1. Character (=trait) evolution

2. Increased in frequency
Character evolution
Fixed for yellow
Increased in frequency
Here are a series of time slices shown schematically. Originally the population is fixed for one trait “blue.” A mutation in some gene causes one individual to manifest the trait “green.” If green individuals have higher fitness they will tend to increase in frequency. At the third time slice shows the the populationsome time later when it is mixed (polymorphic) for blue and green. Eventually, however, the green genes go to fixation causing extinction of the blue trait.
Yellow variant arose
Fixed for white

3. Character evolution white yellow
Once it has been fixed, no descendent will ever bear the original character - it is lost (maybe something that looks like it but it will not be the same). Thus, the novelty will come to mark a species and as that species diverges, a clade. This means that characters tend to come to have hierarchical distributions that match clades. This, a) gives further justification for only concentrating on monophyletic groups, and b) is the basis for how we can go about reconstructing phylogeny (next time).

4. Evolutionary changes are retained by descendants
crest
Tail spines
horns
marks
dewlap

5. A tree is an efficient way to store knowledge of traits
4 species x 4 traits = 16 pieces of info
2 clades + 4 traits + 4 branches = 10
horns
marks
Tail spines
crest
But trees take practice