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 Stabalizing selection in Ipomoea purpurea Increasing fitness Number of seeds produced

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 (h 2 ) 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 Raise offspring from seed Measure height of offspring Field mustard Brassica campestris

Heritability of Height in Field Mustard Field mustard Brassica campestris

Partitioning Phenotypic Variability: A Second View of Heritability (h 2 ) V P – Phenotypic variation V G – Genetic variation V e – Unexplained variation V P = V G + V e

Partitioning Phenotypic Variability: A Second View of Heritability (h 2 ) h 2 = V G / V P

Partitioning Phenotypic Variability: A More Complete View V p = V G + V E + V GxE + Cov(G,E) + V e V E – Variation due to environmental factors: plastic response V GxE – 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 V g in low light V g in high light

Genotype by Environment Interactions VEVE

V GxE – Degree to which lines are not parallel

Partitioning Phenotypic Variability: A More Complete View V p = V G + V E + V GxE + Cov(G,E) + V e Cov(G,E) – non-random association between genetic makeup and local environmental conditions

Sources of Genetic Variation Factors increasing variation Mutation to mutations per base pair per generation to mutations per gene per generation One individual out of 10 per generation

Sources of Genetic Variation Factors increasing variation Mutation Migration Pollen Seeds Population #1 Population #2

Sources of Genetic Variation Factors decreasing variation Natural selection Genetic drift in small populations (<1000) p = frequency (allele A) q = frequency (allele a) p + q = 1 Large population Loss of a allele Loss of A allele Small population

Evidence of Selection in Natural Plant Populations

Selection Among Populations

The Common Garden Experiments of Clauson, Keck and Heiesy (1948)

Differences in phenotype across a gradient: Yarrow (Achiella spp) as an example

What is the source of variation? Different species – genetic variation? Same species – phenotypic plasticity?

Common Garden Experiment Stanford – 100’ Mather – 4600’ Timberline – 10,000’ Step #1: Obtain Plants from Source Populations

Source Plant Clones (e.g., piece of root) Location #1 Location #2 Step #2: Produce Clones Common Garden Experiment

Source Plant Clones (e.g., piece of root) Location #1 Location #2 Step #3: Plant clones in common gardens Common Garden Experiment Common Gardens Location #1 Location #2

Stanford Common Garden

Mather Common Garden

Timberline Common Garden

Interpretation of Results: Pure Plastic Response Source PlantClonesCommon Gardens Location #1 Location #2 ? Location #1 Location #2 ?

Interpretation of Results: Pure Genetic Response Source PlantClonesCommon Gardens Location #1 Location #2 Location #1 Location #2 ? ?

Experimental Outcome: Growth of Mather Achiella Clones Plastic responseGenetic response

Copyright © by Jane Strong and Tom Chester A Second Example Potentilla glandulosa

Lowland Ecotype Lowland Plant ©Brother Alfred Brousseau, St. Mary's College

Montane Plant

Experimental Outcome: Growth of Potentilla Clones

Interpretation Part I Not a pure plastic response Not a pure genetic response What is the relationship between these organisms? Separate experiments show that crosses between different source populations produce viable offspring

Interpretation Part II These are not different species What then are they?

Ecotypes  the middle ground Genetically distinct organisms Phenotypically distinct in terms of Morphology Physiology Phenology Occur in distinct habitats Differences can be traced to ecological differences in home habitat Plants are potentially interfertile (i.e., same biologicial species)