Evolution

1. Evolution What causes evolution? Speciation & hybridization. Uncovering evolutionary history.

2. The four forces of evolution: • Mutation -- spontaneous changes in the DNA of gametes. Mutations are the result of mistakes in DNA replication, exposure to UV or to some chemicals (mutagens) and other causes. Prerequisite to all other evolution. • Natural Selection -- genetically-based differences in survival or reproduction that leads to genetic change in a population. • Gene flow -- movement of genes between populations. In plants this can be accomplished by pollen or seed dispersal. • Genetic drift -- random changes in gene frequency. This is very important in small populations.

3. All these plants are the same species: Brassica oleracea

4. Mutation: Generation of new alleles: Point mutations (changing one base to another, e.g., C-->T) • unrepaired DNA damage, e.g. from UV-light, chemicals • uncorrected copying errors: in any system, error-free transmission of information is a theoretical impossibility • Mutations that are transmitted into gametes are evolutionarily important

5. Sickle cell anemia is an example of a point mutation causing a big change in phenotype.

6. Point mutations are only one of many kinds of chance genetic change: • Indels • (insertions/deletions) • Cause frame-shifts, & usually premature ‘stops’ • Chromosomalmutations • Inversions, translocations, deletions • Geneduplication • May lead to new functions • Polyploidy • May lead to new species in one step • Very common in plants

7. Q: What are the consequences of mutations for an individual’s ability to survive and reproduce?A: Most mutations have no effect or almost no effect. • Why? • 1. Most of the genome seems to be ‘junk’ -- at least it doesn’t code for proteins. We still may have a lot to learn here but the empirical evidence regarding mutations’ effects support this view. • 2. Many mutations within protein-coding genes don’t change the amino acid specified. I.e., there is redundancy in the genetic code For example, 6 different codons specify the amino acide leucine.

8. This distribution of the fitness effects shows: • most ms have no effect (are neutral) • the remainder are usually deleterious the relatively high freq. of lethals is due to missense mutations -- those that cause a premature ‘stop’ in protein synthesis. • Very few ms are beneficial so conservation of genetic variation is extremely important. Fitness

9. The four forces of evolution: • Mutation -- spontaneous changes in the DNA of gametes. Mutations are the result of mistakes in DNA replication, exposure to UV or to some chemicals (mutagens) and other causes. Prerequisite to all other evolution. • Natural Selection -- genetically-based differences in survival or reproduction that leads to genetic change in a population. • Gene flow -- movement of genes between populations. In plants this can be accomplished by pollen or seed dispersal. • Genetic drift -- random changes in gene frequency. This is very important in small populations.

12. The four forces of evolution: • Mutation -- spontaneous changes in the DNA of gametes. Mutations are the result of mistakes in DNA replication, exposure to UV or to some chemicals (mutagens) and other causes. Prerequisite to all other evolution. • Natural Selection -- genetically-based differences in survival or reproduction that leads to genetic change in a population. • Gene flow -- movement of genes between populations. In plants this can be accomplished by pollen or seed dispersal. • Genetic drift -- random changes in gene frequency. This is very important in small populations.

13. Gene flow tends to homogenize populations. Rates of gene flow depend on the spatial arrangement of populations.

14. More models of gene flow

16. The four forces of evolution: • Mutation -- spontaneous changes in the DNA of gametes. Mutations are the result of mistakes in DNA replication, exposure to UV or to some chemicals (mutagens) and other causes. Prerequisite to all other evolution. • Natural Selection -- genetically-based differences in survival or reproduction that leads to genetic change in a population. • Gene flow -- movement of genes between populations. In plants this can be accomplished by pollen or seed dispersal. • Genetic drift -- random changes in gene frequency. This is very important in small populations.

18. Founder effect: Gene flow and genetic drift are responsible for the limited genetic variation on islands, relative to mainland populations.

19. Convergence; similar features in unrelated organisms due to evolution of traits that “work” in similar environments • spiny succulent growth habit in deserts • schlerenchymatous leaves in many families that live in dry habitats • similar flower sizes, shapes & colors for attracting pollinators have evolved in many plant groups. • low prostrate growth for of high altitude plants

20. Convergent structures in the ocotillo (left) from the American Southwest, and in the allauidia (right) from Madagascar.

22. This, believe it or not, is a South African member of the milkweed family.

23. alpine clover and forget-me-nots -- convergence in growth habit

24. Nectar feeders have converged on this hovering long-tongued morphology.

25. Another animal example of convergence.

26. Hybridization (between species) • Well -- what is a species, anyway? • Most species were described by their morphology. • In vertebrates, morphological discontinuities generally correspond to fertility barriers. BSC • In plants, many named species can hybridize. • Hybridization can lead to: • Homogenization of divergent ‘species’ • Production of new species; hybrids are better than parents and/or can’t mate with parents • If hybrids not fit and parents waste resources making them then selection could act to minimize hybridization.

27. Most dandelions are asexual. So the biological species concept doesn’t apply. How can you name species depending on who can mate with whom when the organisms do not mate at all?!

29. These two Calochortus have been named as separate species. But they are interfertile -- should we combine them as one species? Their ranges do not overlap so the chance of hybridization in Nature is very remote.

30. These milkweeds hybridize in the central plains. hybrid A. syriaca A. syriaca A. speciosa

31. Scarlet and Black oaks can hybridize and inhabit the same range -- but they have different microhabitat preferences and so hybridization is rare.

32. These pines can also hybridize but they shed their pollen at different times of the season

33. Speciation by hybridization Hybridization often shows how difficult it is to apply the BSC to plants. The hybrid in this case is a new species. The rearrangements of its chromosomes make it +/- infertile with either parent.

34. Tragopogon pratensis is a new species formed by hybridization between an American Tragopogon and a European Tragopogon that was introduced about 150 years ago. T.p. is a polyploid formed by the union of unreduced gametes -- i.e. 2n x 2n => 4n (Normally n x n => 2n)

35. As the climate becomes drier the desert splits the range of this hypothetical tree species. This reduces gene flow between the now isolated populations and sets the stage for speciation.

36. Geographical isolation leads to genetic differences among the different populations. Theorem: geographic isolation is necessary for new species to arise. Counter-theorem: strong natural selection or big mutations can cause divergence within populations.

41. Taxonomy vs. Systematics • Taxonomy • discovering • describing • naming • classifying • Systematics • Figuring out the evolutionary relationships of species to each other.

42. Taxonomy vs. Systematics • Taxonomy products are • descriptions of new species in journals • Keys • Entries in floras e.g., Flora of Missouri lists all the species found in MO and has keys for identifying plants. • Systematics produces trees that attempt to summarize the evolutionary history of a group. • Usually done with DNA sequences, these days.

43. Phylogenetic trees have more information than a list of names. E.g., the nine animal phyla are hypothesized to have the relationships shown at left.

44. Modern taxonomic groups generally correspond to clades on a phylogenetic tree (=cladogram)

45. plant taxonomy taxon - any group at any rank species genus family order class division (phylum) kingdom

46. discovering describing naming classifying 2 basic rules of naming organisms: - each species name must be a binomial - all scientific names must be in Latin or be “Latinized”

47. " ironweed "

48. " ironweed "

49. Acer rubrum - red maple Genus - always capitalized species - not capitalized - either italicized or underlined Acer rubrum : the scientific name the Latin name the genus &species name

50. Carolus Linnaeus (born Carl von Linné) - wrote Species Plantarum in 1753 - first use of binomial nomenclature - named 7,300 species