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Asexual = efficient and simple, small organisms
with large populations, higher latitudes and disturbed habitats, wider distribution
Sexual = Large organisms with smaller populations
in habitats where "selection may be biotic rather than physical". Sex seen by
Evolutionary Biologists as complicated, needs time and energy, requires a search/encounter
for mate, thought most organisms sexual.
"Evolutionary Biologists fail to find an obvious
general explanation of way natural Selection produces and maintains sex, beyond
the recombination at meiosis."
"Recombination may speed up adaptive evolution,
help purge deleterious mutations from the genome, and may be important in co-evolutionary
struggle between hosts and pathogens." May have to accept Pluralistic explanations
of sexual reproduction, maintained by distinct selective factors in different
organisms...
Reproduction method the major characteristic of
genetic systems. Sex produces genetically diverse offspring. "Reproduction as
the center of biology". Asexual = "Logical" Diplontic and Haplontic (mitotic
division to produce diploid gametes that undergo meiosis to produce haploid
individuals) life cycles. "Sex probably did not evolve for reproduction." as
a derived state "in essence, has nothing to do with reproduction." Less efficient,
complicated, takes longer, more complicated mechanisms, logistic problem of
finding mates, although "very common" among large, multi-cellular organisms.
Variation in Life cycles. Dandelion's apomixes produces
seeds with identical genes, triploidy and hardier at high latitudes. Cyclical
Parthenogenesis practiced seasonally by aphids otherwise sexual.
Patterns of Sexual Distribution: Phylogenetic "asexual
species of recent origin" Mammals = exclusively sexual. Ecological Distribution
of Sex: Asexual in extreme environments and higher latitudes, sexual in more
biotic environments (co evolution of interrelated species?)
Consequences of Sex: Segregation and recombination,
the "prime characteristic"
Indirect consequences: Male and female dimorphism,
sex cell anisogamous (fusing gametes with different morphology and size, big
scarce eggs or ovules. Isogamy "the ancestral condition" no difference and no
male-female distinction among creatures such as ciliates, unicellular algae,
fungi. Some mushrooms with "large numbers" of Mating Types, mating possible
only between "different" mating types.
When and why sex favored over Asex. Twofold cost
of sex: Twofoldness refers to how females produce Asexual females who could
produce the same number of offspring with all the same average fitness, double
the offspring numbers (if no non-productive males in the population)
Recombination creates and destroys favorable gene
combinations, affects the speed of evolution, the elimination of mutations,
repairs genes, affects host-parasite co evolution. Muller's ratchet: mutation
accumulates in small pops, asexual increases "dirtiness". Genetic Parasitism:
Selfish mutations and nuclear meiotic drivers (distort meiosis to their own
advantage) and cytoplasmic parasitic genetic elements. Evolutionary maintenance
of sex (over asex) : Empirical evidence. New Zeeland snails both asexual and
sexual become more sexual when infested with parasitism, or sexual contact passes
the parasites!
Mark Plimsoll: Sex as a cellular cleansing mechanism
to remove the effects and residuals of disease, alien DNA incursions, latent
tumor-provoking mutations, retroviruses, etc. as the alternative to asexual
SOMATIC cell division that passes diseased cytoplasm and DNA to the next generation.
Parthenogenic reproductive germ-cell lines probably cleanse as well.
Can there be one evolutionary explanation for sex?
Book's concession of intellectual defeat, parenthesis mine, in the discussion
of 'good arguments' for a form of weak pluralism: "We are forced to conclude
that sex occurs (only among mammals?) for historic reasons that fixed genomic-imprinting
in the lineage, making the asexual alternative impossible, not because natural
selection favors sex for some reason."
The sex of a gene's parents (the donor sex of
a particular gene or genetic locus, phenotypic attribute or trait system) affects
that trait's expression. Proof of the utility of sex, beyond the obvious dimorphic
potential of either an X or Y chromosome (which isn't the universal sex determinant
either!)
Mark Plimsoll: If one can accept the Life Force
as the universe's expression of a negative entropy, a force that builds order
in opposition to Thermodynamics' 2nd Law of incremental disorder as the natural
state of the physical universe, then continuous ordering extrapolates to increased
complexity (in Mark Plimsoll's concept of Evolution, increased complexity distinguishes
Evolution from natural "Selection" filtration, or even Devolution). Artificial
(Sexual) selection, called "FEMALE CHOICE" (and NOT the mere filtration of so-called
natural selection) between two types seems the simplest mechanism for the expression
of that natural force or impulse toward greater complexity. Perhaps research
on the attractiveness of "exotic" mate choices would offer evidence.
Sexual Selection (Female Choice) Summary:
1. Sexual Selection- natural selection where mating
success trades off with survival.
2. Sexual selection accounts for attractive ornaments
of animals and plants. Sexual selection among plants must be driven by insects,
for plants have little ability to "select"
3. Competition for the scarcer reproductive resource
which would choose the nonlimiting sex "well documented." In plain English,
female choice.
4. Fisherian process of "hitchhiking" genes for
both (female) preference and (male) ornament called "Sexy Son Hypothesis" (why
not Discriminating Daughter Hypothesis?) seen by book's authors as too complicated
and indirect (!) to explain ornaments in Lekking species. They want something
simpler (¿with less female control?) Males form Harems when females "gregarious"?
(from Animal Behavior Chapter Summary, first paragraph)
5. Author's arbitrary "correct order" to explain
sexual dimorphism:
1. Sex differences (size of egg or need for womb,
nesting adaptations?)
2. Ecological sex differences (Differences in each
sex's eating habits?)
3. Choice for direct phenotypic benefit (Anthropomorphic
and probably impossible to discern)
4. Choice for good genes (Selection for HEALTH,
maybe.)
5. The Fisherian Process (explains most non-functional
dimorphism to Mark Plimsoll's satisfaction.)
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E-mail: Mark Plimsoll