1. Chapter 24 – Origin of Species

2. Speciation, the origin of new species, is at the focal point of Darwin’s evolutionary theory.
Evolutionary theory must explain how new species originate and how populations evolve.
Microevolution consists of adaptations that evolve within a population, confined to one gene pool.
Macroevolution refers to evolutionary change above the species level.

3. In the Galápagos Islands Darwin discovered plants and animals found nowhere else on Earth
Figure 24.1 How did this flightless bird come to live on the isolated Galápagos Islands?

4. The Biological Species Concept (BSC) emphasizes reproductive isolation
Biologists compare morphology, physiology, biochemistry, and DNA sequences when grouping organisms.
The biological species concept (BSC) states that a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring; they do not breed successfully with other populations.

5. Similarity between species (Western & Eastern Meadowlark – Sturnella neglecta & magna)

6. Gene Flow Between Populations
EXPERIMENT
Example of a gene tree for population pair A-B
Allele
Population
Gene flow event 1 B
Allele 1 is more closely related to
alleles 2, 3, and 4 than to
alleles 5, 6, and 7.
Inference: Gene flow occurred. 2 A 3 A 4 A 5 B
Alleles 5, 6, and 7 are more closely
related to one another than to
alleles in population A.
Inference: No gene flow occurred. 6 B 7 B
RESULTS
Pair of
populations
with detected
gene flow
Estimated minimum
number of gene flow
events to account for
genetic patterns
Distance between
populations (km)
Figure 24.3 Does gene flow occur between widely separated populations?
A-B 5
340
K-L 3
720
A-C
2–3
1,390
B-C 2
1,190
F-G 2
760
G-I 2
1,110
C-E
1–2
1,310

7. Reproductive Isolation = Barriers to Interbreeding
Reproductive isolation is the existence of biological factors (barriers) that impede two different species from producing viable, fertile offspring.
Hybrids are the offspring of crosses between different species.
Reproductive isolation can be classified by whether factors act before or after fertilization.

8. Reproductive Barriers Between Species
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.
Prezygotic barriers maintain reproductive isolation and include:
Temporal, Habitat, Behavioral, Mechanical, and Gamete Isolation.

9. Reproductive Barriers Between Species
Prezygotic Barriers
Postzygotic Barriers
Habitat
Isolation
Temporal
Isolation
Behavioral
Isolation
Mechanical
Isolation
Gametic
Isolation
Reduced
Hybrid Viability
Reduced Hybrid
Fertility
Hybrid
Breakdown
Individuals of
different
species
Mating
attempt
Viable,
fertile
offspring
Fertilization
(a)
(c)
(e)
(f)
(g)
(h)
(i)
(l)
(d)
(j)
(b)
Figure 24.4 Reproductive barriers
(k)

10. PreZygotic Reproductive 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.

11. Mexican vs. Northern Spotted Owl (Strix occidentalis lucida vs S
Mexican vs. Northern Spotted Owl (Strix occidentalis lucida vs S. occidentalis caurina)

12. The garter snake, genus Thamnophis
Habitat Isolation
Figure 24.4 Reproductive barriers
Water-dwelling Thamnophis
Terrestrial Thamnophis
The garter snake, genus Thamnophis

13. PreZygotic Reproductive 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.
Mechanical isolation: Morphological differences can prevent successful mating.
Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species.

14. Blue footed booby (Sula nebouxii)
Behavioral Isolation Occurs Without Appropriate Mating Rituals
Figure 24.4 Reproductive barriers
Blue footed booby (Sula nebouxii)

15. Mechanical Isolation:
Figure 24.4 Reproductive barriers
Mechanical Isolation:
Bradybaena with shells spiraling in opposite directions

16. PostZygotic Reproductive Barriers
Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult:
Reduced hybrid viability -- weak offspring
Reduced hybrid fertility -- sterile offspring
Hybrid breakdown.

17. Mule: sterile hybrid offspring between horse and donkey
PostZygotic Reproductive Barrier
Figure 24.4 Reproductive barriers
Mule: sterile hybrid offspring between horse and donkey

18. Other Definitions of Species
The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes).
Other species concepts emphasize the unity within a species rather than the separateness of different species.
The morphological species concept (MSC) defines a species by structural features.
It applies to sexual and asexual species but relies on subjective criteria.

19. The ecological species concept (ESC) views a species in terms of its ecological niche.
It applies to sexual and asexual species and emphasizes the role of disruptive selection.
The phylogenetic species concept (PSC): defines a species as the smallest group of individuals on a phylogenetic tree.
It applies to sexual and asexual species, but it can be difficult to determine the degree of difference required for separate species.

20. Speciation can occur in two ways:
Concept 24.2: Speciation can take place with or without geographic separation
Speciation can occur in two ways:
Allopatric speciation: geographic barrier separates populations.
Sympatric speciation: no geographic barrier

22. Speciation (a) Allopatric speciation (b) Sympatric speciation
Fig 24.5 Two main modes of speciation
(a) Allopatric speciation
(b) Sympatric speciation

23. Allopatric (“Other Country”) Speciation
In allopatric speciation, gene flow is interrupted or macroevolution is the cumulative effect of many speciation and extinction events.
reduced when a population is divided into geographically isolated subpopulations … A geographic barrier separates the original population.
Separate populations may evolve independently through mutation, natural selection, and genetic drift.
Reproductive isolation between populations generally increases as the distance between them increases.
Barriers to reproduction are intrinsic; separation itself is not a biological barrier.

