Life History Evolution
1
Examples of tradeoffs
2
Life History tradeoffs
3
1!
Aging - a personal view
What is aging?
!!
Progressive loss of
function with age
Asafa Powell breaks 100
m world record. 2005.
Reuters
Austad and Finch, in Stearns and Koella, 2008
What is aging?
!!
Progressive loss of
function with age,
including
Decreasing fertility
rates
!! Increasing mortality
rates
!!
2!
What is aging?
!!
Progressive loss of function with age,
including
!!
!!
!!
Decreasing fertility rates
Increasing mortality rates
Two Principle questions
!!
Why does aging occur?
!!
!!
We will focus on ultimate reasons, not proximate
mechanisms
Why does aging occur at different rates in
different organisms?
Why not age?
!!
Individuals surviving to reproductive age represent
successful genes and gene combinations (at least in
the current environment)
Why mess with success? Makes sense to keep on
reproducing
!! Immortality could be a means of maximizing individual
fitness
!!
Why does aging occur?
!!
Hypothesis 1: Rate of living
!!
An inevitable outcome of living
!!
!!
Organisms just wear out
Time consumes all things, and age devours all
things with the hard teeth of the years, little by
little, slowly until death. - Leonardo da Vinci
3!
Why does aging occur?
!!
Hypothesis 1: Rate of living
!!
An inevitable outcome of living
!!
Problems
!!
Mookie and Me
!!
Organisms just wear out
Why does aging occur?
!!
Hypothesis 2: Turnover (not in book)
!!
!!
Programmed death to remove older generation
Clear out yesterday’s model to allow room for new,
and possibly more successful, offspring
Why does aging occur?
!!
Hypothesis 2: Turnover
!!
Problems
!! Death from aging is rare
under natural
circumstances
!! Individuals with an
extended or indefinite
lifespan would still have
a selective advantage
4!
Why does aging occur?
!!
Hypothesis 3: Mutation-accumulation
!! Genes that exert deleterious effects only very
late in life are protected from natural selection
!!
Why? The organisms are likely to already be
dead (or at least be done reproducing) by the
time natural selection has a chance to act!!
!!
!!
Selection Shadow
Examples - Huntingtons disease, Alzheimers
disease, some cancers
Why does aging occur?
!!
Hypothesis 3: Mutation-accumulation
Why does aging occur?
!!
!!
Hypothesis 3: Mutation-accumulation
If the Mutation-accumulation hypothesis is correct !
!!
What would happen to rates of aging if I restrict
reproduction to early in life? Why?
5!
Why does aging occur?
!!
!!
Hypothesis 3: Mutation-accumulation
If the Mutation-accumulation hypothesis is correct !
!!
What would happen to rates of aging if I restrict
reproduction to early in life? Why?
Huntington’s Disease
!!
Caused by a dominant allele
!!
!!
Woody Guthrie had it
Woody Guthries mother
had it
Huntington’s Disease
!!
Caused by a dominant allele
Woody Guthrie had it
!! Woody Guthries mother had it
!!
!!
A devastating disease - Will natural selection
be effective in eliminating it?
!!
!!
Woody Guthries mother had five children
Woody Guthrie had six
6!
Why does aging occur?
!!
Hypothesis 3: Mutation-accumulation
!! Genes that exert deleterious effects only very late in
life are protected from natural selection
!! Why? The organisms are likely to already be dead
by the time natural selection has a chance to act!!
!! Late acting deleterious mutations will accumulate in
a wide variety of systems, and will evolve as neutral
genes (drift, etc)
!! At advanced age, these deleterious mutations will
kick in, leading to general system failure
Why does aging occur?
!!
Hypothesis 4: Antagonistic Pleiotropy
!!
!!
Genes can have multiple effects (Pleiotropy)
Some genes will promote early survival/ reproduction at the
expense of longevity
!!
!!
!!
E.g. some tumor suppressing genes increase rates of aging
E.g. most reproductive hormones contribute to health problems
and early death (e.g. cancer)
Neutering promotes a longer (if not necessarily more fulfilling) life.
Neutered cats and dogs live three years longer, on average
Why does aging occur?
!!
Hypothesis 4: Antagonistic Pleiotropy
7!
Why does aging occur?
!!
Hypothesis 4: Antagonistic Pleiotropy
Genes can have multiple effects (Pleiotropy)
Some genes will promote early survival/ reproduction at the
expense of longevity
!! Some genes will promote longevity at the expense of early
reproduction
!!
!!
!!
