1. Processes of Evolution
Chapter 11
The Forces that Change Life

2. Impacts, Issues: Rise of the Super Rats
When warfarin was used to control rats, natural selection favored individuals with a mutation for warfarin resistance– now warfarin is rarely used Fig 18.1

3. 18.1 Individuals Don’t Evolve, Populations Do
Evolution starts with mutations in individuals.
Mutation is the source of new alleles.
Sexual reproduction can quickly spread a mutation through a population.

4. Variation In Populations
All individuals of a species share certain traits.
Individuals of a population vary in the details of their shared traits. Fig 18.1
Population- a group of individuals of the same species in the same area

5. The Gene Pool Gene pool- all the genes found in one population
Alleles- different forms of a gene
- determine genotype and phenotype
Dimorphism- 2 forms of a trait
Polymorphism- many forms of a trait
- usually occur when a gene has 3 or more alleles that persist in a population at frequencies of >1%

6. Table 18.1 Key Events in Inheritance

7. Mutation Revisited
Lethal mutation- mutation that drastically changes phenotype, usually resulting in death
Neutral mutation- mutation that has no effect on survival or reproduction
Beneficial mutation- mutation that conveys an advantage

8. Stability and Change in Allele Frequencies
Allele frequency- relative abundance of allele of a given gene in a population
Genetic equilibrium- a theoretical state which occurs when a population is not evolving

9. Microevolution- small-scale change in allele frequencies
Processes of microevolution:
Mutation
Natural selection
Genetic drift
Gene flow

10. 18.2 A Closer Look at Genetic Equilibrium
Researchers know whether or not a population is evolving by tracking deviations from a baseline of genetic equilibrium.

11. Hardy-Weinberg Equilibrium
Five conditions required for a stable gene pool:
Mutations do not occur
Population is infinitely large
No gene flow (no immigration/no emigration)
Random mating (no sexual selection)
All individuals survive and produce the same number of offspring (no natural selection)
p2 + 2pq +q2 = 1 and p + q = 1
where p is the dominant allele and q is the recessive allele

12. 18.3 Natural Selection Revisited
Natural selection- differential survival and reproduction among individuals of a population that vary in the details of their shared traits
- occurs in different patterns depending on the organisms and their environment Fig 18.4

13. 18.4 Directional Selection
Changing environmental conditions can result in a directional shift in allele frequencies.
Directional selection- allele frequencies shift in a consistent direction
- forms at one end of a range of phenotypic variation become more common over time Fig 18.5

14. Resistance to Antibiotics
Some bacteria carry alleles that allow them to resist the effects of antibiotics.
In the presence of antibiotics, these bacteria will survive and pass on their alleles, so that antibiotic resistance becomes more common.

15. 18.5 Selection Against or in Favor of Extreme Phenotypes
Stabilizing selection is a form of natural selection that maintains an intermediate phenotype.
Disruptive selection is a form of natural selection that favors extreme forms of a trait.

16. Stabilizing selection- an intermediate form of a trait is favored, and extreme forms are not Fig 18.8

17. Disruptive selection- forms of a trait at both ends of a range of variation are favored, and intermediate forms are selected against Fig 18.11

18. Sexual Selection
Sexual selection- some version of a trait gives an individual an advantage over others in attracting mates
e.g. stalk-eyed flies Fig 18.12a
Sexual dimorphism- distinct male and female phenotypes
e.g. bird of paradise Fig 18.12b

19. Balanced Polymorphism
Balanced polymorphism- two or more alleles of a gene persist at relatively high frequencies in a population
- often occurs when environmental conditions favor heterozygotes
e.g. Sickle cell anemia and malaria Fig 18.13
- HbA/HbS heterozygotes survive malaria and do not suffer severe symptoms of sickle cell anemia

20. 18.7 Genetic Drift-- The Chance Changes
Especially in small populations, random changes in allele frequencies can lead to a loss of genetic diversity.

21. Genetic drift- a random change in allele frequencies over time, brought about by chance alone Fig 18.14
- can lead to a loss of genetic diversity, especially in small populations
Fixation has occurred when all individuals in a population are homozygous for one allele.

22. Bottlenecks and the Founder Effect
Bottleneck- a drastic reduction in population size occurs and then a few individuals rebuild the population
e.g. northern elephant seals

23. Founder effect- a few individuals start a new population
Inbreeding- breeding or mating between close relatives who share a large number of alleles
- lowers the genetic diversity of a population
e.g. Amish in Lancaster County, Pennsylvania
1 of 8 people carry the allele for Ellis-van Creveld syndrome

24. 18.8 Gene Flow
Individuals, along with their alleles, move into and away from populations. The flow of alleles counters genetic change that tends to occur within a population.

25. Gene flow- the movement of alleles among populations, caused by individuals moving in or out of a population
e.g. movement of acorns by blue jays Fig 18.15

26. 18.9 Reproductive Isolation
Speciation differs in its details, but reproductive isolating mechanisms are always part of the process.

27. Fig 18.16
Ernst Mayr’s “biological species”- one or more groups of individuals that can potentially interbreed and produce fertile offspring
Reproductive isolation- the end of gene exchange between populations
Speciation- evolutionary process by which new species arise
- begins as gene flow between populations ends

28. Reproductive isolating mechanisms Fig 18.17
Prezygotic
- Temporal isolation- individuals reproduce at different times of the day or year(s) e.g. cicadas
- Mechanical isolation- individuals cannot mate due to physical incompatibilities e.g. sage Fig 18.18
- Behavioral isolation- individuals do not send or receive proper cues for mating e.g. albatross Fig 18.19
- Ecological isolation- individuals live in different areas e.g. manzanita
- Gamete incompatibility- fertilization does not occur e.g. fish

29. Postzygotic
- Reduced hybrid viability
e.g. ligers, tigons
- Reduced hybrid fertility
e.g. mules, hinnies

30. 18.10 Allopatric Speciation
In the most common mode of speciation, a physical barrier arises and ends gene flow between populations.
Allopatric speciation- a physical barrier separates two populations and ends gene flow between them
e.g. llamas, vicunas, and camels Fig 18.20

31. Sympatric Speciation
Sympatric speciation- new species form within the home range of an existing species, in the absence of a physical barrier e.g. palms on Lord Howe Island Fig 18.23
Polyploidy- additional sets of chromosomes; more common in plants e.g. wheat Fig 18.22

32. Table 18.2 Different Speciation Models

33. 18.12 Macroevolution
Macroevolution includes patterns of change such as one species giving rise to multiple species, the origin of major groups, and major extinction events.

34. Patterns of Macroevolution
Microevolution- genetic changes within a single species or population
Macroevolution- large-scale patterns of evolutionary change

35. Coevolution- close ecological interactions between two species cause them to evolve jointly e.g. predator and prey, host and parasite, pollinator and flower Fig 18.25

36. Stasis- a lineage persists for millions of years with little or no change e.g. coelacanth
Exaptation (or preadaptation)- adaptation of an existing structure for a completely different purpose e.g. feathers in dinosaurs and birds
- key innovation- a structural or functional adaptation that allows individuals to exploit their habitat in a new way

37. Adaptive radiation- a burst of speciation that occurs when a lineage encounters a new set of niches Fig 18.26
Extinction- the irrevocable loss of a species from Earth
- mass extinctions- five catastrophic events in which the majority of species on Earth disappeared

38. Evolutionary Theory
Genetic change is the basis of evolution, but many biologists disagree about how it occurs.
Evolutionary biologists try to explain how all species are related by descent from common ancestors.