Extend single-locus multilocus quantitative models
p2:2pq:q2
W0,W1,W2
Mendel's Laws & H-W Theorem
normal
distribution fitness
function
high heritability
Variation can be quantified
mean standard deviation:
variance: 2
Quantitative variation follows "normal
distribution" (bell-curve) iff [read:
"if and only if"]
Multiple loci
involved
Each
locus
contributes equally (variance is additive)
Each locus acts independently:
interaction variance ( σ2GxE )
minimal
Normal Distribution for Mean 100 10
Phenotypic variation has two
sources: genetic (σ2G)
& environmental (
phenotypic variance σ2P
= σ2G
+
additive variance σ2A
= σ2G
+
heritability
h2 = σ2G/ σ2A
= σ2G
/ (σ2G
+
"Heritability in the narrow sense": ignores GxE2 = interaction
variance:
Identical genotypes produce different phenotypes
in different environments.
Ex.: same breed of cows produces different milk
yield on different feed
Contra:
Norm
of Reaction for differential expression within & between
environments
Artificial
breeding indicates that organismal variation
is highly heritable
ex.:
Darwin's pigeon breeding
experiments
Artificial
selection on agricultural species
Commercially
useful
traits
improved by selective breeding
IQ scores in Homo:
h2 ~ 0.7
But: IQ scores improve with education:
Offspring / Mid-parent correlation
Fitness
function expresses relationship between
genotype & fitness
Function is a continuous
variable, rather than discrete values for W0,
W1, & W2
=> Most traits variable &
heritable
Many traits do
respond to 'artificial'
selection (h2 = 0.5 ~
0.9)
Many traits should
respond to 'natural'
selection <=
=> To demonstrate &
measure Natural Selection in nature,
Show experimentally
that heritable
variation has consequences for
fitness <=
What
happens
to
a
normal distribution under Selection?
Compare distributions before [left] and after [right]
selection
Directional
Selection
(I) Fitness function has constant
slope:
Trait mean shifted towards
one extreme of phenotype
trait variance changed
(II) Fitness function normally
distributed
Fitness function shifts mean; variance unaffected
In
single-locus models, limit of selection is
Elimination
of variation by fixation of favored allele
Clinal
variation of B allele in human ABO
system
In quantitative
models, rate limited by
substitutional genetic load:
Fitness "cost" (lost
reproductive potential) to replace disadvantageous
allele
"Soft" selection
Mortality density-dependent
In 'real' populations N(after)
~ N(before)
Survivorship proportional to fitness up to K: more
realistic
Selection affects recruitment to next generation
Ex.: In AS system, deaths from malaria or
sickle-cell are "replaced"
N continually "topped up" to K
Darwin's Finch (Geospiza fortis) adapts to drought:
larger birds survive because of changes in seed size & hardness
Developmental canalization
limits extent of directional selection
Systems
are
controlled
by
multiple epistatic loci:
difficult to select on all loci simultaneously
Organisms have mechanical limits:
e.g. size cannot increase indefinitely
Johanssen bean
experiment exhausted genetic variation within
lineage
(2) Stabilizing
Selection (AKA truncation selection)
Fitness
function
has "peak"
Trait variance reduced around constant trait mean:
optimal phenotype,
or tails of
distribution eliminated (truncated)
Limits: elimination of variant alleles
or, 'weeding out' of disadvantageous variants
homozygosity at multiple loci:
difficult iff variance due to recessive alleles
inbreeding depression: loss
of 'somatic vigor' in inbred lines
Ex.: Birth
weight in Homo
Modal
birth weight has optimum survival
Implications for future human
evolution ?
(3) Diversifying
Selection (two kinds)
There
is a lot of variation: can natural selection explain it?
