1. Macroevolution II

2. The Biological Species Concept There is no perfect definition for a species. However, the one most commonly used is the biological species concept. The biological species concept states that two populations are of the same species if they are able to successfully mate and produce viable (fertile) offspring.

3. 17.9 Reproductive Isolation Speciation differs in its details, but reproductive isolating mechanisms are always part of the process Speciation Evolutionary process by which new species form Reproductive isolating mechanisms are always part of the process Reproductive isolation The end of gene exchange between populations Beginning of speciation

4. Reproductive Isolating Mechanisms Reproductive isolating mechanisms prevent interbreeding among species Heritable aspects of body form, function, or behavior that arise as populations diverge Prezygotic isolating mechanisms prevent pollination or mating Postzygotic isolating mechanisms result in weak or infertile hybrids

5. Prezygotic Isolating Mechanisms With temporal isolation populations can’t interbreed because the timing of their reproduction differs With mechanical isolation, the size or shape of an individual’s reproductive parts prevent it from mating with members of another population

6. Prezygotic Isolating Mechanisms (cont.) Populations adapted to different microenvironments in the same region may be ecologically isolated In animals, behavioral differences can stop gene flow between related species (behavioral isolation) In gamete incompatibility, gametes of different species meet but have molecular incompatibilities that prevent a zygote from forming

7. Behavioral Isolation in Jumping Spiders

8. Postzygotic Isolation Mechanisms Hybrid inviability Extra or missing genes, or incompatible gene products Offspring may be inviable, or have reduced fitness (ligers, tigons) Hybrid sterility Some interspecies crosses produce robust but sterile offspring (e.g. mules) Fertile offspring may have lower fitness with successive generations

9. Individuals ignore or do not get the required cues for sex (behavioral isolation). Individuals live in different places so they never meet up for sex (ecological isolation). Physical incompatibilities prevent individuals from interbreeding (mechanical isolation). Individuals reproduce at different times (temporal isolation). Prezygotic reproductive isolation Different species form and... Stepped Art Interbreeding is successful Hybrid embryos die early, or new individuals die before they can reproduce (hybrid inviability). Postzygotic reproductive isolation Zygotes form and... Hybrid individuals or their offspring do not make functional gametes (hybrid sterility). No fertilization occurs (gamete incompatibility). Mating occurs and... Figure 17-16 p284

10. ANIMATED FIGURE: Reproductive isolating mechanisms To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERECLICK HERE

11. ANIMATION: Temporal isolation among cicadas To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERECLICK HERE

12. 17.10 Allopatric Speciation In allopatric speciation a physical barrier arises and ends gene flow between populations Genetic divergence results in speciation Example: Geographic isolation of Atlantic and Pacific species caused by the formation of the Isthmus of Panama

13. Allopatric Speciation in Snapping Shrimp

14. Allopatric Speciation in Archipelagos Winds or ocean currents carry a few individuals of mainland species to remote, isolated islands chains (archipelagos) such as Hawaii Habitats and selection pressures that differ within and between the islands foster divergences that result in allopatric speciation Example: Hawaiian honeycreepers and thousands of other species of finches are unique to the Hawaiian archipelago

15. Allopatric Speciation on an Archipelago

16. ANIMATED FIGURE: Allopatric speciation on an archipelago To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERECLICK HERE

17. 17.11 Other Speciation Models In sympatric speciation and parapatric speciation, populations speciate even without a physical barrier that blocks gene flow

18. Sympatric Speciation In sympatric speciation, new species form within a home range of an existing species, in the absence of a physical barrier Sympatric speciation can occur in a single generation when the chromosome number multiplies (polyploidy) Example: Common bread wheat originated after related species hybridized, then the chromosome number of the hybrid offspring doubled

19. Sympatric Speciation in Wheat Triticum urartu (wild einkorn) 42 AABBDD28 AABB14 BB14 AA14 DDX14 ABX Aegilops (wild goatgrass, unknown species) Triticum (hybrid) Triticum turgidum (emmer) Aegilops tauschii (goatgrass) Triticum aestivum (bread wheat)

20. Sympatric Speciation in Cichlids Sympatric speciation can also occur with no change in chromosome number Example: More than 500 species of cichlid speciated in the shallow waters of Lake Victoria – they vary in color and in patterning depending on differences in light color and water clarity in different parts of the lake (reproductive isolation)

21. Sympatric Speciation in Cichlids

22. Sympatric Speciation in Warblers A chain of populations of greenish warblers encircles the Tibetan plateau central Asia (a ring species) Gene flow occurs continuously all around the chain, but the two populations at the ends of the chain are different species Individuals of these two populations overlap in range, but do not interbreed because they do not recognize one another’s songs (behavioral isolation)

23. ANIMATED FIGURE: Sympatric speciation in wheat To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERECLICK HERE

24. The Pace of Speciation Classical view: Gradualism – evolution happens slowly, over long periods of time Popularized by Darwin Some fossil evidence (whales, giraffe necks) “New School” view: Punctuated equilibrium – evolution happens in relatively short bursts of rapid change, often following a major environmental change or key innovation Popularized by Stephen Jay Gould Some fossil evidence (Cambrian explosion)

25. Adaptive Radiation Adaptive radiation A burst of speciation that occurs when a lineage encounters a new set of niches Key innovation A structural or functional adaptation that allows individuals to exploit their habitat in a new way

26. Coevolution Two species in close ecological contact act as agents of selection on each other (coevolution) Predator and prey – evolutionary arms race Host and parasite – evolutionary arms race Pollinator and flower – mutualism (mostly) Over time, the two species may come to depend on each other

27. Coevolved Species: Myrmica sabuleti and Maculinea arion. After hatching, the larvae (caterpillars) of the large blue butterfly (Maculinea arion), feed on wild thyme flowers and then drop to the ground Ants (Myrmica sabuleti) pet the caterpillar, eats the honey the caterpillar exudes, and takes the caterpillar back to the ant nest The caterpillar lives in the nest and feeds on ant larvae until it metamorphoses into a butterfly

28. Coevolved Species: Myrmica sabuleti and Maculinea arion. Myrmica sabuleti Maculinea arion