The Process of Speciation

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 (1834-1914) • 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 (1820-1903)

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 spec -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. How did they arise?

17. 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. 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

19. 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

20. 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?

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

22. 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).

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

25. Summary • Biological evolution - 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