Adaptation
“Survival of the Fittest”
• Survival of the fittest was first used by Herbert
Spencer.
I. Natural Selecion
II. Modes of Selection
• At Wallace’s suggestion, Darwin borrowed the
phrase for his second edition of Origin of
Species..
Species
III. Adaptation
• Today, the circularity of this phrase is widely
recognized.
50%
50%
Necessary & Sufficient Conditions for
Evolution by Natural Selection
• Variation
• Heritability
• Differential Reproduction
STARTING POPULATION
17%
83%
ENDING POPULATION
1
Natural Selection at All Biological Levels
...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.
• Genes
g
• Organelles
• Cells
C. Savage in Wolves
• Populations
• Communities
Mitochondrial Evolution
Injection of foreign mtDNA into Host Cell
mtDNA
Preparing to inject
Per cent do
onor mtDNA
Mitochondria of the donor out
out--compete that of the host.
Cell Injection
Natural Selection in Medicine
HIV
Pneumococcus
Generation
Staphylococcus
[Source: Niki et al. 1989, Nature 341, 551551-552]
2
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
Quantifying Directional Selection
y = bx + c
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
y=
x=
b=
c=
Variance
Selection
[Source: Brodie et al. 1995, Trends in Ecology & Evolution 10, 313
313--318]
Empirical Measures of Fitness
relative fitness
trait value
slope (coefficient of directional selection)
intercept
[Source: Endler 1986, Natural Selection in the Wild, Princeton University Press]
Studies Demonstrating Natural Selection in the Wild
Direct measure
• Number of offspring that
survive to reproduce
N um be rs of Spe c ies
A n im als
C o nt.
Indirect measures
•
•
•
•
Growth
Survival
Mating Success
Fecundity
[Source: Cooper 1984, Journal of Theoretical Biology 107, 603603-629]
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]
3
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.
Disruptive Selection in Nature
Number
Number
Probability
of survival
died
survived
African Finch
Pyrenestes ostrinus
Grant 1986
[Source: Smith 1993, Nature 363, 618-620]
Summary
The Imperfection of Selection
•
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).
• 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)
4