15. Evolution, D1-D3 Origin of Life on Earth
D1.1 Describe four processes needed for spontaneous origin of life on Earth
- simple inorganic molecules (in oceans) CONVERT to simple organic molecules
- organic molecules POLYMERIZE to make more complex organic molecules
- self-replication with molecules like DNA, RNA, make inheritance possible
- membrane formation – intracellular and extracellular compartments
D1.2 Outline the experiments of Miller and Urey into the origin of organic compounds
- Scientific evidence suggests that the earth was formed about 5,000 million years ago from a cloud of dust particles surrounding the Sun, and got a metallic core of iron and nickel, surrounded by cooler liquid mantle, on top of which is crust. When earth cooled, gases from hot interior escaped through volcanoes and made atmosphere lacking oxygen.
- ‘Reducing atmosphere’ = lacking oxygen. Prior to origin of life.
- Chemical evolution: prebiological changes that changed simple atoms and molecules into more complex chemicals that life needs
- Chemicals for life (biological synthesis) formed in shallow ocean waters, called ‘chemical soup’.
- Miller and Urey 1953: stimulated conditions on early Earth, does chemical evolution occur?
- Used water (H2O), methane (CH4), ammonia (NH3) and hydrogen (H2). The chemicals were stored in sterile glass tubes and flasks in a loop.
- Water heated, produced steam. Sparks between electrodes stimulated lightning.
- Mixture cooled, water condensed and trickled back into first flask in cycle.
- 15% of the carbon was present in the form of organic compounds
- 13/20 naturally occurring amino acids were detected.
- Base adenine found in high concentration (later research).
D1.3 State that comets may have delivered organic compounds to earth
- Panspermia: life on earth may have originated from introduction of complex organic chemicals or even bacteria via comets (small body orbiting Sun)
- Evidence: bacteria and archaebacteria are resistant to extreme conditions and can survive for long periods of time, and possibly on the surface of icy comets
- Evidence: complex organic molecules can get energy for formation from radiation in space
- Evidence: comet spectras show hydrocarbons, amino acids and peptides
- A shower of comets about 4,000 million years ago could have introduced complex organic molecules and water to earth and made chemical evolution
D1.4 Discuss possible locations where conditions would have allowed the synthesis of organic compounds
- problem with chemical soup theory – it doesn’t explain how amino acids and nucleotides polymerized to form proteins and nucleic acids
- the Miller-Urey experiment produces many substances which would prevent this polymerization
- Black smokers: did the first cellular organisms evolve inside of black smokers on seafloors?
o a hydrothermal vent where superheated water from the Earth’s crust enters the ocean floor. Dissolved sulfides crystallize to make a black chimney around the vent, and inside this vent might be a good environment for formation of biological polymers
- Volcanoes: fix nitrogen
o higher than average level of fixed nitrogen above hot lava lakes
o might have been as important as lightning
o volcanoes under sea bed might have been environment for chemical evolution
- abiogenesis: generation of life from chemicals.
- An opposing viewpoint is that primitive life was formed extra terrestrially, in space, or a nearby planet like Mars. Mars is smaller, cooled quicker, and could have allowed prebiotic evolution while the earth was to hot. Low volcanic activity made atmosphere loss, which stopped evolution.
o Little evidence: only remains of fossilized bacteria
o doesn’t address how life originated, just shifts it to planet/comet
D1.5 Outline two properties of RNA that would have allowed it to play a role in the origin of life
- first molecule with capability to replicate
- ribosomes on RNA: capable of generating first proteins
- RNA replaced by DNA
- different base sequences allow variation during replication
D1.6 Living cells may have been preceded by protobionts, with an internal chemical environment different from their surroundings
- protobionts evolved from coacervate droplets (droplet containing organic molecules) which contained polynucleotides (DNA or RNA)
- protobionts were first precusers to first true cells, which were
o heterotrophic: can’t synthesize own food, dependent on complex organic substances
o anaerobic: lives without needing oxygen
- experiment: artificially prepared coacervate droplets containing enzymes can absorb and concentrate substrate molecules, and release products into the external solution
- e.g. phosphorylase can catalyse polymerisation of glucose phosphate and forms starch
- once a coacervate droplet has a simple genetic code, it has the ability to perform major processes of life
D1.7 outline contribution of prokaryotes to the creation of oxygen-rich atmosphere
- bacteria evolved containing a form of chlorophyll that allowed a simple form of oxygenic photosynthesis to happen
- explosive rise in oxygen levels = oxygen catastrophe
- around 2 billion years ago, this photosynthesis change happened
- irreversible effect – organic chemicals in ocean broken down into carbon dioxide and oxidised sediments. Ozone layer (O3) began forming, stopping UV rays from the sun, which blocked production of new organic chemicals in ocean
D1.8 What is endosymbiotic theory?
