1) The document discusses several processes of microevolution that can cause changes in allele frequencies in a population over time, including nonrandom mating, migration, genetic drift, mutation, and natural selection. 2) It also covers several types of speciation, such as allopatric speciation which occurs when a physical barrier ends gene flow between populations, and parapatric speciation which occurs along a contact zone between populations. 3) Additionally, it discusses patterns of macroevolution like adaptive radiation, where a lineage encounters new niches leading to a burst of speciation, and extinction, particularly mass extinctions where the majority of species disappear.

1. Chapter 17
Processes of Evolution
Individuals Don’t Evolve, Populations Do.
Evolution starts with mutations in individuals; mutation is the source of
new alleles.
Alleles
Different forms of the same gene
Determine genotype and phenotype
Dimorphism and polymorphism
Population
Individuals of the same species in the same area.
Gene pool
All genes found in one population.
17-1

2. Allele frequencies
Relative abundance of alleles of a given gene in a population.
Natural populations are never in genetic equilibrium.
A theoretical state which occurs when a population is not evolving.
Populations Evolve
Individuals of a population differ in which alleles they inherit, and so
they differ in phenotype.
Over generations, any allele may increase or decrease in frequency in
a population.
Such change is called microevolution.
17-2

3. Microevolution
Five processes of microevolution (small-scale changes in a
population's allele frequencies) prevent genetic equilibrium:
1) Nonrandom mating
2) Migration
3) Genetic drift
4) Mutation
5) Selection
natural
artificial
17-3

4. Nonrandom Mating
• Nonrandom mating indicates that individuals of one genotype
reproduce more often with each other.
• Indeed, we marry people similar to ourselves about 80% of the
time.
• Traits that influence our mate choice include:
• - Physical appearance
• - Ethnic or religious preferences
• - Intelligence and shared interests
17-4

5. Nonrandom
Mating
Alters Allele
Frequencies
Figure 15.2

6. Migration
• Individuals migrate and move genes from one area to another.
• - The addition or removal of alleles will alter the genotypic
frequencies.
• Genetic effects of migration are reflected in current populations.
• Changes in allele frequency can be mapped across geographical
or linguistic regions
• Vlax Roma gypsies ---- galactokinase deficiency (cataracts)
autosomal recessive
• Migrated to Bulgaria
17-5

7. Figure 15.5

8. Migration Alters Allele Frequencies
Figure 15.4

9. Genetic Drift
• Genetic drift is the change in allele frequency when a small
group separates from the larger whole.
• Allele frequency changes are unpredictable.
17-6

10. Genetic Drift Alters Allele Frequencies
Figure 15.6

11. Genetic Drift
• Events that create small populations enhance the effect of
genetic drift.
• - Founding a new population
• - Bottlenecks (natural disaster, famine)
17-7

12. Founder Effect
• Occurs when a small group leaves home to found new
settlements.
• The new colony may have different allele frequencies than the
original population.
• - It may, by chance, either lack some alleles or have high
frequency of others.
17-8

14. Examples of Founder Effect
• Dunker community of Germantown, Penn.
• - Descendants of German immigrants who came
between 1719 and 1729
• - Have different distribution of blood types than
the German native and non-Dunker neighbor
population
17-9

15. Table 15.2

16. Genetic Drift and Nonrandom Mating
Small population size increases the probability of homozygosity
Increases recessive phenotypes in population
Example
- Amish and Mennonite populations of Penn. marry
predominantly within their religious groups
- Maintain their original small genetic pool
- Increased incidence of otherwise rare traits
• Consanguinity
• Ashkenazim Jews--------Tay-Sachs
• Arizona Hopi Indians----albinism
• Arranged marriage between cousins 17-10

17. Figure 15.6
Figure 15.7
Ellis-van Creveld syndrome

18. Population Bottlenecks
• Occurs when a large population is drastically reduced in size
• Rebounds in population size occur with descendants of limited
number of survivors
• - Therefore, new population has a much more restricted
gene pool than the large ancestral population
17-11

19. Examples of Population Bottlenecks
• Cheetahs in S. and E. Africa
• - Bottleneck created by changing habitats (after the most
recent ice age) and mass slaughter by humans in the 19th
century
• Ashkenazi Jews
• - Massacres and nonrandom mating between survivors
contributed to high incidence of certain disorders
17-12

20. Population Bottlenecks
Figure 15.8

21. Mutation
• Mutations are a major and continual source of genetic variation
in populations
• Can introduce new alleles
• Can convert one allele to another
• New alleles give rise to differences in details of shared traits
– Lethal mutations usually result in death
– Neutral mutations have no effect on survival or
reproduction
– Beneficial mutations convey an advantage
17-13

22. Mutations Alter Allele FrequenciesMutations Alter Allele Frequencies
Figure 15.9

23. Mutation
• Mutations are a major and continual source of genetic variation
in populations
• Can introduce new alleles
• Can convert one allele to another
• New alleles give rise to differences in details of shared traits
– Lethal mutations usually result in death
– Neutral mutations have no effect on survival or
reproduction
– Beneficial mutations convey an advantage
17-13

24. Natural Selection
• Is the differential survival and reproduction of individuals
with a particular genotype/phenotype
• A driving force of evolution
• Negative selection = Banishment of a dangerous trait
• Positive selection = Retaining an advantageous trait
• Both lead to changes in allele frequencies
17-14

