Short-term measures: "Life Table" parameters
rate of instantaneous increase (r) of a phenotype
dN/dt = rN = rN (K-N)
where K = carrying capacity
net reproductive rate: exp(r)
r is "compound interest" on N
replacement rate (RO):
lifetime reproductive output
~ er(at low density)
components of fitness:
that contribute to survival
Ex.: survivorship (expected survival time)
fecundity (# offspring at age x)
for populations of natural selection on
[cf. adjust / acclimate]
traits that change as a result of selection
are sometimes referred to as "adaptations" or "adaptive characters"
Life table analysis: survivorship and fecundity vary with age
= prob. of survival from
survivorship = probability of survival to age x+1 from age x
mx = fecundity (# offspring) at age x
then (lx)(mx) exp(-rx) = 1 (in a stable population,
x=1 where L = life expectancy)
Ro=(lx)(mx) replacement rate er at low density
is a discrete solution to the continuous
Consider a population with two
These phenotypes correspond to two reproductive 'strategies
iteroparous strategy: offspring produced over several seasons
semelparous strategy: offspring produced all in one season
and fecundity schedule will compare their life histories
life table parameters can be measured experimentally
is 50% / year:
both strategies produce 2 young / female / lifetime
=> both phenotypes are equally 'fit' [and N is stable]
times', survivorship increases to 75% / year:
iteroparous strategy produces 4 young / female / lifetime
semelparous strategy produces 3 young / female / lifetime
=> iteroparous phenotype is 'more fit' [and N is increasing]
times', survivorship decreases to 25% / year:
iteroparous Ro = 0.72, semelparous Ro = 1.00
=> semelparous phenotype is 'more fit' [and N is decreasing]
=> Population phenotypes will adapt to changing conditions
environment, K increases:
e.g., productivity of meadow increases
iteroparity more advantageous, population density increases
environment, r increases:
e.g., severity of winter highly variable
semelparity more advantageous, early reproduction favoured
maintain population size N close to K
long-lived, reproduce late, smaller # offspring, lots of parental care
E.g., many bird species, primates (including Homo)
short-lived, reproduce early, larger # offspring, little parental care
E.g., most invertebrates, some rodents
We can extend single-locus multilocus quantitative models
normal distribution fitness function high heritability
Variation can be quantified
coefficient of variation (CV) = (/) x 100
size effect when comparing variance:
Ex.: Suppose X = whale length Y = tail width
X = 100 1.0 versus Y = 1.0 0.1
CV of X = 1% CV of Y = 10%
Y is more variable, though X is larger
variation follows "normal distribution"
Multiple loci are involved
Each locus has about the same effect
Each locus acts independently
[interaction variance (see below) is minimal]
Variation has two sources: genetic (G2) & environmental (E2) variance
additive variance A2 = G2 + E2
heritability h2 = G2/A2 = G2 / (G2 + E2)
"heritability in the narrow sense": ignores GxE2
Identical genotypes produce different phenotypes in different environments.
Ex.: same breed of cows produces different milk yield on different feed
organismal variation is highly heritable
ex.: Darwin's pigeon breeding experiments
Artificial selection on agricultural species
Commercially useful traits can be improved by selective breeding
IQ scores in Homo: h2 0.7
[But: IQ scores improve with education: GxE2 is large]
Offspring / Mid-parent correlation
CV = 5 ~ 10 %
h2 = 0.5 ~ 0.9
[Read "Suggestions for using the Website" for used in this course]
Function is a continuous variable, rather than discrete values for W0, W1, & W2
=> Most traits vary & are
Many traits do respond to 'artificial' selection.
Many traits should respond to 'natural' selection.
=> To demonstrate &
we must show experimentally that heritable variation has consequences for fitness <=
Organisms typically exhibit engineering criteria of "good design"
variation in Hawai'ian honeycreepers (Drepaninae)
Beak type matches food type
of bat & bird flight
Slow fluttering bats versus fast soaring birds
Wings match aerodynamic principles
Assumption: Form & Function affect survival & reproduction
"Estimated time to extinction"
Are long-lived lineages "better adapted"?
versus modern mammalian orders (3D
Order persisted more than twice as long as any extant order
Ultimately out-competed by Rodentia
(sharks & rays) versus Teleosts
Body form is unchanged in 400 MY
Class is about as diverse now as at anytime in last 250 MY
orders [Hagfish & Lampreys] versus gnathostome
Descendants of Ostracoderms, 500 MYBP (million years before present)
Jawlessness works [ectoparasitism is probably secondary]
"Adaptive characters" cannot be separated from the organisms that bear them
say "Hair & feathers evolved from scales".
