Cichlids in African lakes
and radiations
Lake malawi = 500 spp. of cichlids have evolved from a single CA that lived 5mya.
Lake Tanganyika = CA lived 10mya
Lake victoria = more recent radiation
adaptive landscapes metaphore
combos of genes create adaptive landscapes
- shape of the adaptive landscape depends on the pattern of gene interaction for fitness.
additive gene action gives a simple landscape with a single peak, epistasis gives more complex.
selection usually pushes popualtions to the top of one adaptive hill
from here adaptive and potentially developmental constraints prevent movement to other adaptive landscapes.
smooth adaptive landscape
no epistasis
each new gene adds a set unit
bumpy landscape
positive epistasis
genes are only exceptionally more beneficial when expressed together.
rugged landscape
negative epistasis
genes are beneficial only if in particular combinations
adaptive landscape in baby survival
single peak landscape of birth and weight gestation period in human infacnts
as birth mass and gestation period increases so does survival to a point
after this survival begins to drop (baby too big for birth canal)
adaptive landscape gater snakes
two peaks can be observed in two predator related traits
- reverse wriggle to escape predators
- camouflage with stripes
* when both are present the adaptive landspace drops as survival is low
individually the landscape peaks for each trait
predictability of cancer
cancers can be thought of as evolutionary process = not developmental
individual cancers are their own evolutionary lineage
gliboblastoma gene sequences
- no gene is mutated in all tumours, large diversity at gene level, no gene is essential for progression.
- convergence can be seen at a pathway level, although half of the gliboblastoma mutations in TP53 itself, 83% have altered P53 signalling.
Lenskis experiment
expose. E. coli strains to low nutrient environment
- make 12 replicate e. coli populations from a single cell, serial transfer to new flask occured daily (~7 generation per day) to maintain exponential growth
- new supply of glucose every morning, runs out by afternoon
- samples frozen
characters monitored in lenskis
mean fittness and cell size
- fittness calculated against ancestral
results of lenski
continued for 50,000 generations and still goin
1 mil years to do same in humans
- fitness increased over generation but began to slow
at 10,000 generations
fitness selects for itself.
av. cell size increased up to 1200 then plateus.
- step like increase in both
parrallel evolution in lenskis
sequenced genomes of evolving popualtions at different generations
- identified many potentially causal muutations
- atleast some evolved in the other popualtion too = parallele = ascertainment bias (only looked for them as found elsewhere).
massively replicated thermal evo experiment
al bennet
- lenskis colleague studied thermal adaptation in bacteria.
- evolved 115 lines of lenskis e.coli for 3k generations at 42 degrees
MRTE results
sequened and looked for co-variance between mutation and effects
- few strains shre exactly the same mutations
- homogeneity increases up the hierachy (fraction of shared alleles increase from point mutation, mutation, gene, operon, functional unit etc)
- ** only 33% of strains shared hits in the same functional unit.
- two groups of strains in multilocus space
- suggests adaptive landscape with two peaks
novelty in LTEE
at 31.5k generations one population became cloudy and grew to a higher density of cells
- tests showed popiulation had evolved to use citrate
why did the citrate mutation take so long to evolve?
- rare mutation
- other mutations had to evolve first to fix it
= historical contingency
testing citrate mutation origin
what did lenski do
reran evolution of life again
isolated clones throughout the hisotry fo the population and let them evolve
- asked when new CIT+ strains evolved
- only evolve after generation 31.5k
- so clearly mutations against which CIT+ mutations are adv. must have evolved and fixed firrst = potentiating mutation
what was the mutation that allowed citrate metabolism?
compared genome sequences from 29 CIT+ clones with ancestral
- e. coli lacks citrate transporter
- does have carboxylic acid transporter - expressed in anaerobic metabolism
- rnk promoter from aerobic metabolism was captured and fused to citG producing an aerobic citG hybrid.
- CIT+ oroginally weak, so boosted by increase in copy no.
what happened when CIT+ clone construct was put into other e. coli
did not work
showed there was a potentiating muttion
what are the three steps in evolution
- potentiation: mutation that is neutral or weakly adv in itself but that permits evo. of adv. mutation.
- actualization: an advantageous mutation that confers some weak new function
- Refinement: an advantageuos mutation that boosts the new function.
adaptive landscape of citrate mutation
bumpy landscape
- CIT+ locus is not advantagoues without the potentiating mutation, so cant evolve except by drift.
- potentiating mutation has a small advantage, allowing it to evolve
- once this potentiating mutation has evolved the CIT mutation can evolve too as the advantage occurs when they are together.
secret to cichlid raditaion hypothesis
perhaps they have a special trait: potentiator or key innovation that confers unusual evo flexibility, resistance to extinction or propensity to speciate.
Pharyngeal jaw apparatus in cichlids
hypothesis
- left and lower jaw element fused
- lower jaw suspended in muscular sling
- upper jaw elements articulate with cranium`
unique features of cichlid PJA gave them a particularly powerful bite, permitting evo of many different kinds of feeding apparatus.
