4/15/24 - Lecture 10: Natural Selection II: Basics & Pre-conditions Flashcards
What were Darwin’s 3 inferences?
There is a struggle for existence
There are predictable differences in traits between those that survive and those that do not
Over time, traits that enhance survivability should increase in frequency in the population relative to other traits
Phylogenetic trees
A graphical depiction of the history of relationships among a group of organisms
Descent with modification
Similar to how artificial selection results in changes in a population, natural selection does the same, changing over time and eventually creating new species
Branch point/node
A point where one species branches off from another
Taxa (singular: taxon)
The ends of the branches, typically a species
Clade
A grouping of a common ancestor and all its descendants
Homologous trait
When organisms share a trait due to common ancestry
Analogous trait
Traits that are similar due to similar selective pressures (convergent evolution)
Vestigial structures
Structures that have no apparent function and appear to be residual parts from a past ancestor
The breeder’s equation
R = (h^2)s
R: response to selection
S: selection differential
h^2: heritability
Selection differential (S)
How strong selection is WITHIN a generation. Sign matters!
It is:
(Trait mean in selected population) - (trait mean in whole population)
For example, if the trait is limb length and the selected population has a 7 cm limb length while the whole population has a limb length of 5 cm, the selection differential will be 2 (7-5)
Heritability (h^2)
The measure of how much the offspring have inherited their parents’ traits. It is always less than 1 because no trait is 100% heritable
The squared part doesn’t matter
Response to selection (R)
How strong the response will be in the next generation
Trait (offspring) - trait (whole)
Using the example with the selection differential, if the offspring of the selected population has a limb length of 6 cm, you’d have an R of 1 (6-5)
Pre-conditions for natural selection
- Individuals must reproduce
- Variation among individuals in their traits
- Variation must be heritable
- Differential success due to traits
How to guess whether a trait is homologous or analogous based on a phylogenetic tree
Homologous: look at the common ancestor and count the number of times it would have had to be lost/gained
Analogous: look at the common ancestor and count the number of times it would have had to be evolved separately
The one with less evolutionary transitions (gain/loss) is more likely
