1. BIOL102

2. BIOL102 Origin of Species Part 1 – A few reminders from lecture 2
• Modern Synthesis of Genetics and Evolution
• Hardy-Weinberg Principle
• Factors Changing Allele Frequencies
Source of cover picture: Reece et al. (2010) , Campbell Biology, 9th edition, Pearson Benjamin Cummings, San Francisco (CA), Figure 24.4c

3. BIOL102 Origin of Species Part 2 – Species Concepts • Species
• Biological Species Concept
• Morphological Species Concept
• Ecological Species Concept
• Phylogenetic Species Concept

4. BIOL102 Origin of Species Part 3 – Speciation • Allopatric Speciation
• Sympatric Speciation
• Rates of Speciation
• Dynamics

5. Part 1 – A few reminders from lecture 2
Modern Synthesis of Genetics and Evolution
• A population is the smallest biological unit that can
evolve and is defined as a group of individuals of the
same species that live, interbreed and produce fertile
offspring in a particular geographic area
• A gene pool consists of all alleles (forms of genes)
for all loci in a population and is the source of
genetic variation that produces the phenotypes
and their traits on which natural selection acts
• A population evolves when individuals with different
genotypes survive or reproduce at different rates

6. Part 1 – A few reminders from lecture 2 Hardy-Weinberg Principle
• states that frequencies of alleles and genotypes in a
population remain constant from generation to generation
if certain conditions are met (Hardy-Weinberg equilibrium)
 no mutations
 random mating
 no natural selection
 extremely large population size (no effect of
genetic drift)
 no gene flow (migration into or out of a population)

7. Part 1 – A few reminders from lecture 2
Factors Changing Allele Frequencies
• Hardy-Weinberg equilibrium is a null hypothesis, which
assumes that allele frequencies are not changed
• However, there are at least four mechanisms of evolution,
which cause changes in allele frequencies of populations:
 mutations
 gene flow
 genetic drift
 natural selection

8. Part 2 – Species Concepts
A. Species
• is defined as an evolutionarily independent
population or group of populations
• Biologists commonly use the following four
approaches to identify species:
 the biological species concept
 the morphological species concept
 the ecological species concept
 the phylogenetic species concept

9. Species

10. Part 2 – Species Concepts B. Biological Species Concept
• defines a species as a population or group of populations
whose members have the potential to interbreed and
produce fertile offspring
• considers populations to be evolutionarily independent
if they are reproductively isolated from each other and
no gene flow occurs between them

11. Biological Species Concept Prezygotic and Postzygotic Barriers
• Biologists categorize the mechanisms that stop gene
flow between populations into prezygotic barriers
(before fertilization) and postzygotic barriers (after
fertilization)
 prezygotic barriers: individuals of different
species are prevented from mating
 postzygotic barriers: individuals from different
populations do mate, but the hybrid offspring
produced have low fitness and do not survive or
produce offspring

12. Prezygotic and Postzygotic Barriers
Individuals of
different species
Mating attempt
Fertilization
(zygote forms)
Viable, fertile
offspring
Prezygotic
barriers
Postzygotic

13. Biological Species Concept
Prezygotic Barriers
• block fertilization from occurring by:
 impeding different species from attempting to mate
 preventing the successful completion of mating
 hindering fertilization if mating is successful

14. Prezygotic Barriers
• Habitat isolation: two species encounter each other
rarely, or not at all, because they occupy different
habitats, even though not isolated by physical barriers
• Temporal isolation: Species that breed at different
times of the day, different seasons, or different years
cannot mix their gametes
• Behavioral isolation: courtship rituals and other
behaviors unique to a species are effective barriers

15. Behavioral Isolation
• Floral traits of plants can influence the behavior of
pollinators, and thus whether plants can hybridize
 two species of columbines (Aquilegia) in California
can produce fertile hybrids, but flower structure
determines that one species is pollinated by
hummingbirds, the other by hawkmoths, so
hybridization is rare

16. Behavioral Isolation

17. Prezygotic Barriers
• Mechanical isolation: morphological differences (e. g.,
size and shape of reproductive organs) can prevent
successful mating
• Gametic isolation: sperm of one species may not be
able to fertilize eggs of another species

18. Mechanical Isolation
• In plants, mechanical isolation may involve pollinators
 many orchid flowers look and smell like the females of
particular pollinator species
 male insects attempt
to mate, thereby
transferring
pollen

19. Prezygotic Barriers Prezygotic barriers (a) (c) (e) (f) (b) (g) (d)
Habitat
Isolation
Temporal
Behavioral
Mechanical
Gametic
Individuals of
different
species
MATING
ATTEMPT
FERTILIZATION
(a)
(c)
(e)
(f)
(b)
(g)
(d)
Figure 24.3 Exploring: Reproductive Barriers

20. Biological Species Concept
Postzygotic Barriers
• prevent the hybrid zygote from developing into a
viable, fertile adult due to:
 reduced hybrid viability
 reduced hybrid fertility
 hybrid breakdown
• Hybrids are the offspring of crosses between
different species

