Download Darwin`s Second Idea – Natural Selection

Darwin’s Second Idea –
Natural Selection
• Evolution: controversial?
• Natural selection
– Darwin’s postulates
– Controversial?
– Two examples of NS
Darwin’s Ideas
1. Species evolve over
time: “descent
with modification”
Pattern
Charles Darwin
Darwin’s Ideas
1. Species evolve over
time: “descent
with modification”
Charles Darwin
–
Darwin compiled
abundant data
–
Accepted over 100
years ago
Still Controversial?
“...some scientists
doubt the idea that all
organisms have evolved
from a single common
ancestor.”
-- Steve Meyer &
John Campbell*
San Francisco
Chronicle, December
10, 2004
(after all these years)
*Discovery
Institute,
Seattle, WA
% who think evolution is a scientific
theory well supported by the evidence
This class
68%
Postgrad Ed
65%
52%
College Grad
High School
20%
Darwin’s Ideas
“a naturalists…might come to
the conclusion that each
species … had
descended…from other
species. Nevertheless,
such a conclusion… would be
unsatisfactory until it could
be shown how the
innumerable species
inhabiting this world have
been modified….”
Darwin’s Ideas
1.
Species evolve over time: “descent
with modification”
2. Adaptations arise by
natural selection
Charles Darwin
–
Major evolutionary
mechanism.
–
Controversial until
Modern Synthesis
(1940s).
Adaptation
Traits that increases
fitness of an
individual (ability to
survive or reproduce)
compared to
individuals without the
trait
NS explains:
• The shape of life
•
•
•
•
•
Chisel Bill
Rotated toe
Stiffened tail
Padded brain
Long tongue
Natural selection: 4 Postulates
variability
heritability
“competition”
Fitness
differences
Freq. of long necks increases
1. Variability
• Discrete
• Continuous
Phenotype
The properties of an
organism (morphology,
behavior, physiology,
etc.)
Variability
– Size, color
• Continuous
Frequency
• Discrete
– Size
Size
2. Heritability
Genotype
Alleles of an
individual
inherited from
parents
Phenotype
The properties of an
organism (morphology,
behavior, physiology,
etc.)
Heritability
Genetic basis:
Genotype
The set of
alleles
possessed by
an individual
• Single locus:
– discrete variation
• Polygenic:
– continuous variation
Heritability
• The proportion of phenotypic variation
due to alleles inherited from parents
Genotype
Environment
Phenotype
3. “Competition”
• More individuals are
born than can survive
– “Struggle for
existence”
• Differential
reproductive success
between individuals
– Survival,reproduction
• Relationship between
phenotype and
fitness
Selection
for largersize
size
Selection
for intermediate
Fitness
• Fitness varies among
phenotypes
Frequency
4. Fitness nonrandom
Size
Still controversial?
(after all these years)
• Given 4 postulates, natural selection
is inevitable
• Accepted since the Modern Synthesis
of genetics and evolutionary theory
Artificial Selection
Still controversial?
(after all these years)
“Intelligent design holds that certain features
of the universe and of living things are best
explained by an intelligent cause, not by an
undirected process such as natural selection.”
Phillip Johnson
Michael Behe
Darwin’s Finches
• Galapagos
Islands
• 15 species
• Differences
in beak size,
shape
Darwin’s Finches
Can natural selection
account for
adaptation of beak
size?
• Peter and Rosemary
Grant
• Medium ground
finch
• Study began in 1973
1. Variability: beak depth
•
Continuous variation, a quantitative
trait, many loci
• 2. Heritability
– Parent-offspring resemblance
3. “Competition”: during drought
– Fewer seeds, larger seeds
– Not all survive
Key Point 1
• Natural selection: not forward
looking
• Many selective deaths: lacked the
right bill for the drought conditions
4. Non-random success
Key Point 2
• Natural selection is
not a random,
chance, undirected
process
• Reproductive success
is associated a nonrandom set of trait
values
Darwin’s Finches
Adaptation via natural selection occurred!
Examples of Selection: 2
Flower color in Snapdragons: (Jones &
Reithel 2001)
•
•
Discrete character variation
single locus genetic control: S locus
– SS
homozygote & Ss heterozygote are
white, ss homozygote is yellow.
Example of Selection
Snapdragon example: Variation & Heritability
Fig. 3.3
Example of Selection
Snapdragon example: Competition & Non-random
fitness
Fig. 3.3
Example of Selection
Snapdragon example: Evolution occurred!
Frequency of
S allele
Before: 0.50
After: 0.57
Fig. 3.3
Key point 3
• Individuals experience natural
selection
• Evolutionary change occurs in the
population