DIVERSITY AND EVOLUTION

1. DIVERSITY AND EVOLUTION Chapter 2

2. Diversity of life • Approximately 1.5 million living species described • Likely at least 10 million species today • May represent only 1% of species ever to have lived on earth • 1 billion species presumed to have lived

3. Diversity of body form • Tremendous diversity within each group of plants, animals, fungi, protistans, bacteria • Structural complexity - apparently purposeful adaptation of many characteristics to the environment

4. Reason for this diversity? • Natural selection • Physical environment acts on various characteristics of organisms (variation among individuals of some species) • Sorts out “harmful” ones, leaving individuals with “beneficial” or “neutral” characteristics to produce next generation • Keeps organisms well-suited for survival in their environment

5. Natural selection drives evolution • Broad scale • Development of various “forms” or species to best match the environment • Can best take advantage of variations within that environment

6. History of concept of evolution by natural selection • Lamarck - inheritance of acquired characteristics • Darwin, Wallace - natural selection, but mechanism really unknown • Mendel - genetic understanding of the acquisition of inherited traits

7. Evolution by natural selection - established truths • 1) individuals that form a population of a species are not identical

8. Evolution by natural selection - established truths • 2) some of the variation between individuals is heritable

9. Evolution by natural selection - established truths • 3) all populations are capable of exponential growth, but most individuals die before reproducing, and most others reproduce at less than their maximum rate

10. Evolution by natural selection - established truths • 4) different ancestors leave different numbers of descendents; they do not all contribute equally to subsequent generations

11. SPECIATION Interaction of heritable variation, natural selection, barriers to gene flow

12. Allopatric (Geographic) Speciation • Separating single, interbreeding population into two or more spatially isolated populations • Geographic barrier, remains long enough for speciation • Founder effect, genetic drift (random mutations)

13. Parapatric Speciation • No spatial isolation • Portion of population invades new, adjacent habitat • Little to no movement/interbreeding • Differing natural selection in differing habitats

14. Sympatric Speciation • No spatial isolation • Production of new species within a population • Rare • Most likely to occur in insect parasites of plants, animals • Requires stable polymorphism and under- or unused resource

15. Polyploidy • Abrupt speciation by doubling the number of chromosomes • Most common in plants • Agricultural-wheat, alfalfa, potatoes • Native-birches, willows

16. PATTERNS OF SPECIATION

17. Anagenesis • One species changes into another species over time • Original species “evolves” out of existence and is replaced by new species • Evolutionary extinction

18. Cladogenesis • One species gives rise to one or more additional species while still remaining • Clade-set of species descended from a particular ancestral species (e.g., Darwin’s finches)

19. TEMPO OF SPECIATION

20. Gradualism • Steady change in character(s) resulting in many intermediate forms exhibiting “gradual” shift

21. Punctuated equilibrium • Rapid, abrupt changes that produce quick shifts in character • No intermediate forms

22. REDUCTION IN VARIATION

23. Inbreeding depression • Mating among close relatives produces an increase in expression of recessive traits, many of which are deleterious • Often results from small population size • Mortality may be increased • “Tighter” inbreeding results in more rapid loss of genetic variation within population

24. But…. • Not all populations are harmed by inbreeding • Long-term, small populations (e.g., on islands) may be adapted to inbreeding and survive well even in face of it

25. Outbreeding • Some degree of outbreeding usually beneficial in maintaining genetic diversity • But too much can also be harmful • Too many differences may lead to problems

26. Smaller populations • Genetic variation declines faster in smaller population because of inbreeding • Rule of thumb-50 individuals needed to prevent inbreeding • Problem for saving California condor • Only 26 individuals in 1986

27. Genetic drift • Larger population not subject to inbreeding can lose genetic variation at rates similar to small populations via genetic drift • Some individuals do not mate, not represented genetically in next generation

28. Genetic drift-cont. • Rule of thumb-happens only in populations <500 in size • Genetic drift can be counteracted by minimal levels of immigration into the population

29. Neighborhoods • Even big populations may run into problems if individuals don’t move around much to mate • Some also just don’t reproduce • Effective population size may then be quite small • E.g., grizzly bear in Yellowstone • Actual population ~200 • Effective population ~50 (25%) • Subject to loss of variation

30. Bottlenecks • Can also reduce genetic variation • Bottlenecks - periodic reductions in population size can reduce genetic variation greatly even if average population size is much larger

31. Founder Effects • Can also reduce genetic variation • Founder effects - developing gene pool of growing population is limited by what variation founders had, plus mutation • Pair of founders at most have 4 variations in a gene

32. ORIGIN OF VARIATION

33. Genetic • Increase or decrease variability within a population • DNA - mistakes or mutations during copying of genetic code • Gene or point mutation - most important for enriching the gene pool • Chromosome mutation - most important for rearranging the gene pool

34. Point Mutations • Change in nucleotide base at single location • Change in single amino acid within protein, or entirely different protein • Frameshift mutation - insertion or deletion of single base pair

35. Mutagens and mutations • Mutations usually produced by mutagens (e.g., weak cosmic rays) • 1 mutation per gene in every 100,000 sex cells • Higher organisms have ~10,000 genes • 1 in 10 individuals has newly created mutation

36. Most mutations are harmful, but.. • 1 in 1000 mutations may be beneficial • 1 in 10,000 individuals per generation has a useful mutation • Most individuals have at least one mutant gene (original, or passed down from ancestors)

37. Mutations and Speciation • Estimate - 500 mutations necessary to produce new species from existing one • Rate of new mutations ~1 million times greater than needed to account for known rate of evolution

38. Chromosomal Mutations • No change to variability • Rearrange what is there • Deletions, duplications, inversions, translocations

39. Other changes • Polyploidy - e.g., tetraploid • Failure of gametes to reduce to haploid state during meiosis • 2N + 2N = 4N

40. So… • Mutations produce the variation, and natural selection acts upon the changes • Add in: nonrandom mating, changing environment • End product = EVOLUTION

41. Amount of Variation • Results from protein analyses (electrophoresis) • Within a population - 15-58% of genes exhibit variation • Within individuals - 3-17% of genes exhibit variation

42. Applying this information: 1) Separate populations of organisms with movement of individuals among populations generally exhibit most variation within each population, and very little between or among populations

43. Applying this information: • 2) Reduced movement of individuals among populations produces more variation between or among populations • Populations diverge genetically

44. Applying this information: • 3) Conservation of endangered species which move around very little will require protection of many populations in many different habitats to conserve genetic diversity within the species