Extended Evolutionary Synthesis

Question 1

‘The Modern Synthesis’ (Huxley, 1942)

Answer 1

Presented by Huxley in 1942:
Life evolves through evolution by natural selection, and to some degree genetic drift
• Evidence: paleontology; systematics; & genetics (all dismissed noise generators!)

• Variation in genotype caused by random mutations and recombination creates selective ingredients.
• Those with good ingredients in current environment pass them on more
• Nothing you do in your lifetime can be passed on to offspring (We now know better, active area of research)
• ‘noise’ (variation in phenotype) was brushed under the carpet…

Question 2

Problems with the modern synthesis

Answer 2

• How then do plastically complex traits arise from genes selected for a given trait get constructed during evolution?
• Is phenotypic variation and plasticity just non-adapted noise?
• How do genetic responses to the environment evolve?

Question 3

Gradual Change

Answer 3

Quantitative shifts in mean are common, but not ubiquitous. E.g. How does one explain multi-modality: polymorphisms, polyphenisms, specialisation switches; and life-stage variation (eggs, larvae, juveniles, adults)?

• Any poly-modality should condense to uni-modality through selection or diverge into the generation of new species
• From a gene-centric point of view, natural selection should select for a single fitness peak, there would be little variation/fitness peaks within a population
• But with phenotypic plasticity, you can move from one ‘fitess peak’ to another without negative repercussions from the changing environment/speciality (e.g. tadpole to frog, caterpillar to butterfly)

Question 4

Conservation of development

Answer 4

Change is a deviation from genetic predisposition requiring extreme underpinnings to break co-adapted gene complexes.

• If you are born a human, you will remain a human (can’t develop into a chimp)….
• This is true at a species level, but not a phenotypic level…if you are born a bee you can be a queen or a worker, if you are born an egg, you can live as a tadpole and turn into a salamander, or you can remain as a tadpole and breed as is.
• Is development a facilitator of change or a constraining factor? And if it is a constraining factor, how do you even evolve new species?
• Is it not inevitable that speciation arises through environmental effects on gene switches during development which can create radically different phenotype?

Question 5

Proximate v. Ultimate

Answer 5

Effectively proximate (juvenile) causes of variation have nothing to do with evolution! Only traits that are expressed in adulthood can be under selection, as it is the adult that gives rise to offspring… SO development is often ignored under modern synthesis

Question 6

Continuous vs. discrete variation

Answer 6

Darwin staked his whole idea on the former! If random mutation and selection represent the main evolutionary driver, how can big changes in phenotype evolve so quickly? And how can you get big differences in phenotype without differences in genotype?

Question 7

What is under selection – genotype or phenotype?

Answer 7

We are interested in explaining phenotype, but evolution arises through changing gene frequencies…or does it?
• Take the daphnia example: the difference between the two (one round, one spined) is phenotypic not genetic, caused by the effects of predators on developmental switches. If daphnia were forever exposed to predators, what would happen? You would get a mean change in phenotype without any change in genotype…is this evolution?

Question 8

Filling gaps between modern synthesis and what we now know

Answer 8

Natural selection can only affect genes that are expressed and have measurable phenotypic consequences.
Genes that are not expressed or are phenotypically neutral have their own evolutionary rhythm.
Suggests that selection acts on phenotypes not genes (i.e. outcomes not input).
• We are able to generate variation in phenotype with genes not under selection, because of environmental sensitivity of genes causing changes in expression.

• Novelties arise from re-organisation of developmental pathways given changing environment (Heterochrony…think salamanders and humans) - think epigenetic mechanisms
• Genes are followers, not necessarily leaders!
• Development needs more attention
• Importance of plasticity?

Question 9

Dinosaur evolution after permian extinciton

Answer 9

• Little oxygen in the atmosphere, so running around is a bad idea – also selection for ‘freeing’ lungs and getting them off the ground
• Also selection for eating the abundant plant matter, as active hunting was not a feasibly lifestyle in this atmosphere
• You need to eat a lot of plants to get your quota of nitrogen. Cold blooded animals do not burn off carbs as fast, so it gets stored instead- leading to huge body mass

Question 10

Genetic assimilation

Answer 10

Selection on genes that facilitate the fixing of beneficial developmental pathways, e.g. Waddington 1953 experiments: Calluses on ostrich legs and breasts

Question 11

Phenotypic accomodation

Answer 11

The ability of a phenotype to become ‘adapted’ without changes to existing genotype (e.g. in daphnia), i.e. through substantial plasticity in developmental pathways e.g. Waddington 1953 experiments: Aquired resistance to ether poisoning in flies over time

Question 12

Suggested Extended Evolutionary Hypothesis (West-Eberhard 2003)

Answer 12

1. Trait origin: mutation or environmental change causes expression of a novel trait
2. Phenotypic accommodation: Rearrangement of different aspects of the pehnotype to the new trait, made possible by inherent, pre-existing plasticity of the developmental system
3. Initial spread: Initial spread of variant (assisted by its recurrence in the population, if the initial change is environmental)
4. Genetic assimilation: Fixation by allelic substitution of the novel phenotype, as a result of standard selection
5. Canalisation: Production of a consistent phenotype through seleciton

Question 13

Evolution of eusociality in bees

Answer 13

• If there is a gene for becoming a queen, worker, drone; for policing, passing queen pheromones, making queen versus worker cells, foraging, cleaning… no individual is born deterministically - environment is responsible for the adult behaviours.
• More likely, environmental inputs influence gene expression which determines the myriad of roles in bee society
• Bees can change their ‘role’ during their lifetime (e.g. from worker to defence)
• If it’s predominantly environment that influences phenotype, how to we reconcile that with the modern synthesis, which has a ‘genes are the drivers’ view?
• What came first, the morphology (i.e. genes) or the behaviour (i.e. the selective consequence of phenotype)?
  • Must have been a solitary ancestor – 90% of bees are solitary not eusocial, and social ones generally have no morphological castes.
• Genetic assimilation: the introduction of beneficial genes into existing selection on phenotypes arises through ‘developmental canalization’ (selection on a single outcome)
  • Selection pressure on different groups within the colony allowed those genes to become fixed in different types of bee (drone, worker, queen)… but still very large environmental component (e.g. royal jelly, as it’s not adaptive to become a queen very early in the season when there are no males around – environmental trigger allows flexibility)
• Altruism is very hard to evolve purely by a genetic mutation, you will always lose out to selfish individuals – you need a pre-existing pressure to encourage selection for cooperation and override ‘selfish genes’
• So then a gene for altruism becomes and advantage, rather than a disadvantage, in that environment