21. Postzygotic Barriers
• Reduced hybrid viability: genes of the different parent
species may interact and impair the hybrid’s
development
• Reduced hybrid fertility: even if hybrids are vigorous,
they may be sterile
• Hybrid breakdown: some first-generation hybrids
are fertile, but when they mate with another species
or with either parent species, offspring of the next
generation are feeble or sterile

22. Reduced Hybrid Fertility

23. Postzygotic Barriers Postzygotic barriers Reduced Hybrid Viability
Fertility
Hybrid
Breakdown
FERTILIZATION
VIABLE,
FERTILE
OFFSPRING
Postzygotic barriers
(k)
(h)
(i)
(j)
(l)
Postzygotic Barriers
Figure 24.3 Exploring: Reproductive Barriers

24. Biological Species Concept
Hybrid Zones
• If two formerly isolated populations are reunited
before complete reproductive isolation has developed,
interbreeding can occur with three possible outcomes:
 if hybrid offspring are as fit as those resulting from
matings within each population, hybrids will mate
with individuals of both parental species. The gene
pools will gradually become completely mixed
(no speciation)

25. Hybrid Zones
 if hybrid offspring are less fit, reinforcement may result
in more prezygotic barriers and complete reproductive
isolation may evolve (speciation)
 a hybrid zone may develop in the absence of
reinforcement, or before reinforcement is complete, and
may contain recombinant individuals resulting from
many generations of hybridization

26. Hybrid Zones • Example: two species of European toads have a long
narrow hybrid zone
 the toad hybrids have many defects, some of which
are lethal
 on average, a hybrid toad is significantly less fit as
a purebred individual
 the hybrid zone is narrow, because there is strong
selection against hybrids. But it persists because
individuals of both species continue to move into it
and mate

27. Hybrid Zones
Figure 24.3 Exploring: Reproductive Barriers

28. Biological Species Concept
Limitations
• The criterion of reproductive isolation cannot be evaluated
in fossils or in species that reproduce asexually
 for example, prokaryotic and viral species must be
defined differently
• this concept can only be applied to populations that
overlap geographically
• it also emphasizes absence of gene flow, which can occur
between distinct species
 for example, grizzly bears and polar bears can mate
to produce “grolar bears”

29. Limitations of the Biological Species Concept
Grizzly bear (U. arctos)
Polar bear (U. maritimus)
Hybrid “grolar bear”
Figure 24.4 Hybridization between two species of bears in the genus Ursus.

30. Part 2 – Species Concepts C. Morphological Species Concept
• defines a species by differences in morphological or
structural features
 is based on the idea that distinguishing features are
most likely to arise if populations are independent
and isolated from gene flow
 applies to sexual and asexual species but relies on
subjective criteria
 also cannot identify cryptic species that differ in
non-morphological traits

31. Part 2 – Species Concepts D. Ecological Species Concept
• views a species in terms of its ecological niche
 applies to sexual and asexual species and
emphasizes the role of disruptive selection
 is widely used for viral species (in addition
to genetic homologies)

32. Part 2 – Species Concepts E. Phylogenetic Species Concept
• defines a species as the smallest group of individuals
on a phylogenetic tree (monophyletic group)
 applies to sexual and asexual species, but it can be
difficult to determine the degree of difference
required for separate species
 on phylogenetic trees, an ancestral population plus
all of its descendants is called a monophyletic group
or clade, which is identified by synapomorphies,
homologous traits inherited from a common ancestor
that are unique to certain populations or lineages

33. Phylogenetic Species Concept

34. Phylogenetic Species Concept
• This concept can be applied to any population, but
there are disadvantages:
 phylogenies are currently available for only a
tiny (though growing) subset of populations on
the tree of life
 would probably lead to recognition of many
more species than either of the other species
concepts

35. Part 3 – Speciation
• A key event in the potential origin of a species occurs
when a population is somehow severed from other
populations of the parent species. With its gene pool
isolated, the splinter population can follow its own
evolutionary course and become reproductively
incompatible
• Two modes leading to reproductive barriers can be
distinguished
 allopatric speciation
 sympatric speciation

36. Part 3 – Speciation • Allopatric speciation occurs when geographic
isolation creates a
reproductive barrier
(extrinsic mechanisms)
• Sympatric speciation
occurs when a reproductive
barrier is created by
something other than
geographic isolation
(intrinsic mechanisms)

37. A. Allopatric Speciation
Part 3 – Speciation
A. Allopatric Speciation
• Genetic isolation happens routinely when populations
become physically separated. Physical isolation, in turn,
occurs in one of two ways: dispersal or vicariance.
 dispersal occurs when a population moves to a new
habitat, colonizes it, and forms a new population
 vicariance occurs when a physical barrier splits a
widespread population into subgroups that are
physically isolated from each other
• Speciation that begins with physical isolation via either
dispersal or vicariance is known as allopatric speciation

38. Allopatric Speciation by Dispersal or Vicariance

39. Allopatric Speciation
• Geographic separation prevents species from mating
• Speciation occurs only with the evolution of reproductive
barriers between the
isolated population
and its parent
population

40. Allopatric Speciation
• The definition of barrier depends on the ability of a
population to disperse
 for example, a canyon may create a barrier for
small rodents, but not birds, coyotes, or pollen
• Separate populations may evolve independently
through mutation, natural selection, and genetic drift
 for example, speciation of snapping shrimp (Alpheus)
populations due to separation by the Isthmus of
Panama

41. Physical Isolation and Reproductive Barriers
A. harrisii
A. leucurus
Figure 24.6 Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon.

