Natural Selection in the Laboratory LIFS 691 Advanced Evolution

The Power of Selection in the Laboratory 1) Quantify heritability and genetic correlations (Falconer 1989; Intro. to Quantitative Genetics) 2) Link hypothetical selective environments to specific traits (Bennett & Lenski 1999, Integr. Comp. Biol.) 3) Increase the frequency of novel phenotypes for tests of hypotheses about adaptation (Conner 2003, Ecology)

Types of Selection Experiments Artificial SelectionArtificial Selection Investigator breeds selected phenotypes from a population for multiple generations. Semi-natural selectionSemi-natural selection Investigator maintains populations in controlled environments for multiple generations; the artificial environments select phenotypes.

Design Considerations Control lines Units of replication Minimizing genetic drift

Design Considerations Control lines All selection experiments require an appropriate control. For artificial selection, control lines are selected randomly with respect to the phenotype. For semi-natural selection, control lines are maintained under the same conditions as were the ancestral population. Units of replication Minimizing genetic drift

Design Considerations Control lines Units of replication The unit of replication is the population (or line), not the individual or family; a good rule of thumb is to use at least 4 lines per selection regime. Minimizing genetic drift

Design Considerations Control lines Units of replication Minimizing genetic drift Genetic drift within lines can cause a spurious result during selection; a good rule of thumb is to have at least 50 male- female pairs per selection line.

Design Considerations When sample size is limited, the optimal experimental design is constrained by a tradeoff. Little drift but low power High power but much drift Tradeoff

Replicate 2Replicate 1 Replicate 2Replicate 1 Random Control linesSelection lines Random Ancestral Population Initiating Selection Lines

Mechanics of Artificial Selection Selection LinesSelection Lines Step 1: score trait(s) in each individual and breed and randomly mate a subset of individuals with the extreme phenotypes. Step 2: Repeat step 1 using the offspring. Control LinesControl Lines Step 1: randomly mate a subset of individuals in the same manner as for the selection lines (traits are not scored so selection is random with respect to the trait). Step 2: Repeat step 1 using the offspring.

[Source: D. M. Hillis, unpublished data]

How strongly should one select? Too weakToo weak Response to selection will be undetectable because of mutations and genetic drift. Too strongToo strong Response to selection will be brief because additive genetic variance will be eroded. The trick is to maintain an intermediate level of selection; therefore, most people select 30-50% of the population for mating.

The Power of Selection in the Laboratory

If R=h 2 S, then heritability equals R/S. h2h2 Cumulative selection differential Response (Generation mean) [See Falconer 1989, Intro. to Quantitative Genetics. Wiley, New York]

The Power of Selection in the Laboratory

[Source: Coyne & Beecham 1987, Genetics 117: ] In many organisms, northern genotypes are larger than southern genotypes. A Bergmann’s Cline in Drosophila melanogaster

Semi-natural Selection in Drosophila melanogaster Three replicates of each selection lineThree replicates of each selection line Initial population sizes were 2000Initial population sizes were 2000 Held at 16.5° or 25°C for 5 years (number of generations differed between treatments)Held at 16.5° or 25°C for 5 years (number of generations differed between treatments) [Source: Partridge et al. 1994, Evolution 48: ]

Selection at low temperature resulted in the evolution of large size. ● cold lines ○ hot lines ♀♀ ♂♂

The Power of Selection in the Laboratory

[Source: McCabe & Partridge 1997, Evolution 193: ] Is large size adaptive in cold environments? Creation of Novel Phenotypes through Artificial Selection Selection at 25°C

[Source: McCabe & Partridge 1997, Evolution 51: ] Being big prolonged life in cold environments but not in hot ones.

[Source: Reeve et al. 2000, J. Evol. Biol. 13: ] Being big conferred greater fecundity in cold environments but not in hot ones.

The Power of Selection in the Laboratory 1) Quantify heritability and genetic correlations 2) Link hypothetical selective environments to specific traits 3) Increase the frequency of novel phenotypes for tests of hypotheses about adaptation