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Evolution ( lecture 7), Mutations (Lecture 7), Evolution, Cancer Evolution…
Evolution ( lecture 7)
Definition of Evolution: changes in the gene pool of a population of organisms over successive generations.Darwin's definition: “Descent with modification”
Two important conditions:
- Changes must be stably inherited - passed on from one
generation to another i.e. genetic change must occur
- Any genetic change must spread through population – all
individuals are mutants compared to parents but unless change spreads through population, it is not evolution.
Understanding evolution has practical
implications.
- Example for infectious disease –
helps us to understand.
- How do pathogens evolve to
evade the immune system?
- What is the source of
emerging (new) pathogens?
- How do pathogens become
antibiotic resistant
How we came to understand facts in regards to evolution.
- Early biological thinking: species were fixed natural types ( created by god)
- Mid 18th Century: idea that species can change- incl. descent w/modification.
Palaeontology: fossil evidence that life
has changed (evolved)
- Fossil hunters collected important evidence.
- fossils often resembled ,modern species e.g. mammoth.
- many past species are extinct/replaced by newer species.
- Recognition by geologists that different rock layers (of different ages) contain distinct fossils
Anatomy: Homologous traits
- Homologous appendages in
different organisms share underlying similarities due to inheritance from a common ancestor.
Homology is existence of
shared ancestry between pair of of structures, or genes, in different species.
- Homologous structures may
share common function (human & chimpanzee arms) or may differ (human arm & bat ring)
Lamarck's theory of "transmutation“ - Important but incorrect - thought adaptation to environment
occurs by inheritance of acquired changes.
- e.g. “proto-giraffe” stretches neck reaching for high leaves –
passes on long neck to offspring
Charles Darwin: 1809-1882
- English - travelled around the world as naturalist on HMS Beagle - collecting fossils & observing flora & fauna - famously in
Galapagos islands
- Observed diversity in species of e.g. finches – what could explain this variety.
- On return to England, over next 20 yrs he formulated Theory of Evolution.
Chares Darwin's theory
- Patterns observed in biology: homologies, fossil record etc. best explained by inheritance of features from common ancestors with modification i.e. Evolution.
Mechanisms For Evolution
- Gene pool: set of all genes (alleles) in a population, usually of a particular species.
How the gene pool changes occurs
- Mutations (with selection)
- genetic drift
- gene flow.
Mutations (Lecture 7)
Genes & Alleles
- Gene: discrete genetic unit controlling hereditary characteristic.
- Alleles alternative forms of a gene
Mutation
- Heritable change (abnormality) in the nucleotide sequence of an organism.
- Mutant' An individual, organism, or new genetic character (allele) arising or resulting from mutation
Molecular Basis for Mutation - Errors in DNA replication (copying)
- DNA damage and errors in repair.
- Transposition (insertional mutagenesis by transposons).
- Recombination (e.g. rearrangements between similar sequences)
How novel, adaptive, mutations become established in the gene pool.
- Artificial selection (i.e. breeding)
- Natural selection (i.e. survival of the fittest)
Artificial (Un-Natural) Selection Breeding:
- human intervention in reproduction i.e. artificial selection of mating pairs
example 1: diversity of dog breeds.
example 2: diversity of crop plants selected from wild mustard.
Natural selection
- There must be variation in phenotypes (& underlying genotypes) of population
Competition for resources affects **reproductive
success,**
- Fly lifecycle only few weeks, each ♀ lays 100 of eggs.
But, Earth is not covered in flies – why not?
- Phenotypes (& underlying genes) that aid survival & reproduction become more common; phenotypes that hinder survival and reproduction become rarer.
Survival of the fittest.
- Darwin & Alfred Russel Wallace both independently formulated the idea of natural selection
Sexual Selection.
- Darwin proposed another mechanism for evolution: sexual selection is a type of natural selection.
- Intersexual selection: members of one biological sex choose mates of the other sex with whom to mate
- Intrasexual selection: competition between members of same sex to reproduce with members of opposite sex.
- Result: some individuals have better reproductive success than
others within population e.g. more “attractive”
Modern Evolutionary Synthesis - Modern synthesis = Darwinism + other mechanisms.
- Speciation.
- Mendelian genetics.
- Genetic drift: Darwin’s theory expanded to include effect of random changes to the gene pool resulting from chance events / random sampling of individuals for reproduction
(consider “marbles” simulation)
- Microevolution happens on a small scale (e.g. within a single population): breeding of dogs; antibiotic resistance in bacteria.
- Macroevolution refers to evolutionary change at or above the level of species e.g. speciation
- Evolution at both levels relies on same, established mechanisms of evolutionary change.
- Mutations.
- Gene flow/ migration
- Genetic drift.
- Natural selection.
Speciation:
formation of new species Allopatric mechanisms involve geographical isolation of 2
populations i.e. when two populations of same species are separated e.g. by mountain range, can no longer interbreed & evolve apart
- Sympatric mechanisms involve reproductive isolation i.e.
when 2 (initially interbreeding) evolving groups live in same geographical area (& regularly encounter each other)
- Relies on non-random mating based on phenotypic features
Sympatric speciation in fruit flies
- Apple maggot fly Rhagoletis
pomonella once laid eggs on fruit of hawthorn trees, but less than 200 years ago some began to lay eggs on apples instead.
- This led to ecological speciation - adaptation to different host plants fruiting at different times of year drives reproductive isolation.
