1. Chapter 24
The Origin of Species

2. Overview: The “Mystery of Mysteries”
Darwin explored the Galápagos Islands
And discovered plants and animals found nowhere else on Earth
Figure 24.1

3. The origin of new species, or speciation
Is at the focal point of evolutionary theory, because the appearance of new species is the source of biological diversity
Evolutionary theory
Must explain how new species originate in addition to how populations evolve
Macroevolution
Refers to evolutionary change above the species level

4. Concept 24.1: The biological species concept emphasizes reproductive isolation
Is a Latin word meaning “kind” or “appearance”

5. The Biological Species Concept
1. Defines a species as a population or group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring but are unable to produce viable fertile offspring with members of other populations

6. Two basic patterns of evolutionary change can be distinguished
Anagenesis
Cladogenesis
Figure 24.2
(b) Cladogenesis
(a) Anagenesis

7. 2. Patterns of speciation
a. anagenesis: accumulation of heritable changes altering a species
b. cladogenesis: branching evolution, new species form from parent species. This is a basis for biodiversity.

8. Similarity between different species
Similarity between different species. The eastern meadowlark (Sturnella magna, left) and the western meadowlark (Sturnella neglecta, right) have similar body shapes and colorations. Nevertheless, they are distinct biological species because their songs and other behaviors are different enough to prevent interbreeding should they meet in the wild.
(a)
Diversity within a species. As diverse as we may be in appearance, all humans belong to a single biological species (Homo sapiens), defined by our capacity to interbreed.
(b)
Figure 24.3 A, B

9. Reproductive Isolation
3. Reproductive isolation: essential for formation of distinct species.
Is the existence of biological factors that impede members of two species from producing viable, fertile hybrids
Is a combination of various reproductive barriers

10. 4. Barriers causing isolation
a. Prezygotic barriers
Impede mating between species or hinder the fertilization of ova if members of different species attempt to mate
g. Postzygotic barriers
Often prevent the hybrid zygote from developing into a viable, fertile adult

11. Prezygotic barriers
b. habitat: snakes living in a pond vs. on land
c. behavioral: boobies differ in courtship dance
d. temporal: skunks mate different seasons
e. mechanical: different morphology (shape, structure, etc) attracting different pollinators
f. gamete: sea urchin sperm of red won’t fertilize purple sea urchin

12. Individuals of different species
Prezygotic and postzygotic barriers
Figure 24.4
Prezygotic barriers impede mating or hinder fertilization if mating does occur
Individuals of different species
Mating attempt
Habitat isolation
Temporal isolation
Behavioral isolation
Mechanical isolation
HABITAT ISOLATION
TEMPORAL ISOLATION
BEHAVIORAL ISOLATION
MECHANICAL ISOLATION
(b)
(a)
(c)
(d)
(e)
(f)
(g)

13. Reduce hybrid viability Reduce hybrid fertility Hybrid breakdown
Viable fertile offspring
Reduce hybrid viability
Reduce hybrid fertility
Hybrid breakdown
Fertilization
Gametic isolation
GAMETIC ISOLATION
REDUCED HYBRID VIABILITY
REDUCED HYBRID FERTILITY
HYBRID BREAKDOWN
(h)
(i)
(j)
(k)
(l)
(m)

14. Postzygotic barriers
h. hybrid inviability: frail salamanders, aborts at varied stage of development or result in frail offspring
i. hybrid sterility: horse & donkey result in sterile mule

15. Limitations of the Biological Species Concept
The biological species concept cannot be applied to
Asexual organisms
Fossils
Organisms about which little is known regarding their reproduction

16. Other Definitions of Species
The morphological species concept
Characterizes a species in terms of its body shape, size, and other structural features
The paleontological species concept
Focuses on morphologically discrete species known only from the fossil record
The ecological species concept
Views a species in terms of its ecological niche
The phylogenetic species concept
Defines a species as a set of organisms with a unique genetic history

17. 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
Sympatric speciation
(a)
Allopatric speciation. A population forms a new
species while geographically isolated from its parent population.
(b)
Sympatric speciation. A small population becomes a new species without geographic separation.
Figure 24.5 A, B

18. Allopatric (“Other Country”) Speciation
5.a.In allopatric speciation
Gene flow is interrupted or reduced when a population is divided into two or more geographically isolated subpopulations

19. Sympatric (“Same Country”) Speciation
5.b.In sympatric speciation
Speciation takes place in geographically overlapping populations

