Chapter 7, Section 7.2:  The Adaptive Significance of Sex

• Sexual vs. asexual reproduction
• Asexually reproducing population:  all individuals are females, meiosis is bypassed and eggs are diploid.  These develop into embryos without need of fertilization.  In absence of meiosis (and not taking into account mutation), daughters are genetically identical to their mothers.
• Sexually reproducing population: some individuals are female, others are male.  Eggs and sperms are haploid and are produced by meiosis.  [This is condition for animals; the plant condition is more complex].  Due to recombination and independent assortment, all of the gametes produced by an individual are genetically distinct, and all of the offspring of a mating will be genetically distinct from the parents.
• The problem:
• Sexual reproduction is the most common form of reproduction, but is not universal.  There are many examples of species that reproduce asexually, either obligatorily or facultatively.
• With the exception of the Bdelloid Rotifers (350 species, all asexual), asexual species are "taxonomically isolated" from each other, suggesting that asexual reproduction has arisen independently in these species, and is derived from sexual reproduction.
• The sexual to asexual shift does not involve major developmental change (diploid egg develops into embryo).
• Maynard Smith's "null model" (p.276, Fig. 7.17) shows that if number of offspring and survival of offspring are the same for sexual and asexual females in the same population, then the population will evolve toward universal asexual reproduction.
• There are other costs attached to sexual reproduction: courtship, display, attraction of pollinators (plants), territory defense, female must mate with male of unknown fitness, etc.
• So - Why do species reproduce sexually?
• Explanation 1:  Sexual reproduction reduces mutational load.
• Muller's ratchet (Fig. 7.20)
• In an asexual population, class of individuals with lowest number of loss-of-function mutations is lost by drift.  (Class with highest number of mutations [e.g. "5" in the figure] is continually being replenished by mutations in individuals in class with lower number of mutations [e.g. "4" in the figure]).  This is a one-way trip, hence the term "ratchet".
• As this happens, average number of mutations per individuals increases.   Ultimately this high "genetic load" drives population to extinction.
• In a sexual population, recombination and independent assortment reconstitute low-mutation genotypes.  For example, if allele a is lethal, Aa X Aa crosses will produce some viable AA offspring and natural selection will remove aa offspring.
• Experimental tests of Muller's ratchet:
• Andersson and Hughes: subjected lineages of bacteria to periodic bottlenecks (creating drift).  Result - loss of fitness in some strains, no strain evolved higher fitness.
• Lambert and Moran: compared mutations in rRNA genes in bacteria endosymbiotic in insect guts to free living relatives.  Endosymbiotic bacteria are subject to founder effect. Result - more mutations in endosymbiotic bacteria than in free living relatives.
• Muller's ratchet turns rapidly in small populations (more opportunity for drift), more slowly in large populations.  In large populations the reproductive advantage of asexual reproduction may compensate for accumulation of mutations, causing asexual reproduction to predominate.
• Explanation 2:  Sexual reproduction increases fitness in changing environments.
• Recombination increases genetic variance in a population. It creates (but also destroys) favorable gene combinations.
• "In a constant environment asexual reproduction is a safer bet [than sexual reproduction" - p. 282.  A female who has reproduced in an environment has proven fitness in that environment.  Why should she gamble with the fitness of her offspring?
• Problem: what aspect of the environment is changing fast enough so that the benefits of sexual reproduction offset its costs (e.g. 50% reduction in reproduction).  Answer - parasites and disease: "host-parasite arms race".
• Tests of hypothesis that sexual reproduction increases fitness in changing environments:
• Fish in desert pools (Top minnows, Poeciliopsis spp., native to American southwest).
• There are sexual and asexual species, all infested by flatworms.
• Asexual species are more infested than sexual species.
• In asexual species the most common clone is the most severely infested.
• Natural experiment: 1976 drought caused pools to dry up resulting in local extinction of populations.  In 1978 pools were recolonized by  a few individuals (opportunity for bottleneck effect).
• After 1978 sexual species with lowest genetic variance were the most severely infested.
• Experimental introduction of heterozygous individuals to sexual  populations with low genetic variance reduced infestation rate.
• Freshwater snails in Japan (Potamopyrgus antipodarum).  The species contains sexual and asexual populations.
• Snails are infested by several species of trematode worms which consume gonads of males.
• Fig 7.23 shows that populations most challenged by parasitism have the highest rates of sexual reproduction.  Explanation:  if parasite populations are rapidly evolving, sexually reproducing snails will have higher fitness because at least a few of their offspring will be able to resist parasites.
• Anecdotal evidence in support of the "changing environment" hypothesis:
• In species with "cyclical parthenogenesis" asexual reproduction prevails while environmental conditions are stable, switch to sexual reproduction is associated with uncertainty, e.g. dispersal to new host (aphids), formation of overwintering structure (Daphnia)
• In angiosperm groups with asexual species, these are most common in physically challenging environments (e.g. the arctic), not in environments where biotic stresses (herbivory, competition) are more important.