- chloroplasts and mitochondria have evolved from larger prokaryotes, after surviving in cytoplasm and evolving into chloroplast and mitochondrion
- mitochondria – evolved from proteobacteria
- chloroplasts – evolved from cyanobacteria
- supporting evidence: organelles contain DNA similar to bacteria and different from nucleus
- both organelles surrounded by membranes resembling composition of a prokaryotic cell
- new organelles formed by process resembling bacterial binary fission
- internal structure/biochemistry of chloroplasts similar to cyanobacteria
- DNA sequence analysis
Evolution of earth?
1. atmosphere of CO2, methane, hydrogen, ammonia and water on earth
2. simple organic molecules (amino acids, adenine, ribose) formed
3. organic molecules in oceans are mostly at certain locations, e.g. hydrothermal vents
4. simple organic molecules polymerised. coacervates and microspheres formed
5. enzymes catalyse more polymerisation … smaller coacervates
6. lipid layers form around coacervates, which have RNA self-replicating molecules, later DNA. protein synthesis develops
7. primitivce anaerobic prokaryotic cells evolve
8. oxygen producing anaerobic autotrophs evolve – ozone layer forms
9. aerobic prokaryotic cells develop
10. eukaryotic cells develop (endosymbiosis)
11. colonial forms (e.g. slime moulds) develop
12. multicellular organisms evolve from these
13. adaptive radiation makes many different species, some on land
Geological time scale, history of life- 5 billion years ago – origin of earth
- 3.5 billion years ago – life
- bacteria
- eukaryotes
- sponges
- non-vertebrate phyla
- 500 million years ago – vertebrates, jawless fish
- jawed fish
- amphibians (spiders)
- 350 million years ago (reptiles, insects)
- 225 million years ago dinosaurs
- 195 million years ago dinosaurs dominant, birds and mammals
- dinosaurs go extinct, modern fish and mammals come
- adaptive radiation of humans
- 2 million years ago – humans!
D.2 Species and Speciation
Define allele frequency and gene pool
- Allele frequency: what proportion of all gene copies are in this allelic form, in this population?
- Gene pool: total collection of alleles in sexually reproducing population. Change often – mutations, introduction of new genes, natural selection removes unfavorable alleles
Evolution involves a change in allele frequency in a population’s gene pool over generations.
- New combinations of alleles make unique genotypes… when they become expressed as phenotypes natural slection determines which genes go on to next generations
- three types of natural selection: stabilising selection, directional selection, disruptive selection
Discuss definition of term species
- species: many definitions, based on aspect
- breeding: can interbreed and make fertile offspring
o hybrid animals (mules and tigrons) cant mate with one of their own kind, but can produce offspring when mated with members of a parent species
- ecological: share same ecological niche
- genetic: group of organisms with identical karyotype
o doesn’t apply to single-celled organisms
- evolutionary: share unique collection of structural/functional characteristics
- cladistic: share an ancestor, become separate species when members diverge from each other
- species are REAL BIOLOGICAL UNITS, the relationship between the same species organisms is different than the relationship between two organisms in the same kingdom, for example.
