Week 3 Flashcards
sex parity
50/50 divide is maintained by negative frequency dependent selection
Fisher’s principle
Parental strategies should evolve towards equal investment in offspring of two sexes
Assuming each sex is equally costly to produce
Maintained by negative frequency dependent selection - maintains equilibrium
genetic mechanisms of sex determination
Haplodiploid - happens in some wasps, sex is determined by number of sets of chromosomes an individuals receives. Haploids become males and diploids become female
Sex chromosome - xy sex determination and zw sex determination
environmental mechanisms of sex determination
Sex might be determined by photoperiod (some shrimp)
Temperature
Environmental social determined sex (clown fish)
parisite sex determination
Sex manipulating microorganisms
Bacteria are maternally inherited - increasing fitness of female hosts helps them
Turn wasps into an all female species
definition of aging
Physiological deterioration that results in decreased reproductive output and reduced survival fitness
what selection keeps sex parity?
Negative frequency dependent selection
can natural selection shape sex ratio? Trivers and Willard Hypothesis of sex allocation
natural selection should bias parents in a way that depended on the mother’s body. A healthy mother should produce the sex that is more costly to produce
In polygynous species, one male monopolises all the mating females
Deer
Most males have no success
All females will reproduce at least once - less variance in female reproductive success
If a female gains high fitness through producing sons then she is going to need to produce extremely fit sons that can outcompete other males
Some females are in high body condition, some are in lower body condition. Therefore high body condition females should produce males and low body condition should produce females
Still about 50:50 as a population but depends on ratio of high to low ratio females (50:50 if normal distribution)
Sex bias occurs in the individuals
Do intermediate condition produce at parity?
Assumes it takes different amounts of energy to make high quality males or females - not the case in all species
Assuming maternal condition differs (normal distribution)
Assumes offspring condition depends on mother’s condition
Assumes that maternal affects on offspring condition carry through to adulthood
Assumes body condition of males is more consequential to breeding success than female body condition
Insight: selection can shape the offspring sex ratio of individuals but the population will remain at parity
case study in red deer
Takes males about 6 years to grow big antlers to fight
Males will remain at the top of the reproductive success for only a few years
Higher social ranking females have higher condition
Females of higher rank (better body condition) produce more sons
Strong support for Trivers and Willard hypothesis
A lot of noise but a lot of signal too
how is this possible when male deer at XY and females can only give X?
sn’t sex determined by males??
Females may be selective of the sperm that fertilises the egg - is there some genetic mechanism that does this?
Selective abortion - in cases where low condition female has a son?
Higher glucose levels (in high condition females) might have an affect on the success of male blastocysts implanting
sex determination system should…
influence the degree to which females can bias their sex ratio
sex allocation in birds
Often produce 10 or so males or females in a row
This is at odds of the 50/50 chance of producing male or females
Sex determining system: ZW where males are ZZ and females are ZW
Females are heterogametic - possibly more scope for females to alter sex bias
sex allocation in Seychelles warblers
Males are more likely to disperse, leave home and get their own habitat
Parents in low quality territories might not want to produce females because they will stay at home and use resources - male bias
Parents in high quality territories have resources and might produce more females - female bias
80% males being produced in low quality habitats
Medium quality habitats produce at parity
High quality territory with 2 or more female helpers, parents began producing males
This might because of ZW
classic evolutionary theory of aging
Magnitude of natural selection decreases with age as most reproduction happens earlier in life
Harmful mutations that arise later in life are less likely to be selected against and are passed on through reproduction. These mutations accumulate in the population and lead to aging
Examples: alzheimers, many cancers, Huntington’s, Parkinson’s
why do reproductive probability distributions have a normal distribution?
The chance of surviving later life is reduced by extrinsic mortality hazards - accidents, disease, war
Death still occurs regardless of aging - this selects for reproducing early in life
Natural selection reproducing early in life and this results in accumulation of aging mutations later in life
Populations with higher extrinsic mortality might have earlier reproduction but shorter longevity
fruit fly experiment on extrinsic mortality
Increased extrinsic mortality = earlier reproduction = shorter life
Two treatments: high extrinsic mortality in one, low in the other
How? Random flies would be randomly picked off from the treatments
5 years of evolution was carried out
Early fecundity was more prevalent in the population with high extrinsic mortality and also had higher aging
theory of aging: mutation accumulation
Peter Medawar’s mutation selection balance
Aging is caused by mutations that accumulate
High equilibrium number of mutations that can be maintained in the gene pool when expressed in later age classes
Prediction:
One prediction that is specific to the MA hypothesis is that traits such as reproductive success should exhibit increased levels of genetic variation with advancing age
antagonistic pleiotropy
George Williams
Pleiotropy - when a gene can affect more than one trait
Mutations that are beneficial in early life but they come at a cost later in life
Increase survival to reproduction (so selected for) even though they have later harmful effects
Involves genetic trade-offs
Selection favours reproduction over lifespan
Aging can therefore be adaptive
Prediction:
Find mutations that affect reproduction at the expense of longevity
Methuselah gene in fruit flies gives them long life but they have much lower fecundity
which aging hypothesis?
not mutually exclusive. probably a bit of both
controversial topic in science:
eukaryotes evolved sexual reproduction but some are asexual reproducers too (tho rare and evolved late in evolutionary time)
costs in sexual reproduction
- energy finding a mate/sexual selection
- energy allocation to producing offspring and zygotes
- parental investment
- lower reproductive rate and generation times
- sexually transmitted disease
benefit of asexual reproduction
asexual females produce twice as many child-bearing offspring as sexual reproducers can
why does asexual reproduction not out evolve sexual reproduction? benefits to sexual reproduction
- genetic variation
- less competition for resources (fewer individuals)
- more likely to survive in changing environments (adapt)
- more resources invested per individual
benefits of recombination
- crossing over
- remove deleterious mutations without the whole gene being lost (mutation based)
- creates genetic variation for selection (red queen hypothesis)
sex and mutation based theories
- recombination
sex and adaptation to new stressful environments theories
- increases variation and adaptation
sex and parasites - red queen hypothesis
- parasites and pathogens reduce host fitness
- susceptibility depends on genotype
- coevolution where the host must keep evolving to be immune and parasite must keep evolving to be invasive
- in order to keep pace, sex evolved to allow for increased variation
