Evolution How Natural Selection Shapes Populations Chapter 17 Miller Levine Honors Biology NNHS...
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Transcript of Evolution How Natural Selection Shapes Populations Chapter 17 Miller Levine Honors Biology NNHS...
Genotype Phenotype Alleles
Describe how natural selection affects genotypes by acting on phenotypes. (Hint- THINK BUNNIES!)
Evolution & Natural Selection Work via Genetics & Heredity
Definitions of Evolution
Gradual process (over millions of years!) by which modern organisms have descended from ancient ancestors
Evolution, in genetic terms, involves a change in the frequency of alleles in a population over time.
Mutation – random changes in the DNA sequence
Gen Shuffling- during sexual reproduction Creating haploid cells Crossing over
Sources of genetic variation
Can lead to changes in allele frequencies and, thus, to changes in phenotype frequencies
Think of those Naked Bunnies!
Natural selection acting on asingle gene trait
Polygenic traits
If a trait is influenced by more than one gene, it is referred to as a polygenic trait.
Polygenic traits often produce a normal distribution curve of variation.
In these cases, we refer to the phenotype as a continuous trait.
Natural selection can act on traits produced by multiple genes in Directional selection Stabilizing selection Disruptive selection
Natural selection acting ona polygenic trait
Directional Selection
Changes the trait in one direction or the other – bigger or smaller, darker colored or lighter colored, faster or smaller, etc.
3 Ways Natural Selection Affects Genotypes
1. Directional Selection- favors individuals at one end of the phenotypic
range. Most common during times of environmental
change or when moving to new habitats. Example: Greyhounds
3 Ways Natural Selection Affects Genotypes
Disruptive Selection Favors extreme over
intermediate phenotypes. Occurs when
environmental change favors an extreme phenotype.
If pressures are too strong and last long enough they can cause the curve to split.
3 Ways Natural Selection Affects Genotypes
3. Stabilizing Selection favors intermediate over extreme
phenotypes. Reduces variation and maintains the current
average. Example : Siberian Husky
Genetic Drift
Natural selection is NOT the only source of evolutionary change.
Smaller populations do not always follow laws of probability. Individuals may leave more
descendants than other individuals by chance
A number of chance occurrences can cause an allele to become common in a population.
Genetic equilibrium – if a population is NOT experiencing natural selection then the allele frequencies in a population will remain the same
IF nothing is disturbing equilibrium then: p2 + 2pq +q2 = 1.0
5 conditions that can disturb genetic equilibrium: Nonrandom mating (sexual selection) Small population size (genetic drift) Immigration or emigration Mutations Natural selection
The Hardy-Weinberg Principle
Applying Hardy-Weinberg to Alleles
Example: In a certain breed of fish, blue is dominant to yellow
Blue fish can have the following genotype(s): BBBb
Yellow fish can have the following genotype(s): bb
Genetic Variables: p= the frequency of dominant allele B (blue)(frequency = total # of B alleles/total number of alleles) q= the frequency of recessive allele b (yellow)
(frequency = total # of b alleles/total number of alleles)
Since there are only two alleles in the population and everyone in the population has one of the two alleles, the frequencies of the two must add up to one. p+ q = 1 (remember 1 --> 100%)
Hardy-Weinberg Equation
Heterozygous fish Ex:
The equation: 1.0 = p2 + 2pq + q2
Where:p2 = frequency of BB genotype2pq = frequency of Bb genotypeq2 = frequency of bb genotype
p
p
q
q
p2 pq
q2pq
B
B
b
b
BB Bb
bbBb
Let’s Watch a Video: http://ed.ted.com/lessons/five-fingers-of-evolution
The Five Causes of Evolution (take notes!) 1. Natural Selection
2. Small Populations or GENETIC DRIFT
3. Non-Random Mating
4. Mutation
5. Gene Flow (Immigration and Emmigration)
What is a species?
A species is a population or group of populations that can interbreed and produce fertile offspring
Speciation is the formation of a new species.
Occurs thanks to Natural Selection Chance Events
How does this happen?
Isolating Mechanisms
The gene pool of two populations become separated.
Reproductive Isolation When members of two species cannot
Interbreed Produce fertile offspring
These two populations will respond to natural selection and genetic drift as separate units