E.g. devoting energy to repair will take energy away from
reproductive output
Why does aging occur?
!!
Hypothesis 4: Antagonistic Pleiotropy
Why does aging occur?
!!
!!
!!
Wear and tear? Probably not
Turnover? Tough to explain
Mutation accumulation? A likely factor
!!
!!
Deleterious genes acting after likely age of reproduction
will not be selected against
Antagonistic Pleiotropy? Also likely
!!
Natural selection may favor genes with late acting
deleterious effects but beneficial early effects on
reproduction
8!
The common message ...
!!
!!
The mutation accumulation and antagonistic
pleiotropy theories both imply that aging rates are
influenced by natural selection
Why, then, do organisms age at different rates?
The common message ...
!!
!!
The mutation accumulation and antagonistic
pleiotropy theories both imply that aging rates are
influenced by natural selection
Why, then, do organisms age at different rates?
!! Depends on risk of death due to other factors
!! Organisms with high extrinsic mortality rates should
favor early reproduction over longevity
!! Organisms with low extrinsic mortality rates should
favor longevity more
Experimental evidence
!!
Drosophila
!!
Increases in life span can be produced by
!!
!!
!!
!!
Restriction of reproduction to late in life
Direct selection for longevity
Reduction in the extrinsic
mortality rate
Why?
9!
Experimental evidence
!!
Consider the trade-offs
!!
Increases in life span can be produced by
!!
!!
!!
!!
Restriction of reproduction to late in life
Direct selection for longevity
Reduction in the extrinsic
mortality rate
Why?
Experimental evidence
!!
Consider the trade-offs
!!
Selection on late acting deleterious alleles becomes
stronger because they are more likely to be expressed and
to influence reproduction
!!
No longer in the selection shadow
Experimental evidence
!!
Opossums
Two populations of opossums. One on the mainland, one
on Sapelo Island near Georgia
!! Populations experience similar conditions, except
!!
!!
!!
High rate of predation
on mainland, no
predators on the island
Predictions for aging
rate?
10!
Comparative evidence
!!
Correlation between protection from predation and
low aging rates
!!
E.g. large mammals, turtles, birds, bats, spiny critters
A final example, your task !
!!
Coho salmon
!!
!!
Two size classes – “Hooknose” and “Jacks”
Males compete for proximity to females
!!
!!
Hooknose males are better fighters, are more attractive to females
Jacks are sneaky, come back a year earlier
A final example, your task !
!!
Masou salmon
!!
Dwarf males !
11!
Darwinian Medicine
34
Quotes about Darwin/Med
!!
!!
!!
Pro and con
From Eignor, Discovery Institute
The single incontrovertible Darwinian contribution to
the field of medical genetics was eugenics, which is
the Darwinian theory that humans can be bred for
social and character traits, like animals. The field of
medical genetics is still recovering from eugenics,
which was Darwin’s only gift to medicine.
35
Epidemiology
!!
Diseases sometimes caused by organisms
!!
Implications?
36
12!
Organisms evolve
Including disease organisms!
!!
Influenza
Organisms evolve
Including disease organisms!
!!
Phylogenies inform us about
the history of disease
organisms
!!
Including routes of transmission!
Organisms evolve
Including disease organisms!
!!
Phylogenies inform us about
the history of disease
organisms
!!
!!
Including routes of transmission!
And origin
13!
Antibiotic resistance
!!
!!
Evolution of penicillin resistance
Why the dip at the end?
Antibiotic resistance
!!
!!
Evolution of penicillin
resistance
Why the dip at the end?
!!
Resistance comes at a cost
!!
!!
Sensitive bacteria outcompete
resistant bacteria in the
absence of streptomycin
BUT !!
!!
Compensatory mutations might
occur!
The Evolution of Virulence
!!
What is Virulence?
!!
What is the proximate cause of virulence?
!!
!!
Deaths per infection
Take over the host’s cellular machinery to make copies of
itself. More copies, more damage
14!
The Evolution of Virulence
!!
Coincidental Evolution hypothesis
!!
!!
Virulence not directly selected, but related to traits that
are useful in another context
Short sighted Evolution Hypothesis
!!
Evolution within host
The Evolution of Virulence
The Evolution of Virulence
!!
!!
What about evolution in general?
Common Wisdom
Virulent diseases will eventually evolve towards reduced
virulence and benign coexistence
!! Why? What’s bad for the host is bad for the disease
organism
!! Do you agree?
!!
15!
Evolution of Virulence
!!
Trade-off hypothesis
!!