But: It is more accurate to say:
"Reptiles (with scales) evolved into mammals (with hair) and birds (with feathers)."
[and this isn't completely accurate either]
class, we might say
"The carnassial pair evolved from the P4/M1 combination."
But: it is more accurate to say
"Carnivorous mammals evolved from insectivorous ancestors.
The carnassial pair is adapted for slicing meat."
distribution can be described as a bell curve
with a particular mean & variance:
distribution under Selection?
(1) Directional Selection
function has constant slope:
Trait mean shifted towards favored phenotype
trait variance unaffected
models, the limit of selection is
Elimination of variation by fixation of favored allele
rate is limited by
substitutional genetic load:
"cost" of replacing non-favored allele ( "intensity" of selection)
Mortality is density-dependent
In 'real' populations: N(after) N(before)
fitness up to K: more realistic
Selection will affect recruitment to next generation
Ex.: If the first-born dies of malaria, s/he will be replaced.
More births occur such that N is continually "topped up".
Birth of succeeding offspring will maintain N near K
artificial selection on agricultural species
(Aristelliger) has "suction pad" feet:
lamellar scale counts increase with age
(Geospiza fortis) adapts to drought:
larger birds survive because of changes in seed size & hardness
(recall that size is heritable)
limits extent of directional selection
Systems are controlled by multiple epistatic loci:
it is difficult to select on all loci simultaneously
Organisms have mechanical limits:
size cannot increase indefinitely
Johanssen's bean experiment
Skull volume versus birth canal diameter in Homo
Phenotypes are not infinitely plastic:
[But: Eozostrodon lineage evolved into whales & bats]
Selection (AKA truncation selection)
Fitness function has a "peak"
Trait variance reduced around (existing) optimal phenotype,
trait mean unaffected
elimination of variant alleles
or, 'weeding out' of disadvantageous variants
homozygosity at multiple loci:
difficult iff variance due to recessive alleles (Lab #1)
inbreeding depression: loss of 'health' in inbred lines
Lab #1: Elimination of non-cryptic pepper moths (Biston)
Dark variants are eliminated rapidly in light environments
Light variants are reduced (more slowly) in dark environments (why?)
[This may look like an example of directional selection: why isn't it?]
shock in house sparrows (Passer) (Bumpus 1898)
Animals that die are at extremes of distribution
in Homo (Karn & Penrose 1951)
Modal birth weight is optimum for survival
Selection (two kinds)
There is a lot of variation: does selection explain it?
Fitness function has more than one peak (multi-modal)
Trait variance increases
polymorphic["strict sense"]: variation maintained within populations
Ex.: cornsnakes, tomatoes, bell peppers, snails, scallops
variation distributed among populations
Ex.: shell patterns in Cepaea snails
fraction of dark / banded shells varies with substrate
segregational genetic load:
loss of reproductive potential due to production of less fit homozygotes
In Lab #1, Exercise #2, about 1/3 of population "dies" in malarial environment
have superior fitness at a locus
because different alleles are favoured in different environments
sickle-cell hemoglobin in Homo ('Contradictory' selection)
Leucine Aminopeptidase (LAP) & salinity tolerance in Mytilus mussels
multimeric enzymes with polypeptides from different alleles
often show wider substrate specificity, kinetic properties (Vmax & KM)
myoglobin in diving mammals
heterozygosity at multiple loci improves general fitness
Hybrid vigour: crossbreeding of inbred lines improves fitness in F1
epistasis: high 'Hobs' is 'good
Ex.: correlation between phenotype & genotype: antler points in Odocoileus deer
Ex.: fluctuating asymmetry: Acionyx cheetahs are lopsided
Maintaining polymorphic phenotypic variation by selection
phenotypes favored in different environments
crypsis: Cepaea land snails match background (Fig. 13-06)
'Tasty' mimics converge on 'distasteful' models
Viceroy butterflies (Limenitis) converge on Monarch (Papilio) butterflies
Distasteful models converge on each other,
different combinations evolve in different parts of range
Heliconius butterflies (Futuyma 1997)
aposematic (warning) colouration warns off predators (Mertensian mimicry]
Ex.: scarlet kingsnake (nonvenomous) mimics
coral snake (highly venomous) [black / red / yellow pattern]
Fitness value of phenotype varies with frequency
predation on Cepaea
'search image' changes when prey type becomes rare
male' effect: females prefer "different" male
Male zebra finches with artificial crest get more copulations (Fig. 20-13)
Selection (Darwin 1871):
'exaggerated' phenotypes are disadvantageous somatically
but are favoured in competition for mates
Sexual dimorphism in mallards, peafowl, & lions
Antlers in Cervidae are used in male-male combat
Tail displays in peacocks attract mates
sexual selection': the Madonna
/ Ozzy Osborne Effect
Females choose males on basis of some distinctive trait
Offspring have exaggerated trait (males) & preference for trait (females)
selection reinforces trait & preference for trait simultaneously
New phenotype spreads rapidly in population
Fitness function is a valley
Trait variance increases (like balancing), BUT polymorphism is unstable
[Try NatSel with: q = 0.5, N = 9999, W0 = 1.0, W1 = 0.7, W2 = 1.0]
can usually be maintained only temporarily:
One of the phenotypes will outcompete the other
unless different phenotypes choose different niches (Ludwig Effect)
[and then this becomes Balancing Selection]
Drosophila (Thoday & Gibson 1962)
Selection for 'high #' versus 'low #' lines
=> 'pseudo-populations' with reduced interfertility
Might disruptive selection contribute to speciation?