20. 6. The Grand Canyon is IN THE WAY
Once geographic separation has occurred
One or both populations may undergo evolutionary change during the period of separation
Figure 24.6
A. harrisi
A. leucurus

21. In order to determine if allopatric speciation has occurred
Reproductive isolation must have been established
Figure 24.7
Initial population of fruit flies (Drosphila Pseudoobscura)
Some flies raised on starch medium
Some flies raised on maltose medium
Mating experiments after several generations
EXPERIMENT
Diane Dodd, of Yale University, divided a fruit-fly population, raising some populations on a starch medium and others on a maltose medium. After many generations, natural selection resulted in divergent evolution: Populations raised on starch digested starch more efficiently, while those raised on maltose digested maltose more efficiently. Dodd then put flies from the same or different populations in mating cages and measured mating frequencies.

22. RESULTS
When flies from “starch populations” were mixed with flies from “maltose populations,” the flies tended to mate with like partners. In the control group, flies taken from different populations that were adapted to the same medium were about as likely to mate with each other as with flies from their own populations.
Female Starch Maltose
Female
Same population
Different populations
Male Maltose Starch
Male
Different populations
Mating frequencies in experimental group
Mating frequencies in control group 22 8 9 20 18 12 15
CONCLUSION
The strong preference of “starch flies” and “maltose flies” to mate with like-adapted flies, even if they were from different populations, indicates that a reproductive barrier is forming between the divergent populations of flies. The barrier is not absolute (some mating between starch flies and maltose flies did occur) but appears to be under way after several generations of divergence resulting from the separation of these allopatric populations into different environments.

23. Ring species: salamanders
7. species with a geographic distribution that forms a ring and overlaps at the ends. The many subspecies of Ensatina salamanders in California exhibit subtle morphological and genetic differences all along their range. They all interbreed with their immediate neighbors with one exception: where the extreme ends of the range overlap in Southern California, E. klauberi and E. eschscholtzii do not interbreed. So where do we mark the point of speciation?

24. Habitat Differentiation and Sexual Selection
Sympatric speciation
Can also result from the appearance of new ecological niches

25. Allopatric and Sympatric Speciation: A Summary
In allopatric speciation
A new species forms while geographically isolated from its parent population
In sympatric speciation
The emergence of a reproductive barrier isolates a subset of a population without geographic separation from the parent species

26. Adaptive Radiation Adaptive radiation
Is the evolution of diversely adapted species from a common ancestor upon introduction to new environmental opportunities
Figure 24.11

27. 8. Island chains encourage adaptive radiation:
Islands and organisms are physically diverse; there are multiple “invasions” and both allopatric & sympatric speciation events have ignited an explosion of adaptive radiation of novel species.

28. Adaptive Radiation on Islands
Island chains frequently produce many new species.
Islands chains provide barriers that facilitate invasion and re-invasion by different species.
This is the probable mechanism for the proliferation of Darwin’s finches on the Galapagos.
The Hawaiian islands once supported thousands of unique Drosophila flies that probably evolved by a similar mechanism

29. Argyroxiphium sandwicense
The Hawaiian archipelago
Is one of the world’s great showcases of adaptive radiation
Figure 24.12
Dubautia laxa
Dubautia waialealae
KAUA'I
5.1
million
years
O'AHU
3.7
LANAI
MOLOKA'I
1.3 million years
MAUI
HAWAI'I
0.4
Argyroxiphium sandwicense
Dubautia scabra
Dubautia linearis N

30. Sympatric Speciation
9. Sympatric speciation results from intrinsic factors, such as chromosomal changes and nonrandom mating.
Sympatric populations become genetically isolated even though their ranges overlap.

31. Polyploidy Polyploidy
10. Is the presence of extra sets of chromosomes in cells due to accidents during cell division
Has caused the evolution of some plant species

32. An autopolyploid
Is an individual that has more than two chromosome sets, all derived from a single species
Figure 24.8
2n = 6
4n = 12 2n 4n
Failure of cell division in a cell of a growing diploid plant after chromosome duplication gives rise to a tetraploid branch or other tissue.
Gametes produced by flowers on this branch will be diploid.
Offspring with tetraploid karyotypes may be viable and fertile—a new biological species.