- problem with fossils for species identification: don’t say about interbreeding or fertility of offspring
- biologists usually use morphological/cladistic definition
- concept of ring species: variation within a single species, but it is a scale from one end to the other, and two ends of the scale in two rings can’t interbreed. this is a strong evidence for evolution, because individuals within a species are very different
- physical impossibility of mating between large and small dogs, but still dogs (Canis familiaris)
Describe three examples of barriers between gene pools
- speciation: process by which one or more species arises from previously existing species
- genetic isolation is most common requirement for specation to happen. they aquire different allele frequencies and eventually behave as separate species because of genetic incompatability
- THREE BARRIERS BETWEEN GENE POOLS
- barriers to formation of hybrids:
- temporal isolation: two species mate or flower at diff. times of year
o In california, two species of pine flower in February and April. they can hybrise
o In N. America, four frog species of genus Rana differ in times of peak breeding ability, but there are overlaps
- ecological isolation: two species in similar regions but diff. habitats
o not so common in animals, but in plants. Closely related species may differ in their flowering seasons or requirements for soil type or climate. Breeding is rare, though hybrids are fertile. An example is lions (open grassland) and tigers (forest) who could have made ‘tigrons’ or ‘ligers’ but didn’t.
- behavioural isolation: courtship behaviour has to be accepted between the sexes
o eg. pheromones secreted by female moths and detected by antennae of males, each species has own pheromone. or cricket songs (wings) or bird songs (also to defend territory)
- mechanical isolation: genital differences
- barriers to hybrids:
- hybrid inviability: hybrids produced but don’t develop to maturity
o male horse has 64 (2n) chromosomes while female donkey has 62, so the mule has 63, which can’t pair up in meiosis and the mule is sterile
- hybrid infertility: cant produce functional gametes
- hybrid breakdown: F1 (generation) fertile but later generations not
- geographic isolation: Galapegos islands, charles darwin, the ‘Darwin’s finches’ evolved into slightly different, e.g. beak shape, darwin says because of mutation from an undersea volcanic eruption… fourteen new species evolved
How does polyploidy contribute to speciation?
polyploidy: condition where an organism’s cells have more than two homologous sets of chromosomes
triploid (three sets)
tetraploid (four sets)
polyploidy makes new combinations of genes, it is well tolerated in many plants.
polyploids don’t survive well amongst animals because extra chromosomes makes normal gamete formation in meiosis unprobable, and pairing of sex chromosomes is disrupted. plants don’t have separate sexes, so its fine. animals also cant do asezual reproduction usually or self-fertilisation. but some animals can do polyploidy, like goldfish, salamanders, and salmon
- two types of polyploidy
- autopolyploidy: increase in number of chromsomes in a species, e.g. in replication the cytoplasm doesn’t cleave, a tetraploid can be formed. usually autopolyploids with an odd number of chromosomes are sterile.
- allopolyploidy: chromosome number in sterile hybrid becomes doubled and makes fertile hybrids.
Compare allopatric and sympatric speciation
- speciation: one or more species arise from a previous species
- intraspecific speciation: single species gives rise to new species
- interspecific hybridisation: two diff. species make new species
- allopatric speciation: INTRAspecific speciation while the populations are physically separated
o geographical barrier such as mountain range, sea or river produces barrier to gene flow, so they cant meet and reproduce
o adaptations to new environment change allele and genotype frequencies
- sympatric speciation: speciation occurs while populations are in the same geological area or range
What is adaptive radiation?
adaptive radiation is a principle where a group of organisms shares a homologous structure (evolutionarily similar structure – bat wing- man arm) which are diffrentiated to perform variety of different functions.
adaptive radiation happens within all taxanomic groups, except at species level.
e.g. all organisms in the same taxanomic class share a number of modified features, which adapt them to their particular habitat.
e.g. mouthparts of insects have same bsaic structures, but some are larger and modified for certain feeding, or vice versa
compare convergent and divergent evolution
convergent evolution: process where distantly related organisms evolve similar traits because of similar environments / ecological niches
- similarity a result of pressure of natural selection and environment
divergent evolution = adaptive evolution: two or more adaptations have a common evolutionary origin, but have diverged over time.