42. Physical Isolation and Reproductive Barriers
Figure 24.6 Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon.

43. Physical Isolation and Reproductive Barriers
Figure 24.6 Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon.

44. Allopatric Speciation
• Regions with many geographic barriers typically have
more species than do regions with fewer barriers
• Reproductive isolation between populations generally
increases as the distance between them increases
 however, barriers to reproduction are intrinsic;
separation itself is not a biological barrier

45. Allopatric Populations and Reproductive Isolation
EXPERIMENT
Initial population
of fruit flies
(Drosophila
pseudoobscura)
Some flies raised
on starch medium
Mating experiments
after 40 generations
Some flies raised on
maltose medium
Figure Inquiry: Can divergence of allopatric populations lead to reproductive isolation?

46. Allopatric Populations and Reproductive Isolation
RESULTS
Female
Starch
Maltose
Male
Number of matings
in experimental group 22 9 8 20
population 1
population 2
in control group 18 15 12
Figure Inquiry: Can divergence of allopatric populations lead to reproductive isolation?

47. B. Sympatric Speciation
Part 3 – Speciation
B. Sympatric Speciation
• In sympatric speciation, speciation takes place in
geographically overlapping populations
• can occur if a genetic change produces a reproductive
barrier between mutants and the parent population
• may be the result of:
 polyploidy
 extreme habitat differentiation
 sexual selection

48. Sympatric Speciation

49. Sympatric Speciation Polyploidy
• is the presence of extra sets of chromosomes due to
accidents during cell division
 an autopolyploid is an individual with more than
two chromosome sets, derived from one species
 an allopolyploid is a species with multiple sets of
chromosomes derived from different species
• is much more common in plants than in animals
 many important crops (oats, cotton, potatoes,
tobacco, and wheat) are polyploids

50. Autopolyploidy
Figure 24.6 Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon.

51. Allopolyploidy
Figure 24.6 Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon.

52. Allopolyploidy
Figure 24.6 Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon.

53. Extreme Habitat Differentiation
Sympatric Speciation
Extreme Habitat Differentiation
• Sympatric speciation can result from the appearance
of new ecological niches
 for example, populations of the North American
maggot fly prefer to live either on native hawthorn
trees or on more recently introduced apple trees
 although they are not yet separate species on the
basis of any species concept, apple flies and
hawthorn flies are diverging

54. Extreme Habitat Differentiation
Figure 24.6 Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon.

55. Sympatric Speciation Sexual Selection
• Sexual selection can drive sympatric speciation
 such selection for mates of different colors
has likely
contributed
to speciation
in cichlid fish
in Lake
Victoria
Normal light
Monochromatic
orange light
P. pundamilia
P. nyererei

56. Comparison with Allopatric Speciation
Sympatric Speciation
Comparison with Allopatric Speciation
• In allopatric speciation, geographic isolation restricts gene
flow between populations
 reproductive isolation may then arise by e. g., natural
selection or genetic drift, in the isolated populations
• In sympatric speciation, a reproductive barrier isolates a
subset of a population without geographic separation
from the parent species
 sympatric speciation can result from polyploidy,
natural selection, or sexual selection

57. Part 3 – Speciation C. Rates of Speciation
• differ among organisms and can occur in time scales
 a slow rate of speciation evidenced by a living horseshoe
crab (13 species) and a 300 million year-old fossil
 a rapid rate of speciation evidenced by Galapagos finches
which have diversified into 13 species within the last
100,000 years

58. Part 3 – Speciation D. Dynamics • Gradual model
 traditional evolutionary
trees diagram the
descent of species as
gradual divergence

59. Dynamics • Punctuated equilibrium  is a contrasting model
of evolution
 states that species most
often diverge in spurts
of relatively sudden change
 accounts for the relative
rarity of transitional fossils
and hence appears to be a
more accurate view of
speciation dynamics

60. BIOL101 Introduction to Biology B
Learning Objectives and Check of Understanding
• Compare and contrast the different species concepts?
• Distinguish prezygotic and postzygotic barriers.
• Differentiate allopatric and sympatric speciation.
• Compare and contrast the models describing the
dynamics of speciation.

61. BIOL101 Introduction to Biology B
Reading Assignments
• Campbell: Chapter 24
• Sadava: Chapter 23

62. Brief Outline of the Upcoming Lecture Classification and Phylogeny
Part 1 – A few reminders from lecture 3
Part 2 – Classification and Taxonomy
Part 3 – Phylogeny or Systematics 62