Lecture 3 Flashcards
What is epigenome?
Set of chemical marks in an individual’s genome that affects in gene expression. This set of marks is heritable but can be modified by environmental exposures
Epigentic factors (mechanisms)
- non coding RNA
- histone modifications
- histone variants
- chromatin remodeling factors
- DNA methylation
Why is there an epigenome?
- cell differentiation
- tissue differentiation
- life history events (e.g. puberty)
What’s in an epigenome? A calico cat
- a mixture of red - and black-based colours
- are always females (XX)
- One X chromosome has to be active, the second one is turned off
- X chromosome’s silencing in each cell appears to be random which results in their unique patchwork coat
Why is there an epigenome? What potential adaptive advantage could the epigenome provide?
- It allows for a changes in the expression of the, otherwise, static genome to respond to environmental conditions
- epigenetic changes associated with environmental stressors
- said changes are reversible
In plants they have been shown to temporarily activate genes allowing them to adapt to challenges
Life History Theory (LHT)
- suggests that ontogeny has been shaped by natural selection
- on average, organisms are expected to follow resource allocation patterns across their life span that result in an optimisation of their overall biological fitness
what is ontogeny?
Development from the moment of conception
What is overall fitness?
overall fitness = individual fitness + inclusive fitness
Individual fitness = offspring produced by self
Inclusive fitness = positive effects on the fitness of relatives
inclusive fitness
Inclusive fitness = positive effects on the fitness of relatives
To calculate if a trait (e.g., a behavioural trend) would have a positive selective value Hamilton developed the following formula:
rb > c
c = reproductive cost to self
b = reproductive benefit to the recipient
r = coefficient of relatedness
The problem is usually in estimating the reproductive cost and benefits
How do individuals maximise fitness?
By optimising resource allocation throughout the life span
- resource allocation is key to understanding Human Life History (LHT)
What is the principle of allocation?
(at any given life stage) resources (energy, time, etc) used for one purpose, cannot be used for another
Where and when to invest energy?
Natural selection expected to favor the evolution of optimal allocation patterns
- optimal allocation implies trade-offs
What do we need to allocate energy to?
- growth
- maintenance
- reproduction
What are the five main human life stages?
- prenatal
- childhood
- adolescence
- reproductive life
- post-reproductive life: menopause and andropause
What is prenatal life?
1st trimester: fertilisation to 12 weeks - organogenesis
2nd trimester: 4th to 6th month, rapid growth in length
3rd trimester: 7th month to birth, rapid growth in weight and organ maturation
What is postnatal life?
Infancy: birth to 3 years of age
Childhood: 3 to 7 years old
Juvenile period: 7 to 10 years old (girls), 7 to 12 (boys)
Adolescence: 5 to 10 years AFTER puberty
Reproductive: 18 years old until end of childbearing
Senescence: from end of childbearing to death
what does human childhood look like?
- relatively small body size
- comparatively slow and steady rate of growth
- dependent feeding (immature dentition) higher RMRs than any other mammalian species
- A large, expensive, fast-growing brain (87% of newborns RMR dedicated to it - twice that of chimps)
- motor and cognitive immaturity
- Adrenarche (shared with chimps) and mid-growth spurt (unique to humans)
What differentiates us from other apes?
Delays in:
- eruption of the first permanent molar
- menarche
- first birth
- shorter birth intervals
- we can rear multiple offspring simultaneously
what explains human childhood?
more time for developmental plasticity –> more precise tracking of socio-ecological conditions –> greater offspring survival rate —> greater life time fertility for the offspring
Anything special about human adolescence?
The growth pattern is quantitatively different in terms of amount, rate, and duration of growth
- human skeletal growth spurt is not accompanied by a similar rate of soft tissue growth as it tends to happen in other primates species (particularly in the case of males)
Length of time between age at puberty and age at first birth ~ 10 years
Why do boys experience adolescence?
- considerable delay between age at which viable sperm are produced at which men have their first child
- very few cultures consider adolescent boys as viable husbands and fathers
Time to learn/practice hypothesis
Ecological risk aversion hypothesis
Proven value hypothesis
Resource accumulation hypothesis
A prolonged sub-fertile juvenile period - why?