Natural selection is ordinarily
differential survival & reproduction of individuals:
Can selection operate on other biological units?
Can such selection 'oppose' individual selection?
Differential survival & 'reproduction' of alleles
t-alleles in Mus
tt is sterile (W = 0)
Tt is 'tail-less' (cf. Manx cats) (W < 1)
t alleles are preferentially segregated into gametes (80~90%)
=> f(t) is high in natural populations (40~70%)
even though it is deleterious to individuals
Differential survival & reproduction of related (kin) groups (families)
alleles:r = coefficient
of relationship [see
offspring & parents are 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
= direct fitness of i + indirect fitness of relatives j,k,l,...
Wi = ai + (rij)(bij) summed over all relatives j,k,l,...
ai = fitness of i due
bij = fitness of j due to i's phenotype
rij = coefficient of relationship of i & j
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
value in a kin group?
Wbrothers = 0.0 + [(0.5)(1.0) + (0.5)(1.0)] = 1.0
Wcousins = 0.0 + [(0.125)(1.0)] = 1.0
Such behaviors can evolve among related individuals in (extended) family groups
"I would lay down my life for two brothers or eight cousins."
'Broken wing' display in mother birds
Mother sacrifices herself for (at least two) offspring
( "unselfish concern
'Alarm calls' in Belding ground squirrels (Spermophilus)
females warn more in related groups
Can behaviours to help unrelated individuals evolve?
Haplodiploidy: females diploid, male drones haploid
Females workers are sterile (Wi = 0): what is the selective advantage?
related to queen or offspring by 1/2
related to sisters by 3/4
Care for sisters, don't have offspring
Natural Selection may be the most
concept in biology.
It is ...
(1) Not "Survival
of the Fittest"
Herbert Spencer (1820 - 1903) "Social Darwinism"
the "naturalistic fallacy": 'is' = 'ought'
[Darwinian theory was accepted in part because
it could be read to support British imperial ambition]
not predation (nor inter-species competition, usually)
not "Nature red in tooth and claw"
Darwin: plants in desert 'struggle' for water
not equivalent to population growth:
population declined in semelparous example
(2) Not equivalent to evolution
Selection may conserve existing types (stabilizing
Evolutionary change ultimately requires new variation (mutation).
Migration, population structure, genetic drift are important.
(3) Not a tautology (a self-evident statement; a circular argument)
How do you know they're fit? Cuz they survive..."etc.
a syllogism (an if /
a logical consequence):
(2 & W & h2) => q
[cf. physics: F = M A depending on how Force, Mass, & Acceleration are defined
arithmetic: 1 + 2 = 3 because I and II make III]
(4) Not "Mother Nature"
a force, not a thing that acts
[We don't say, "Arithmetic causes one plus two to equal three."
We might say, "One plus two equals three. That's arithmetic.]
not good or bad (amoral)
/ verb / object distinctions
[In most languages, "nouns verb objects"
i.e., objects perform actions on other objects. Not.]
(5) Not teleological (goal-directed):
does not have "goal", "direction", or "purpose"
(Homo sapiens are not the endpoint of evolution!)
such phrases as "Natural Selection acts ..."
"in order to ...",
"for the purpose of ...",
"so that ...",
"because its trying to ..."
Text material © 2012 by Steven M. Carr