Evolution as genetic change

Evolution: Genetic definition Evolution is any change in the relative frequencies of alleles in the gene pool of a species Sexual reproduction alone cannot cause such a change

Natural selection on single gene traits Harmful mutations Fewer copies of the allele are passed on because the individual does not survive or does not reproduce Allele becomes less common in the population

Natural selection on single gene traits Helpful mutations Carriers of the allele produce more offspring that survive than non-carriers Relative frequency of this allele increases in a few generations  An allele with no negative effects will be found after several generations in nearly all members of the species

Natural selection on polygenic traits Variation of polygenic traits is a normal distribution Fitness of individuals near each other in the distribution is similar BUT can vary considerable from one side of the curve to the other

Natural selection on polygenic traits Natural selection can affect this distribution in three ways: 1. Stabilizing selection 2. Directional selection 3. Disruptive selection

Stabilizing selection Organisms near the center of the curve are more fit than those at the ends Look at figure on page 489 Example: human baby weight at birth Evolution is minor or absent

Directional selection Organisms at one end of the curve have higher fitness than those in the middle or the other end Entire curve moves as the trait changes Evolution occurs, change in allele frequency Example: finches

Disruptive selection Organisms at the ends of the curve have higher fitness than those in the middle Single curve can split into two curves if the selective pressure is high Example: insects that resemble poisonous insects

Other mechanisms of evolution Role of chance In small population allele frequency can change due to chance Random change in allele frequency is called genetic drift Bottleneck and founder effects Look at figure on page 490 for example

Speciation (1) A new species evolves when: A population changes enough (genetic changes) so that breeding cannot occur between members of the original population and the changing population

Speciation (2) One major way is when two populations are isolated from each other geographically Other ways: Behavioral isolation Temporal isolation

Pace of evolution Gradualism Punctuated equilibrium Evolutionary change takes long periods of time Darwin proposed this Punctuated equilibrium Evolution proceeds rapidly followed by periods of stability, then more evolution, then stability Modern view, along with gradualism

How do you test if evolution is happening? Scientific method again….. Hypothesis is evolution is not happening (= allele frequencies are not changing) Measure allele frequencies over time and see if they change or not Five conditions can result in allele frequency change

Hardy Weinberg principles The five condition are: Non random mating Immigration or emigration Small population sizes Mutations Natural selection