EvolutionHow Natural Selection Shapes Populations

Chapter 17 Miller LevineHonors Biology NNHS 2015

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.

Polygenetic Trait- distribution of phenotypes

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 drift

Bottlenecks Founder effect

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)

Break save for next time…

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

Types of Reproductive Isolation

Behavioral isolation Geographic isolation Temporal isolation