Download Evolution: Pt I

What is a Species?
• The biological species – “Members of a group of
populations that interbreed, or potentially interbreed,
with each other under natural conditions to produce
viable offspring”
Individual characteristics vary within a species
Sources of Variation
(within and among species)
• Phenotypic variation
• Morphology
• Behavior
• Physiology
• Etc.
Sources of Variation
(within and among species)
• Phenotypic variation
• Genotypic variation
• Information contained
in DNA: the genome
• Starting point for expression of phenotypic
variation
Components of Natural Selection
•
Phenotypic variation
• Fitness differences – phenotypic
differences must influence fitness
to some extent for there to be
natural selection on the trait
• Heritability – phenotypic differences
must have a genetic basis if there is
to be evolution through natural selection
Components of Fitness in Plants:
A Few Examples
• Fecundity – number of gametes produced
• Fertilizing ability – male reproductive success
• Fertility – number of offspring produced
• Survivorship – chance of surviving
Phenotypic selection
• The effect of fitness differences related to
differences in phenotype
Directional selection
in Danthonia spicata
Total number of spikelets
Phenotypic selection
• The effect of fitness differences related to
differences in phenotype
Directional selection
in Danthonia spicata
Increasing fitness
Phenotypic selection
Number of seeds produced
Stabalizing selection
in Ipomoea purpurea
Increasing fitness
Phenotypic Selection in Multiple Traits
Number of fruits produced
Sea rocket: Cakile edentula
Stabilizing
Directional
Components of Natural Selection
•
Phenotypic variation
• Fitness differences – phenotypic
differences must influence fitness
to some extent for there to be
natural selection on the trait
• Heritability – phenotypic differences
must have a genetic basis if there is
to be evolution through natural selection
Heritability (h2) defined
“The amount of resemblence
among relatives that is due to
shared genes”
Heritability of Height in Field Mustard
Experimental design
• Choose pairs of plants
• Pollinate one plant with
pollen from the other in the pair
• Cover flowers to prevent
other pollen from fertilizing
• Collect seeds
• Measure height of parents
Field mustard
Brassica campestris
• Raise offspring from seed
• Measure height of offspring
Heritability of Height in Field Mustard
Field mustard
Brassica campestris
Partitioning Phenotypic Variability:
A Second View of Heritability (h2)
VP = VG + Ve
VP – Phenotypic variation
VG – Genetic variation
Ve – Unexplained variation
Partitioning Phenotypic Variability:
A Second View of Heritability (h2)
h 2 = VG / V P
Partitioning Phenotypic Variability:
A More Complete View
Vp = VG + VE + VGxE + Cov(G,E) + Ve
VE – Variation due to environmental
factors: plastic response
VGxE – Variation due to an interaction
between genotype and the
environment that influences
phenotype
Genotype by Environment Interactions
Response by a single
genotype
Genotype by Environment Interactions
Vg in high light
Vg in low light
Genotype by Environment Interactions
VE
Genotype by Environment Interactions
VGxE – Degree to which lines
are not parallel
Partitioning Phenotypic Variability:
A More Complete View
Vp = VG + VE + VGxE + Cov(G,E) + Ve
Cov(G,E) – non-random association between
genetic makeup and local
environmental conditions
Sources of Genetic Variation
Factors increasing variation
•Mutation
10-8 to 10-10 mutations per base pair per generation
10-5 to 10-7 mutations per gene per generation
One individual out of 10 per generation
Sources of Genetic Variation
Factors increasing variation
•Mutation
•Migration
Population #1
Population #2
Sources of Genetic Variation
Factors decreasing variation
•Natural selection
•Genetic drift in small populations (<1000)
Small population
Loss of a allele
Large population
p+q=1
p = frequency (allele A)
q = frequency (allele a)
Loss of A allele