5 - Evolution

5 - Evolution

1. Basics

   • Evolution : change over time
   • Biological evolution : cumulative changes of heritable characteristics over successive genertaions
   • Change in allele frequency of a population's gene pool over successive generations

2. Proofs of evolution

   • Fossil record
     • Fossil: preserved remains or traces of any organism from the remote past
       • Remains: body fossils --> direct evidence of ancestral forms - bones, teeth, shells, leaves, etc
       • Traces --> indirect evidence of ancestral forms - footprints, faeces, burrows
     • Law of fossil succession
       • Dossils can be dated by determining the age of rock layer (strata) in which the fossil is found.
         • Lower layers --> older -- higher layers --> newer
         • Each strata represents a length of time; according to a geological time scale
       • Different kinds of organisms are found in rocks of particular ages ina consistent order, indicating a sequence of development
         • Prokaryotes before eukaryotes Ferns before flowering plants Inverterbrates before vertebrates
       • Chronoligcal sequence of complexity by which characteristics appear to develop
     • Transitional fossils
       • Fossil record is incomplete
         • Fossilisaiton requires an unsusual set of specific circumstances --> very few organisms become fossils
         • Only fragments of remains are discovered:
         • Evolutionary petterns are hard to discern --> missing links
       • Common trait to both ancester and predicted descendant
       • Archaeopteryx : dinosaurs and birds
       • When new fossils are discovered; new evolutionnary patterns emerge and old assumptions are challenged
     • Example of fossil evidence = Hominn skeletons
       • Australopithecus (4 millions years ago) and Homo sapiens
         • Transition to bipedalism : S-shpaed curvature of spin; larger heelo bone
         • Changed dietary requirements: smaller teeth, narrower jaw
         • Altered birthing patterns: lower and broader pelvis
     • Fossil records provides evidence by revealing the change in features of living organisms from an ancestor for comparison against living descendants.
   • Selective breeding
     • Artificial selection: man intervenes in the breeding of species to produce desired traits in offsprine
     • Evidence for evolution : targeted breeds show wignificant variation in short period -- deisred trait's frequecny becomes more common in successive generations
     • Examples
       • Plants: Brassica --> different food; broccoli (flower buds); cabbage (leaf buds), kale (leaves)
       • Domesticated animals: horse (race or draft horses); cow (milk or Belgian Blue); dog (hunting; herding; racing; toy)
   • Comparative anatomy
     • Homologous structures: anatomical features similar in basic strucure but different use
       • The more similar, the more closely related
     • Adaptative radiation: several species rapidly diversify from an ancestral source; with each new species adapted to utilise a specific unoccupied niche
     • Example: petadactyl limb of mammals, birds, amphibians, reptiles -- five digit limb
       • Human - tool manipulation
       • Bird and bat wings - flying
       • Horse hooves - galloping
       • Whale and dolphin fins - swimming

3. Speciation

   • Genetic variation within a population of a species : typically continuous - follow a normal sitrivution surve (rate is gradual and cumulative
   • 2 populations of a species geographically sparated (different ecological conditions) -- the 2 populations will adapt to environmental conditions and graudally diverge over time
   • Degree of divergence greater with longer time and/or bigger geographical separation
   • Time passing by
     • Genetic divergence increases; genetic compatibility consequently decreases
     • Eventually : the populations diverge too much to be able to interbreed if returned to shared environment -- speciation
   • Evolutionary process by which two related populations diverge into seprate species (offspring is not viable or fertile

4. Example : peppered moths (Biston betularia)

   • 2 polymorphic forms : lighter or darker melanic variant
   • Environment
     • Unpolluted : trees covered by pale coloured lichen -- camouflage for lighter moth
     • Polluted: sulphur dioxide kills the lichen, soot blackens the bark -- camouflage for darker moth
   • Conditions change
     • Before industrial revolution : unpolluted, lighter moth surival advantage
     • After industrial evolution : polluted environment, darker moth survival advantage
     • Rcent environmental policies in Europe, reduced pollution levels, frequency of two populations altered

5. Extra content

   • Other Evidences for evolution
     • Vestigial structures
       • Functionless (because changes to the environment) and reduced remnants of organs once present in their ancestors
       • Demonstrate evolutionary divergence of a species form a past activity
       • Example: pelvic bone inwhales - ancestors : terrestrial mammals
     • Biogeography
       • Distribution of lifeforms over geographical areas, poth past and present times
       • Related species are in physical proximity (gradual divergence)
         • Fossils found in a region closely resemble modern organisms of the region
       • Proof for evidence : suggests that closely distributed species share common lineage
       • Examples
         • Modern maruspials found in Australia (70% of extant species)
         • Australia : few placental mamals compared to South America, even though similar environemental condition
       • Exceptions explained by continental drift
         • Over 250 millions years ago : single continental landmass (Pangaea) which split into 6 land regions
         • Closely related species were spearated by landmass breaking, now loclaised in regions once connected
           • Exmaple : ratites (flightless birds) distributed according to regions that were once in Gondwanaland
     • Comparative Embryology
       • Growing embryo in animals or plants shows that closely related species go through similar stages of development
         • Similaraties in embryonic development suggest common evolutionary pathway
       • Examples
         • All terrestrial animals have non functionnaing gill slits (pharyngeal slits) as early embryos (aquatic origin)
         • Many vertebrates demonstrate primitive tail at early embryonic stage
   • Fossilisation
     • Conditions
       • Hard body parts (bones teeth shells) - soft body parts will not fissilise (leave traces)
       • Preservation of remains (no eorsion, scavenging or environmental damage)
       • High pressure to promote mineralisation of remains
       • Anoxic (low oxygen) to protect against oxygen damage and decomposition of saprotrophs
     • Stages
       • 1. Death and decay - soft body parts are decomposed or scavenged
       • 2. Deposition - hard remains are covered with silt and sand, over time more layers deposite
       • 3. Permineralisation - pressure from covering layers of dirt/rock cause hard organic material to be replaced by minerals
       • 4. Erosion/exposure - movement of earth plates displace fossil and return it to the surface for discovery
   • Geological time scale
     • Earth formed 4.6 billion years ago, earliest forms of life appeared 3.5 billion years ago
     • 4 units : Epoch < Periods < Era < Ean
     • Based on occurence of significant geoligical event (ex: mass extinction)
   • Radioactive Dating (aka absolute dating)