2. Warm-up- 5 minutes
Explain the biological species concept.

3. Today’s structure Warm-up Lecture Solve it! With a partner
Finish clover case study

4. Homework Read lab manual Due Friday Bring book on Friday for review
Read pp
MasteringBiology HW, due Friday midnight

5. The Biological Species Concept
The biological species concept: 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
Gene flow between populations holds the phenotype of a population together

6. (a) Similarity between different species
Figure 24.2a
Figure 24.2 The biological species concept is based on the potential to interbreed rather than on physical similarity.
(a) Similarity between different species

7. (b) Diversity within a species
Figure 24.2b
Figure 24.2 The biological species concept is based on the potential to interbreed rather than on physical similarity.
(b) Diversity within a species

8. Reproductive Isolation
Reproductive isolation is the existence of biological factors (barriers) that impede two 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

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

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

11. Speciation
Process by which one species splits into one or more species

12. Limitations of the Biological Species Concept
The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes)
The biological species concept emphasizes absence of gene flow
However, gene flow can occur between distinct species
For example, grizzly bears and polar bears can mate to produce “grolar bears”

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

14. Other Definitions of Species
Other species concepts emphasize the unity within a species rather than the separateness of different species
The morphological species concept defines a species by structural features
It applies to sexual and asexual species but relies on subjective criteria

15. The ecological species concept 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 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

16. Allopatric (“Other Country”) Speciation
In allopatric speciation, gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations
For example, the flightless cormorant of the Galápagos likely originated from a flying species on the mainland

17. The Process of 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

18. A. harrisii A. leucurus Antelope squirrels, Grand Canyon Figure 24.6
Figure 24.6 Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon.
Antelope squirrels, Grand Canyon

19. Reproductive isolation may arise as a result of genetic divergence
Separate populations may evolve independently through mutation, natural selection, and genetic drift
Reproductive isolation may arise as a result of genetic divergence
For example, mosquitofish in the Bahamas comprise several isolated populations in different ponds
(a) Under high predation
(b) Under low predation

20. Evidence of Allopatric Speciation
Figure 24.8
Evidence of Allopatric Speciation
15 pairs of sibling species of snapping shrimp (Alpheus) are separated by the Isthmus of Panama
These species originated 9 to 13 million years ago, when the Isthmus of Panama formed and separated the Atlantic and Pacific waters
A. formosus
A. nuttingi
Atlantic Ocean
Isthmus of Panama
Figure 24.8 Allopatric speciation in snapping shrimp (Alpheus).
Pacific Ocean
A. panamensis
A. millsae

21. 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
For example, reproductive isolation increases between dusky salamanders that live further apart

22. Degree of reproductive isolation
Figure 24.9
2.0
1.5
Degree of reproductive isolation
1.0
0.5
Figure 24.9 Reproductive isolation increases with distance in populations of dusky salamanders.
Geographic distance (km)

23. EXPERIMENT Initial population of fruit flies (Drosophila
Figure 24.10
EXPERIMENT
Initial population
of fruit flies
(Drosophila
pseudoobscura)
Some flies raised
on starch medium
Some flies raised on
maltose medium
Mating experiments
after 40 generations
RESULTS
Female
Female
Starch
population 1
Starch
population 2
Starch
Maltose
Figure Inquiry: Can divergence of allopatric populations lead to reproductive isolation?
Starch
population 1
Starch 22 9 18 15
Male
Male
population 2
Starch
Maltose 8 20 12 15
Number of matings
in experimental group
Number of matings
in control group

24. Sympatric (“Same Country”) Speciation
In sympatric speciation, speciation takes place in geographically overlapping populations

25. Solve it! 15 minutes
You have two populations of birds from the same species. These two populations are geographically separated by a mountain range. One population is much smaller than the other.
1) How would you determine if these two populations’ genetic compositions were changing from one generation to the next?
2) What mechanisms would most likely lead to this evolutionary change in each of these populations? Explain these mechanisms.
3) How would you test whether or not these two populations have become two different species?
4) What type of speciation would this be categorized as?

26. Homework Read lab manual Due Friday Bring book on Friday for review
Read pp
MasteringBiology HW, due Friday midnight

27. Finish Clover Case Study