The learning hypothesis
- brain growth and learning necessary for success
- ecological risk aversion hypothesis - Avoid competing w adults for food and other resources - better to keep a “low profile” until ready
Why do girls experience adolescence?
practice of parenting skills? –> greater lifetime fertility
Reduction in infant mortality rates: 50-60% of first borns die in infancy among non human primates VS 39-44% in hunter-gather societies
Why are adolescent girls sub-fertile?
Adolescent sub fertility - reproductive immaturity, skeletal immaturity
Earlier births involve higher risks (e.g. LBW, prematurity and birth complications associated with narrow birth canals)
- Full fertility achieved about 5 years after menarche (average 12.7 years old)
Reproductive maturity - two fundamental trade-offs
Optimal allocation implies trade-offs
- Current vs future reproduction
- quantity vs quality
Current vs future reproduction
“costs of reproduction”
Do I reproduce now or wait for a better time?
At the species/population level, this determines major demographic patterns e.g. age at first reproduction, inter-birth intervals (IBI), fertility rate
quantity vs quality
with the same effort: many cheap offspring or few expensive ones?
At the species/population level, this determines life history traits such as duration of lactation, duration of the juvenile stage, total fertility
Quantity vs quality of offspring: Lacks principle and Blurton Jones study
Lacks principle: optimal number of chicks: intermediate brood size lead to the highest number of surviving offspring
Blurton Jone’s study: optimal interbirth interval among the Kung: IBI that led to the highest number of surviving offspring for the Kung parents was also the most common IBI in the population
does timing and intensity of reproduction have consequences for both moms and children?
yes
Senescence is associated with a broad range of changes such as?
- hormones
- immune system
- cardiovascular system
- body composition
- bones
- brain function
- reproductive system (menopause, andropause)
Three hypothesis has to why women stop reproducing
- follicular depletion (proximate)
- timing of senescence (ultimate)
- grandmother hypothesis (ultimate)
Timing of senescence hypothesis - ultimate explanation
End of reproduction set by females age specific life expectancy and duration of infant dependency
- reproduction senescence occurs at appropriate age of investment in the last offspring
grandmother hypothesis - ultimate explanation
Selection may have favoured early termination of childbearing in favor of investing in grown children and grandchildren
Patriarch hypothesis
Once males became capable of maintaining high status and reproductive access beyond their peak physical condition, selection favoured the extension of maximum life span in males
- shifts to less strength-based male-male competition
- this ability in human males to keep reproducing after their physical prime would have selected for longevity
- such selection would have been on autosomal genes - thus passed on to females as well
Embodied capital hypothesis (ECH)
- focuses on the co-evolution of brain capacity and longevity
- humans invest in brain-based capital, which yield high returns later in life
- a prolonged lifespan is necessary for reaping the rewards of those investments
key concepts:
Compared to other apes, humans have:
- extreme brain size
- prolonged juvenile period
- intergenerational transfers
- long life span
All co-evolved in response to a dietary shift
humans have specialised on the consumption of calorie-dense, low fibre foods
Our intelligence is associated with our long developmental period how?
- extended period of prenatal production of cortical neurons
- extended period of myelination
- extended period of dendritic development
- cognitive development not completed until the 20s
- associated with slowed aging of brain
expensive brains linked to higher quality but more difficult to acquire food resources
calorie rich underground plants storage organs (e.g. tubers) require extraction skills not mastered until individuals reach their twenties
- big game fishing and hunting require complex skills that take time to acquire
- even gathering can be risky and involve complex skills
humans have a unique age profile of energy production
- long dependency period
- energy production peaks in later ages (learning and acquiring skills takes time)
- payoffs offset the costs of the prolonged dependency period
- transgenerational transfers are required to support this profile
Embodied capital hypothesis summary
- exploration of high-quality, difficult to acquire resources
–> high levels of knowledge and skill
–> large brain and long (and low productivity) period of development
–> productivity increases with age -> lower mortality rates and greater longevity
–> compensation of low early productivity during the adult period via intergenerational transfers
Modern Life
- The human feeding niche and parental investment system depends upon cooperation between men, women, their extended families and larger social units
- This cooperative system has resulted in a complex web of specialisation and cooperative associations that encompasses complex modern economies
In this system education based embodied capital determines income and social capital. The latter two forms of “capital” affect survival, growth, maintenance, and reproduction
