Organic Evolution

ORGANIC EVOLUTION

• MODERN ORGANISMS ARE EXTREMELY VARIED
• ANCIENT ORGANISMS HAVE VARIED THROUGH TIME
• LATER ORGANISMS HAVE DESCENDED FROM EARLIER FORMS

CHARLES DARWIN

• PROVIDED THE KEY CONCEPT: EVOLUTION PROCEEDS THROUGH NATURAL SELECTION ("The Origin of Species by Means of Natural Selection"; 1859)
  • 1. there is natural variation among the individuals of a given species
  • 2. some variations are more favorable than other variations
  • 3. organisms produce many offspring that die before maturity
  • 4. individuals surviving to reproduce must be those with the most successful combinations of variable characteristics
• Co-developed with ALFRED R. WALLACE

JEAN BAPTISTE DE LAMARCK

• 1ST ACCEPTED THEORY OF ORGANIC EVOLUTION - INHERITANCE OF ACQUIRED CHARACTERISTICS
  • organism acquires traits during lifetime & passes them on to descendants

EVIDENCE FOR ORGANIC EVOLUTION

• EMBRYOLOGY
  • similarities early in development (tails in humans); divergence in later stages
• COMPARATIVE ANATOMY
  • homologous structures (tetrapod limbs modified to wings or fins), as opposed to analogous structrures (bird/bat wings vs. fly wings)
  • vestigial organs [non or partially funtional organs] - horse toes & mammalian dewclaws, whale & snake pelvises, human appendix & wisdom teeth
  • activator genes
• BIOCHEMISTRY
  • DNA, ADP, blood proteins
• SMALL SCALE EVOLUTION
  • industrial melanization
  • insecticide/antibiotic resistance
• FOSSILS
  • fossils are observed to change through time
    • SIMPLEST & LEAST SIMILAR TO MODERN IN OLDEST ROCKS
    • COMPLEX & MOST SIMILAR TO MODERN IN YOUNGER

GENETICS (originally discovered by Gregor Mendel; unknown to Darwin)

• MECHANISM FOR TRANSMISSION OF HERITABLE CHARACTERISTICS FROM ONE GENERATION TO THE NEXT
• CHARACTERISTICS ARE CARRIED BY GENES, WHICH ARE SEGMENTS OF DNA
  • DNA is a long molecule consisting of 2 linked strands
  • genes are also segments of larger units called Chromosomes, the central axis of which is a DNA molecule
  • Alleles are alternate forms of genes that control the same trait
• VARIABILITY IS INTRODUCED BY GENETIC RECOMBINATION & MUTATION
  • Genetic recombination occurs during sexual reproduction
    • MITOSIS - normal cell reproduction: a single parent cell reproduces via 1 cell division into 2 genetically-identical daughter cells - NO recombination
    • MEIOSIS - cell reproduction in reproductive organs: a single parent cell divides by 2 cell divisions into 4 granddaughter cells (called gametes), each containing half the number of chromosomes of the original parent cell
    • genetic recombination occurs when male & female gametes from different individuals fuse to form a fertilized egg that grows into a new individual with recombined genes
  • Mutation = a change in the structure of a gene or chromosome

SPECIES

• A POPULATION OF INDIVIDUALS THAT ARE BASICALLY ALIKE IN STRUCTURAL & FUNCTIONAL CHARACTERISTICS & INTERBREED (OR POTENTIALLY INTERBREED) & PRODUCE FERTILE OFFSPRING IN NATURE
  • [note: horses/donkeys & goats/sheep - interbreed in captivity, but mules are sterile, although the offspring of goats & sheep are fertile]
• PRESENTLY-LIVING SPECIES ARE RELATIVELY EASILY DISTINGUISHED
• EXTINCT SPECIES ARE RELATIVELY MORE DIFFICULT TO DISTINGUISH BECAUSE PALEONTOLOGISTS CANNOT TELL IF TWO PRESUMED SPECIES REALLY DIDN'T INTERBREED

FORMATION OF NEW SPECIES = SPECIATION

• A GIVEN SPECIES EXISTS FOR AN AVERAGE OF 5-10 MILLION YEARS
• FOR A GIVEN SPECIES THERE IS A FREE FLOW OF GENES WITHIN THE GENE POOL
  • gene pool - the sum of all genes defining a species
• NO GENE EXCHANGE BETWEEN DIFFERENT SPECIES BECAUSE OF BARRIERS:
  • geographic = physical separation by water or mountains
  • reproductive = lack of fertilization, can't mate (size difference, behavior), or sterile offspring

