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Evolutionary History
In this lecture:
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Major transitions
Origins of life
Multicellularity
Kin selection and eusociality
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Properties of major transitions
• 1. Smaller entities coming together to form larger
entities. (e.g. eukaryotes, multicellularity, colonies...).
• 2. Smaller entities become differentiated as part of
larger entity. (e.g. organelles, anisogamy, tissues,
castes...).
• 3. Smaller entities are often unable to replicate without
the larger entity (e.g. organelles, tissues, castes...).
• 4. The smaller entities can disrupt the development of
the larger entity, (e.g. meiotic drive, parthenogenesis,
cancer...).
• 5. New ways of transmitting information arise (e.g.
DNA-protein, germline vs soma, indirect fitness...).
Maynard Smith and Szathmary 1995
Early life
last universal common ancestor (LUCA)
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Origins
• Organic molecules ≠ Life
• (Abiogenesis ≠ Natural Selection)
• Early life:
Hereditary replication
Compartments
First hereditary information?
Miller-Urey
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Which came first?
First hereditary information?
• Probably RNA: Genetic information (that can
be copied)
• + Enzymatic activity.
• An RNA world?
• “a period in the early history of life when RNA,
carried out most of the information processing
and metabolic transformations needed for
biology to emerge from chemistry.”
Higgs & Lehman, 2015
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First hereditary information?
• Probably RNA: Genetic information (that can
be copied)
• + Enzymatic activity.
Early life
• First hereditary information/replicators?
• Probably RNA: Genetic information (that can
be copied)
• Co-factors that would speed up reactions:
Amino-acids (initially); other functions (later)
• Selection for stability
• Selection for correct dosage AND reduced
competition between replicators ➡Linkage of
replicators (chromosomes):
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Prokaryote to Eukaryote
Origins of sex
• Evolution of sex
• Sexual selection
• Wait for week 5
• Integrate notes!
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Multicellularity
Multicellularity
• Benefits to being in groups with specialists
(protection, efficiency) but hard to determine.
• Put a single celled algae (Chlamydomonas
reinhardtii) that had never had multicellular
ancestor in conditions that would promote
group living.
Ratcliffe et al., 2012
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Changes
• Produced extracellular matrix to hold cells
together.
• “clusters develop clonally by daughter cells
‘staying together’ after mitotic reproduction”.
Model
• Some similarities to other relatives.
• E.g. Basichlamys has clusters of 4 cells held
with a matrix that then separate and divide
into new clusters.
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Eusociality
• Solitary lifestyle --> Eusociality
• 1. Reproductive division of labor
• 2. Overlapping generations (older offspring
help younger offspring)
• 3. Cooperative care of young
• Esp: ants, bees, wasps, termites.
• But also: naked mole rats, a beetle, a shrimp...
Question
• If it really came down to it, and you had to
make a choice would you sacrifice yourself to:
• Save your family?
• Save an equal number of strangers?
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Why?
Kin Selection
• How to explain altruism.
• Group selection – ‘save the species’
• Replaced by kin selection
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Eusociality
• Hamilton’s rule: genes for altruism increase in
frequency when:
• General framework: Kin selection: can favor the
reproductive success of an organism's relatives (i.e.
indirect fitness), even at a cost to the organism's own
survival and reproduction.
• J.B.S. Haldane:
“I would gladly lay down
my life for two brothers or
eight cousins.”
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Support
• Hamilton’s Rule has been broadly
demonstrated in a number of behavioural
studies.
• Closer relatives are more likely to help each
other than distant relatives.
• E.g. cooperative breeding
• Do animals know how related they are?
Eusociality
• Extreme version of kin selection – individuals
sacrifice breeding (and even themselves) for a
close relative.
• Occurs when individuals are very highly
related.
• Known in bees, ants, wasps, termites, beetles,
shrimp and mammals.
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Common traits
• Division of labour and specialist forms.
• Single reproductive female (queen).
• Main workers may be clones of each other
and / or the queen.
• Cooperative care of young.
• Overlapping generations.
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Forelius pusillus
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Dorylus – Driver ants
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Oecophylla – Weaver ants
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Leaf cutters
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Important organisms
• Animal biomass – Amazon rainforest Brazil
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Conclusions
• Some major evolutionary trends towards
increasing size and complexity.
• Selection pressures can drive these trends.
• Multicellularity and eusociality are strong
examples.
Further Reading
• http://www.nature.com/nrg/journal/v16/n1/f
ull/nrg3841.html
• http://www.nature.com/articles/ncomms374
2?WT.ec_id=NCOMMS-20131113
• http://www.eoht.info/page/John+Haldane
• https://zookfoster.zoo.ox.ac.uk/sites/default/files/files/Fo
ster%20TREE%20online%20early.pdf
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