Polygenic traits are Many Gene Traits Trait value Number of individuals Gene 4230 Trait Value Individuals Gene Individuals

Polygenic traits are Many Gene Traits Trait Value Number of individuals + Positive Allele + increases Trait Value Negative Allele – Decreases Trait Value -

Natural Selection on Polygenic traits Trait value Number of individuals Trait value Number of individuals Natural Selection Natural selection by death of small individuals by death of small individuals Before Natural Selection After Natural Selection

Phenotype: Body Size Frequency SmallIntermediate Large Distribution of a Phenotype in Population BEFORE NATURAL SELECTION 0 1 Variance Average

Phenotype: Body Size Frequency SmallIntermediate Large Distribution of a Phenotype in Population BEFORE NATURAL SELECTION 0 1 Variance Average Peak is Average Value Spread around the Peak is Variance

Phenotype: Body Size Frequency SmallIntermediate Large Distribution of a Phenotype in 2 Populations: Black and Red 0 1 Variance Average Variance Peaks are equal: Average = Average Spread around Peaks Are unequal: Variance > Variance

Disruptive Selection Stabilizing SelectionDirectional Selection Body size (g) Body size (g) Body size (g) Selection for small and large individualsSelection for larger individualsSelection for mid-size individuals 0 Peak gets narrowerPeak shifts 25 Two peaks form 0 25 Number of individuals Three kinds of Natural Selection Pink individuals die; Blue individuals live. Die Before Natural Selection After Natural Selection

An Example of Stabilizing Natural Selection Natural Selection on Larval Body Size in the goldenrod gall fly. The Ecology involves 5 species: 1. The golden rod gall fly. 2. The golden rod plant. 3. A parasitoid wasp predator. 4. Two avian or bird predators: chicadees and woodpeckers.

Goldenrod, the ‘host’ plant, Solidago altissima (hexaploid). 130 species of goldenrod in N. America. All Pictures from Dr. Warren Abrahamson or Paul Heinrich

Solidago altissima is a clonal plant. That is, a cluster of stems that all belong to the same individual. Each stem is called a ramet and they are connected underground by a ‘root’ called a rhizome. All of the ramets connected to the same rhizome underground and are genetically identical.

Solidago altissima is a clonal plant. That is, a cluster of stems that all belong to the same individual. Each stem is called a ramet and they are connected underground by a ‘root’ called a rhizome. All of the ramets connected to the same rhizome underground and are genetically identical. Ramets = Stems Connected underground

Eurosta solidaginis, goldenrod gall fly FemaleMale This is the species that is experiencing evolution by Natural Selection. The kind of Natural Selection is Stablizing Selection.

Eurosta solidaginis female ovipositing into a goldenrod ramet. When the fly larva hatches, the maggot’s saliva causes the plant to make a gall. The fly larva will feed on the plant tissue inside the protective gall.

Gall, three weeks after egg.Gall, six weeks after egg.

Galls or swellings on the ramets of S. altissima. These galls are made by the plant, but are induced by chemicals in the saliva of the goldenrod gall fly maggot. In Winter: the above-ground Plant dies, But the maggot Lives on inside the gall!

The adult fly has no chewing mouthparts, so it cannot ‘chew its way out’ of the gall. The fly larva or maggot chews an exit hole in the late fall, when the above-ground plant is dead. When the maggot becomes an adult, it escapes out of the gall through the exit hole. The gall is an imperfect defense for the fly larva.

Why does the fly maggot live inside the gall? The gall is a defense against predators BUT The gall is an imperfect defense for the fly larva.

Eurytoma gigantea, is a parasitoid wasp that preys only on gall fly larvae. This is a female wasp on a green gall getting ready to oviposit her egg into the gall. Her egg will hatch and her larvae will eat the gall fly maggot. Agent of Natural Selection

Because her ovipositor is short, the female wasp attacks the smaller galls. Maggots in the smaller galls are eaten by wasp larvae. Fly larvae in the bigger galls are protected from wasp attack by the thick walls of the gall. Maggot survival strategy: make a BIG gall to escape wasps. Agent of Natural Selection ovipositor

In the winter, downy woodpeckers and the black capped chickadees eat the gall fly larvae. As many as 60% of all galls are attacked by birds during the winter. MORE Agents of Natural Selection

In order to get to the ‘chewy center’ of the gall where the maggot lives, a bird must peck through the protective outer wall of the gall.

The birds seek the largest food reward for their pecking effort and they attack the larger galls. Fly larvae in smaller galls are protected from bird attack.

Maggot survival strategy: make a SMALL gall to escape birds. Note: this is the opposite of the best strategy for escaping wasps!

