Speciation is the process by which new biological species evolve. It often occurs when a population becomes geographically isolated from another population, resulting in different selective pressures in each area that can drive the evolution of reproductive barriers between the populations over time. There are two main patterns of speciation: anagenesis, where a single lineage gradually transforms into a new species, and cladogenesis, where a new species branches off from an existing species. Geographic isolation is the most common cause of speciation, but reproductive isolation can also drive populations apart even without a physical barrier between their ranges.

1. Speciation The Process of Evolution

2. Speciation Formation of a new species Species : a population that can breed freely and produce fertile offspring Speciation often occurs when part of the population is isolated from another part Selective pressures of the environment in one area may be different from pressures in another area

3. What is a Species? Definition : Morphospecies - based on appearance Biologic species - a population that can breed freely and produce fertile offspring The largest unit of population in which gene flow is possible Limitations: doesn’t work for asexual organisms extinct life forms populations that are geographically isolated - sometimes call subspecies No clear answer; idea is arbitrary

4. Patterns of Speciation Fossil record shows 2 patterns: Anagenesis ( phyletic evolution) the transformation of an unbranched lineage of organisms, sometimes creating an organism different enough to be a new species Cladogenesis branching evolution; budding of one or more new species from a parent species that continues to exist.

5. Anagenesis vs. Cladogenesis

6. Causes of Speciation Speciation often occurs when part of the population is isolated from another part Geographic Isolation most common a physical barrier develops (changing course of a river; separation of an island) Selective pressures in one area are different from pressures in another area Reproductive Isolation another form of isolation

7. Isolation

8. Geographic Isolation Biogeography of Speciation Classified according to geographic relationship between new and old species Sympatric population becomes reproductively isolated in the midst of the parent population ranges of new and old species overlap. Allopatric species are physically separated more likely in small populations Adaptive radiation is allopatric : emergence of numerous species from a common ancestor that spreads to several new environments.

9. Allopatric vs. Sympatric

10. Allopatric Barriers

11. Geographic Isolation

12. Reproductive Isolation Example: organisms breed at different times Reproductive barriers are of 2 types: Prezygotic before the formation of fertilized eggs impedes mating or fertilization Postzygotic after

13. Reproductive Isolation

14. Prezygotic Isolation Impedes mating or fertilization Habitat isolation not geographically separated, but occupy different niches within an area, e.g. trees versus ground Temporal isolation breed at different times Behavioral isolation don’t produce appropriate courtship signals Mechanical isolation anatomically incompatible Gametic isolation mating occurs but gametes rarely fuse to form zygotes

15. Behavioral Isolation: Courtship Barrier

16. Postzygotic Barriers Hybrid inviability offspring don’t make it Hybrid sterility e.g. mules Hybrid breakdown F 2 are sterile or weak

17. Introgression Alleles pass a reproductive barrier when a fertile hybrid mates with a parent species Increases variation Rare 2 species remain distinct

18. Post Speciation Evolution Divergent Evolution Process by which related organisms become less alike occurs after speciation at first 2 new species are very similar, but over time become more & more different. Adaptive radiation is a special type of divergent evolution Many new species from a single parent species

19. Adaptive Radiation

20. Timing of Evolution Most scientists accept natural selection as the process of evolution The timing is controversial Gradualism the traditional view a slow, steady accumulation of changes, leads to new species Punctuated Equilibrium long periods of inactivity followed by big jumps Fossil record provides evidence that the pace of evolution varies The same evidence is used to support different ideas Could be some of both

21. Gradualism vs. Punctuated Equilibrium