Announcements Movie tonight, 7pm, 151 Evrt: Social Climbers (Life of Mammals)

Overview Some of the costs & benefits of cooperation. Altruism & selfishness. Relatedness & kin selection.

The Major Transitions Maynard Smith & Szathmáry Replicating molecules --->Molecules in protocells 2. Independent replicators ---> Chromosomes 3. RNA as gene and enzyme ---> DNA genes, protein enzymes 4. Bacteria (prokaryotes) ---> Eukaryotes (organelles) 5. Asexual clones ---> Sexual populations 6. Single-celled organisms ---> Multicellularity 7. Solitary individuals ---> Eusocial colonies 8. Primate societies ---> Human societies (language)

Unicellularity --> Multicellularity Many occurred simultaneously, during the early Cambrian (~ mya). Occurred independently in plants, animals, and fungi (and others). Suggests that something happened: developmental constraint solved. environmental conditions changed. Allows division of labor, and specialization on certain tasks (differentiation). Transition 6

Problems that needed solving Inheritance of cell identity in differentiated tissues. Differential gene expression Pattern formation. relative position of cells vs. chemical gradients Soma vs. germ line Why be sterile?

free-living amoeba multicellullar slug fruiting body spores The social amoeba, Dictyostelium discoideum

spores fruiting body free-living amoeba multicellullar slug

The social amoeba, Dictyostelium discoideum Cheating can be prevented if: Non-relatives are recognized and excluded. or Even mixing occurs within slugs.

Used microsatellite markers to genotype sections of slugs. Mixed two clones together in equal proportions. Strassmann et. al (2000) Nature 1. Do unrelated clones mix together to form slugs? Social cheating in Dictyostelium

Strassmann et. al (2000) Nature Social cheating in Dictyostelium 2. Are clones equally represented in the stalk vs. spore? Genotyped and compared prestalk vs. prespore.

Social cheating in Dictyostelium Some clones appear to be specialized cheaters, but require wild-type clones for stalk formation. Cheating is not prevented: Non-relatives are not excluded. and Even mixing does not occur within slugs.

The basics of cooperation & conflict (among cell lineages) Cheaters often do not do well alone (become parasitic). Bottlenecks increase genetic similarity, and align interests. In some systems, cheating can - and does - arise. Control can be imposed by previous generation (maternal effects).

Cell lineages within multicellular organisms In most organisms, germlines are determined via: Somatic Embryogenesis Why be a leaf if you can be an acorn?

Cell lineages within multicellular organisms In most organisms, germlines are determined via: Somatic Embryogenesis Not everyone can become an acorn - plant would die

In most of the organisms we are familiar with, germlines are preformed or formed early

The unicellular bottleneck Because all cells are genetically identical, r = 1. This aligns interests. When cheaters go through bottleneck, they are non-viable (all acorns). Other potential benefits: dispersal diapause expose bad alleles Potential costs: ecological vulnerability costs and risks of development Why break down a costly, multicellular organism each generation?

Dispersal is risky Death Finding resources (food, home) Finding mate When the costs of ecological constraints are high, offspring do better by staying home. Solitary individuals to social groups Transition 7

Competing with relatives Limited resources Inbreeding Conflict over reproduction Risks of not dispersing: Family-based social groups So an individual’s decision should be determined by the balance of these costs and benefits.

Potential benefits of sociality Pooled resources/shared defenses. Increase indirect fitness (by helping relatives reproduce) Potential costs of sociality Parasitism Cheaters Division of labor. Shared resources

Shared defenses. dilution effect - prey group together in large numbers to overwhelm the feeding capacity of the predator. selfish herd - prey found on the inside of a clump will tend to survive, those on the outside are eaten. Prey constantly moving towards middle of a group and appear flock or school.

Caraco and Wolf Packer Shared resources

Fieldfare The larger the colony, the higher probability that some offspring will die of starvation. Shared resources

Social spiders (Anelosimus) Female fitness increases with group size intially, but then declines in largest groups.

Relatedness and indirect fitness. Some re-occurring themes Why not cheat? What happens when cheaters succeed? Conflict between selection at different levels. New levels of individuality (cells, organisms, colonies) can arise when selection at lower levels is repressed.