33. 11. Allopolyploid hybrids are usually sterile due to abnormal meiosis, chromosomes are not homologous and cannot pair during meiosis.
An allopolyploid
Is a species with multiple sets of chromosomes derived from different species
Figure 24.9
Meiotic error;
chromosome
number not
reduced from
2n to n
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
with 7 chromosomes
Viable fertile hybrid
(allopolyploid)
Normal gamete
n = 3
Species A
2n = 4
Species B
2n = 6
2n = 10

34. 12. Hugo DeVries discovered “mutations”

35. 13. Two factors demonstrating sympatric speciation in cichlids of Lake Victoria:
Specialized exploitation of food resources & non-random mating.

36. In cichlid fish
Sympatric speciation has resulted from nonrandom mating due to sexual selection
Figure 24.10
Researchers from the University of Leiden placed males and females of Pundamilia pundamilia and P. nyererei together in two aquarium tanks, one with natural light and one with a monochromatic orange lamp. Under normal light, the two species are noticeably different in coloration; under monochromatic orange light, the two species appear identical in color. The researchers then observed the mating choices of the fish in each tank.
EXPERIMENT
P. nyererei
Normal light
Monochromatic
orange light
P. pundamilia
Under normal light, females of each species mated only with males of their own species. But under orange light, females of each species mated indiscriminately with males of both species. The resulting hybrids were viable and fertile.
RESULTS
The researchers concluded that mate choice by females based on coloration is the main reproductive barrier that normally keeps the gene pools of these two species separate. Since the species can still interbreed when this prezygotic behavioral barrier is breached in the laboratory, the genetic divergence between the species is likely to be small. This suggests that speciation in nature has occurred relatively recently.
CONCLUSION

37. Studying the Genetics of Speciation
The explosion of genomics
Is enabling researchers to identify specific genes involved in some cases of speciation

38. The Tempo of Speciation
The fossil record
Includes many episodes in which new species appear suddenly in a geologic stratum, persist essentially unchanged through several strata, and then apparently disappear
Niles Eldredge and Stephen Jay Gould coined the term 14. punctuated equilibrium to describe these periods of apparent stasis punctuated by sudden change

39. The punctuated equilibrium model
14. Contrasts with a model of gradual change throughout a species’ existence: gradualism
Figure 24.13
Gradualism model. Species
descended from a common
ancestor gradually diverge
more and more in their
morphology as they acquire
unique adaptations.
Time
(a)
Punctuated equilibrium
model. A new species
changes most as it buds
from a parent species and
then changes little for the
rest of its existence.
(b)

40. 15. Macroevolutionary change
Concept 24.3: Macroevolutionary changes can accumulate through many speciation events
15. Macroevolutionary change
Is the cumulative change during thousands of small speciation episodes
Microevolution: one generation below the species level; change in the genetic makeup of a population from generation to generation.

41. Evolutionary Novelties
Most novel biological structures
Evolve in many stages from previously existing structures

42. Evolution of the Genes That Control Development
Genes that program development
Control the rate, timing, and spatial pattern of changes in an organism’s form as it develops into an adult

43. Changes in Rate and Timing
16. Heterochrony
Is an evolutionary change in the rate or timing of developmental events
Can have a significant impact on body shape

44. Heterochrony
Has also played a part in the evolution of salamander feet
Ground-dwelling salamander. A longer time
peroid for foot growth results in longer digits and
less webbing.
Tree-dwelling salamander. Foot growth ends
sooner. This evolutionary timing change accounts
for the shorter digits and more extensive webbing,
which help the salamander climb vertically on tree
branches.
(a)
(b)
Figure A, B

45. 16. And also Allometric growth
Is the proportioning that helps give a body its specific form
Figure A
Newborn Adult
(a) Differential growth rates in a human. The arms and legs
lengthen more during growth than the head and trunk, as can be seen in this conceptualization of an individual at different ages all rescaled to the same height.
Age (years)

46. 17. Mollusk eye demonstrates:
That complex structures have evolved independently; mollusks eyes range from simple to camera-like.