Divergent evolution:
- 2 or more species share common ancestor but evolve different characteristics
Convergent evolution:
- 2 or more species do not share common ancestorr but evolve similar characteristics through adaption to similar environmental conditions.
An example of divergent evolution - Darwin’s finches diversified beak shapes to adapt to different food sources.
- Adaptive radiation: process in which organisms diversify rapidly from ancestral species into multitude of new forms
Evolution
Convergent Evolution: microscopic
- Bacterial, eukaryotic (& archaeal) flagella all used for motility.
Eukaryotic flagella (cilia)
- Microtubule-based structures, anchored at cell membrane by basal bodies or centrioles, covered in plasma membrane.
- Undulating motion powered by dyneins (ATPase motor proteins)
Bacterial flagella
- Hollow protein tube - helically-arranged flagellin subunits.
- Rotary nanomachine – like electric motor - proton flux into cell powers rotation
Evidence of evolution
- Antibiotic resistance -
- Anatomical evidence
- Molecular sequence evidence.
Origin of antibiotic- resistance.
- Evolution loosely defined as genetic change over time.
- Driven by mutation (random changes in
genes) + natural selection for particular alleles.
- Antibiotics can apply strong selective pressure.
- Resistance mechanism can evolve from scratch.
- Variations in genes (and encoded proteins or RNAs) that
confer some level of resistance will be selected-for
- Evolution may proceed in a stepwise way:
Evidence of evolution:
- Homologous structures show how similar species have changed from their ancient ancestors
- Analogous structures show how different species have evolved to become more similar.
- Vestigial structures show how organs / features have reduced as they have lost function(s)
- e.g. the human coccyx corresponds bone-for-bone to base of monkey’s tail, but is much reduced
Evidence of evolution- molecular.
- Sequence similarity between same gene / protein in different organisms.
- Sequence similarity indicates homology.
- Homology: shared ancestry between a pair of genes in different species
Evolution of complex characteristics stepwise evolution of complex structures (eye, flagellum)
- Unlikely to occur by mutation alone, but much more likely if possessing individual components of “final” structure confers selective advantage
- e.g. in evolution of an eye, even the most basic
light-sensing ability might confer a selective advantage
Flu life cycle
- Replication (copying) of viral genetic material (RNA) in host nucleus.
Due to rapid error prone replication of RNA genome, mutations arise frequently / quickly. these mutations may be :check:
- Harmful: causing reproduction to decreases
- Beneficial: causing reproduction to increase.
Viral strains with beneficial mutations increase in
frequency through natural selection
- Mutant viral strain no longer recognised by the immune system.
- leads to greater reproduction of mutant.
- mutant dominates population in subsequent generations.
Natural selection creates diversity.
- Influenza H1N1 pandemic strain emerged in 2009
- cientists analysed nucleotide
sequence of samples collected during the first few months of the outbreak.
- Sequence comparison allowed "tree" of relationship to be constructed.
- Tips of branches are sampled strains.
Viral reassortment allows flu strains to infect new hosts.
- Genes of H7N9 influenza virus derived from four different bird strains: 2 (domestic) chicken, 2 wild bird.
- H7N9 capable of infecting humans (& still evolving)
When new strains become transmissible by humans, results can be devastating. Major flu pandemics.
- 1918: Killed more than 50 million.
- 1957-1958 Asian flu killed 1.5 million.
- 1968-1969 Hong Kong flu killed 1 million.
- 2009: 280,000 killed.
Understanding of influenza virus evolution allows rational & effective response to threat of emerging pandemic strains
Key concepts
- Natural selection favors flu variants that evade immune system.
- Reassortment can result in strains that are both deadly & highly infectious.
- By continual monitoring, scientists can observe evolution of new strains as they emerge (in real time)
- This allows appropriate informed control measures
Cancer Evolution
Cancers
- are diseases where mutant cells proliferate in an uncontrolled way.
- Evolution of cancer is multistage process: (inherited predisposition +) series of somatic mutations.
- If mutant cell proliferates faster (fitter) then it will tend to dominate (natural selection)
How evolution is studied.
- Morphology (comparative anatomy, palaeontology)
- Biological sequence data: gene sequences; encoded protein (amino acid) sequences
- Similarities in sequences suggest homology.
- Direct observation: usually in bacteria / viruses e.g. Lenski’s LTEE
Are human evolving
- Evolution in Homo sapiens – e.g. lactase persistence.
- All infants can digest lactose (milk sugar) due to milky diet using lactase enzyme (encoded by LCT gene)
- Most mammals lose this ability into adulthood.
- some human adults retain the ability to digest lactose: those who don't are lactose intolerant.
- Result of regulatory mutation that occurred relatively recently (1000s years ago):
- single nucleotide mutation in promoter region of LCT gene determines if adult is lactase persistent or lactose intolerant.
- Adaptive advantage to those with persistence - ability consume milk as adult probably provided Neolithic farmers with alternative calorie source.
- Once lactase persistence trait evolved, it quickly spread across N. Europe (lactose intolerance <5% vs >90 % in some Asian countries.
- What about modern humans? – the power of natural selection may be reduced, but evolution has not gone away!
The origin(s) of life Requirements / Major Steps
- Formation of simple organic molecules.
- Self-replicating molecules (with variation) – probably RNA (RNA world)
- Boundary (cell membrane) to prevent dispersal of “cellular” components
hen natural selection takes over / evolution can begin.
- Modern metabolic processes out-compete older forms of metabolism.
- Multicellularity is a relatively late arrival
-