- similarity a result of common origin
- e.g. vertebrate limb – paddles in whale, forelimb of birds, hand of human
parallel evolution: two species maintain same degree of similarity while undergoing evolutionary change along similar path
Discuss ideas on the pace of evolution including gradualism and punctuated equilibrium.
punctuated equilibrium: claim that many species will change little over geological history but have rapid spurts of change. explains gaps in fossil record. Rapid evolutionary change occurs during speciation, and during these times the rate of change is high.
- common in fossil record because of incompleteness
- extreme model
- change is accelerated by major environmental changes, in factors such as pressure from predators, parasites, food supply, global climate. natural selection favours genetic varieties that were previously at disadvantage. also meteors, comets, volcanic eruptions, ice ages.
gradualism: evolutionary changes happen slowly, doesn’t have to be uniform rate, but transforms one species graudally into another
- trilobites from Wales, whose rib count increased gradually, with partially developed ribs, etc. the generations were always intermediates of each other
- extreme model (truth is maybe in between?)
stabilising selection: operates when environmental conditions are favourable to particular phenotype, competition isn’t severe. this eliminates extreme phenotypes.
directional selection: pressure to move the mean phenotype to a new mean, and will stabilise afterwards
disruptive selection: conditions like season and climate (environment) favour the presence of two phenotypes within population, so the population is pushed into two extremes. this can split a population into sub-populations, and also cause speciation. an example is Darwin’s finches, with long and short beaks for different food, and eventually became new species.
Describe one example of transient polymorphism
polymorphism: existence of two or more forms o the same species within a population.
two forms:
1. transient polymorphism: a polymorphism in which one allele is in process of displacing another
2. balanced polymorphism: a polymorphism maintained by selection in favor of the heterozygote phenotype
Example: moth species have natural selection during Industrial revolution in Britain. The British peppered moth. Until around 1840, all individuals were creamy-white with black dots and darkly shaded areas. In 1848 a black form or variety was recorded, and by 1895 most of the population of the black peppered moth was black. This variety had a strong selective advantage because the Industrial revolution resulted in acid rain which produced sulfur dioxide and killed off lichens on trees in industrial areas.
In 1956 Britain passed the clean air act, which has decreased numbers of the melanic form.
Other examples have been found in pathogenic bacteria and malarial mosquitos. Mutation and natural selection have favoured strains of bacteria resistant to antibiotics, and malarial mosquitos resistant to insecticides.
Describe sickle-cell anemia as an example of balanced polymorphism.
Balanced polymorphism occurs when two different forms or varieties coexist in the same population in a stable environment. Sickle-cell anaemia is a mutation that causes substitution, which changes a base in the genes encoding hemoglobin. This produces sickle-cell hemoglobin. This hemoglobin is non-polar or hydrophobic, while the normal hemoglobin amino acid is polar/hydrophilic. This makes the hemoglobin S crystallise at low oxygen concentrations, so they are distorted and appear sickle-shaped. Thus they can carry less oxygen, which causes anaemia.
In heterozygous conditions, the red blood cells appear normal, and about half is abnormal. But in exercise they become sickle-shaped. This makes mild anaemia, and PREVENTS CARRIERS OF THE TRAIT FROM CONTRACTING MALARIA as the red blood cells are so short lived.
this is an example of balancing selection or BALANCED POLYMORPHISM. this as a heterozygote advantage, where the homozygous sickle cell people are severe anemics and die, while the homozygous healthy get malaria and die.
so more of the heterozygotes will go onto the next generation – wont suffer sickle-cell disease, and are resistant to malaria.
D3 Human Evolution
Define half-life
http://www.youtube.com/watch?v=-kHK3rF7R7M&feature=related
Outline method for dating rocks using radioisotopes, with reference to 14C and 40K.
C-14 and half life: http://www.youtube.com/watch?v=81dWTeregEA&feature=related
C-14: http://www.youtube.com/watch?v=ErgdpG_N9vQ
K-40:
carbon-half life: 5,730 years
potassium-argon half life: 1.3 billion years
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