No single optimal from:
trait variance increases
polymorphic: variation
maintained within populations
Ex.: Genetic variation in corn
snakes, tomatoes, bell peppers, snails
Ex.: shell
patterns in Cepaea snails
combinations of dark / light, banded / unbanded
shells varies with substrate
polytypic:
variation distributed among populations
Ex.: Clinal variation in Cepaea
snails
patterns
of banded / unbanded shells vary over short distances
Limits:
segregational genetic load:
Fitness "cost"
(loss of reproductive potential) from production of less
fit homozygotes
Overdominance: heterozygotes
have superior fitness
because
different
alleles
favored in different environments
Examples:
sickle-cell hemoglobin in Homo ('Contradictory'
selection)
Leucine Aminopeptidase (LAP) affects salinity
tolerance in mussels (Mytilus)
Heterodimers:
multimeric enzymes with polypeptides from different
alleles
often show wider substrate specificity, kinetic properties (Vmax
& KM)
Myoglobin in
diving mammals
Heterosis: heterozygosity at
multiple loci correlated with overall fitness
Ex.: correlation between phenotype
& He: antler points in deer
(Odocoileus)
Ex.: Hybrid
vigor: crossbreeding
of inbred lines in maize (Zea)
improves vigor in F1
Maintaining polymorphic phenotypes by selection
Alternative phenotypes favored in different environments
Batesian mimicry:
'Tasty' mimics converge on 'distasteful' models
Ex.: Viceroy butterflies (Limenitis) converge on Monarch
(Papilio) butterflies
Müllerian mimicry:
Distasteful models converge on each other,
Different combinations evolve in different
parts of range
Ex.: Heliconius
butterflies
Mertensian mimicry:
aposematic
(warning) coloration discourages predators
Ex.: non-venomous scarlet king snakes
mimic
venomous coral snakes with black / red / yellow
pattern
Frequency-dependent selection:
apostatic predation: thrush
predation on Cepaea
'search image' changes when prey type becomes rare
Sexual Selection (Darwin 1871):
'Exaggerated'
ornamentation disadvantageous to individual as survival
phenotype
but
favored in competition for mates (reproductive phenotype)
Sexual dimorphism in birds
& mammals
Antlers in deer (Cervidae)
used in male-male
combat
Runaway sexual selection
Females
choose
males
on
basis of secondary sex ornamentation
Offspring have exaggerated trait (males) & preference
for trait (females)
selection reinforces trait &
preference for trait simultaneously
New phenotype spreads
rapidly in population
(B) Disruptive
selection
Fitness function is a valley
Trait variance increases (like balancing selection), but
polymorphism unstable
Polymorphism can usually be maintained only temporarily:
One
of
the
phenotypes
will out-compete the other
unless different phenotypes choose different niches (Ludwig
Effect)
then this becomes Balancing Selection
or frequency
selection occurs
Scutellar
bristles in Drosophila
Selection for 'high #' versus 'low #' lines
=> 'pseudo-populations'
with reduced inter-fertility
Might
disruptive
selection
contribute
to speciation?
Darwinian natural selection defined
as
differential
survival & reproduction of individuals:
Can
selection operate on more inclusive biological
units?
Kin
(Interdemic) Selection
Differential survival & reproduction
of related (kin) groups (extended families)
Related
individuals share alleles: r =
coefficient of relationship [derivation]
offspring
&
parents
related
by r = 0.50 [They share
half their alleles]
full-sibs
"
"
r = 0.50
half-sibs
"
"
r = 0.25
first-cousins
"
"
r = 0.125
Inclusive fitness (WI) of
phenotype for individual i
= direct fitness of i + indirect
fitness of relatives j,k,l,...
WI = ai + (rij)(bij) summed over all relatives j,k,l,...
where: ai = fitness of i
due to own phenotype
bij = fitness of j
due to i's phenotype
rij =
coefficient of relationship of i &
j
If i & j are unrelated
warn: Windividual
= 0.0 + (0.0)(1.0) = 0.0
don't warn: Windividual = 1.0 + (0.0)(0.0) = 1.0
Such behaviors should not
evolve among unrelated individuals
What
is
the
fitness
value in a kin group?
Wbrothers = 0.0
+ [(0.5)(1.0) + (0.5)(1.0)] = 1.0
Wcousins =
0.0 + [8][(0.125)(1.0)] = 1.0
Such behaviors can evolve among related
individuals in (extended) family groups
JBS Haldane (1892 -
1964):
"I would lay down my life for two
brothers or eight cousins."
Humans evolved
in kin groups, function today in unrelated groups:
do evolved (Primate) behaviors persist?
Parenting behaviour:
'Broken wing'
display in mother birds
Mother sacrifices herself for
(at least two) offspring
[See also active
defense of offspring] [Warning:
language!]
Altruistic behaviour ( "unselfish concern
for others")
Alarm
calls in ground squirrels (Spermophilus)
females
warn
more
in
related groups
Can
behavior
help
unrelated individuals evolve?
Eusocial insects (Hymenoptera, Isoptera)
Haplodiploidy: females diploid, male drones
haploid
Females
workers
sterile
(Wi = 0): what selective advantage?
related
to
queen
by 1/2
related to sisters by 3/4
Care for sisters, don't have offspring
Text material © 2024 by Steven M. Carr