AP BIO- Unit 7: Natural Selection

What is natural selection

* An individual with a more favorable trait is more likely to survive, reproduce and pass on its genes to their offspring

What are the four different conditions for natural selection?

1. Variation (there has to be a favorable trait)
2. Competition (if every organism can get every resource, no unfavorable traits will be lost)
3. Adaptations (there has to be a specific trait/advantage that allows an organism to survive longer)
4. Selection (needs to be changes in allele frequency; evolution taking place)

Differential Survival

How organisms successfully(or fail at) survive changes in their environment

What is evolutionary fitness?

The ability of the organism to survive and produce fertile offspring

How does environmental stability impact how populations evolve

* Populations in stable environments are more likely less evolution because there has been no change in the environment which favors on trait over the other (vice versa)

How does genotypic variation lead to phenotypic variation?

Changes in the genes will lead to changes in the phenotype

What are selective pressures?

* Biotic or abiotic factors(climate, food availability, predation, disease) that affect how a population survives
* Changes in environment introduce different selective pressures

Artificial Selection

Humans have decided which traits are favorable and which are not meaning they have selectively bred different organisms for their favorable traits.

Convergent Evolution

* Organisms possess the same trait because they live in similar environments not because they have a common ancestor
* Organisms have ANALOGOUS STRUCTURES because they have the same function
* Due to similar selective pressures unrelated species can evolve similarly (similar structures)

What is a mutation? (how does it change the phenotype)

A change in the DNA, which leads to a change in the protein, which leads to a change in the phenotype

Genetic drift

Changes in allele frequency in a population due to random chance

* Bottleneck effect, founder effect
* Affects smaller populations more than larger populations

Bottleneck Effect

A rapid decrease in a population that changes the allele frequency by chance

* This change is not representative of the whole population
* The whole allele frequency is changed

Founder Effect

A small group of organisms get isolated from the rest of the population and set up a new population by chance

* This change also in not representative of the whole population
* The whole allele frequency has changed

Gene flow

The movement of alleles between populations

(immigration and emigration of alleles)

Types of selection: directional, stabilizing, disruptive

Directional selection: A certain extreme is favored over all traits (darker moths are favored over lighter and medium moths)

Stabilizing selection: The average trait is favored over all traits(too small and too big babies are bad)

Disruptive selection: Both extremes are favored and the average trait is unfavorable(light mice can live in sandy area, dark mice can live in dark area, gray mice can’t live anywhere)

What is Hardy Weinberg Equilibrium?

The population’s allele and genotypic frequencies are constant unless there is some evolutionary force acting upon them

What conditions must be met for the Hardy Weinberg Equilibrium to exist?

* No selection: natural selection doesn’t affect the organism’s fitness
* No mutations: every organism has the same traits
* No migration: frogs can’t go in or out
* Large population: makes it less vulnerable to genetic drift
* Random mating: organisms mate w/o a choice

What equation is used to solve for allele frequencies?

p + q = 1

p= dominant allele frequency. q= recessive allele frequency

60% of the alleles are dominant. p= 0.6 q= 0.4

What equation is used to solve for genotypic frequencies?

p^2 + 2pq + q^2 = 1.

p^2= homozygous dominant. 2pq= heterozygous. q^2= homozygous recessive

125/500 frogs are homozygous recessive. q^2= .25 p^2= .25 2pq= .5

Evidence for Evolution (4)

* Geographical: live in same area
* Geological: fossils document evolution and environmental changes
* Physical: similar phenotypes
* Biochemical: similar amino acid sequences

Divergent Evolution

Organisms have a common ancestor, but diverge into their own varied species and have homologous structures (same structure, different function)

Vestigial Strucutres

Serve no purpose to the organism (human tailbone)

Evidence that supports common ancestry in eukaryotes?

* Membrane bound organelles
* Linear chromosomes
* Genes that contain introns

Endosymbiotic Theory

Supports common ancestry in eukaryotes

* mitochondria and chloroplasts share similar structures which means they probably evolved from the same starting cell

Mechanisms of genetic change (causes continuing evolution)

* Changes in DNA (mutations)
* Cell division (increases genetic diversity)
* Environmental disruptions (changes in allele frequencies)
* Fossils (changes in the fossil record show that the organism is evolving, new selective pressures may also change fossils)

What does resistance in organisms lead to?

* Resistance can be caused by mutations
* If the resistant phenotype is favorable it will lead to evolution among the species

What is a pathogen?

* An infectious agent that causes disease and mutates often(generates genetic diversity)

Phylogenetic Trees vs Cladograms

Phylogenetic Trees: show the ancestry of organisms and is backed by molecular evidence

Cladograms: a hypothesis of how organisms are related by looking at their shared traits

What is the out-group in a cladogram?

* The organism that is the least related to the others

Speciation

The formation of a new species through evolution (allopatric/sympatric)

Node vs Root (cladograms)

* Node: wherever the lines meet represents the most common recent ancestor
* Root: a common ancestor for all the species

Derived character

A trait in a recent species that has evolved from an ancestral trait

Allopatric Speciation

* The gene flow is prevented by geographical isolation (allows new species to form because organisms can’t reproduce and have fertile offspring)

Sympatric Speciation

* The evolution of a new species from its ancestor while both continue to live in the same area(not very common)
* Can result from genetic mutations or sexual selection
* This is influences by prezygotic and postzygotic barriers

Prezygotic barriers

* Hinders fertilization if mating were to occur


1. Habitat Isolation- Two similar species live in different parts of the habitat(land and sea)
2. Temporal Isolation- Different species may breed at a different season, day or time
3. Behavioral Isolation- different mating songs to isolate species
* If mating were attempted


1. Mechanical Isolation- The organisms try to mate but the parts don’t fit
2. Gametic Isolation- The organisms’ sperms’ and eggs’ won’t fertilize each other

Postzygotic barriers

Prevent a hybrid from passing on genes


1. Reduced viability- Offspring produced by two different species are very weak
2. Reduced fertility- Offspring produced by two species can’t reproduce(sterile)
3. Hybrid breakdown- Offspring can’t develop because there is genetic incompatibility

Punctuated Equilibrium

Rapid speciation will occur after a long period of stasis (periods of time w/ little to no change)

Gradualism

Evolutions occurs slowly over hundreds of thousands of years

Adaptive Radiation

* An ancestral species gives rise to many new species
* This happens when new habitats become available and organisms can fill up new niches/roles

What are extinctions caused by?

* Changes in the environment such as solar flares, asteroid impacts, rising sea levels, volcanic eruptions

How does extinction impact diversity

It decreases diversity

What things/factors can cause extinction (5)

* Human activity(poaching)
* Global Warming
* Pollution
* Habitat Degradation
* Invasive Species

What is a niche? How does extinction impact different niches?

* A role an organism plays in their environment
* Once many species go extinct (around the world), new niches open causing rapid speciation and adaptive radiation
* Basically many new species arise from a mass extinction

Which populations are more resilient to genetic change

Genetically diverse populations (one who haven’t face mass extinction)

* These populations must be able to withstand environmental pressures

Deleterious vs Adaptive Traits

Deleterious: Decrease the chance of survival

Adaptive: Increase the chance of survival

What is the theory about the origin of life that includes inorganic and organic molecules?

Complex organic compounds can be formed from inorganic molecules

RNA World Hypothesis

RNA is the oldest genetic code

How do selectively bred organisms respond to environmental changes?

* Selectively bred organisms result in lower genetic diversity meaning they will respond poorly to environmental changes