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