47. Some complex structures, such as the eye
Have had similar functions during all stages of their evolution
Figure A–E
Pigmented cells
(photoreceptors)
Epithelium
Nerve fibers
Pigmented
cells
Patch of pigmented cells.
The limpet Patella has a simple
patch of photoreceptors.
Eyecup. The slit shell
mollusc Pleurotomaria
has an eyecup.
Fluid-filled cavity
Cellular
fluid
(lens)
Cornea
Optic nerve
layer (retina)
Optic
nerve
Pinhole camera-type eye.
The Nautilus eye functions
like a pinhole camera
(an early type of camera
lacking a lens).
Lens
Retina
Complex camera-type eye. The squid Loligo has a complex
eye whose features (cornea, lens, and retina), though similar to
those of vertebrate eyes, evolved independently.
(a)
(b)
(d)
(c)
(e)
Eye with primitive lens. The
marine snail Murex has
a primitive lens consisting of a mass of
crystal-like cells. The cornea is a
transparent region of epithelium
(outer skin) that protects the eye
and helps focus light.

48. 18. Evolution Is Not Goal Oriented
The fossil record
Often shows apparent trends in evolution that may arise because of adaptation to a changing environment
Figure 24.20
Recent
(11,500 ya)
Pleistocene
(1.8 mya)
Pliocene
(5.3 mya)
Miocene
(23 mya)
Oligocene
(33.9 mya)
Eocene
(55.8 mya)
Equus
Hippidion and other genera
Nannippus
Pliohippus
Neohipparion
Hipparion
Sinohippus
Megahippus
Callippus
Archaeohippus
Merychippus
Parahippus
Hypohippus
Anchitherium
Miohippus
Mesohippus
Epihippus
Orohippus
Paleotherium
Propalaeotherium
Pachynolophus
Grazers
Browsers
Key
Hyracotherium

49. Different allometric patterns
Contribute to the contrasting shapes of human and chimpanzee skulls
Figure B
Chimpanzee fetus
Chimpanzee adult
Human fetus
Human adult
(b) Comparison of chimpanzee and human skull growth. The fetal skulls of humans and chimpanzees are similar in shape. Allometric growth transforms the rounded skull and vertical face of a newborn chimpanzee into the elongated skull and sloping face characteristic of adult apes. The same allometric pattern of growth occurs in humans, but with a less accelerated elongation of the jaw relative to the rest of the skull.

50. Changes in Spatial Pattern
Substantial evolutionary change
Can also result from alterations in genes that control the placement and organization of body parts

51. 19. Evolutionary trends:
a. convergent evolution: different species evolve to have similar morphology
b. analogous traits: similar due to convergent evolution NOT common ancestors
c. parallel evolution: different species, same adaptation
d. co-evolution: 2 interacting species evolve in ways that complement their symbiotic relationship

52. In paedomorphosis
The rate of reproductive development accelerates compared to somatic development
The sexually mature species may retain body features that were juvenile structures in an ancestral species
Figure 24.17

53. Homeotic genes
Determine such basic features as where a pair of wings and a pair of legs will develop on a bird or how a flower’s parts are arranged

54. The products of one class of homeotic genes called Hox genes
Provide positional information in the development of fins in fish and limbs in tetrapods
Chicken leg bud
Region of
Hox gene
expression
Zebrafish fin bud
Figure 24.18

55. The evolution of vertebrates from invertebrate animals
Was associated with alterations in Hox genes
Figure 24.19
The vertebrate Hox complex contains duplicates of many of the same genes as the single invertebrate cluster, in virtually the same linear order on chromosomes, and they direct the sequential development of the same body regions. Thus, scientists infer that the four clusters of the vertebrate Hox complex are homologous to the single cluster in invertebrates. 5
First Hox
duplication
Second Hox
Vertebrates (with jaws)
with four Hox clusters
Hypothetical early
vertebrates (jawless)
with two Hox clusters
Hypothetical vertebrate
ancestor (invertebrate)
with a single Hox cluster
Most invertebrates have one cluster of homeotic genes (the Hox complex), shown here as colored bands on a chromosome. Hox genes direct development of major body parts. 1
A mutation (duplication) of the single Hox complex occurred about 520 million years ago and may have provided genetic material associated with the origin of the first vertebrates. 2
In an early vertebrate, the duplicate set of genes took on entirely new roles, such as directing the development of a backbone. 3
A second duplication of the Hox complex, yielding the four clusters found in most present-day vertebrates, occurred later, about 425 million years ago. This duplication, probably the result of a polyploidy event, allowed the development of even greater structural complexity, such as jaws and limbs. 4

56. According to the species selection model
Trends may result when species with certain characteristics endure longer and speciate more often than those with other characteristics
The appearance of an evolutionary trend
Does not imply that there is some intrinsic drive toward a particular phenotype