chapter 11 evolution of populations. darwin knew traits had to be heritable, but didnt know how...
DESCRIPTION
Dominant or recessive have nothing to do with frequency, its just how many times the allele appears Allele frequency is how common an allele is in the population To find frequency: # of time allele occurs Total # of allelesTRANSCRIPT
Chapter 11– Evolution of Populations
Darwin knew traits had to be heritable, but didn’t know how heredity worked scientists started to connect Darwin and Mendel’s work for a better understandingGenetic variation is seen in
populations since they share common genes
Gene pool = all the genes/alleles in a population
Dominant or recessive have nothing to do with frequency, its just how many times the allele appears
Allele frequency is how common an allele is in the population
To find frequency:
# of time allele occursTotal # of alleles
Genetic variation has 2 main sources:1. Mutations – random DNA changes2. Recombination – results from meiosis and sex
How would the fitness of this alligator be affected by his mutation?
Natural selection acts on the phenotypes produced by the alleles
Single gene traits will only have 2 phenotypes; dominant or recessive
Polygenic traits have many phenotype options since they are controlled by more than one gene
Polygenic traits usually show a normal, or bell-like shape distribution
This usually indicates an equal chance of survival for all phenotypes
Mean/average
High extremeLow extreme
An environmental change can cause natural selection to favor certain phenotypes this changes their
distribution
Possible causes: Limited resources, new element introduce to environment
1. Directional selection favors phenotypes of either extreme
Causes the whole curve to shift in 1 direction
2. Stabilizing selection occurs when individuals with the average phenotype are favored Causes the curve to slim around the
mean
Example
Example
3. Disruptive selection favors both extremes and selects against the average
If natural selection is strong enough, the curve fully splits forming a new species
Causes the curve to start to split
Sometimes natural selection isn’t the only cause of change in a population
Gene flow = alleles move from one pop to anotherThis means new alleles in a pop
and more genetic diversity
Genetic drift = the random change in allele frequency due to chance
Generally affects small populations
2 common causes are the bottleneck effect and the founder effect
Bottleneck effect = a much smaller
population size after a major disruption
Founders effect = small number of
individuals colonize a new area
Sexual selection = when certain traits increase mating success
The traits aren’t always adapted for survival, but seem to stick around
Male frigate bird
Just like scientists compare experimental results to a control, they often compare population data to models
The Hardy-Weinberg equilibrium model is a go to comparison and prediction It says a pop will remain in
equilibrium if certain requirements are met Weinberg
Hardy
5 conditions for Hardy-Weinberg (remember, if all 5 are met no
evolution)1. Large population
2. No moving in or out of the pop
3. No mutations
4. Random mating
5. No natural selection
If one pop is isolated from another, they no longer share genes, they could become 2 different speciesSpeciation = the formation of a
new species from an existing species
There are several ways populations can become isolated1. Behavioral isolation - caused by different
courtship/mating rituals
2. Temporal isolation – caused by mating at different times
Certain geographic barriers may not separate all species
3. Geographic isolation - caused by physical barriers that separate pops
Why might a river geographically isolate a lizard population, but not a bird population?
4. Reproductive isolation - when different pops can no longer mate successfully
Considered final stage of speciation other isolation types feed into this
Macroevolution looks at evolution on larger scale 6 major topics/trends have been
noticed1. Convergent evolution – evolution toward similar characteristics in unrelated species
Ex. Wings in birds, insects, bats (analogous)
2. Divergent evolution – related species evolve in different directions
Generally due to different environments
Kit Fox Red Fox
3. Coevolution – 2 species evolve in response to changes in each other over time
4. Extinction – a species no longer existsBackground extinctions occur constantly, but at a low rateMass extinctions happen on large scale and are usually sudden
5. Punctuated equilibrium – a burst of evolution, followed by a long period of stability
6. Adaptive Radiation – the diversification of 1 ancestral species into many descendent species