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 understanding Genetic 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 occurs Total # of alleles

Genetic variation has 2 main sources: 1. Mutations – random DNA changes 2. 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 extreme Low 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

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 another This 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 species Speciation = the formation of a new species from an existing species

There are several ways populations can become isolated 1. 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 noticed 1. 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 exists Background extinctions occur constantly, but at a low rate Mass 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