Natural Selection


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  • E. coli. with 30 minute division would weigh more than the earth in less than a week. Elephants ... 19 million in 750 years
  • Evolution by natural selection need not occur.
  • Natural selection acts on individuals but its consequence occur in populations. It does not, however, work for “the good of the species.”
  • Fitness applies to the here-and-now, not the future.
  • Natural selection is not forward looking and does not lead to perfection or necessarily progress. Adaptations need not be “perfect” in any sense.
  • Natural Selection

    1. 1. Implications Mechanism Pathway Fact
    2. 2. EVOLUTION “Biological (or organic) evolution is change in the properties of populations of organisms or groups of such populations, over the course of generations. The development, or ontogeny, of an individual organism is not considered evolution: individual organisms do not evolve. The changes in populations that are considered evolutionary are those that are ‘heritable' via the genetic material from one generation to the next. Biological evolution may be slight or substantial” Douglas J. Futuyma (1998) Evolutionary Biology
    3. 3. 7mya! 6mya! 5mya! 4mya! 3mya! 2mya! 1mya! Now! Kenyanthropus! platyops! Homo erectus! Homo ergaster! A. garhi! Homo! “Archaic”! rudolfensis! Homo! sapiens! Ardipithecus! Ardipithecus! Australopithecus! kadabba! ramidus! afarensis! Sahelanthropus! tchadensis! Homo! habilis! Modern! Humans! Australopithecus! anamensis! Australopithecus! Orrorin! africanus! tugenesis! Paranthropus! robustus! Neandertals! Paranthropus! aethiopicus! Paranthropus! boisei!
    4. 4. EVOLUTION AS PATH Degree of relatedness of modern species Timing of splits among lineages Characteristics of extinct ancestors
    5. 5. “LAWN OF LIFE”
    6. 6. “ORCHARD OF LIFE”
    7. 7. Natural Selection Economy of Tree Thinking Nature Gradual Genealogical Change Classification Darwin Biogeographic Coevolution Distribution Sexual Selective Selection Extinction Deep Time
    8. 8. DARWIN’S VIEWPOINT Fact Pathway: Establishment of genealogical relationship (“tree thinking”) with common descent and multiplication of species via splitting or budding Mechanism: Natural selection and other mechanisms
    9. 9. Instinct Classification Fossil Record Evolution Embryology Morphology Geographic Distribution
    10. 10. FACT #1 Potential exponential increase of populations (“superfecundity”) Source: Thomas Malthus (1798), William Paley (1802), observation
    11. 11. E. coli
    12. 12. FACT #2 Steady-state stability of most populations Source: observation
    13. 13. FACT #3 Limitation of resources Source: observation
    14. 14. INFERENCE #1 Struggle for existence among individuals Author of inference: Malthus (1798)
    15. 15. Linear Growth Food Production Exponential Growth Population Growth Malthusian Catastrophe
    16. 16. FACT #4 Variation between individuals Source: observation
    17. 17. FACT #5 Heritability of much of this variation Source: animal breeders
    18. 18. INFERENCE #2 Some of this variation is advantageous in certain environments therefore differential survival, i.e. natural selection, will occur. Inference held by: Darwin, Alfred Russel Wallace, and others
    19. 19. Patrick Matthew (1831) Edward Blyth (1835)
    20. 20. INFERENCE #3 Over many generations, provided selection pressure is maintained, evolution will occur Inference unique to Darwin & Wallace
    21. 21. ALTERNATIVELY • If there are variations in a replicating entity, and • If these are inherited, and • If one variant is more suited to some task than the others, and • Ifthat task directly affects survival and therefore reproduction of the entities, • Then selection will result in evolutionary change in the population of entities.
    22. 22. Non- Random Random Variation Selection NATURAL SELECTION IS NOT A RANDOM PROCESS
    23. 23. Non- Random Random Variation Selection NATURAL SELECTION IS NOT A RANDOM PROCESS
    24. 24. NATURAL SELECTION “A non-random difference in reproductive output among replicating entities, often due indirectly to differences in survival in a particular environment, leading to an increase in the proportion of beneficial, heritable characteristics within a population from one generation to the next.” (Gregory, 2009)
    25. 25. FITNESS Fitness is a measure of the reproductive output of an organism with a particular genotype with respect to that of other genotypes in a particular environment. For a trait to be evolutionarily relevant, it must affect reproduction - it is not enough to affect survival.
    26. 26. ANALYZING SELECTION Is the population variable? Is some of the variation among individuals within the population heritable? Do individuals vary in their success as surviving or reproducing? Are survival and reproduction non-random? Did the population change over time?
    27. 27. T. RYAN GREGORY (2008) “Natural selection is considered by many to be the prime component of evolutionary theory and is the only workable mechanism ever proposed that is capable of accounting for the adaptive features of organisms. At the molecular level, nonadaptive mechanisms are recognized as highly significant.” (p. 50)
    28. 28. PREDICTING THE FUTURE No need to be able to predict long-term course of evolution. Evolution is analogous to a poker tournament.
    29. 29. DARWIN “I have just been writing an audacious little discussion, to show that organic beings are not perfect, only perfect enough to struggle with their competitors.” Letter to J.D. Hooker, 9/11/1857
    30. 30. NATURAL SELECTION Nature “cares not for mere external appearances; she may be said to scrutinize with a severe eye, every nerve, vessel & muscle; every habit, instinct, shade of constitution, - the whole machinery of the organization. There will be here no caprice, no favoring: the good will be preserve[d] & the bad rigidly destroyed.” “By nature, I mean the laws ordained by God to govern the Universe.”
    31. 31. ORIGIN, 2ND ED. “A celebrated author and divine has written to me that ‘he has gradually learnt to see that it is just as noble a conception of the Deity to believe that He created a few original forms capable of self-development into other and needful forms, as to believe that He required a fresh act of creation to supply the voids caused by the action of His laws.’ ”
    32. 32. LETTER TO ASA GREY (1860) “There seems to me too much misery in the world. I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of caterpillars or that a cat should play with mice... On the other hand, I cannot anyhow be contented to view this wonderful universe, and especially the nature of man, and to conclude that everything is the result of brute force. I am inclined to look at everything as resulting from designed laws, with the details, whether good or bad, left to the working out of what we may call chance.”
    33. 33. DARWIN “The old argument of design in nature, as given by Paley, which formerly seemed to me to me so conclusive, fails, now that the law of natural selection has been discovered. We can no longer argue that, for instance, the beautiful hinge of a bivalve shell must have been made by an intelligent being, like the hinge of a door by a man. There seems to be no more design in the variability of organic beings and in the action of natural selection, than in the course which the wind blows. Everything in nature is the result of fixed laws.”
    34. 34. SEXUAL SELECTION Inter-male competition Female choice
    35. 35. PALEY A: X is intricate and well suited to a task T W1: X is a product of intelligent design W2: X is a product of random physical forces Paley claims that the likelihood of W1 given A exceeds that of W2, i.e. P(A|W1) >> P(A|W2)
    36. 36. DARWIN A: X may or may not be intricate or well-suited to a task T. W1: X is a product of intelligent design W2: X is a product of a non-random natural mechanism Darwin claims that the likelihood of W2 given A exceeds that of W1, i.e. P(A|W2) >> P(A|W1)
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    40. 40. CONTROVERSIES • Is evolution always gradual? • Can complex features be regained if lost? • Balance between chance and non-random mechanisms • Does “junk” DNA have a function? • At what levels can selection work? • Did mammals diversify because of the extinction of the • Are genomic duplications dinosaurs? common and are they associated with major • What are the direct ancestors evolutionary changes? of Homo sapiens?
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