Chapter 7 Flashcards
Should see 3:1 brachydactylous:wild type in population
Dominant will be selectively advantageous
Or at least take over
incorrect beliefs
relative commonness or rarity of alleles
allele frequency
genetic variation in sexually reproducing species
recombination into heterozygotes or homozygotes
Depends on relative abundances of alleles
Genetic Variation
Alterations in gene frequencies in one generation
Genetic Variation
Will alter the alleles carried to next generation
Genetic Variation
When genotypes have predicted frequencies
Hardy-Weinberg Equilibrium
Was the original population in H-W quilibrium
Chi Square
Genotype frequencies obtain H-W values after how many rounds of random mating
one
If something caused a population to not be in equilibrium
One round of random mating would obscure that
Allele frequencies don’t change from one generation to the next so a new mutation would remain rare
true
is when populations violate the assumptions of H-W
Evolution
H-W assumptions
- No natural selection- all individuals have equal chances at survival and reproduction.
- Mating is random- no sexual selection
- No migration
- The population is effectively infinite (no changes by chance alone) - No Genetic drift - random change in small populations
a value for how much survival is decreased for a particular phenotype
s
Need Selection Coefficient
The larger the selection coefficient is, the stronger the…
action of natural selection \
-so when s is a bigger number, the A1 allele approaches fixation earlier.
An allele is consistently favored
Directional Selection
Will lead to fixation – will be only allele in population (well, just about at least).
Directional Selection
Rate differs whether allele is dominant or recessive
-recessive is slower to reach fixation
Directional Selection
Why is directional selection slow when recessive gene is expressed?
- Most A1 are in heterozygotes and phenotype not expressed
- Slow to build up more successful homozygotes
In directional selections this reaches fixation faster
Codominant
In directional selections this receives some advantage from the A1 allele
Heterozygotes
In directional selections this reaches fixation slower
Dominant A1
In directional selections this is hard to remove from a population
A2
-If A1 is rare, there will be an increase in the gene
I-f it is common, there will be a decrease because more, less fit homozygotes
overdominance
a stable polymorphism
Balancing selection- this is very rare
Sickle-cell (S allele vs. A)
AS suffer slight anemia, SS severe anemia
But normal, AA, have higher mortality from malaria
Blood cells of heterozygotes broken down more rapidly limiting chance of survival
example of overdominance
Heterozygote has lower fitness
Underdominance
If an allele is rare, it will be lost because most are in heterozygotes
RARER
Underdominance
Diversifying Selection
Underdominance
Black-bellied seedcrackers - bimodal distribution of bill depth
Probably not one gene
Diversifying Selection
Heterozygotes have severe autoimmune disease
Underdominance
Costs and benefits of a trait depend on how many individuals with such a trait are present in the population
Frequency Dependent Selection
fitness increases as number of individuals with the trait increase in population.
Positive Frequency Dependent Selection
Flat Land Snails – mate face to face
Can only mate with snails whose shells coil in same direction
Higher the frequency of either type, the greater the success
Positive Frequency Dependent Selection
Fitness of a trait decreases as frequency of the trait increases
Scale-eating cichlid Perissodus microlepis attacks from behind
Right-turned (dextral) and left-turned (sinistral) mouths
Negative Frequency Dependent Selection
Number of offspring produced
May survive fine, but not produce offspring
Crossed wild and domestic sunflowers
-fitness decreased in hybrid!!
Fecundity Selection
reproduce once and then die
Semelparity
reproduce more than once
It gets more complex
Iteroparity
Usually ecologists just count daughters
Iteroparity
Offspring produced at an earlier age increase fitness more
Life History - Population Increase
A genotype that reproduces earlier and has a shorter generation time and higher fitness (as measured by r) than a genotype that reproduces later
Life History - Population Increase
r = per capita rate of population increase R = er
Life History
If one allele mutates into another
the allele will continuously appear in the population
Shift from H-W equilibrium
Mutation
80% are new mutations inherited from one parent
Achondroplasia
mate with own genotype or phenotype
Assortative Mating
mate with those of different genotype or phenotype
Disassortative Mating
gene copies may be identical by descent
Inbreeding – Assortative
Offspring between closely related individuals have lower fitness
Inbreeding Depression
increased with inbreeding – people form Croatian islands
Hypertension
20% of marriages between genetically related individuals
Inbreeding Depression
Increase in heterozygotes over that predicted by H-W
Disassortative Mating
Prefer mates with different Major Histocompatibility Complex
Disassortative Mating
could drastically alter allele frequencies
Migration from a large population to a small one
Will bring new alleles into a population
Migration
