Download Adaptation II. Modes of Selection III. Adaptation “Survival of the Fittest”

Adaptation
I. Natural Selecion
II. Modes of Selection
III. Adaptation
“Survival of the Fittest”
• Survival of the fittest was first used by Herbert
Spencer.
• At Wallace’s suggestion, Darwin borrowed the
phrase for his second edition of Origin of
Species..
Species
• Today, the circularity of this phrase is widely
recognized.
Necessary & Sufficient Conditions for
Evolution by Natural Selection
• Variation
• Heritability
• Differential Reproduction
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50%
50%
STARTING POPULATION
17%
83%
ENDING POPULATION
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...most authorities now agree that all dogs,
from chihuahuas to dobermans are descended
from wolves which were tamed in the Near
East ten or twelve thousand years ago.
C. Savage in Wolves
Natural Selection at All Biological Levels
• Genes
g
• Organelles
• Cells
• Populations
• Communities
Mitochondrial Evolution
mtDNA
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Injection of foreign mtDNA into Host Cell
Preparing to inject
Cell Injection
Per cent do
onor mtDNA
Mitochondria of the donor out
out--compete that of the host.
Generation
[Source: Niki et al. 1989, Nature 341, 551551-552]
Natural Selection in Medicine
HIV
Pneumococcus
Staphylococcus
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HIV inserts its DNA into immune cells. It’s DNA
mutates faster than that of any other organism.
Natural Selection in Engineering
• Variation
• Heritability
• Differential
Reproduction
Adrian Thompson
University of Sussex
Fitness
Fre
equency
Fitness
Before
After
Fre
equency
Fre
equency
Fitness
Modes of Selection on Continuous Characters
Trait
Trait
Trait
Directional
Selection
Stabilizing
Selection
Disruptive
Selection
Variance
Selection
[Source: Brodie et al. 1995, Trends in Ecology & Evolution 10, 313
313--318]
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Quantifying Directional Selection
y = bx + c
y=
x=
b=
c=
relative fitness
trait value
slope (coefficient of directional selection)
intercept
[Source: Endler 1986, Natural Selection in the Wild, Princeton University Press]
Empirical Measures of Fitness
Direct measure
• Number of offspring that
survive to reproduce
Indirect measures
•
•
•
•
Growth
Survival
Mating Success
Fecundity
[Source: Cooper 1984, Journal of Theoretical Biology 107, 603603-629]
Studies Demonstrating Natural Selection in the Wild
N um be rs of Spe c ies
A n im als
C o nt.
P lan ts
D is c.
33
4
34
71
4
0
10
14
37
4
44
85
P hysio log ica l
7
0
2
9
8
7
3
18
15
7
5
27
10
10
2
2
Tw o o r M o re
To ta l
—
—
Bo th
To ta l
C o nt.
D isc.
A ll Sp ec ies
C o nt .
M o rp ho log ica l
Bioc he m ica l
D isc.
—
—
Bo th
To ta l
—
—
B oth
To ta l
12
12
0
8
1
9
4
4
0
8
4
12
1
17
40
12
47
99
16
11
15
42
56
23
62
14 1
[Source: Endler 1986, Natural Selection in the Wild,
Wild, Princeton University Press]
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Frequ
uency
In most cases, directional selection was not very strong.
Directional Selection Coefficient
[Source: Kingsolver et al. 2001, The American Naturalist 157, 245245-261]
Small beaks cannot crack large, hard seeds.
Grant 1986
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Disruptive Selection in Nature
Number
Number
Probability
of survival
died
survived
African Finch
Pyrenestes ostrinus
[Source: Smith 1993, Nature 363, 618-620]
The Imperfection of Selection
• Natural selection modifies a preexisting
design.
• Natural selection produces designs that
are successful in the past environment.
• Natural selection is not progressive.
(e.g., selection has eliminated many organs of parasitic
species)
Summary
•
If heritable variation enables some individuals to
achieve greater reproductive success than
others, evolution must occur.
•
There is abundant evidence for natural selection
of morphological, physiological, and biochemical
traits in nature.
•
The process of natural selection causes the
spread of traits that enhance the reproductive
success of organisms in their current
environment (i.e., adaptation).
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