Two Ways of Seeing Selection BA Selection occurs  0 =2  1 =3  1 =1  2 =4  2 =2  3 =3 Here,  i and  i give the fitness of an A type and B type, respectively, in a group with i A types  0 =6  1 =7  1 =1/7  2 =8  2 =1/2  3 =9  0 =0)  3 =1) Here,  i gives the productivity of a group with i A types and  i gives the fraction of that productivity claimed by the A type. Selection occurs BA Individualistic or contextual perspective Multi-level or collective perspective

Translation  2 =4  2 =2 Contextual  2 =8  2 =1/2 Collective Contextual Perspective Collective Perspective

Perspectives, Parameterizations and Pluralism The two crucial components to any trait-group model are: –The group frequency distribution –The fitness structure The fitness structure can be represented in different ways: –Contextual perspective (assigns group- dependent fitnesses to individuals) –Collective perspective (assigns fitnesses to groups as well as how the productivity is shared between individual types) These two parameterizations are simply different ways to package the same information. They are mathematically interchangeable. There can be benefits from culturing an ability to switch perspectives: “gestalt-switching” pluralism –Analogies, visualization, understanding difficult terms, sorting causal stories, and even practical benefits W.E. Hill, 1915

Levels of Selection Lecture Outline History of the levels of selection debate Pluralism in the group selection debate Artificial selection at higher levels Origin of new levels Summary

Poultry Breeding: The Individualistic Approach The standard approach to increasing the egg yield of a hen is to select the hen that produces the most eggs as a breeder (this focuses on  and , not  ). The expectation is that over generations, egg yield will increase. In some cases, using this selection regime actually led to decreases in egg production! What is going on?! breeder

Hens Live in Groups In the current poultry industry hens are raised in groups. It turns out that the hens with the highest productivity of eggs were the hens that were the most aggressive (e.g., pecking at others, stealing food, etc.). Thus, as you select for good egg-layers, you are indirectly selecting for nasty hens. As more nasty hens fill a coup, every hen’s productivity decreases. How could we address this problem?

Poultry Breeding: The Group Approach Muir (1995) decided to select instead breeders based on group (not individual) productivity (this is a  parameter, not an  or  ). Annual egg production was increased by 160 percent using this group selection regime! “If this [group-evolved] strain becomes widely used in the poultry industry, the projected annual savings will far exceed the money spent by the U.S. government for basic research in evolutionary biology.” (Sober & Wilson, 1998) mostly nice hensmostly mean hens Coup 1Coup 2

Artificial Ecosystem Selection Swenson and colleagues performed artificial selection on ecosystem phenotypes (e.g., biomass of a plant growing in an evolving microbial community). Selection for Low Biomass Selection for High Biomass No Selection for Biomass W. Swenson D.S. Wilson R. Elias

Ecosystem Response to Selection In 2 of 3 experiments, the authors reported that the ecosystem phenotype responded to selection in a sustained way. Different lines had different chemical profiles, suggesting that the microbes evolving had affects on abiotic character of the environment. Just as Darwin made a case for natural selection through analogy with artificial selection, the authors propose that natural ecosystem selection may occur. There may be practical applications (e.g., bioremediation, agriculture, biofuels, etc.) generation dry weight (mg) high low Question: What would be required for natural ecosystem selection to occur?

Levels of Selection Lecture Outline History of the levels of selection debate Pluralism in the group selection debate Artificial selection at higher levels Origin of new levels Summary

The Origin of New Levels Emergence of a new level in the biological hierarchy is termed a “major transition”: –Genes to chromosomes –Prokaryotes to Eukaryotes –Single cells to multi-cellular –Solitary individuals to societies What are the common themes for these transitions? – Lower level units cooperate to ensure the functioning of the higher level unit – There is often division of labor between the lower level units – There is a sense of “common fate”: lower-level units “go down together with the higher-level ship” prokaryote eukaryote solitary insectsocial insect

Potential for Major Transition in Bacteria? Rainey proposed an interesting hypothesis: –Cheats might serve as a type of propagule in a “multicellular mat life cycle” in his Pseudomonas fluorescens system. Question: How does this life cycle compare with cycles from multicellular organisms? Wrinkly Spreader (WS) The life cycle: 1. WS forms mat. 2. Mat grows. 3. And grows. 4. SM mutants arise. 5. SM mutants rise in frequency. 6. Mat collapses & SM are liberated. 7. Back mutation to WS occurs. Smooth Morph (SM)

The Real-Time Evolution of Multicellularity Ratcliff, Travisano, and colleagues performed an elegantly simple experiment with budding yeast: 1.Growth under shaken conditions 2.Culture sits undisturbed for 45 min. 3.Culture’s bottom 1% is propagated. After a handful of transfers, they found yeast that had evolved a “snowflake” clustering phenotype. These snowflakes: –Form by post-division adhesion. –Have a selective advantage under gravity selection (but not otherwise). –Reproduce by fission. ancestor evolved W. RatcliffM. Travisano evolved ancestor gravity no gravity fitness minutes of growth

Selection at the Emerging Level One snowflake isolate was subjected to varying degrees of selection: Larger clusters with a faster settling rate evolved under strong selection. A higher rate of apoptosis (cell death) evolved under strong gravity selection. These dead cells are found where the cluster splits– this is the origin of somatic tissue! “We conclude that selection was acting on the reproduction and survival of individual clusters rather than that of their component cells.” selection strength Weak Int. Strong 25 min. 15 min. 5 min. early snowflakelate snowflake

Levels of Selection Lecture Outline History of the levels of selection debate Pluralism in the group selection debate Artificial selection at higher levels Origin of new levels Summary

Few topics in evolutionary biology have been as contentious as the debate over the units/levels of selection. Darwin invoked selection at levels above individuals; this process was formalized by V. C. Wynne-Edwards and criticized by George Williams. Higher level selection was resurrected by D. S. Wilson and colleagues in the context of multi-level selection theory. There have been artificial selection experiments targeting units above the level of the individual (e.g., chicken coups and microbial ecosystems). There have been real-time experiments on the origins of new levels of individuality (e.g., multicellularity in budding yeast).