24. Allopatric Speciation
Figure 24.6 Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon
A. harrisIi
A. leucurus
White-tailed Antelope Squirrel (Ammospermophilus harrisii & A. leucurus)
Ammospermophilus harrisii & leucurus
Harris's antelope squirrel in the Grand Canyon

26. Continental Drift Caused Allopatric Speciation
Mantellinae
(Madagascar only):
100 species
Rhacophorinae
(India/Southeast
Asia): 310 species
Other Indian/
Southeast Asian
frogs
100 80 60 40 20 1 2 3
Figure 24.7 Allopatric speciation in frogs
Millions of years ago (mya) 1 2 3
India
Madagascar
88 mya
65 mya
56 mya

28. Sympatric (“Same Country”) Speciation
In sympatric speciation, speciation takes place in geographically overlapping populations.
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.
Polyploidy is common in plants. Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids.

29. Sympatric Speciation via Polyploidy is Common in Plants
Failure of cell
division after
chromosome
duplication gives
rise to tetraploid
tissue.
Gametes
produced
are diploid..
Offspring with
tetraploid
karyotypes may
be viable and
fertile.
Fig Sympatric speciation by autopolyploidy in plants

30. Sympatric Speciation - Polyploidy --> Allopolyploid
Species B
2n = 4
Unreduced
gamete
with 4
chromosomes
Unreduced
gamete
with 7
chromosomes
Hybrid
with 7
chromosomes
Meiotic
error
Normal
gamete
n = 3
Viable fertile
hybrid
(allopolyploid)
2n = 10
Figure One mechanism for allopolyploid speciation in plants
Normal
gamete
n = 3
Species A
2n = 6

31. Sympatric Speciation: Habitat Differentiation and Sexual Selection
Sympatric speciation can also result from the appearance of new ecological niches.
For example, the North American maggot fly can live on native hawthorn trees as well as more recently introduced apple trees.
Sexual selection can drive sympatric speciation.
Sexual selection for mates of different colors has likely contributed to the speciation in cichlid fish in Lake Victoria.

32. Allopatric and Sympatric Speciation May Cause Reproductive Isolation: A Review
In allopatric speciation, geographic separation restricts gene flow between populations.
Reproductive isolation may then arise by natural selection, genetic drift, or sexual selection in the isolated populations.
Even if contact is restored between populations, interbreeding is prevented.
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.

33. Concept 24.3: Hybrid zones provide opportunities to study factors that cause reproductive isolation
A hybrid zone is a region in which members of different species mate and produce hybrids.
A hybrid zone can occur in a single band where adjacent species meet.
Hybrids often have reduced fitness compared with parent species.
The distribution of hybrid zones can be more complex if parent species are found in multiple habitats within the same region.

34. Hybrid Zones
EUROPE
Fire-bellied
toad range
Hybrid zone
Fire-bellied toad,
Bombina bombina
Yellow-bellied
toad range
Yellow-bellied toad,
Bombina variegata
0.99
0.9
Fig A narrow hybrid zone for B. variegata and B. bombina in Europe
Allele frequency (log scale)
0.5
0.1
0.01 40 30 20 10 10 20
Distance from hybrid zone center (km)

35. Hybrid Zones over Time
When closely related species meet in a hybrid zone, there are three possible outcomes:
Reinforcement -- Strengthening of reproductive barriers reducing gene flow.
Fusion -- Weakening of reproductive barriers with eventual fusion into one species.
Stabilizing -- Continued formation of hybrid individuals.

36. Hybrid Zones Over Time Possible outcomes: Reinforcement Fusion into
Isolated population
diverges
Hybrid
zone
Reinforcement
of gene flow barrier. OR
Fusion into
one species.
Gene flow
Hybrid OR
Figure Formation of a hybrid zone and possible outcomes for hybrids over time
Barrier to
gene flow
Population
(five individuals
are shown)
Stability:
Continued formation
of hybrid individuals.

37. The Time Course of Speciation
Broad patterns in speciation can be studied using the fossil record, morphological data, or molecular data.
The fossil record includes examples of species that appear suddenly, persist essentially unchanged for some time, and then apparently disappear
Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibrium to describe periods of apparent stasis (no change) punctuated by brief periods of rapid change.
The punctuated equilibrium model contrasts with a Darwinian model of gradualism: slow continuous change over time in a species’ existence.

38. Patterns in Speciation
Punctuated Equilibrium
pattern
Change / Time
Gradualism pattern
Figure Two models for the tempo of speciation

39. Studying the Genetics of Speciation
The explosion of genomics is enabling researchers to identify specific genes involved in some cases of speciation.
Depending on the species in question, speciation might require the change of only a single allele or many alleles.
From Speciation to Macroevolution:
Macroevolution is the cumulative effect of many speciation and extinction events.

40. Speciation Review Allopatric speciation Sympatric speciation
Original population
Fig. 24-UN1
Allopatric speciation
Sympatric speciation

41. Speciation Ancestral species: AA BB DD Triticum monococcum (2n = 14)
Wild
Triticum
(2n = 14)
Wild
T. tauschii
(2n = 14)
Product:
Fig. 24-UN2
AA BB DD
T. aestivum
(bread wheat)
(2n = 42)

42. You should now be able to:
Define and discuss the limitations of the four species concepts.
Describe and provide examples of prezygotic and postzygotic reproductive barriers.
Distinguish between and provide examples of allopatric and sympatric speciation.
Explain how polyploidy can cause reproductive isolation.
Define the term hybrid zone and describe three outcomes for hybrid zones over time.