In order to study Natural Selection acting on gall size, we collect 50 to100 galls from a single field in the early spring, after predation by wasps and birds has occurred. Then, we (1) measure the size of each gall; (2) open each gall and observe its contents (if any); And, (3) Sort the galls into four categories: Live maggot, bird predation, wasp predation, ‘other.’

Type of Gall Average Gall Size Gall-Fly in Galls mm Wasp-Eaten Galls mm Bird-Damaged Galls mm Average Gall Size mm Typical Data Set

Type of Gall Average Gall Size Inference Gall-Fly in Galls mm Flies survive in Intermediate size galls Wasp-Eaten Galls mm Wasps attack smaller galls Bird-Damaged Galls mm Birds attack larger galls Average Gall Size mm Average gall Size: all galls Conclusion: Stabilizing Natural Selection by the combination of predators favors fly larvae which induce intermediate size galls.

Type of Gall Average Gall Size Interpretation Gall-Fly in Galls mmHighest Fitness Wasp-Eaten Galls mmLower Fitness Bird Attacked Galls mmLower Fitness Average Gall Size mmAverage Fitness Conclusion: Natural selection by predation favors those fly larvae which induce intermediate size galls.

Fly Phenotype: Gall Size in mm Frequency SmallIntermediate Large Distribution of Gall Sizes BEFORE NATURAL SELECTION mm 0 1 Variance

Fly Phenotype: Gall Size in mm Frequency SmallIntermediate Large Distribution of Gall Sizes AFTER NATURAL SELECTION by Wasps Wasp predation eliminates many of these flies Wasp predation on the smaller galls, pushes the mean toward larger size. Natural Selection by wasps favors large gall size: Directional Selection. Average gall size with live Fly is larger.

Fly Viability Fitness: Relative Probability of Survival Fly Phenotype: Gall Size in mm SmallIntermediate Large mm 0 1 Wasp Predation Alone Low Fitness High Fitness Directional Selection: Trait values as one extreme have the Highest Fitness. Trait values at the other extreme have the Lowest Fitness

Fly Phenotype: Gall Size in mm Relative Fitness: w Fly Phenotype: Gall Size in mm Frequency X multiply two curves Fly Phenotype: Gall Size in mm Frequency Phenotype Frequency Distribution AFTER SELECTION Phenotype Frequency Distribution BEFORE SELECTION Relative Fitness Distribution = SELECTION 1 WASP PREDATION and SELECTION

Fly Phenotype: Gall Size in mm Frequency SmallIntermediate Large Distribution of Gall Sizes AFTER NATURAL SELECTION by Birds Bird predation eliminates many of these flies Bird predation on the larger galls, pushes the mean toward smaller size. Natural Selection favors small gall size: Directional Selection.

Fly Phenotype: Gall Size in mm Frequency SmallIntermediate Large Distribution of Gall Sizes AFTER NATURAL SELECTION by Birds Bird predation on the larger galls, pushes the mean toward smaller size. Natural Selection by bird predation favors small gall size: Directional Selection.

Fly Viability Fitness: Relative Probability of Survival SmallIntermediate Large mm 0 1 Fly Phenotype: Gall Size in mm Directional Selection: One extreme phenotype has the Highest Fitness. The other extreme has the Lowest Fitness Bird Predation Alone Low Fitness High Fitness

Relative Fitness Distribution = SELECTION Fly Phenotype: Gall Size in mm Relative Fitness: w 1 Fly Phenotype: Gall Size in mm Frequency Phenotype Frequency Distribution BEFORE SELECTION Fly Phenotype: Gall Size in mm Phenotype Frequency Distribution AFTER SELECTION Frequency BIRD PREDATION and SELECTION X multiply two curves

Fly Phenotype: Gall Size in mm Frequency SmallIntermediate Large Distribution of Gall Sizes AFTER NATURAL SELECTION by Birds AND by Wasps Bird predation has eliminated many of these flies Wasp predation has eliminated many of these flies Variance

Fly Viability Fitness: Relative Probability of Survival SmallIntermediate Large Distribution of Relative Fitness in relation to Gall Size mm 0 1 Variance Fly Phenotype: Gall Size in mm High Fitness Low Fitness Low Fitness

Fly Viability Fitness: Relative Probability of Survival SmallIntermediate Large mm 0 1 Variance Fly Phenotype: Gall Size in mm Stabilizing Selection: Intermediate phenotypic values have the Highest Fitness, extreme phenotypic values have the Lowest Fitness.

Relative Fitness: w Fly Phenotype: Gall Size in mm Frequency Phenotype Frequency Distribution BEFORE SELECTION Fly Phenotype: Gall Size in mm Phenotype Frequency Distribution AFTER SELECTION Frequency BIRD and WASP PREDATION and SELECTION Note: Variance is Reduced by Stabilizing Selection X multiply two curves

Stabilizing Selection on Birth Weight in Humans Mortality at Birth is HIGH For Very Small and For Very Large babies