1. 22
The Origin of Species 1

2. Overview: That “Mystery of Mysteries”
In the Galápagos Islands Darwin discovered plants and animals found nowhere else on Earth

3. Figure 22.1

4. Speciation is the process by which one species splits into two or more species
It explains the features shared between organisms due to inheritance from their recent common ancestor

5. Speciation also forms a conceptual bridge between microevolution and macroevolution
Microevolution consists of changes in allele frequency in a population over time
Macroevolution refers to broad patterns of evolutionary change above the species level

6. Concept 22.1: The biological species concept emphasizes reproductive isolation
Species is a Latin word meaning “kind” or “appearance”
Biologists compare morphology, physiology, biochemistry, and DNA sequences when grouping organisms

7. The Biological Species Concept
The biological species concept 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
Gene flow between populations holds the populations together genetically

8. Reproductive Isolation
Reproductive isolation is the existence of biological 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 barriers act before or after fertilization

9. Figure 22.3
Prezygotic barriers
Postzygotic barriers
Habitat
isolation
Temporal
isolation
Behavioral
isolation
Mechanical
isolation
Gametic
isolation
Reduced
hybrid
viability
Reduced
hybrid
fertility
Hybrid
breakdown
VIABLE,
FERTILE
OFF-
SPRING
MATING
ATTEMPT
FERTILI-
ZATION
(a)
(c)
(e)
(f)
(g)
(h)
(i)
(l)
(d)
(j)
(b)
(k)

10. Prezygotic Barriers
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

11. Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers
Example: 2 species of garter snake live in the same area, but 1 lives on land, the other in water

12. Temporal isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes
Example: Eastern and western spotted skunks live in same area, but eastern mates in winter, western in summer

13. Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers
Example: Blue footed boobies mate only after a courtship display unique to their species

14. Mechanical isolation: Morphological differences prevent successful mating
Example: Different species of damselflies have different shaped penises

15. Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species
Example: 2 different species of sea urchin have differing gamete proteins and their gametes will only fuse with those of the same species

16. Postzygotic Barriers
Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult by
Reduced hybrid viability
Reduced hybrid fertility
Hybrid breakdown

17. Reduced hybrid viability Reduced hybrid fertility Hybrid breakdown
Figure 22.3c
Postzygotic barriers
Reduced
hybrid
viability
Reduced
hybrid
fertility
Hybrid
breakdown
VIABLE,
FERTILE
OFFSPRING
FERTILIZATION
(h)
(i)
(l)
(j)
(k)

18. Reduced hybrid viability: Genes of the different parent species may interact and impair the hybrid’s development or survival
Example: Salamanders of the genus Ensatina may interbreed but most hybrid don’t complete development, or are extremely frail if they do

19. Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile
Example: Horses and donkeys can interbreed to produce mules, but the mules are sterile

20. Hybrid breakdown: Some first-generation hybrids are fertile, but when they mate with another species or with either parent species, offspring of the next generation are feeble or sterile
Example: A strain of cultivated rice has vigorous hybrids in the first generation but plants in the next generation are small and sterile

21. 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” 21

22. Grizzly bear (U. arctos) Polar bear (U. maritimus)
Figure 22.4
Grizzly bear (U. arctos)
Hybrid “grolar bear”
Polar bear (U. maritimus) 22

23. 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 23

24. 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 24