How Evolution Occurs Genetically

 Evolution occurs in _populations_ not _individuals.   Why? Within the _lifespan_ of one  individual, new features cannot evolve in  response to _natural_ selection. Only as  traits are inherited, can the _environment select for them

 _gene_ _pool_ is the entire collection of genes in a population.

 If you know the genotypes of a population, you can calculate the _allelic  _frequencies, or the percentage of an allele in a population.   When the frequency of alleles is not changing from generation to generation the  population is in _genetic_ _equilibrium.  When allelic frequencies are changing, the population is evolving.

 Hardy-Weinberg postulated that populations WILL NOT change allelic frequenices if these five conditions are met:  A. No Mutation  B. Large population  C. No migration  D. Random mating  E. No natural selection

 _Mutation___ is any change in DNA. While most  often _harmful in some cases it can be _beneficial_.   Causes: a. _chemicals____  b. _radiation___   c. _DNA replication errors__

 2._genetic _drift is random change of allelic   frequencies due to _chance. More important in  _small_ _populations_ than large ones.   For example, there is village in Spain that is  polydactyl, that recessive gene shows up in a   much larger percentage_ than normal due to chance.  Continued inbreeding in the same population _concentrates_ the trait.

 3. _Migration___   Individuals _enter_ or _leave_ a population  either adding or subtracting alleles from a population.

 4. Sexual selection is non-randoming mating, which can result in dramatic evolution of certain traits.

 5. Natural selection will always cause allelic change due to survival differences.

 Variations tend to go over a _range.   Ex – Height of people.   Selection occurs when _one area__ of traits are favored

 1. Stabilizing __selection__ is when  average individuals in a population are favored. This reduces __variation_ in a population

 2. directional _selection is when one  extreme form of the trait is favored. This can lead to rapid _evolution in a population.

 3. disruptive _selection_ is when selection favors both the extreme forms of a variation. May lead to new _species_. 

 A _species are organisms that basically look  similar and can have fertile offspring.   _speciation_ is the evolution of a new species.

 1. geographic isolation is when a physical  Barrier separates two groups leading to _unique gene pools. _Australia__  is considered a good example of this

 2. _reproductive_ isolation is when  Formerly interbreeding organisms no longer produce viable offspring. Happens in two ways:   _reproductive, genetic differences lead to death of embryo.   seasonal, animals start breeding at different times of the year based on their environment.

 3. polyploidy is when, during meiosis, extra  chromosomes are added to the genome. This is seen in plants. Perhaps _half_ of   all flowers_ and _crops_ are polyploid. It  leads to new species instantly.

 1.._gradualism is the idea that species  develop through the slow buildup of  adaptations. This was _Darwin’s_ view.  However, many species have been amazingly stable over time.

 punctuated equilibrium says that living  Things have long periods of stability with speciation that occurs in rapid bursts. This is   caused by changes in the environment or introduction of new alleles.

. convergent evolution is when two species  evolve similar adaptations in different locations. This happens with geographic isolation  Ex. Rabbits and patagonian hares_

 4. divergent evolution is when two species grow less alike over time due to reproductive isolation.  Ex. African and Asian elephant