1. The Origin of Species
Chapter 22

2. The Nature of Species
The concept of species must account for two phenomena:
The distinctiveness of species that occur together at a single locality
The connection that exists among different populations belonging to the same species

3. The Nature of Species
Speciation: the process by which new species arise, either by
transformation of one species into another,
or by the splitting of one ancestral species into two descendant species

4. The Nature of Species
Sympatric speciation: the differentiation of populations within a common geographic area into species
Species that occur together:
Are distinctive entities
Are phenotypically different
Utilize different parts of the habitat
Behave separately

5. The Nature of Species
Population: any group of individuals, usually of a single species, occupying a given area at the same time
Exhibit geographic variation
Subspecies: within a single species, individuals in populations that occur in different areas may be distinct from one another

6. Geographic Variation of the Milk Snake

7. The Biological Species Concept
Ernst Mayr’s biological species concept defines species as…
“…groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.”
In short: members of a population mate with each other and produce fertile offspring

8. The Biological Species Concept
Reproductively isolated: populations whose members do not mate with each other or who cannot produce fertile offspring
Reproductive isolating mechanisms: barriers to successful reproduction
- Geographic
- Ecological
- Behavioral
- Temporal
- Mechanical
Gamete fusion
Postzygotic

11. The Biological Species Concept
Prezygotic isolating mechanisms prevent the formation of a zygote
Ecological isolation
Utilization of different portions of the environment
Do not encounter each other
Example: lion and tiger (Tiglon)

12. The Biological Species Concept
Lions and tigers are ecologically isolated

13. The Biological Species Concept
Tiglon in zoo
In captivity lions and tigers can mate and reproduce offspring that survive

14. Biological Species Concept
Behavioral isolation: species differ in their mating rituals

15. Biological Species Concept
Sympatric species avoid mating with members of the wrong species in a variety of ways, including differences in:
Visual signals
Sound production
Chemical signals: pheromones
Electrical signals: electroreception

16. Biological Species Concept
Differences in courtship songs of Lacewings

17. Biological Species Concept
Temporal isolation: species reproduce in different seasons or at different times of the day
Mechanical isolation: structural differences between species prevent mating
many plants require specific insect to fertilize & insect will only stop at these plants
Prevention of gamete fusion: gametes of one species functions poorly with the gametes of another species or within the reproductive tract of another species

18. Biological Species Concept
Postzygotic isolation prevents normal development into reproducing adults
Hybridization: mating between two different species with a zygote being formed
Hybrids often:
Do not develop into adults
Do not develop into fertile adults
Example: mule

19. When Leopard Frog inter-breed, defective embryos form

20. Biological Species Concept
Criticisms of biological species concept:
Interspecific hybridization
50% California plant species, in one study, not well defined by genetic isolation

21. Biological Species Concept
Hybridization is not uncommon in animals
10% of bird species have hybridized in nature
Hybrid offspring of Galápagos finches appeared to be at no disadvantage for survival or reproduction
Reproductive isolation may not be the only force for maintaining the integrity of species

22. Ecological Species Concept
Theories & criticisms
Hybridization has little effect because alleles introduced into one species’ gene pool from other species are quickly eliminated by natural selection
Difficult to apply biological species concept to populations that are geographically separated in nature
Many organisms are asexual and do not mate

23. Reproductive Isolation
Cladogenesis: one ancestral species becomes divided into two descendant species
If species are defined by the existence of reproductive isolation, then
the process of speciation is identical to the evolution of reproductive isolating mechanisms

24. Reproductive Isolation
Populations can become geographically isolated

25. Reproductive Isolation
The formation of species is a continuous process
Two populations may only be partially reproductively isolated
If isolating mechanisms have not evolved, then two populations will interbreed freely
If populations are reproductively isolated, no genetic exchange will occur, two populations will be different species

26. Reproductive Isolation
The intermediate state:
Hybrids are partly sterile
Hybrids are not as well adapted to the habitat
Selection would favor any alleles in the parental populations that prevent hybridization
Reinforcement: incomplete isolating mechanisms are reinforced by natural selection until they are completely effective

27. Reproductive Isolation
European Flycatchers
If 2 species occur sypatrically they change colors so each species knows who to mate with

28. Reproductive Isolation
Gene flow may counter speciation
Reinforcement is not inevitable
Incompletely isolated populations have gene flow
Hybrids may be inferior but serve as a conduit of genetic exchange
Two populations will lose their genetic distinctiveness
A race between complete reproductive isolation evolution and gene flow

29. Genetic Drift Random changes may cause reproductive isolation
Genetic drift in small populations
Founder effects
Population bottlenecks
Hawaiian Islands: Drosophila differ in courtship behavior
Changes in courtship behavior between ancestor and descendant population may be the result of founder events

30. Genetic Drift
Given time, any two isolated populations will diverge because of genetic drift
Random divergence may affect traits responsible for reproductive isolation
-Speciation may occur

31. Genetic Drift Adaptation can lead to speciation
Wet conditions vs dry conditions
Natural selection produces a variety of differences in physiological and sensory traits
Promotes ecological and behavioral isolation

32. Genetic Drift Genetic drift may act on mating behavior
Anolis lizards and dewlap color
Ability to see dewlap depends on color and environment
Light color: reflects light in dark forest conditions
Dark color: more visible in bright glare of open habitats

33. Genetic Drift

34. Genetic Drift (Cont.)

35. Genetic Drift (Cont.)

36. Genetic Drift (Cont.)