[Home] [FAQ] [ShortWorks] [Blog] [Books] [Quotes] [About] [Contact]
"Darwin thought sexual selection acted through male=male
competition and female choice. Those remain the principal mechanisms."
Two types of Interactions that lead to Sexual Dimorphism:
Choice = a signal receiver system where intrinsic
qualities of one individual increases probability it will mate with some partners
more than others. Consequences "more subtle and surprising" as one partner (usually
female) receives signal about the other's control of a resource (territory,
nest, food, defense) or potential for parenting- in general, selection for HEALTH
due to "reliable, honest signals that must be costly" to avoid sick cheaters
using such signals.
Competition for Mates: Among males who compete
for females (who chose, or agree, to copulate with them), and among females
who compete with each for the most attractive males (to chose from).
How did sexual selection originate? How does Sexual
Selection work? Any evidence for Sexual Selection? What determines its strength?
How does Sexual Selection work in plants? Do alternative explanations for sexual
dimorphism exist? Ecological differences and primary sexual "characters?"
Merely a consequence of anisogamy, different sizes
of gametes Egg and Sperm. In other words, eggs expensive, sperm cheap.
Unfortunately, this begs the question of the functional reason for that anisogamy.
Maybe the quality control of only one source for the all-important inheritance
of the cell's cytoplasm and organelles including mitochondrial DNA made possible
only through the Egg.
Mate Selection driven by Inbreed avoidance and frequency-dependent
selection. Some ancient lines of Ciliates represent over forty "mating types"
that must find a "not me" type to mate. Reduction to two mating types necessary
for evolution of anisogamy. (chicken or egg?) "Larger gametes for more energy,
smaller for great numbers of agile, motil egg-seekers to find "large, pheromone-producing
gametes."
Sex Pheromones, "chemical substances" produced by
females (butterflies the most famous, but people too) detectable by males with
extraordinary sensitivity to these chemicals, which implies over long distances.
Competition and Choice.
Competition: Contests (harem control or resource
defense which may not be selection, but protection), Scrambles (Plant pollen),
Endurance Rivalry "persistence" (book's caveat, italics mine: "competition co-occurs
with Mate Choice whenever females use competitive ability as the criterion for
choice."
Larger female dimorphisms infer selection for greater
fecundity. "Mate competition is the main explanation for sexual SIZE dimorphism
and weapons, like mammalian pinnipeds (male seals six times heavier) "striking
examples" (sic) where the males "control" the females, yet almost one-third
of all males practice sneak copulations through "Female Choice"! The girls "cheat"
under the nose of the harem master.
"Mate Choice" (from a male's perspective?)
1. Females prefer honest costly signals, good genes,
handicap hypothesis. A. stresses fitness and B. emphasizes costliness of honest
signal of "indicator mechanism."
2. Females preference passed down to offspring females
along with preferred trait in offspring males, Fisherian self-reinforcement,
"sexy-son" ("discriminating daughter") hypothesis.
3. Sensory Bias: Females inherit sensory capacities
from ancestors that bias traits selected. A female's Color-blindness makes her
"prefer" striking contrasts, not striking complementary colors of similar shades
(which she can't distinguish). Sensory bias hypothesis, might better be called
Specific Pattern Recognition. Book disregards other non-sexual causes for sensory
bias.
First Evolutionary Basis Hypothesis (Free Bogus Science section)
Indicator Mechanisms: Females "select males with traits that signal
health or resistance to parasites and disease." Four Necessary Conditions for
Evolution of Female Preferences Proven ONLY in Guppies, Pheasants, and Barn
Swallows:
1. Male "fitness" (reproduction) should decrease
with increased parasite infection (bad health)
2. Ornament condition should worsen with bad health
3. Health must be heritable (disease/parasite resistance)
4. Females should choose best ornamented and least
parasitized males (healthiest).
The above illustrates Anthropomorphic, overly-mathematical
and fundamentally illogical reasoning by virtue of neglect of pheromones and
other signals, and failure to examine the parsimonious and obvious, that females
may also select for "excitation" based upon the more vigorous, healthier male's
display of larger ornament, or more 'beautiful' ritual behavior. (This might
be called the Dandy Factor, evidenced by the barn swallow female's preference
for longest tails, even artificial ones.)
Female Promiscuity
One third of the blue tit nests's offspring sired
by more than one daddy. Females paired with more attractive males tend to stray
less (stayed home to fight off wives of less-attractive neighbors?) Simplified
paraphrase of book's illegible grammar: "If extra-pair (bastard) offspring of
attractive males also had better survival … it would be (easy) to explain… in
terms of direct phenotypic reward."
"Firm empirical evidence for genetic indicator processes
is still lacking." Better evidence based on benefits from parental care or resources.
Duh.
Female Choice
"No test supports Fisherian selection and excludes
the alternatives or estimates relative importance." Under similar constraints,
maybe Gravity doesn't exist either.
The much-touted Guppy experiment in Trinidad and
Venezuela, with predated populations moved upstream beyond waterfalls in area
with different predators, and "manipulated predation". After eleven years, or
30 - 60 generations, "results met expectations". Later, scientists explain without
details of a "rapid change." The two opposing pressures of Natural "Filtration"
due to predation against orange spots, and Female Choice (Fisherian Process)
of selection for orange spots. In other words, given free reign without undo
environmental filtration, female choice tends to 'beautify' through preferred
ornamentation(?).
Male Mortality
Male mortality in a natural Environment, due to
female preferences for ornaments that lessen the male's survival rate, would
stop or limit ornamentation. (Extravagantly ornamented animals can occur in
domestic or 'protected' situations, even when not 'as viable' in nature.)
