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Biology 105 - Evolution Dr. Theodore Garland, Jr. 3 Dec. 2015:

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Biology 105 - Evolution Dr. Theodore Garland, Jr. 3 Dec. 2015:

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  1. Biology 105 - Evolution • Dr. Theodore Garland, Jr. • 3 Dec. 2015: • "Extinction & Evolutionary Trends" Accompanies Chapter 15 in: Bergstrom, C. T., and L. A. Dugatkin. 2012. Evolution. W.W. Norton and Company.

  2. Rates of Evolution

  3. A "rate" is something expressed per unit of time. It is a ratio with a numerator and a denominator. Examples: miles per hour grams per million years # of new species per million years

  4. Darwin understood that rates of evolution vary: "Species of different genera and classes have not changed at the same rate, or in the same degree." Stasis The only figure in The Origin of Species, 1859

  5. However, Darwin (1859) emphasized that rates of evolution were generally slow: "I do believe that natural selection will always act very slowly, often only at long intervals of time …" Nevertheless, he argued that: "… this very slow, intermittent action of natural selection accords perfectly well with what geology tells us of the rate and manner at which the inhabitants of this world have changed."

  6. G. G. Simpson (1953) noted that rates vary among characters, among organisms, and over time. Also proposed terminology. B&G2012 Fig. 15.33, p. 528

  7. Living Fossils:slow rates over long times Living (extant) horseshoe crab Fossil from 450 million years ago

  8. But remember that even "living fossils" have likely evolved at the molecular, biochemical, physiological, and/or behavioral levels ... we just cannot see that in the fossil remains.

  9. This raises interesting questions about how whole organisms evolve as coordinated entities: diversity within constraint is a general principle. Example: bacteria have huge metabolic diversity, but much less morphological diversity as compared with plants.

  10. Selection Experiments:fast rates over short times Darwin (1859) discussed selection experiments at length, and noted: "Slow though the process of selection may be, if feeble man can do much by his powers of artificial selection, I can see no limit to the amount of change … which may be effected in the long course of time by nature's power of selection."

  11. ?s About Evolutionary Rates: How fast can evolution proceed? What are the rates observed empirically? What do these imply about evolutionary mechanisms?

  12. Three Time Scales of Interest in Evolutionary Biology: 1. generational, involving change from generation-to-generation 2. microevolutionary, involving patterns on scales from about 2-100 generations 3. macroevolutionary, involving patterns on scales of hundreds or more generations Gingerich, P. D. 2001. Rates of evolution on the time scale of the evolutionary process. Genetica 112-113:127-144.

  13. Quantifying Evolutionary Rates: J. B. S. Haldane (1949) proposed a rate calculation in a unit called the darwin that uses two quantities: 1) the difference between means of two samples of natural-logged measurements (e.g., body size); 2) the time interval between the samples, measured or estimated in millions of years. rate in darwins = ln(x2) - ln(x1) ------------------ time interval This rate is expressed per million years.

  14. Sidebar on Phylogenetically Independent Contrasts: Felsenstein, 1985 They are ratios: 1) the difference between means of two samples (e.g., body size of two species); 2) the square root of the sum of the branch lengths connecting them (branch lengths could be time in millions of years. So, independent contrasts can be used to study rates of evolution by comparing clades.

  15. Evolutionary Rates: Phil Gingerich, a mammalian paleontologist at the University of Michigan, has compiled evolutionary rates from many different studies.

  16. Observed Evolutionary Rates in darwins Ridley, 1994, page 594: The next slide graphs the data …

  17. Observed Evolutionary Rates in darwins Gingerich, 1983 lab selection experiments historical coloniza-tions ln-ln scale raw scale youngerfossils olderfossils

  18. General Conclusions 1: Observed evolutionary rates in darwins decline in proportion to the time interval over which they are measured. Once you account for this time scaling, you do not need to invoke some special process for short or long time scales relative to each other: they are on a continuum. This contrasts with data on molecular evolution, which shows that rates of evolution can sometimes be fairly constant over long time periods.

  19. Refining Evolutionary Rates: Rates measured in darwins do not account for the fact that: 1. Organisms have different generation times. 2. Traits have different degrees of within- population variability. So, Haldane (1949) also proposed a calculation in terms of proportional change divided by elapsed time, in units called haldanes. This rate is expressed in terms of phenotypic standard deviations per generation, and makes more sense in terms of evolutionary processes.

  20. Observed Evolutionary Rates in haldanes = S.D. per generation Macroevolutionary Early discussions of evolutionary rates were based on examples from the fossil record, which reinforced the view that evolution is usually very slow(~10-7 to 10-3S.D. per generation). Panel D in Figure 8 from:Gingerich, P. D. 2001. Rates of evolution on the time scale of the evolutionary process. Genetica 112-113:127-144.

  21. General Conclusions 2: Observed evolutionary rates in haldanes also decline in proportion to the time interval over which they are measured. This decline is not just an artifact of using a ratio or some other mathematical/statistical issue.

  22. Observed Evolutionary Rates in haldanes = S.D. per generation B A On different time scales: A) selection experiments in lab & field (e.g., Ted Garland's mice in the lab, David Reznick's guppies in the field); B) 'microevolution'; C) historical field study of deer mice (Peromyscus gracilis); D) from the fossil record All predict (extrapolate to) rates ~10-1 to 100 (0.1 to 1) S.D. per generation on a time scale of one generation (100; the time scale of the evolutionary process). Figure 8 from:Gingerich, P. D. 2001. Rates of evolution on the time scale of the evolutionary process. Genetica 112-113:127-144. D C

  23. General Conclusions 3 & 4: Evolutionary rates witnessed in the fossil record on a geological timescale are compatible with those witnessed on biological timescales, and even with maximal rates under artificial selection. But they don’t last that long. Again, no additional evolutionary process is required to reconcile rates over these different temporal scales.