What do disease organisms need to do?
Evolution of Virulence
!!
Trade-off hypothesis
!!
What do disease organisms need to do?
The Evolution of Virulence
!!
Rhinovirus - the common cold
Relies on host mobility for transmission
Infects only those areas that promote transmission
!! Coughing and sneezing are not by-products of an
infection - they are the virus way of using us to
transmit their genes!
!! High virulence would be related to more reproduction, but
would lower transmission if the host was immobilized
!! Colds are relatively mild
!!
!!
16!
The Evolution of Virulence
!!
Small Pox - A deadly disease
!!
Plague of Antonius, Rome. A.D. 165-180
!!
Arrived in Mexico in 1520
!!
Killed millions
!! By 1618 the Native Population
had dropped from 20 million to 1.6 million
!!
Devastating throughout New
World after arrival of Europeans
!!
One Mandan Village went from 2000 to 40 in a few
weeks
The Evolution of Virulence
!!
Small Pox - How can it be so virulent?
Employs a sit and wait strategy
A durable pathogen - can remain viable for 10 years
outside host
!! Even if host is killed, it can infect new hosts
!!
!!
!!
E.g. through bedding, etc.
The Evolution of Virulence
!!
!!
Vectors promote the evolution of virulence
How?
17!
The Evolution of Virulence
!!
Animal Vectors - E.g. Nurses/E. coli
!!
Nurses handle babies, especially sick babies
The Evolution of Virulence
!!
Cultural Vectors - E.g. water/cholera
!!
!!
Causes severe diarrhea
and death
Virulence of cholera
strains varies
!!
Contamination of drinking
water favors virulence.
!!
Why?
Evolution of Virulence
!!
Cultural Vectors - E.g. water/cholera
Causes severe diarrhea and death
Virulence of cholera strains varies
!! Contamination of drinking water favors virulence.
Why?
!!
!!
!!
!!
!!
Reduces cost of immobilizing or killing host
Feces in water provide an efficient and indirect
transmission route
Virulent form of cholera was widespread in India
!!
Was displaced by less virulent form after water
purification programs in the 1960s
18!
Evolution of Virulence
What about human evolution?
!!
Process – Examples? More later !
!!
Pattern – Today
!!
“Mismatch to Modernity” - Humans have a history
What about human evolution?
!!
Do you have (or need) glasses, contacts, or
surgery to correct your vision?
Yes
!! No
!!
19!
What about human evolution?
!!
Do you have (or need) glasses, contacts, or
surgery to correct your vision?
Yes
!! No
!!
!!
Concordant
Pairs
Discordant
Pairs
Identical Fraternal
Twins
Twins
36
18
19
36
Myopia
!!
!!
!!
Myopia is very common
Myopia has negative effects
Myopia has a heritable component!
What about human evolution?
!!
Myopia
Why hasn’t natural selection eliminated the genes
contributing to myopia?
!! Genes and environment interact!!
!! Barrow Alaska
!! Few adults over 35 had attended formal school
!! Age 6-35 : 42% myopic; Age 36-88 : 5% myopic
!! Norm of reaction. Phenotype depends on genes and
environment!
!! Myopia is a recent disease arising from old genes
being expressed in a new environment
!!
What about human evolution?
!!
!!
Myopia
Breast cancer
Deadly, and often strikes early
12% of women in NA. Why is it so common?
!! Risk of breast cancer related to exposure to reproductive
hormones
!! Trade-off between reproductive success and survival;
AND !
!! Modern women live in unnatural conditions
!!
!!
20!
What about human evolution?
!!
!!
Myopia
Breast cancer
Deadly, and often strikes early
12% of women in NA. Why is it so common?
!! Risk of breast cancer related to exposure to reproductive
hormones
!! Monthly menstrual cycling is not the historical norm
!! Historically, women had many fewer (<1/3) menstrual
cycles than modern women
!! Spent much more time pregnant or lactating
!!
!!
What about human evolution?
!!
!!
Myopia
Breast cancer
What about human evolution?
!!
!!
!!
Myopia
Breast cancer
Sickle-cell anemia
!!
A recessive allele with very serious effects when homozygous
!!
Why does it persist?
!!
Heterozygote advantage
!!
!!
!!
q=0.2 in some African populations
Heterozygotes are more resistant to malaria
Frequency of heterozygotes (e.g. 2pq = 0.32) is very much
greater than the frequency of recessive homozygotes (e.g. q2 =
0.04)
21!
Darwinian Medicine
22!