4 - Evolution And Speciation Flashcards
Three main mechanisms of micro evolution
Natural selection
Genetic drift
Gene flow
Micro evolution def
Changes in allele frequencies in a population of a species over time
Postulates by Rosemary and Peter Grant to prove Darwin’s theory (1976-1978)
1 - for any particulate trait, individuals within a species are variable
2 - some of this variation is heritable
3 - reproduction is not random; individuals that reproduce the most are those with the most favourable variations - survival of the fittest
Evidence for postulate 1
Variable beak depth for different ground finches
Evidence for postulate 2
- parents with small beaks tend to have offspring with shallow beaks
- parents with deep beaks tend to have offspring with deep beaks
- this there is a large genetic component to determination of beak depth
Evidence for postulate 3
- a drought reduced the number of seeds available - many birds died
- the seeds that were produced were larger and harder in the drought than they were before or after
- selective pressure arised
- the next generation of birds had a larger selection for deeper, stronger beaks
How do populations adapt
Adapt genetically to the environment
Micro evolution by natural selection is therefore a theory and a fact
Evolution def
Change in inherited traits of a population through successive generations: the genetic content of a population changes over time
- via mutations
Dominant advantageous alleles visibility
Heterozygous and homozygous dominant (Aa and AA) are both visible
- these alleles will rapidly rise in frequency of population
Recessive advantageous allele visibility
Only visible to natural selection as homozygous recessive
- take a long time to accumulate, so fixation in a population takes longer
How mutations with no selective advantage change in frequency in populations
Doesn’t occur by natural selection
Occurs by genetic drift
Modelling genetic drift by sampling error
Not all gametes contribute to next generation
- so expect sampling error
Modelling suggests genetic drift will fix alleles in a population
Richard Lenski, long term evolution experiment (LTEE)
E. Coli can use glucose as a C source in aerobic conditions, but cannot use citrate in aerobic conditions
Why E. Coli can’t use citrate in aerobic conditions
- anaerobic conditions: expresses a citrate transporter, can take up citrate from growth medium
- aerobic conditions: citrate transporters not expressed, so E. Coli cannot utilise external citrate as energy source (Carbon source)
How E. coli in LTEE evolved to use citrate in aerobic conditions
Mutation in Cit strain
- duplication of gene encoding citrate transporter
- de-regulated expression
- so citrate transporter expressed in aerobic growth
- Cit strains now can utilise as well as glucose as C and energy source
- gives them a selective advantage
Gene flow def
The movement of alleles between previously separate populations
How do alleles move/shift in a population
Migration of adults and subsequent mating movements of gametes (e.g. pollen) and subsequent fertilisation
Three main mechanisms of population divergence - that leads to speciation
- Genetic drift - removes genetic variation within demes (sub populations) but leads to differentiation between demes
- all by random change in allele frequencies - Gene flow introduces new alleles into demes with a metapopulation
- by itself can lead to genetic homogeneity between demes - Selective and reproductive isolation
Species def - eukaryotic level
A population of organisms that can potentially or actually interbreed, giving viable fertile offspring
- species are reproductively isolated from other groups - different species cannot reproduce together
The Tube mosquito case study
Experimental evidence of speciation
The Tube mosquito
Tree frogs and chromosomal duplication
Male Geospiza conirostris flying to Dapne Major from Isla Espanola - Big Bird Lineage
Double line meaning on a family tree meaning
Consanguinity - two offspring mated with each other
