Transcript PowerPoint Presentation - Running With the Red Queen

Running with the Red Queen
Why is there sexual reproduction?
Why is there a problem?
• Loss of adaptive gene combinations
– Let us assume that in a particular
environment the genotype AaBBCc has
the highest fitness.
– What happens when two individuals
with this genotype mate?
Why is there a problem?
AaBBCc AaBBCc
1AABBCC: 1AABBcc:1aaBBCC:1aaBBcc:
2AABBCc: 2AaBBCC: 2AaBBcc: 2aaBBCc:
4 AaBBCc
• Only 25% have the high fitness genotype
Why is there a problem?
• What happens when an individual
that is AaBBCc reproduces
asexually?
• 100% of the offspring have the
high fitness genotype!
It gets worse!
• The asexual individual produces
only female offspring. What are the
consequences of that?
• Let’s assume any female (sexual or
asexual can produce only 4
offspring) and that on average
sexual females produce two males
and two females.
It gets worse!
Generation
1
2
3
4
5
Sexual
2 female + 2 male
4 female + 4 male
8 female + 8 male
16 female + 16 male
32 female + 32male
Asexual
4 female
16 Female
64 female
256 female
1024 female
• Asexual Females leave more offspring
The Cost of Males
• This large numerical advantage for
asexual females is known as the “Cost of
Males” or “Cost of Meiosis”
• When the Cost of Males and the cost of
loss of adaptive genotypes are put
together there must be some advantage
very large advantage to sex for it to exist
at all.
Possible Solutions:The Vicar of Bray
• Is genetic variation a bad thing?
• Survival of a species in a changing
environment requires lots of
flexibility
• Speed of Evolution
Possible Solutions - The Vicar of Bray
Possible Solutions: The Vicar of Bray
• This is very plausible and was the first
and most common explanation given
for sexual reproduction.
• However it ignores a basic principle of
evolution, natural selection acts on
individuals not groups! An individual
which varies its offspring will always
be at a disadvantage.
Possible Solutions - Muller’s Ratchet
• When a deleterious mutation arises in
an asexual individual it will remain.
Over time an asexual population will
amass lots of mutations and
ultimately all individuals will be
carrying deleterious genes
• In sexual populations recombination
gives the possibility of creating
mutation free individuals.
Possible Solutions - Muller’s Ratchet
• Unfortunately this over simplifies the
case.
• In order for it to work we need two
assumptions or the advantage of sex is
not high enough:
– A high mutation rate (no evidence of this)
– Interaction between deleterious mutations
(no conclusive evidence of this.)
Possible Solutions: The Red Queen
• Other organisms are killed by the
direct or indirect actions of other
organisms more often than they are
killed by physical factors.
• Parasites and their hosts are in a
perpetual genetic arms race.
The Red Queen - Our best Explanation
“Well in our country,” said Alice, still panting a
little. “you’d generally get to somewhere else-if you
ran very fast for a long time as we’ve been doing.”
“A slow sort of county!” said the Queen. “Now,
here, you see, it takes all the running you can do to
keep in the same place. If you want to get
somewhere else, you must run at least twice as fast
as that!”
The Battle of the Sexes
• Once there are two sexes then
what are the implications.
• Since the number of eggs a
female can produce is finite and
the number of sperm a male can
produce is not, their strategies
for maximizing fitness differ.
Who has the Babies?
• The Hermaphrodites Dilemma
Why do some females eat their mate?
• It’s all about energy
How not to get eaten
• Help to rear the
babies
• Male investment in
offspring
– Find food
– Build nest
– Protection (women
and children first)
Choosing a Mate
• Since the female maximizes her fitness by having
high quality offspring she needs to choose a mate
that has superior fitness characteristics.
Tail of a Swallow
• Parasites and tail Length a test of Darwin’s
hypothesis
– Parasites should reduce the survival of a female's
young
– Parasite resistance should be heritable
– Parasite infection should lead to a visible sign in
the male's ornament
– Female's should prefer males which show an
indicator ornament that establishes his clean bill
of health
But it may not be that Simple!
• In experiments in a maze swallows with long tails
performed better than swallows with shorter tails.
• Females have long tails too.
A convoluted tale(tail)?
• Are tail streamers of females sexually or naturally
selected?
– Streamer length may reflect female quality (sexual
selection)
– Streamers may be selected as flight aids (natural
selection)
– Females may only have streamers because of a
genetically correlated response to their possession by
male swallows
A convoluted tale(tail)?
• Are tail streamers in males:
– A measure of male quality (sexual selection)
– The result of selection for precision flight (natural
selection)
– Sexual selection on a pre-existing naturally selected
trait.
Why be altruistic?
• If selection acts at the level of the individual
altruistic behavior would seem to reduce fitness
and therefore should be selected against.
Manipulation
• In this case altruism is deleterious e.g. brood
parasitism in birds.
Individual Advantage
• Aggregation - protection against predators
Kin Selection
• Hamilton’s Rule
rb  c
• Where r is relatedness b is benefit and c is cost
Kin Selection
• Florida Scrub Jays. Pairs may have up to six
helpers.
• Helpers are either full or half-sibs of the the
young thy are helping to rear.
• There are an average of 1.8 helpers per nest.
• Mumme (1992) experimentally removed helpers
from 21 nests.
Kin Selection
Initial Sample
Size
% Surv. from
egg to hatch
% Surv. from
hatch to fledge
% Surv. Fledge
+ 60 days
% Surv. Egg to
Fledge + 60
Helpers Removed
Control
45
63
67
68
30
63
33
81
7
35
Kin Selection
• Using this data we can calculate the benefit,
• Given 2 helpers on average then the benefit per
helper is.
35 7
b
14%
2
Kin Selection
• Cost of helping c
• Lower bound 0 - In the highly competitive
environment the Scrub Jay’s live in young birds
may not be able to mate
• Upper bound 7 - Value of reproducing without
helpers.
Kin Selection
• Since the helping behavior is to sibs vs. their own
offspring we must modify Hamilton’s rule as
follows.
rsib b  roff c
Kin Selection
• Since the genetic relatedness of full sibs and
offspring is the same 1/2 both the lower and upper
bound estimate predict that “altruism” will
increase fitness.
• Lower Bound
1 14  1  0
2
2
• Upper Bound
1 14  1  7
2
2