  24. So why are evolutionary rates lower over longer time scales? • strong directional selection does not last for many generations • direction of selection fluctuates • organisms evolve and diversify within limits ("constraints") determined by their basic body plan ("Bauplane") or set of characteristics (diversity within constraint)

  25. Relative Low Rate Overall But some much higher rates over shorter time intervals! 2. How Fluctuating SelectionAffects Evolutionary Rates: Mean Trait Value for a Popu-lation Time

  26. 3. How Limits or ConstraintsAffect Evolutionary Rates: Lower Rate Higher Rate Upper Limit Mean Trait Value for a Popu-lation Lower Limit Time

  27. Diversification occurs within constraints on what is possible for an organism of a given type (basic morphology, physiology, biochemistry). "Bauplane" = fundamental body plan or set of characteristics. To illustrate what we mean by this, what's wrong with these movies?

  28. The Amazing Colossal Man (1957)

  29. The Amazing Colossal Man (1957)

  30. The Amazing Colossal Man (1957) He took it pretty philosophically for a while, but you just knew this was going to end badly …

  31. The Amazing Colossal Man (1957)

  32. Attack of the 50 Foot Woman (1958)

  33. Them! (1954)! The earliest atomic tests in New Mexico cause common ants to mutate into giant man-eating monsters that threaten civilization. I hate it when that happens!

  34. The human or ant bauplane cannot work at gigantic sizes because they would collapse under ther own weight. As something gets larger, mass increases as length cubed, whereas strength of skeletal elements or muscles increases with length squared (i.e., as cross-sectional area). If shape and the strength of what you are made of does not change, then you get weaker as you get larger.

  35. The Tempo and Mode of Evolution The gradualist model of evolution suggests that character change is slow and continuous, and that nothing unusual happens during speciation events. This is certainly reasonable for selectively neutral traits.

  36. Fossil coelacanth, Cretaceous ~80 mya Modern coelacanth, Indian Ocean But, some species show “stasis” (See Darwin's figure back on slide 4.) "living fossil" Time Morphology

  37. Perfect stasis would be surprising 1. Environments are always changing. 2. Random genetic drift is always occurring.

  38. Eldredge & Gould argued we should take a more literal view of the fossil record: believe what the data seem to be showing, take them more at face value. For example, stasis is real and common. Eldredge, N., and S. J. Gould. 1972. Punctuated equilibria: An alternative to phyletic gradualism.Pages 82-115 in T. J. M. Schopf, ed. Models in paleobiology. Freeman, Cooper and Co., San Francisco. Gould, S. J., and N. Eldredge. 1977. Punctuated equilibria: The tempo and mode of evolution reconsidered. Paleobiology 3:115-151. Niles Eldredge Stephen J. Gould

  39. In particular: • Stasis (absence of change) was more common than we recognize • Change was concentrated around speciation events

  40. Punctuated Equilibrium Model Eldredge & Gould said that the gradualist model should be replaced with one in which changes were infrequent, but rapid. ancestralformremains ancestralformchanges B&G2012 Fig. 15.33, p. 528

  41. No transitional forms Transitional forms If Punctuated Equilibrium is often true, then it could explain the frequent lack of transitional forms in the fossil record.

  42. Under Phyletic Gradualism, the rate of phenotypic evolution is independent of speciation rates. Under Punctuated Equilibrium, however, more speciation means greater phenotypic evolution. Speciation (cladogenesis) is followed by stasis.

  43. Slow Common Various Not correlatedwith speciation Altered Rapid Rare Allopatric speciation Correlated with speciation Retained Comparing the Two Models Characteristic Gradualism Punc. Eq. • Rate of change • Frequency of change • Cause(s) of change • Phenotypic divergence • Ancestral form

  44. Cheetham’s Miocene bryozoa are consistent with punctuated equilibrium: stasis, speciation & rapid change, persistence of ancestor. 11 of 17 species in stasis for 2-6 million years B&G2012 Fig. 15.34, p. 529

  45. Ridley 2004Fig. 21.8, p. 605 Sheldon's Ordovician trilobites are consistent with phyletic gradualism in pygidial rib number. Mean Rib No

  46. Criticisms of Puncuated Equilibrium (1) Because the fossil record is so “condensed” (lots of missing strata), a gradual change across many generations can look almost instantaneous.

  47. Because the fossil record is so spotty, a gradual change in a single taxon can look like an "instantaneous" appearance of a new form. Criticisms of Puncuated Equilibrium (2) Most recent fossil coelacanth: 80 mya! Missing links This is known as pseudoextinction

  48. The verdict? • Empirical support exists for multiple evolutionary patterns. • The rate of speciation is not fixed, either within or among lineages. • Rate of evolution may or may not increase around speciation events. • All patterns, however, are consistent with what we already know about evolution. • Current information does not allow us to rule out microevolutionary processes as the cause of macroevolutionary patterns of change.

  49. Some "Rules" have been observed in the fossil record:

  50. Cope's Rule: A common pattern in gradual change = tendency for the membersof a clade (evolutionary lineage)to get larger over time, asevidenced by the fossil record Named for Edward Drinker Cope(1840-1897) Current evidence indicates that it applies for some lineages but not for others …