1. Evolution of Populations
Genetic variation= wide range of phenotypes
Why is variation in a population beneficial?
Genetic variation in a population increases the chance that some individuals will survive
Gene pool- all genes, including different alleles, that are present in a population
Allele frequency- # of times that the allele occurs in a gene pool, compared to other alleles for same gene
Expressed as %
Nothing to do with allele being recessive or dominant
What are the main sources of genetic variation in a population?
Mutations and gene shuffling from sexual reproduction

2. Sources of Genetic Variation
Main sources of genetic variation= mutations and genetic shuffling from sexual reproduction
Mutations- any change in sequence of DNA
May affect organism’s fitness or ability to survive and reproduce
May have no effect on fitness
Gene shuffling- occurs during production of gametes
Crossing over
Sexual reproduction- major source of variation within many populations

3. Natural Selection in Populations
Natural selection acts on distributions of traits
Traits not undergoing natural selection have normal distribution
Natural selection can change the distribution of a trait in one of three ways
Microevolution- evolution within a population
Observable change in allele frequencies
Can result from natural evolution

4. Natural Selection in Populations
Natural selection can take one of three paths
Directional selection- individuals at one end of curve have higher fitness than individuals in the middle or at end

5. Natural Selection in Populations
Stabilizing selection- takes place when individuals near the center of curve have higher fitness than individuals at either end

6. Natural Selection in Populations
Disruptive selection- individuals at the upper and lower ends of the curve have higher fitness than individuals near middle

7. Other Mechanisms of Evolution
Natural selection- not only source of evolutionary change
Gene flow- movement of alleles between populations
Individuals join new populations and reproduce
Keeps neighboring populations similar
Low gene flow increases chance that two populations evolve into different species

8. Other Mechanisms of Evolution
Genetic drift is a change in allele frequencies due to chance
Causes a loss of genetic diversity
Most common in small populations
How do populations become small enough for genetic drift to occur?
Bottleneck effect- an event greatly reduces the size of a population- overhunting
Founder effect- small number of individuals colonize a new area- Old Order Amish community

9. Other Mechanisms of Evolution

10. Other Mechanisms of Evolution
Sexual selection occurs when certain traits increase mating success
Why does it happen?
Due to higher cost of reproduction for females
Females are more limited in potential offspring each cycle- they want a good return
Two types of sexual selection
Intrasexual selection: competition among males
Intersexual selection: males display certain traits to female

11. Evolution Versus Genetic Equilibrium
Hardy-Weinberg principle- allele frequencies in a pop will remain constant unless one or more factors cause those frequencies to change
Genetic equilibrium- allele frequencies remain constant
Conditions required to maintain genetic equilibrium
Random mating: no sexual selection
Population must be large: no genetic drift
No movement into or out of population: no gene flow
No mutations: no new allele added to gene pool
No natural selection: all traits aid equally in survival
Real populations rarely meet all five conditions

12. Speciation Through Isolation
Speciation- formation of new species
What must happen for a species to evolve into two new species?
Gene pools of two populations must become separated
Reproductive isolation- members of two pop cannot interbreed and produce fertile offspring
Behavioral isolation
Geographic isolation
Temporal isolation

13. Temporal isolation- two or more species reproduce at different times
Behavioral isolation- two populations have differences in courtship rituals or other reproductive strategies that involve behavior
Eastern and western meadowlarks
Geographic isolation- two pops are separated by geographic barriers, like rivers, mountains, bodies of water
Abert and Kaibab squirrel
Do not guarantee formation of new species
May separate some organisms but not others
Temporal isolation- two or more species reproduce at different times
Orchids in rain forest

14. Patterns in Evolution
Evolution through natural selection is not random
It can have direction, results cumulative
Different species often must adapt to similar environments
Convergent evolution- evolution toward similar characteristics in unrelated species
Divergent evolution- closely related species evolve in different directions, become increasingly different

15. Patterns in Evolution Species can shape each other over time
Coevolution- two or more species can evolve together
Species evolve in response to changes in each other

16. Patterns in Evolution Species can become extinct
Extinction- the elimination of a species from Earth
Background Extinction
Caused by local changes in environment
Mass extinction
More rare but more intense
At least five mass extinctions in last 600 million years

17. Patterns in Evolution Speciation often occurs in patterns
Punctuated equilibrium- episodes of speciation occur suddenly in geologic time
Followed by long periods of little evolutionary change
Adaptive radiation- many species evolve from one species
Ancestral species diversify many descendent species
Usually adapted to wide range of environment

18. Why is understanding evolution important?
Drug resistance in bacteria and viruses
Pesticide resistance in insects
Evolutionary theory helps us understand and respond to these changes in ways that improve human life