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Lecture 10

BIOL 103 Lecture Notes - Lecture 10: Genetic Load, Mate Choice, Genetic Variation

Course Code
BIOL 103
Virginia K Walker

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Biology Week 10
Sexual Selection
o Darwin envisioned two distinct forms of sexual selection:
Intrasexual selection generally male-male combat for control of females
Intersexual selection generally female choice of agreeable males based upon their charms
Batemans Classic Experiment
o Angus Bateman sought a universal principle explaining differences in behavior between male and female
o Measured relationship between # of matings and reproductive success in Drosphila
o More matings does not = more offspring
o A male is deemed more successful if he finds more mating partners
o The variance in male fitness is higher than the variance in female fitness
o Males should be eager to make with undescrimination
o Females should be passive with discrimination
Polygyny and Fitness
o A male that secures many breeding partners can have very high relative fitness
o In many species, males take extraordinary risks in pursuit of reproductive stress
o If a few males gain all the copulations, there would be many unsuccessful males
o Although the variation is greater, average male fitness is still equal to average female fitness
Trivers and role reversal
o Assurance of paternity worth it to the male seahorse to confirm that the babys are his (bc he is the one who gets pregnant)
o ole eesig speies hee ales do the paetig poide a test of the Batea Piiple
o The higher investing sex (M) should be the choosier and the other being more strongly sexually selected
o The Sex that does more parenting is choosier rather than the sex that is chillin is more flighty
o In shorebirds there is a great variety in sex-specific parenting responsibilities
Where females tend the nest, males tend to be showier and perform aerobatic displays
Where males tend the nest, the females tend to be larger and more colourful and more aggressive
Polyandry and Paternity
o When male nest care is limited, a female may need to convince several males to accept her eggs males will be choosey
o Of prime importance to the male will be paternity of the chicks he looks after
Invested energy into rearing another males offsring is worse than not breeding at all
Male care may often evolve as a way to assure paternity
o R.A. Fisher suggested that exaggerated traits could be explained by a mechanism he called runaway sexual selection
o If females prefer a trait carried by a male, genes for the preference will be passed to offspring, and offspring will also inherit
the genes for the trait itself
o Preference of genes and trait genes become linked
o Linkage means that offspring carrying the preference and the trait breed with other animals carrying these genes
o This feedback loop creates an evolutionary synergy that drives trait exaggeration
Direct benefits:
o When there are direct benefits, it is often easy to understand mate choice
o In many mating systems, males provide nuptial gifts to females that are of considerable value
Glandular secretions (crickets)
Prey items (insects and some birds)
The male hunts the bug, the female is super impressed with the males bug hunting abilities and then the
male lets the female see the bug and eat the bug (only a little), and then THE MALE HAS SEX WITH THE
o Other benefits include:
Parental care
Good territory
Indirect benefits: Why be choosey?
o Females expand considerable amounts of time evaluating males in many species with no direct benefits
o Mate searching is costly as suggested by mate choice copying
o Suggests there may be an indirect (genetic) benefit
o Indirect benefits could take three forms:
1. Universally good genes a mate has a lower mutation load or displays higher condition because the geneotype it
carries promotes survival
2. Compatible genes the chosen mate has a genotype that compliments ones own genotype
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o Evidence for good genes: offspring of the choosy survive better
Mate preference:
o Female grey tree frogs prefer males that have longer and more complex calls
o Frogs chorus to attract females
o Long calling males offspring grew faster towards maturity than the
offspring of shorter calling males
Do opposites attract?
o The simplest criterion for compatibility might be dissimilarity:
By choosing a genetically dissimilar mate, organisms can
increase heterozygosity
But there must be limits to this, or else other species would
be favoured
o Across animal species, a meta-analysis by Jiang et al. of mating pair
likeness across 254 species showed that positive assortative mating
was common and negative assortative mating was rare
o Positive assortment, if underlain by genes promotes lower
heterozygosity within breeding groups
o Positive assortment is so strong in humans that D.M Buss referred to
hua ate hoie as assotatie aissis
Two peas in a pod
o Humans choose their pets that look like themselves
o Photos of humans and dogs were taken and shown to undergrad students with a chice between two dogs
o The undergrads could choose which dog belonged to which human
o And it only really worked with purebread dogggos
I love you for your immune system
o Several studies have reported preference for MHC dissimilarity in humans and other animals:
Major Histocompatibility Complex is an important part of the immune system so outbreeding could result in
offspring with better immune function
“tudies hae used stik t-shit tests fo odou attatieess
Chaix et al. compared MHC of spouses to rest of the genome and found:
No difference in an African population (MHC dissim=genome dissim)
Strong diffence in European americanns (MHC dissim>>>>genome dissim)
Oage is the e…oage
o Some studies of female Trinidadian guppies show that they tend to prefer males with strong orange clours
o Is this a reflection of good genes?
o If the bright coloration signals ability evade predators, to find rare carotinoid pigments in the diet or is difficult to produces it
could be an indicator of genetic equality
o Or maybe it makes the male look like a YUMMY piece of fruit and the attraction represents sencsory exploitation
o If females seek the orange foods, like the calabash fruit, and her senses are turned into finding the fruite when the males of
colouring may stand out
o Maybe its an adcantage because it is or was rare at the time of the earlier studies
Na Bule’s Zea Finches
o Male finches articifially ornamented with leg band jewelry had higher pairing success
o Females that pair with them have lower rates of extra pair copulations and provide more nest care for young
o Ornamented males reduce nest care and solicit extra pair copulation
o Females mated to ornamented males invest more energy and testosterone in male eggs and bias their clutches towards sons
Alternative male Reproductive strategies
o Guppy males have at lease two distinctive mating strategies
1. Colourful displays
2. Sneak copulation is an alternative mating strategy maintained by negative frequency dependence
More cryptic males use surprise in mating with females
The guppy gonopodium is their intomittent organ (penis). Barbed, if a male can thrust his gonopodium into
the females gongopore then he may be able to latch on and inseminate
o I salo, the seake ales ae alled Jaks
Jacks mature early, at a small size gaining an advantage by their inconspicuousness
Prosper by their precious life history, even through discriminated against by fighter males and females
Lizards three colour morphs in Uta Stansburiana corresponding to genetically-mediated behavioral mating strategies
It does’t ed ith opulatio
o Pake’s ok ith the dug fl “athophaga i the ’s ad ’s opeed aaeess of post-copulatory sexual selection
o Early models saw sperm competition as a numbers game
I a fai affle odel, the ale ith the ost tikets stood to e the ie
Relative testis size is a good indicator of the intensity of sperm competition
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