Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Two Ways of Seeing Selection B A Here, ai and bi give the fitness of an A type and B type, respectively, in a group with i A types Selection occurs a2=2 b0=2 b1=3 B a1=1 a3=3 b2=4 A Here, pi gives the productivity of a group with i A types and fi gives the fraction of that productivity claimed by the A type. Selection occurs p0=6 (f0=0) p1=7 f1=1/7 p2=8 f2=1/2 p3=9 Individualistic or contextual perspective (f3=1) Multi-level or collective perspective Contextual b2=4 a2=2 Translation p i ia i ( n i ) b i ia i fi ia i ( n i ) b i Contextual Perspective Collective p2=8 f2=1/2 Collective Perspective ai bi fip i i (1 fi )p i ni 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 groupdependent fitnesses to individuals) – Collective perspective (assigns fitnesses to groups as well as how the productivity is shared between individual types) W.E. Hill, 1915 • 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 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 a and b, not p). • 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 p parameter, not an a or b). • 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) Coup 1 Coup 2 mostly nice hens mostly mean hens Artificial Ecosystem Selection • Swenson and colleagues performed artificial selection on ecosystem phenotypes (e.g., biomass of a plant growing in an evolving microbial community). W. Swenson Selection for High Biomass Selection for Low Biomass D.S. Wilson R. Elias No Selection for Biomass • • • 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. Question: What would be required for natural ecosystem selection to occur? • There may be practical applications (e.g., bioremediation, agriculture, biofuels, etc.) dry weight (mg) Ecosystem Response to Selection high low generation 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 prokaryote eukaryote solitary insect social insect • 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” 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. • 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. Question: How does this life cycle compare with cycles from multicellular organisms? Wrinkly Spreader (WS) 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. W. Ratcliff evolved ancestor • After a handful of transfers, they found yeast that had evolved a “snowflake” clustering phenotype. • These snowflakes: M. Travisano – Form by post-division adhesion. – Have a selective advantage under gravity selection (but not otherwise). – Reproduce by fission. fitness evolved ancestor minutes of growth gravity no gravity Selection at the Emerging Level • One snowflake isolate was subjected to varying degrees of selection: selection strength Weak 25 min. Int. 15 min. Strong 5 min. • 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.” early snowflake late 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 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).