1. CHAPTER 24
The Origin of Species

2. Biologists estimate that there are 5–30 million species of organisms currently on Earth
North America and Europe are drifting apart at a rate of about 2 cm per year

3. MACROEVOLUTION AND THE DIVERSITY OF LIFE
Encompasses the major biological changes evident in the fossil record
Includes the multiplication of species
Speciation
Is the focal point of macroevolution
May occur based on two contrasting patterns

4. In nonbranching evolution (anageneisis), a population changes and becomes a new species
Figure 14.2a

5. In branching evolution (cladogenesis), one or more new species branch from a parent species that may continue to exist
(b) Branching
evolution
Figure 14.2b

6. THE ORIGIN OF SPECIES
Species is a Latin word meaning “kind” or “appearance”

7. What Is a Species? Ernst Mayr
Led an expedition into New Guinea and found a great diversity of birds
Distilled his observation into the biological species concept
Figure 14.3

8. The biological species concept defines species as “groups of interbreeding natural populations that are reproductively isolated from other such groups”
(a) Similarity between different species
Figure 14.4
(b) Diversity within one species

9. The biological species concept cannot be applied in all situations, for example, with fossils and asexual organisms
Conspecific: members of a species are united by being reproductively compatible

10. Reproductive Barriers Between Species
Individuals of
different species
Pre-zygotic
barriers
Pre-zygotic barriers impede mating between species or hinder fertilization of eggs
Temporal isolation: Mating or flowering occurs at different seasons or times of day
Habitat isolation: Populations live in different habitats and do not meet
Behavioral Isolation: Little or no sexual attraction between males and females
Mating
Mechanical isolation: Structural differences in genitalia or flowers prevent copulation or pollen transfer
Gametic isolation: Female and male gametes fail to unite in fertilization
Fertilization
(zygote forms)
Post-zygotic
barriers
Hybrid inviability: Hybrid zygotes fail to develop or fail to reach sexual maturity
Hybrid sterility: Hybrids fail to produce functional gametes
Viable, fertile offspring
Figure 14.5

11. Pre-zygotic barriers include
Temporal isolation: breed during different times of the year, day, or season
Habitat isolation: live in different habitats
Mechanical isolation: attempt to mate, but anatomically incompatible
Gametic isolation: sperm of one species may not be compatible

12. Behavioral isolation
i.e. courtship rituals
Figure 14.6

13. Post-zygotic barriers
Are backup mechanisms that operate should interspecies mating actually occur and produce hybrid zygotes
Hybrid inviability
Hybrid sterility
Horse
Mule (hybrid)
Donkey
Figure 14.7

14. Introgression
Fertile hybrids mate successfully with one of the parent species
i.e. corn and wild grass/teosinte

15. Mechanisms of Speciation
A key event in the potential origin of species occurs when a population is somehow severed from other populations of the parent species

16. The two modes of speciation are
Allopatric speciation
Sympatric speciation
Allopatric speciation
(b) Sympatric speciation
Figure 14.8

17. Allopatric Speciation
Geological processes
Can fragment a population into two or more isolated populations
Can contribute to allopatric speciation
Figure 14.9

18. Speciation occurs only with the evolution of reproductive barriers between the isolated population and its parent population

19. (a) Speciation has not occurred (b) Speciation has occurred
Populations
become sympatric
again but do not
interbreed
Geographic barrier
Populations
become
sympatric
again and
interbreed
Population
becomes
allopatric
(a) Speciation has not occurred
(b) Speciation has occurred
Figure 14.10

20. Sympatric Speciation
Sympatric speciation occurs if a genetic change produces a reproductive barrier between mutants and the parent population
Polyploidy, a mechanism of sympatric speciation, was first observed by Hugo de Vries
Figure 14.11

21. Polyploids Originate from accidents during cell division
Can lead to sympatric speciation
Gametes
n = 2
Hybrid (usually sterile; chromosomes cannot pair during meiosis)
Gametes
n = 5
Fertile
hybrid
2n = 10
Species A
2n = 4
Self-
fertilization
Meiosis
Mitotic error
produces polyploid
condition
2n = 10
n = 3
n = 5
Species B
2n = 6
Figure 14.12

22. Many domesticated plants are the result of sympatric speciation
Triticum monococcum
(14 chromosomes)
Wild Triticum
(14 chromosomes)
Many domesticated plants are the result of sympatric speciation 1
Sterile hybrid
(14 chromosomes)
Cell division error;
self-fertilization 2
T. turgidum
Emmer wheat
(28 chromosomes) 3
T. tauschii
(wild)
(14 chromosomes)
Sterile hybrid
(21 chromosomes)
Cell division error;
self-fertilization 4
T. aestivum
Bread wheat
(42 chromosomes)
Figure 14.13

23. What Is the Tempo of Speciation?
Traditional evolutionary trees diagram the descent of species as gradual divergence

24. (a) Gradualist model
Time
Change
Figure 14.14a

25. Punctuated equilibrium
Is a contrasting model of evolution
States that species most often diverge in spurts of relatively rapid change
Accounts for the relative rarity of transitional fossils

26. (b) Punctuated equilibrium model
Time
Change
Figure 14.14b

27. Adaptation of Old Structures for New Functions
Birds
Are derived from a lineage of earthbound reptiles
Developed from flightless ancestors, but how?
Wing claw
(like reptile)
Teeth
(like reptile)
Feathers
Long tail with many vertebrae (like reptile)
Figure 14.15

28. Exaptation
Involves a structure that evolves in one context gradually becoming adapted for other functions
Is a mechanism for novel features to arise gradually