Sensory Bias
Illogically separates sensory capabilities to only
sexual purposes.
Genetic variation for fitness. Illogical assumption:
"fitness is certainly under directional selection to increase, and under continued
directional selection the genetic variation for fitness should be reduced to
near zero." Fitness does not comprise one quality, but rather the variability,
the potential "reaction norm" of that individual's genotype, so that the female
would probably select for the variability, the more exotic, amongst the HEALTHY
male representatives of the genome.
"Three factors to maintain genetic variation for
fitness to allow sexual selection to work: Spatial, Temporal, and Mutational
variation that affects fitness." Unless species or population's genome restricted
by bottleneck, (Cheetahs) the genome should increase in variation. A steady
flow of favorable mutations into the population through gene flow and frequent
environmental change should filter and re-arrange the fitness rank of potential
phenotypic expressions (reaction norms) within the same genome.
Any evidence for Sexual Selection?
Female choice in 132 out of 232 cases of selection
seen in 186 species. 30 cases of male choice. 58 cases where males contested
access to female. In sum, 132 + 58 = 190 out of 232 species = 82% FEMALE CHOICE.
GUPPIES: Opposing pressures of natural "Filtration"
due to predation and Female Choice for orange spots. Katydids (Males produce
large, nutritious spermatophores. When food scarce, females limited by lack
of spermatophores so they compete and fought for males, and males chose females),
Redwing Blackbirds ("extravagant ornamentation … maintained by male-male interactions"
- homosexual parthenogenesis?!??), African Widowbirds (female choice preference
for artificially elongated tails)
What determines strength of Sexual Selection?
Parental care and mating systems give one sex "greater
potential reproduction" which influences operational sex ratio, the local ratio
of males (ready to mate) to females (ready to mate) which determines the strength
of sexual selection. Whew.
Darwin saw a relationship between parental care,
mating system, and strength of sexual selection.
Potential reproductive rates and operational sex ratio.
General principle for more complex mating systems: The sex with the higher
potential reproductive rate will compete more strongly for mates and be subject
to stronger sexual selection.
Monogamy:
Monogamy, when one male and one female form a "pair
bond" for one or more breeding seasons and neither monopolize more than one
member of the opposite sex (obviously, each other). Therefore, from the scientist's
viewpoint, monogamy occurs when it looks like a sexual couple always hangs out
together, not that they cannot stray in secret. This suggests that Monogamy
functions as a social, political, legal, or natural phenomenon that has little
"faithful" reality in Nature nor society. More evidence of scientific Male chauvinism:
"Occasionally, females monopolize males, as in resource-defense polyandry or
female-access polyandry, but such cases of polyandry are restricted to relatively
few species." In other words, simple monogamy means a male dominates the female,
never the other way around!
Sex-role reversal: Males care for young,
female reproductive success increases with number of mates, but males does not.
Females arrive early and fight for breeding grounds to attract greater number
of males.
Association of one male to one female at a time.
In areas where feed or nest resources difficult to monopolize, monogamy likely,
while defensible habitats favor polygyny.
Polygyny:(Polygamy includes polygyny and polyandry)
Male competition to mate with more than one female,
usually implies female-only parental care.
Polygynandrous where males guard nests filled
with several promiscuous females' eggs.
Resource-Defense polygyny Males control access
to females indirectly by monopolizing critical resource. Polygyny Threshold
- point where female prefers male with good resources over unmated poor boy.
Female-defense polygyny
Males directly control access to females (females
often grouped for other reasons)
Male-dominance polygyny (Male Chauvinist misnomer)
Females select mates from aggregations or leks "based
on quality of male's display or territory / resources. Mating success highly
skewed for a small number of males in the lek." Older? more experienced? Better
displays? Tendency for FEMALES TO COPY OTHER FEMALE'S choice? Seems to me that
the male's continuous potential for promiscuous insemination of multiple females
means that any male in a "pair-bond" subordinates his genes to the females,
i.e. female dominates his "genetic resource."
Although only anecdotal and without value
as supporting evidence, my wife agrees with this.
Male Botflies of genus Cuterebra fight mid-air with
wing-damaging collisions. In aerial aggregations or swarms, males move chaotically
and search for "incoming females"
Scramble Polygyny
Males search for mates
without overt competition
Polyandry
Association of one female to two or more males at
a time. Females arrive on natal breeding grounds early, fight for access to
males. Philopatric when female mammals mate with more than one male, or when
littermates have more than one father, system referred to as promiscuous (rather
than polyandrous), as when males also mate with multiple females and provide
little or no offspring care nor lasting bond to female.
Alternative Reproductive Tactics
"Mate Poaching" nonaggressive parasitic, or satellite,
male that lurks at edge of dominant male's territory and attempts to copulate
with females. Sneak copulations. Best tactic for an individual depends on its
status. Salmon- two males types, hooknose (a year longer to mature, red, hooked
mandibles and cartilage for fighting) that competes for females, and jacks=
smaller, but larger larvae tend to become jacks, use refuges to get close to
spawning females and fertilize their eggs.
Side-blotched lizards, Uta stansburiana, play rock-paper-scissors
game, Orange-throats aggressive polygynous with large territories- lots of testosterone.
Blue-throat milder and monogamous with smaller territories (lose to Orange),
Yellow resemble females in color and behavior and sneak-mate with Orange's females,
but Blue males "evict" them. Orange beats Blue beats Yellow beats Orange.
This work is licensed under a Creative
Commons Developing Nations license.
E-mail: Mark Plimsoll