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Summarize Darwin's theory of evolution by means of natural selection.
- Differential survival survival and reproduction of individual
- 4 testable postulates
- 1. Individuals are variable within a population
- 2. Variation passed from parents to offspring (traits are inherited)
- 3. Individuals more successful that others at surviving/reproduction are selected
- 4. Over time, variations accumulate to form new varieties and new species
Can a trait evolve by natural selection if it does not change the lifetime reproductive success (RS) of an individual possessing the trait? Give example
- No, a trait can only evolve by NS if it makes a positive contribution to RS or survival of the individual that bears it
- EX- longehaired horse tail couldn't have evolved to increase horse's comfort, BUT it may have decreased mortality caused by fly-borne diseases
Why do the effects of genetic drift, non-random mating and gene flow act on the entire genome, but the effects of NS act only on the portions of the genome that are directly translated into proteins?
- Drift, nonrandom mating, and gene flow necessarily affect every gene available
- Mosaic evolution: Different traits evolve at different rates
- NS acts independently at different loci (changes some traits and holds others constant)
Be able to draw a graph depicting how trait variation is usually distributed in natural populations.
- Graph: frequency of trait in population (%) vs trait (std deviations)
- Graph is a normal distribution w/ 68% of ind within 1 stdev, and 95% of ind within 2 stdev
What is the evolutionary significance of a trait exhibiting h2 of .85? How does this number relate to VP=VG+VE?
- As h2 reaches 1, trait controlled more by genes, less by environment
- h2 can be determined using linear regression of a plot (offspring values vs mid-parent value)
- A trait exhibiting .85 heritability indicates that nearly all variation in the trait is controlled by genes (only .15 due to environment)
How is statistical regression analysis used to study h2? What does it mean if the slope of line = .85?
- The slope of the linear regression of a plot (offspring values vs. mid-parent value) is equivalent to h2
- If the slope of the line is .85 then 85% of variation in a trait is controlled by the genes
- An increased h2 indicates more potential for trait to evolve by NS
If regression analysis indicates that trait h2=.02, will this trait respond much if NS acts upon it?
- An increased h2 indicates more potential for trait to evolve by NS
- In this case, the variation is controlled almost entirely by the environment, and likely won't be responsive to NS pressures
Does NS act on the phenotypic expression of a single trait, or the expression of the sum of all traits in an individual?
- NS acts on the expression of the sum of all traits in an individual, but may affects different traits in different ways
- NS will always select for individuals who are more fit, but in some cases these individuals may include "bad" traits
- If the overall phenotypic result is more fit then it will be selected
What was Darwin referring to when he discussed "over-production", and how is this phenomenon related to competition within species? What is an "r-selected" species and what is a "K-selected" species? Examples
- Life tables indicate that there are more young being produced than survive, and that many will not survive to reproductive age
- This literally creates competition to survive
- R: many kids, low investment
- K: few kids, high investment
- *note- R and K are always in comparison to another species
Describe how overprodution, variation, natural selection, and inheritance work together/are related.
- Overproduction: populations produce more offspring than can survive, creating competition
- Variation: some variations are more favorable than others
- Natural selection: different traits influence which individuals survive and reproduce (results from over production and variation)
- Inheritance: over time favorable combinations of traits leave more offspring and these genes are put into future generations
How can thinking about examples of artificial selection throughout history increase our understanding of NS?
- Selective breeding: controlled breeding of domesticated plants and animals
- can result in astounding range of phenotypic variation
- In artifical selection we are selecting for (just as the environment would) desired traits. Heritability is still a factor and must be taken into account.
- size and production traits have high heritability (.5-.7), but fecundity traits have low heritability (.05-.3)
- Important to provide the proper environment to increase the fecundity traits (exercise, food, water, keeses)
What are the three modes of natural selection? Be able to draw appropriately labeled graphs depicting how trait variation changes in a population as the result of each
*note- only one mode covered for this test
- Directional selection: selection occurs against less adapted individuals at one tail of the distribution
- Before selection there is a broad range of variation in the population (graph- phenotype frequency vs character value; normal distribution)
- After selection (and some generations after) the distribution narrows at one end of the range while favoring phenotypes at the other end (graph- phenotype frequency vs character value; taller, thinner, and offset to one side [broad on that side]).
- *note- image on 311
How does evolving pesticide resistance in insect populations serve as an example of directional selection? What is the selective pressure?
- Pesticides select against genetically susceptable individuals
- after initial sprays few survivors remain (most resistant)
- over time, repeated sprays leave behind highly resistant individuals
- future sprays require higher dose rates to control the same pest, because the entire population is mostly resistant
How does evolving antibiotic resistance in bacterial populations serve as an example of directional selection? What is the selective pressure?
- Antibiotics inhibit bacterial growth
- with increased antibiotic use after WWII, bacteria quickly developed resistance (E. coli, staphylococcus, enterobacter, M. tuberculosis)
- besides antibiotic pressure, spontaneous mutations are caused by replication error
- bacterial cells can acquire resistant genes through conjugation (sex pilus) or transformation (naked DNA)
How does evolving beak size in Darwin's finch populations serve as an example of directional selection? What is the selective pressure?
- Drought-mediated selection for larger beak size
- terrible drought on Daphne major in 1977 (24mm vs normal 130mm)
- plants made few flowers/seeds (decline in bird food)
- finches did not breed that year
- 84% of finches died over next 20 months
- Once small, soft seeds were gone large/hard fruits became key food item
- Only large finches w/ big beaks and strong muscles could crack the touch seeds
- Smaller, weaker birds expended too much time and energy trying to crack them and died at an increased rate
- *note- beak size trait QUICKLY shifted in population until rains resumed