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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 1995

1. 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 earlyCambrian (~560-600 mya).

Occurred independently in plants, animals, andfungi (and others).

Suggests that something happened:developmental constraint solved.environmental conditions changed.

Allows division of labor, and specialization oncertain 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 lineWhy be sterile?

free-livingamoeba

multicellullarslug

fruitingbody

spores

The social amoeba, Dictyostelium discoideum

The social amoeba, Dictyostelium discoideum

spores

fruitingbody

free-livingamoeba

multicellullarslug

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 representedin the stalk vs. spore?

• Genotyped and compared prestalkvs. prespore.

Social cheating in Dictyostelium

Some clones appear to be specialized cheaters,but require wild-type clones for stalkformation.

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 aligninterests.

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, theyare 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 organismeach generation?

Dispersal is riskyDeathFinding 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 relativesLimited resourcesInbreedingConflict 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.