evolution and its implications for astrobiologydns/frs126/evolution.pdf · evolution and its...

Evolution and Its Implications for Astrobiology

David Spergel

Sunday, April 17, 2011

Evolution

• “Survival of the Fittest”

• Evolution selects for ability to survive and reproduce

• Evolution at gene level, individual level, group level?


Experimental Evolution

• Lenski (MSU) has been following 12 populations of E coli for over 50,000 generations. Long enough and large enough sample that every single point mutation typically occurred multiple times.

• Selected to grow fast in a minimal growth medium.

• In 2008, a mutation occurred in one of the populations that allowed the bacteria to utilize citrate as an energy source (seems to have required two mutations-- historical contingency.

All 12 population show increase in cell sizein Lenski’s experiment.

Also see “Beak of the Finch”- Jonathan WeinerPeter and Rosemary Grant


Evolution in the Fossil Record

• Fossil record is the history of evolution

• Limited by “missing links” and challenges in constructing evolutionary pressures

• Evolution of horses:

• shift from forest to grassland

• foliage to grasses

• faster, bigger horses


Evolutionary Mechanisms

• Variation in population properties:

• mutations, sex, etc.

• Natural Selection

• Genetic Drift (Important in small populations). Hardy-Weinberg Theorem applies in large population limit.


Mutation

• Transcription errors

• Much higher in RNA (e.g., flu virus)

• Much lower in R. Durans

• Cosmic ray or chemical “carcinogens”

• Almost all mutations are negative or neutral

• Negative mutations are selected against relatively rapidly


Sex and Recombination

• Sex is very popular among Eukaryotes!

• Evolutionary advantage of recombination is that it brings together different favorable alleles.

• Sex produces rare genotypes. Advantage of diversity versus uniformity sensitive to rate of change in environment


The cost of males

• Clonal females could rapidly dominate the population• Males don’t produce off-

spring. Evolutionary very high cost• Sex must be worth it!

Co-evolution with parasites? (Red Queen Hypothesis)


Red Queen Hypothesis

• “It takes all the running you can do to keep in the same place”

• Constant evolutionary arms race

• Co-evolution and Cooperation: Production of tetrodotoxin (zombie poison) in rough skinned newt and evolution of resistance in its predator, common garter snake. Actually produced by symbiotic bacteria (Psuedoalteromans tetraodonis) that are in the newts. (Same poison in pufferfish).


NZ Snails as a Laboratory for Studying Sex

• Sexual and asexual females can coexist in the same population

• More sexual females seen in lakes with larger parasite population

• Clone frequency oscillates with infection rate

• Parasites adapt to the local populations

Potamopyrgus anipodarum (4-7 mm) High density in a wide range of freshwater habitats throughout NZ. Most populations are asexual females but many populations also contain sexual females and males.


Sex isn’t the only way: Horizontal Gene Transfer

• Organisms can incorporate genese from other species without being offspring

• Virus can transfer genes between bacteria

• Bacteria can transfer genes through direct cell contact

• Common in prokaryotes. Seen in spread of drug resistance.

• Gene transfer from chloroplasts to nuclei. Some evidence for horizontal gene transfer in eukaryotes


Evolutionary Pressures

• Constant competition

• parasites, predators and prey

• “Only the Paranoid Survive” [Grove]

• within species

• evolution in constantly changing landscape

• Environmental Changes


Evolution Rates

• Remarkable stability of species

• tadpole shrimp: unchanged for 200 million years

• horseshoe crab: unchanged since the Ordovician period (445 Myr ago). [Remember multicellular animals appear in fossil record only 600 Myr ago]

• Significant genetic changes can occur in very short timescales (e.g., dog breeds)


Mass Extinctions

• Mass extinctions create “opportunities”. Dinosaurs rise at the end of Triassic extinction and disappear at K-T boundary event

• Without Jupiter, the Earth would have a cataclysmic event every 10 Myr. The Earth has had ~1 event every 100 Myr.

• Snowball Earth event was 635 Mya

• Mutation rate? Escalation hypothesis?

• Basaltic events? Snowball earth

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The Great Dying

• Permian-Triassic Event 251.4 MYa. 96% of all marine species and 70% of all terrestrial vertebrate species become extinct. Even Insects.

• Impact? Increased Volcanism? Methane hydrate gasification? 13C/12C ratio shows suden change at boundary.


Cambrian Explosion

• 580 Myr ago, life was simple...

• In the following 70-80 Myr, huge growth in diversity> Much of the diversification in 10 Myr

• Burgess shale:”Its a Wonderful Life” Gould

• Many innovations like armor


Why did the Cambrian Explosion Happen?

• End of snowball earth? Increase in Oxygen? Timing is not right

• Ecological explanations: arms race

• Devonian explosion:

• evolution of land plants (Devonian 400 Mya)

• land plants spread very quickly-- no herbivores. Did drop in CO2 produce late Devonian extinction of shallow marine life?


Punctuated Equilibrium


Appearance of New Species

• Is species level evolution rapid or gradual? Role of punctated equilibria

• What draws species boundaries? (Human and Neaderthals)

• Recent evidence of human and neanderthal mating in Europe about 30-40,000 years ago.


Long-term Evolution of Diversity

• Life seems to get dramatically more diverse with time.

• Specialization?

• Extinction events are small “bumps” in the road.


Ribosomes

• “reads” the information in RNA and creates proteins.

• ribosomes in bacteria, archaea and eukaroyotes are all distinct, but 29 “core” proteins accross all cellular life

• mitochondrial ribosomes similar to bacteria

• basis of tree of life


Tree of Life

• Our ability to sequence the genetic record of many different species have enabled the construction of phylogenic trees that link genetic related species.

• Challenges:

• limited ancient DNA

• Horizontal gene transfer (mitigated by focus on ribosomal DNA)


Life Spreads Quickly

• Evolutionary pressures reward individuals that find new “fertile lands”

• Spread of humans and “introduced species”

• Spread of life through the galaxy?


Is life necessary for evolution?

• Crystal growth

• Most stable crystals are preferred. Propagate in medium.

• Stability as a criterion for “natural selection” in cellular automoton


Evolution of “Memes”

• Spread of ideas

• Richard Dawkins: “Unit of Cultural Transmission”

• Spread of religion

• an example that Dawkins might not like

• selected for ability to propagate

• “be fruitful and multiply” (failure of Shakerism)

• advantage of multiple transmission mechanisms:

• sword (Islam)

• missionaries (Mormonism)

• Evolutionary principles in business, ideas, etc.


Evolution on Other Planets

• Basic evolutionary dynamics does not require any particular biochemistry

• Does seem to require “lego-like” life

• Ratio of time-scales

• stellar lifetimes and age of universe (Gyr)

• species differentiation (Myr)

• reproduction (tens of minutes to Years)


Spread of Life through the Galaxy

• Travel time to nearest habitable planets

• D ~ 10 parsecs

• v ~ 10 km/s - 300,000 km/s

• t ~ 30 yr - 1 Myr

• Time to cross Milky Way

• t~30,000 yr - 1 Gyr

• age of Milky Way ~ 10 Gyr

• Where are They? - Enrico Fermi

Freshman Seminars on Other Earths


The Great Silence...


Possible Solutions to the Fermi-Hart

• Few other civilization exist• Civilization is rare• Civilization is short lived• Communication is difficult• Too expensive to travel• Humans aren’t listening properly• They are not talking to us• Zoo Hypothesis• Dangerous to communicate• We aren’t interesting enough• Too alien• Non-technological• They are here, but we haven’t noticed


evolution and its implications for astrobiologydns/frs126/evolution.pdf · evolution and its...

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