2400: Major transitions

what has increased over time?

body size

multicellular complexity

slime molds

eat white fungi

some cells give up individuality for the community

in forests

help “clean” forests

group benefits

1. individuals give up being able to reproduce independently, and form a larger group that reproduces collectively

2. individuals that aggregate into higher-grouping can take advantage of economics of scale and efficiencies of specialization

3. efficiencies of specialization

4. aggregation and specialization facilitate organisms to develop new and more efficient ways to acquire, process, transmit, and store info

economies of scale

when a group performs a task more efficiently or does things one individuals couldn’t

efficiencies of specialaization

groups benefit form division of labor, allowing individuals to specialize in different tasks

major transitions from viewpoint of natural selection

larger benefits as a group

groups reproduce faster

division of labor

improved info processing

maintenance of group benefits

1. policing mechanisms

2. genomic imprinting

policing mechanisms

higher-level individuals that evolve ways to suppress cheating are favored in competition with other higher-level organisms

genomic imprinting

alleles are expressed differently when they’re inherited from the mother than from the father

prokaryotes

lack membrane bound organelles

DNA not enclosed in nucleus

evolved > 3 billion years ago

less complex

eukaryotes

membrane bound organelles

DNA enclosed in nucleus

evolved between 1-2 billion years ago

more complex

endosymbiotic theory

may explain the origin and evolution of the mitochondria and chloroplasts

engulfed aerobic bacteria may have become mitochondria

engulfed photosynthetic bacteria evolved into plastids

over time, symbiotic relationship became obligate: endosymbionts no longer able to live alone

endosymbiosis

a mutually beneficial relationship where one organism lives within the body (or cell) of another

supporting the endosymbiosis theory as

1. mitochondria and chloroplasts have diff genomes to that found in the nucleus

2. organellar genomes are single circular chromosomes (like bacteria)

3. chloroplast RNA is more closely related to cyanobacteria than eukaryotes

4. mitochondrial genes in eukaryotes resemble those of alpha-proteobacteria more closely

5. evidence for two main events of genes transferred from bacteria and eukaryotes that correspond with the timing of the origin of chloroplasts and mitochondria

archaea or bacteria: og host?

eukaryotic “informational” genes associated with transcription + translation → archaea

eukaryotic “operational” genes associated with metabolic processes, cell membrane, formation, amino acid production → bacteria

“operational” genes associated with the cytoskeleton → archaea

bacteria derived genes seem to be higher that archaea derived genes, but they’re more variable across eukaryotic genomes

3 domain model

biologists have favored this model for decades

archaea is monophyletic

eukaryota and archaea are sister domains

elaborate membranes, vesicle trafficking, cytoskeleton

2 domain model

new sequencing evidence suggests archaea may be paraphyletic and eukaryota is a subclade

first taxa identified in this group- Lokiarchaea- and others are extremely closely related to the eukaryotes

contains eukaryotic signature proteins not found in other archaea or bacteria

• cytoskeleton, trafficking

eukaryotes may be branch within the asgard superphylum or a sister group

asgard archaea

superphylum of anerobic archaea

lokis castle

hydrothermal vent

evolution of multicellularity

multicellularity across the tree of life reflects convergent evolution

coming tg and staying tg model

staying together: yeast and multicellularity

experimentally manipulated selective conditions that might favor multicellularity in yeast by selecting yeast that clump tg and sink

yeast evolved the ability to cluster into snowflake assemblages

favored characteristics: heavier, larger, and more hydrodynamic and spherical to settle faster

biologists believe more common than coming tg

coming together: slime molds and multicellularity

cAMP is released in regions with high cell concentration

newly arrived cells adhere to the multicellular slug by producing “sticky” proteins

slug uses the cAMP signaling process to orient and move

other benefits of colonial living

slug produces a slime sheath around itself to protect itself from predators

slime sheath is less expensive to produce as a slug than many individual sheaths

can reach new food sources quicker

can reproduce together

evolution of individuality

integrated and indivisible wholes that can reproduce and pass on heritable variations to their offspring

key transitions:

1. fitness is transferred from an individual cell to a higher level of organization

2. cells become differentiated, increasing complexity (impossible in single-celled organisms)

volvocine algae

diverged from their unicellular ancestor ~ 230 mya

model organism to study evolution of individuality due to their exceptional variation

• unicellular

• multicellular (unspecialized)

• specialized germ and somatic cells (evolved in 3 separate occasions)

volvox carteri

somatic cells

germ cells

movement is essential to stay close to the surface where nutrient abundance is high

mutants somatic cells that don’t produce flagella don’t compete to reproduce well

somatic cells will specialize in survival and growth for the colony

regA genes suppresses chloroplast proteins ~ somatic vs germ cells

regA

ris1 gene that regulates cell division in unicellular organism in response to environmental cues may have evolved to become this

regulates differentiation between somatic and germ cells

evolution from solitary to group living

benefits

• economies of scale

  • foraging

  • predator

requires

• some degree of sociality

• new levels of coordination and communication

group

a set of conspecific inds who effect each others fitness

foraging efficiency

foraging success per ind increases in groups by flushing out more prey

experimental las studies show a positive relationship between group size and amt of food per fish

passive benefit

• same foraging behavior but a positive aggregation impact

increase group size = increase food per capita

Chimpanzee

show cooperation through complex but subtle social rules that regulate access to fresh kills

• inds involved in hunt

• inds not involved

group living costs

1. proximity effect

2. cheaters

3. transmission of parasites

proximity affect

conspecifics as natural competitors for food and resources

cheater

inds that try to take resources from others in a group

cliff swallows

long term field study

group size increases the prob of eggs hatching and survival

parasitized by blood-sucking insect

scientists fumigated some bird nests and measured mortality

mortality higher in unfumigated nests

parasites increased with group size

overall fitness is higher in larger groups, not cost free