1. The Process of Speciation
Every individual alive today, the highest as well as the lowest, is derived in an unbroken line from the first and lowest forms
August Frederick Lopold Weismann
( )
From the remotest past which Science can fathom, up to the novelties of yesterday, that in which Progress essentially consists, is the transformation of the homogeneous to the heterogeneous
Hebert Spencer ( )

2. The Process of Speciation
We wish to ask:
What is biological evolution?
How are the theories of microevolution and macroevolution related?
What is a species, and what are the different ways is can be defined?
How is reproductive isolation important to speciation, and what forms can it take?
Why should natural selection reinforce reproductive isolation?
Can species be formed in ways other than geographic isolation?

3. Biological Evolution Defined
Changes in the genetic composition of a population with the passage of each generation
Micro-evolution
Small, gradual changes
The gradual change of living things from one form into another over the course of time, the origin of species and lineages by descent of living forms from ancestral forms, and the generation of diversity
Macro-evolution and taxonomic hierarchy
May not be so gradual

4. Species formation
To ask, “how are species formed”, it helps first to ask, “What is a species?”
Two main definitions
Morphological species
Biological species

5. Morphological Species Concept
“members of a species are individuals that look similar to one another”
-based on appearance and professional judgment
-good enough for Linneaus
Criticisms
-arbitrary
-may fail to discriminate (e.g. mimicry complexes)

6. Biological Species Concept
“a species is a group of actually or potentially interbreeding individuals who are reproductively isolated from other such groups”
-critical test of ‘species-hood’ –ability to produce successful, fertile offspring
-species is evolutionary unit
Criticisms
-useless with fossils, museum specimens
-can not use with spatially disjunct populations
- hybridization

7. How does Reproductive Isolation Arise?
First, what are reproductive barriers ?
Pre-zygotic isolating mechanisms
- ecological
- temporal
- behavioral
- mechanical
Post-zygotic isolating mechanisms
- hybrid invariability
- hybrid sterility
- hybrid less fit

8. Courtship displays are a behavioral, pre-zygotic isolating mechanism

9. By adapting to breed at different times (temporal isolation), these four frog species achieve at least partial pre-zygotic isolation.

10. Post-zygotic reproductive isolation

11. A sheep-goat chimera produced by injecting a goat inner cell mass into a sheep blastocyst. The chimera was carried to term in a sheep recipient female. Dept Animal Science, UC-Davis

12. Speciation by Geographic Isolation
Intact population becomes separated by geographic barrier (note link to earth processes)
Different geographic regions have different environments, competitors, predators
Separated populations are genetically independent (different mutations)
Given enough time, the two populations may diverge enough to become distinct species
If reproductively isolated,  two species

13. How geographic isolation may lead to the splitting of one species into two, following the allopatric model of speciation

14. How might geographic isolation occur?
Dispersal
Continental drift
Climate fluctuations, including glaciation

15. Drifting Continents and Biogeography
                                                                                                                                  
100-million- year old Rugops primus
Freshwater and land dinosaurs became isolated on different continents as plates drifted apart.

16. Five species of toucans occur in the rain forest of South America
Five species of toucans occur in the rain forest of South America. How did they arise?

17. Contemporary rainfall
Contemporary rainfall. Areas receiving > 2500 mm annually are suggested Pleistocene forest refugia
Proposed pleistocene refugia for forest-dwelling plans and animals
Savannahs occur where rainfall is < 1500 mm annually

18. Adaptive radiation on island chains
Adaptive radiation is the evolution of many diversely adapted species from a common ancestor
Example is Darwin’s finches in the Galapagos
Multiple events of colonisation, adaptation, speciation and recolonisation

19. Conditions favouring allopatric speciation
A small, isolated population is more likely to change substantially enough to become a new species than a large one
The geographic isolation of a small population usually occurs at the fringe of the parent population’s range
Peripheral isolates are good candidates for speciation, although most peripheral isolates do not survive long enough to speciate

20. Sympatric speciation
Sympatric speciation is the formation of a new species within the range of the parent population
Reproductive isolation without geographical isolation
Many plant species have evolved through polyploidy:
Autopolyploids have chromosomes derived from a single species
Allopolyploids arise from two different species:
More common that autopolyploidy

21. Polyploidy in plants

22. Species Formation and the Hierarchy of Life
Speciation results in the splitting of one ancestral species into two (or more) descendent species
Extended over great expanses of time, this same process gave rise to the entire tree of life
The link from micro-evolution to macro-evolution is time – lots and lots of time

23. Species Formation and the Tree of Life
Time + speciation => diversity
Outcome depends on:
- strength of selection pressure
- length of time
Explains why species is not an unambiguous entity
- continuum of differentiation

24. Species formation (Revisited)
Cladogenesis – the splitting of one lineage into two
Geographic (allopatric) isolation is the prerequisite condition for reproductive isolation to evolve (animals)
Except in plants, where hybridization and/or chromosome doubling can instantly produce reproductively isolated, new species (see notes)
How can we re-construct phylogenetic trees?

25. A B C D E F G
Living species
Fossil species
Time
Ancestral (common) species
A phylogenetic tree

26. Clocks in Molecules
"One of the most startling things that has come to light is that molecular evolution seems to happen at a surprisingly constant rate. If you plot molecular changes in a particular lineage of organisms against time they seem to accumulate these changes at a remarkably constant rate, and that has come to be known as the molecular clock.“ Linda Partridge
This suggests that molecular evolution is constant enough to provide a molecular clock of evolution.
This means that the amount of molecular change between two species measures how long ago they shared a common ancestor.
Figure: the rate of evolution of hemoglobin. Each point on the graph is for a pair of species, or groups of species. From Kimura (1983).

27. A phylogenetic tree constructed from similarities between cytochrome c molecules in various organisms.

29. Summary Biological evolution Species definitions
- genetic change (microevolution)
- change from one form into another over time (macroevolution)
Species definitions
- morphological, biological
Reproductive isolation
- central to biological species concept
Allopatric speciation
- main mechanism in animals
- long-term geographic isolation is prerequisite
Diversity and the tree of life
- repeated splittings over great expanse of time