EXTINCTION

• DISAPPEARANCE OF A TAXONOMIC GROUP
  • Psuedoextinction - evolution of a parent species into a descendent species
  • Terminal extinction - a species dies out without giving rise to a descendent species
• MASS EXTINCTION
  • extinction of large numbers of species & higher taxonomic groups
  • most severe mass extinction at the end of the PERMIAN
  • less severe at the end of the Cretaceous
  • important mass extinctions at or near the ends of the Cambrian, Ordovician, Devonian, & Triassic
  • humans have caused & are causing at an accelerating rate the extinction of many animal & plant species
    • THE RESULT BY THE END OF THE NEXT CENTURY MAY BE A MASS EXTINCTION RIVALING THAT AT THE END OF THE PERMIAN
      • Is there any guarantee that we ourselves will survive such a mass extinction?
• MAJOR PROPOSED CAUSES
  • 1. formation of supercontinents through plate tectonic activity
    • DRIER & COLDER CLIMATE
    • REDUCED SHALLOW-MARINE CONTINENTAL SHELF ENVIRONMENT
      • lower sealevel, reduced perimeter & provinciality
  • 2. meteorite impacts - shown by Iridium-rich layers
    • REDUCTION OF SUNLIGHT FOR SEVERAL MONTHS FROM DUST CLOUDS
  • 3. disruption of food chains

ADAPTIVE RADIATION

• A FEW SPECIES ADAPTED TO A LIMITED ENVIRONMENT EVOLVE TO MANY SPECIES ADAPTED TO MANY DIFFERENT ENVIRONMENTS
  • Galapagos finches; Mesozoic reptiles; Cenozoic mammals; Cambrian to Paleozoic to Modern Faunas
• OCCURS WHEN NEW ENVIRONMENTS OR NICHES BECOME AVAILABLE
• A TYPE OF DIVERGENT EVOLUTION (SIMPLE BRANCHING OF PHYLOGENETIC TREE)

SPECIATION & THE RATE OF EVOLUTION

• ALLOPATRIC SPECIATION - NEW SPECIES FROM ISOLATION OF A SMALL POPULATION THAT BECOMES GENETICALLY DISTINCT FROM ITS PARENT SPECIES
  • concensus mechanism by which speciation occurs
  • well documented from islands
  • disagreement as to rate that new species evolve
• PHYLETIC GRADUALISM - SLOW ACCUMULATION OF MINOR CHANGES BRINGING ABOUT A TRANSITION FROM A PARENT SPECIES TO ITS DESCENDANT SPECIES
• PUNCTUATED EQUILIBRIUM - RAPID BURSTS OF CHANGE FOLLOWED BY LONG PERIODS OF LITTLE OR NO CHANGE
• PROPONENTS OF PUNCTUATED EQUILIBRIUM ARGUE THAT THE FOSSIL RECORD SUPPORTS THEIR HYPOTHESIS; PROPONENTS OF PHYLETIC GRADUALISM ARGUE THAT THEIR HYPOTHESIS COINCIDES WITH THE PRINCIPLE OF UNIFORMITARIANISM & ALSO ARGUE THAT THE FOSSIL RECORD GENERALLY MAKES EVOLUTION SEEM PUNCTUATED
• THERE ARE EXAMPLES OF BOTH PHYLETIC GRADUALISM & PUNCTUATED EQUILIBRIUM

OTHER EVOLUTIONARY PATTERNS

• CONVERGENT EVOLUTION - SIMILAR MORPHOLOGY DEVELOPED BY DISTANTLY-RELATED GROUPS, USUALLY BY ADAPTATION TO SIMILAR ENVIRONMENTS
  • shark (fish) - ichthyosaur (reptile) - dolphin (mammal); clam (mollusk) - brachiopod; modern marsupials in Australia - placental mammals elsewhere; Cenozoic marsupials & placentals of N. & S. America
• PARALLEL EVOLUTION (SOMETIMES CALLED ITERATIVE EVOLUTION) - SIMILAR MORPHOLOGY DEVELOPED BY CLOSELY-RELATED GROUPS
  • Cretaceous & Neogene keeled foraminifers; Oligocene-Miocene & Pleistocene saber-toothed cats [note - development of saber-like canines in creodonts & marsupials is convergent evolution]

EVOLUTIONARY TRENDS (based on PHYLOGENY - EVOLUTIONARY HISTORY)

• INCREASING/DECREASING SIZE (BODY, TEETH, BRAIN, HORNS)[COPE'S RULE]
• REDUCTION IN NUMBER (TOES, TEETH, JAW BONES)
• INCREASING COMPLEXITY (SUTURES)
• MOSAIC EVOLUTION - DIFFERENT RATES OF CHANGE FOR DIFFERENT BODY PARTS
  • Archaeopteryx (the earliest bird) - feathers, but otherwise a small dinosaur
• LIVING FOSSILS = GROUPS EXHIBITING LITTLE CHANGE
  • Lingula , opossum, Caelacanth (Latimeria )

Study Questions

1. Define Organic Evolution.

2. What was Charles Darwin's contribution?

3. Define natural selection.

4. How does natural selection work?

5. What was Jean Baptiste de Lamarck's contribution to the development of the theory of Organic Evolution?

6. Summarize the biological & fossil evidence for evolution.

7. What are homologous organs? Give some examples.

8. What are vestigial structures? Give some examples.

9. What is the role of genetics in the theory of Organic Evolution?

10. Define genes, genetic recombination & mutation.

11. What are the differences between mitosis & meiosis?

12. Define species.

13. Define mass extinction.

14. Summarize the proposed causes of mass extinction.

15. Define adaptive radiation & give examples.

16. What is the mechanism by which speciation occurs?

17. What are the differences between phyletic gradualism & punctuated equilibrium? Give examples of each.

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