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Natural selection (1858)
Darwin
1809-1882
Wallace
1823-1913
Natural Selection
Natural selection is a process in which
Natural Selection
Natural selection is a process in which
If a population has:
a. variation among individuals in some attribute or trait;
b. a consistent (nonrandom) relationship between that trait and mating ability, fertility,
fecundity, or survivorship (’component of fitness’)
c. inheritance of the trait
Natural Selection
Natural selection is a process in which
If a population has:
a. variation among individuals in some attribute or trait;
b. a consistent (nonrandom) relationship between that trait and mating ability, fertility,
fecundity, or survivorship (’component of fitness’)
c. inheritance of the trait
Then:
1. the trait frequency will differ among age classes or life-history stages, beyond that
expected from ontogeny;
2. the trait distribution of offspring in the population will be different from that of the
parent population.
J. A. Endler, Natural Selection in the wild (1986)
Natural Selection
Natural selection is a process in which
If a population has:
a. variation among individuals in some attribute or trait;
b. a consistent (nonrandom) relationship between that trait and mating ability, fertility,
fecundity, or survivorship (’component of fitness’)
c. inheritance of the trait
Then:
1. the trait frequency will differ among age classes or life-history stages, beyond that
expected from ontogeny;
2. the trait distribution of offspring in the population will be different from that of the
parent population.
But this is natural selection and evolution, whereas it is better
to distinguish the two
Natural Selection
Natural selection is a process in which
If a population has:
a. variation among individuals in some attribute or trait;
b. a consistent (nonrandom) relationship between that trait and mating ability, fertility,
fecundity, or survivorship (’component of fitness’)
c. inheritance of the trait
Then:
1. the trait frequency will differ among age classes or life-history stages, beyond that
expected from ontogeny;
2. the trait distribution of offspring in the population will be different from that of the
parent population.
Natural Selection
“Natural selection acts on phenotypes. It is ineffective unless it favours one
genotype at the expense of another, but it may occur without doing so.
We (…) judge the intensity of Natural Selection (…) by a comparison of the actual
parents of the next generation with the population of which they are a sample.”
J. B. S. Haldane (1954)
Example of directional natural selection
Zbefore
S = Zafter – Zbefore = 1
Zafter
ΔZoffspring = h2S = 0.2
Zoffspring
Phenotypic trait
1a. Direct measurement - Observation
Selection on medium ground finch in the Galapagos
P. T. Boag and P. R. Grant (1981)
Intense Natural Selection in a Population of Darwin's
Finches (Geospizinae) in the Galápagos. Science 214:
Selection on medium ground finch in the Galapagos
P. T. Boag and P. R. Grant (1981)
Intense Natural Selection in a Population of Darwin's
Finches (Geospizinae) in the Galápagos. Science 214:
Directional selection differential S = Zafter – Zbefore
Gibbs & Grant 1987 Nature
1b. Direct measurement - Experimental
Mimicry in Heliconius butterflies
postman
rayed
H. melpomene
H. erato
http://en.wikipedia.org/wiki/File:Heliconius_mimicry.png
H. Eltringham (1916). Heliconius
butterflies. Trans. Ent. Soc.
London.
Races of Heliconius butterflies
across the Neotropics
Models: H. erato and relatives
Mimics: H. melpomene
2. Non-random evolution
Classic study: natural selection in the peppered moth
http://www.thetimes.co.uk/tto/science/biology/article2987019.ece
http://www.genetology.net/index.php/92/biologie/
The peppered moth: continuing studies
Map of northwest UK showing sampling
locations (circles) and urban areas (shaded).
The transect is the dashed straight line.
Frequency of the dominant
melanic phenotype
carbonaria along the transect
from WSW to ENE
1964-1975
2002
Prediction based on
selection and dispersal
Saccheri et al (2003). PNAS 105: 16212-16217
Natural selection is common
Endler (1986) survey
Direct demonstration of natural selection on
heritable traits in wild populations
Kingsolver et al (2001) 1984-1997 survey
How selection on discrete traits was measured using “s”
in Endler (1986) survey
Strength of selection (s) on discrete traits
Undisturbed populations
Perturbations field cages,
stressful environments
Selection at one life stage
may be counteracted by selection at another
http://animal.discovery.com/guides/wild-birds/s-y/song-
Selection may oscillate between years
Gibbs & Grant 1987 Nature
Long-term study of selection on medium ground finches
Standardized selection differentials (S),
calculated for each sample surviving
from year x to year x + 1.
From Grant and Grant (2014) “40 Years
of Evolution”.
Forms of natural selection
Directional selection: Average fitness is higher on one side of the population mean value than
the other.
Stabilizing selection: The maximum fitness lies within the range of trait values in the
population (between the extremes)
Disruptive selection: The minimum fitness lies within the range of trait values in the population
(between the extremes)
Endler, 1986
Forms of natural selection
Fitness
This population experiences
...only directional selection
...only stabilizing selection
...both types of selection
Trait
Forms of natural selection
“correlational” selection in song sparrows
Correlational selection: stabilizing or disruptive selection on a combination of traits
“correlational” selection in garter snakes
Directional selection differentials in the medium ground finch
S = Zafter – Zbefore
weight(g)
beak length (mm)
beak depth (mm)
beak width (mm)
1977-78 drought
Selection differential (S)
S
weight (g)
0.62
beak length
0.49
beak depth
0.60
beak width
0.49
Directional selection gradients in the medium ground finch
S = Zafter – Zbefore
weight(g)
beak length (mm)
beak depth (mm)
beak width (mm)
1977-78 drought
Selection differential (S)
S
β
weight (g)
0.62
0.51
beak length
0.49
0.17
beak depth
0.60
0.79
beak width
0.49
-0.47