Lecture 1 - Natural Selectoin and Evolution Flashcards
Types of natural selection resulting in evolution
- Stabilizing selection (Mean/Average)
- Directional selection (one end of spectrum)
- Disruptive selection (Opposite ends of spectrum)
Toba Volcano Theory
- Example of Human population bottleneck
- Supervolcanic eruption 70,000 years ago
- 6-10 year long global volcanic winter
- population reduced to 3-10k individuals
Sum of allele frequencies
=1
Stabilizing Selection
- Favors average individuals
- Mean does not change but variation is reduced
q
recessive allele
1 - p = q
Process: Evolution by Natural Selection
Variant for a trait
–>
Differential survival or reproductive success (fitness) depending on trait value
–>
transmission of trait value to next generation (trait variation must be heritable)
–>
Change in allele frequencies
Population Bottleneck
- Normally large population may pass through a period in which only a small number of individuals survive
- Composition of population is changed
- Genetic variation is reduced (natural disasters, epidemic diseases, hunting and habitat destruction)
Peppered Moth
Example of natural selection
- Initial: most moths had light coloration, camouflaged with the light trees
- Industrial revolution turned trees black
- Light moths were disadvantaged, dark moths survived with camouflage
- Post rev: Dark moths flourished
How Evolutionary Theory is observed
- Genetic mutations arise
- Affects form and function of organisms
- New genetic variation spreads through a population
Observations for Evolutionary Theory (5)
- All species have the capacity, in terms of fertility, to increase without bounds
- Populations tend to be relatively stable in size
- Resources are limited
- Individuals vary in their characteristics
- Some of this variation is heritable
Non-random mating (4 scenarios)
- individuals in population do not choose mates at random
- self-fertilization
- mating preferentially with individuals of same genotype
- mating preferentially of individuals with different genotype
- Sexual selection - mating preferentially with individuals with a specific trait
Natural Selection
Differential survival and reproduction of individuals in a population based on variation in their traits
Mutations
- Change in nucleotide sequence of an organisms DNA
- Occur randomly
- Most are harmful or neutral
- a few are beneficial
- Rate is so low that they only account for very small deviation from HW by themselves
- Can create genetic variation on which selection can act
Mechanisms of Evolutionary Change (5)
- Mutation
- Gene flow
- Genetic drift (small pops, bottleneck, founder effect)
- Non-random mating
- Natural selection
Inferences of Evolutionary Theory
- Only a fraction of offspring survive
- “More fit” individuals are more likely to survive
- This leads to a gradual accumulation of individuals with more favorable traits
Hardy-Weinberg Establishment
- Founder population will have different genotype frequencies, but establish p and q
- subsequent generations will have predicted genotype frequencies because of random mating
Hardy Weinberg Equilibrium (conditions)
- Absence of evolution
- Allele frequencies are constant
- p and q do not change
Genetic Drift - Founder Effect
- A few pioneering individuals colonize a new region
- Unlikely to have all alleles found in members of its source population
ex: European fruit fly came to Chile
Genetic Drift
- Random changes in allele frequencies in small populations from one generation to the next (small pop size)
- Can produce large changes in allele frequencies over time (May result in loss of alleles from a population)
Gene Flow
- Migration of individuals (few populations are completely isolated)
- New individuals can add new alleles to the population’s gene pool
- Can change frequency of existing alleles
Founder Population
- First generation
- Established frequency of each allele
- will not follow a specific ration because collection is random
- Offspring will have predictable ratio of genotypes
- Hardy Weinberg Equilibrium
Evolutionary Theory
“entire body of work on the understanding and application of a field of knowledge”
- Using what we know to make predictions about what will happen
- NOT an untested hypothesis
Evolutionary Mechanisms vs. Adaptive Changes
- Not all are adaptive
Evolutionary mechanisms = change in allele frequency (neutral, beneficial or harmful)
Adaptive evolutionary changes = becoming more fit
- Natural selection is only that is absolutely adaptive
Evolutionary Mechanism - Natural Selection
Individuals that differ in heritable traits survive and reproduce with different degrees of success
“fitness” - a phenotype’s relative rate of survival and reproduction
Evolution as heritable variation
Individuals do not evolve, populations do
Evolution - In terms of what happens genetically
- change in the genetic composition of a population across generations
- Change in allele frequency across generations
Ellis-Van Creveld Syndrome in Amish
- Founder effect example
- Amish in Easter PA descended from 200 colonists, one founder had the disease
- Frequency in amish is .07 compared to .001 in other populations
Dominant Allele Freq (Equation)
2 x N(AA) + N(Aa)
Diversity of Galapagos Finches
Example of natural selection
- Striking differences between finches on different islands
- Most found nowhere else, but similar to those on mainland SA
Disruptive Selection
- favors individuals that vary in both directions from the mean of the pop
- opposite extremes contribute more offspring than individuals close to mean
Ex: finch beaks
- large bills good for hard seeds but not soft
- small bills good for soft seeds but not large
- medium does neither
Directoinal Selection
- Favors individuals that vary in one dorection from the mean of the pop
- individuals of one exptreme produce more offspring
- Average value of that character is shifter to that direction
Deviation from HW Equilibrium
Evolution is occurring
Typically the case
Conditions for Hardy-Weinberg Equilibrium
- Mating is random
- Population size is infinite
- There is no gene flow (No movement of individuals in or out, no reproductive contact with other populations)
- No mutation (no change to alleles in gene pool, no new alleles are added to hange gene pool)
- No differential survival of individuals with different genotypes (no selection)
Charles Darwin
- Theory of natural selection
- Pub “Origin” at 50 yrs old
Bottleneck example - Cheetah
- Bottleneck events have crated unusually low genetic variability
- Of big cats, least able to adapt to new environment
- Do not thrive
Artificial Selection
- Informs us about natural selection
- Helps us understand the variation and heritability of traits
- Selection acting on genetic variation leads to new phenotypes
- Great diversity of traits breeders can achieve
- Selection by breeders vs. selection by nature
Alternate Hypothesis to Toba Volcanic Theory
- Long bottleneck where numbers in sub-Saharan Africa could have dropped as low as 2,000 for up to 100,000 years before expanding in late stone age
What happens to allele frequencies if evolution is occurring?
Frequencies change across generations (p and q will change)
What happens to allele frequencies if no evolution is occurring?
Stay the same from generation to generation
Adaptation - Process
Process by which useful characteristics evolve by natural selection
Adaptation - Characteristics
Characteristics that make it more likely for an organism to survive and reproduce
2 main causes of genetic drift
- Population Bottleneck
2. Founder effect
“p”
- frequency of allele A (dominant)
p = (# of copies of the allele in the pop) /
sum of alleles in the pop
“N”
- number of individuals with a particular genotype
N(AA)
N(Aa)
N(aa)
Total # indivs = N = N(AA) + N(Aa) + N(aa)
“More Fit”
- Individual whose heritable traits “fit” the environment
- Not necessarily “biggest, strongest, etc”
- Contingent upon environment and being best suited for it
“2N”
Total number of alleles present
Causes of population bottleneck
- Genetic variation is reduced (natural disasters, epidemic diseases, hunting and habitat destruction)
Genetic Drift vs. Gene Flow
Genetic Drift: A portion of a population becomes isolated or a whole population is reduced –> changes allele frequencies
Gene Flow: Different populations introduce alleles to one another