25. Natural Selection Alters Allele Frequencies
Figure 15.10

26. Natural Selection
• Is the differential survival and reproduction of individuals
with a particular genotype/phenotype
• A driving force of evolution
• Negative selection = Banishment of a dangerous trait
• Positive selection = Retaining an advantageous trait
• Both lead to changes in allele frequencies
17-14

27. Predation and Peppered Moths
• Light color is adaptive in areas of low
pollution; dark color is adaptive in areas of high
pollution

28. Balanced polymorphism
A state in which natural selection maintains two or
more alleles at relatively high frequencies
Occurs when environmental conditions favor heterozygotes
Example: Sickle Cell Disease and Malaria
The beta hemoglobin gene exhibits balanced polymorphism
Sickle cell allele causes the recessive sickle cell anemia trait
(when homozygous) and is therefore under negative selection
Sickle cell allele helps protect heterozygotes from malaria
therefore under positive selection 17-15

29. Artificial Selection
• Controlled breeding with the intent of
perpetuating individuals with a particular
phenotype
• Examples:
• - Crop plants
• - Pets
Figure 15.11

30. Speciation
Reproductive Isolation
• Speciation
– Evolutionary process by which new species form
– Reproductive isolating mechanisms are always part of the
process
• Reproductive isolation
– The end of gene exchange between populations
– Beginning of speciation
17-16

31. Reproductive Isolating Mechanisms
• Reproductive isolating mechanisms prevent interbreeding
among species
– Heritable aspects of body form, function, or behavior that
arise as populations diverge
– Prezygotic isolating mechanisms prevent pollination or
mating
– Postzygotic isolating mechanisms result in weak or infertile
hybrids
17-17

32. Prezygotic Isolating Mechanisms
• Temporal isolation
• Mechanical isolation
• Behavioral isolation
• Ecological isolation
• Gamete incompatibility
7-18

33. Postzygotic Isolation Mechanisms
• Reduced hybrid viability (ligers, tigons)
– Extra or missing genes
• Reduced hybrid fertility (mules)
– Robust but sterile offspring
• Hybrid breakdown
– Lower fitness with successive generations
17-19

34. Speciation
• Allopatric speciation
– A physical barrier arises and ends gene flow between
populations.
– Genetic divergence results in speciation.
17-20

35. The Inviting Archipelagos
• Winds or ocean currents carry a few individuals of mainland
species to remote, isolated island chains (archipelagos) such as
Hawaii
• Habitats and selection pressures that differ within and between
the islands foster divergences that result in allopatric speciation
17-21

36. Allopatric Speciation
on an Isolated Archipelago

37. Speciation
Other Speciation Models
• Populations sometimes speciate even without a physical
barrier that blocks gene flow
– Sympatric speciation
– Parapatric speciation
17-22

38. Speciation
Sympatric Speciation
• In sympatric speciation, new species form within a home range
of an existing species, in the absence of a physical barrier
• A change in chromosome number (polyploidy) can cause
instant speciation
17-23

39. Speciation
Parapatric Speciation
• In parapatric speciation, populations in contact along a
common border evolve into distinct species
• Hybrids in the contact zone are less fit than individuals on either
side
17-24

40. Different Speciation Models

41. How Species Arise
• Speciation varies in its details, but it typically starts after
gene flow ends
• Microevolutionary events that occur independently lead to
genetic divergences, which are reinforced as reproductive
isolation mechanisms evolve
17-25

42. Evolutionary Theory
• Evolutionary biologists try to explain how all species are related
by descent from common ancestors
• Genetic change is the basis of evolution, but many biologists
disagree about how it occurs
17-26

43. Macroevolution
• Macroevolution
– Large-scale patterns of evolutioary change
– Includes patterns of change such as one species giving
rise to multiple species, the origin of major groups, and
major extinction events
17-27

44. Macroevolutionary Patterns
• Patterns of genetic change that involve more than one
species are called macroevolution
• Recurring patterns of macroevolution include exaptation,
adaptive radiation, and extinction
17-28

45. Stasis and Exaptation
• Stasis
– A lineage exists for millions of years with little or no change
(e.g. coelacanth)
• Exaptation (preadaptation)
– Some complex traits in modern species held different
adaptive value in ancestral lineages (e.g. feathers in birds
and dinosaurs)
17-29

46. Adaptive Radiation
• Adaptive radiation
– A burst of speciation that occurs when a lineage encounters a
new set of niches
• Key innovation
– A structural or functional adaptation that allows individuals
to exploit their habitat in a new way
17-30

47. Extinction
• Extinction
– The irrevocable loss of a species from Earth
• Mass extinctions
– Extinctions of many lineages, followed by adaptive
radiations
– Five catastrophic events in which the majority of species on
Earth disappeared
17-31

48. Fig. 18-26a, p. 297
platypus,other
monotremes
kangaroos,other
marsupials
elephants,other
proboscideans
anteaters
armadillos
shrews,other
insectivores;
bats
carnivores
whales,
dolphins
deer,other
artiodactyls
horses,other
perissodactyls
primates
rodents
rabbits
Eomaiascansoria
Cenozoic
Mesozoic
manatees