather than Runaway selection as the explanation of exaggerated male traits)
    • Chase-away selection is often uncovered because the exaggerated trait is expected to be more attractive to congeneric females than to conspecific females
    • Conspecific females are under selection to avoid mating and, so, to not respond to the exaggerated male trait
    • Congeneric females, which may have the same preference, are not under selection to avoid its attractions, and so may maintain their preference for the exaggerated male trait

Alternative Mating Strategies

In situations where males directly contest one another for mates, there is often an alternative strategy that avoids all conflict in favor of "sneaky" matings (Sneaky male strategy)

Some male contests take place in an arena, called a Lek, that is observed by females, who chose among the males displaying or contesting one another at the Lek

When size strongly affects mating or reproductive success, hermaphroditism is often sequential

• In some fish, males fight for mates and size is important for success
  • These fish are sometimes female when small and become males when larger
  • The sneaky males may also be present as small males - they don't bear the costs of fighting but also don't mate as often
• In other hermaphroditic fish, males do not need to be big to mate and the number of eggs produced is larger in larger fish, so small individuals are male (small males can still produce excess sperm in a given mating) and larger individuals are female
• Crepidula sp., the slipper limpet - a hermaphroditic gastropod (many snails are hermaphrodites), often forms social groups where one organism attaches itself to a larger congener and, in these situations, the smaller individuals are males and remain male until they becom larger  or the larger females die or leave (they move, but remain stacked as much as possible)

Last updated April 13, 2011