Fossils are the preserved remnants or impressions left by organisms that lived in the past'

1
CHAPTER 25 PHYLOGENY AND SYSTEMATICS
The Fossil Record and Geological Time

• Systematics is the study of biological diversity
  in an evolutionary context.
• A major goal of evolutionary biology is to
  reconstruct the history of life on earth.
• Part of the scope of systematics is the
  development of phylogeny, the
  evolutionary history of a species or group of
  related species.

• Fossils are the preserved remnants or impressions
  left by organisms that lived in the past.
• In essence, they are the historical documents of
  biology.
• The fossil record is the ordered array in which
  fossils appear within sedimentary rocks.
• These rocks record the passing of geological time.

2
Sedimentary rocks are the richest source of
fossils

• Sedimentary rocks form from layers of sand and
  silt that settle to the bottom of seas and
  swamps.
• As deposits pile up, they compress older
  sediments below them into rock.
• The bodies of dead organisms settle along with
  the sediments, but only a tiny fraction are
  preserved as fossils.
• The organic material in a dead organism usually
  decays rapidly, but hard parts that are rich in
  minerals (such as bones, teeth, shells) may
  remain as fossils.

3

• The organic material in a dead organism usually
  decays rapidly, but hard parts that are rich in
  minerals (such as bones, teeth, shells) may
  remain as fossils.
• Under the right conditions minerals dissolved in
  groundwater seep into the tissues of dead
  organisms, replace its organic material, and
  create a cast in the shape of the organism.

Fig. 25.1c
4
Paleontologists use a variety of methods to date
fossils

• When a dead organism is trapped in sediment, this
  fossil is frozen in time relative to other strata
  in a local sample.
• Younger sediments are superimposed upon older
  ones.
• The strata at one location can be correlated in
  time to those at another through index fossils.
• These are typically well-preserved and
  widely-distributed species.
• By comparing different sites, geologists have
  established a geologic time scale with a
  consistent sequence of historical periods.
• These periods are grouped into four eras the
  Precambrian, Paleozoic, Mesozoic, and Cenozoic
  eras.
• Boundaries between geologic eras and periods
  correspond to times of great change, especially
  mass extinctions, not to periods of similar
  length.
• The serial record of fossils in rocks provides
  relative ages, but not absolute ages, the actual
  time when the organism died.

5
(No Transcript)
6

• Radiometric dating is the method used most often
  to determine absolute ages for fossils.
• This technique takes advantage of the fact that
  organisms accumulate radioactive isotopes when
  they are alive, but concentrations of these
  isotopes decline after they die.

• For example, the radioactive isotope, carbon-14,
  is present in living organisms in the same
  proportion as it occurs in the atmosphere.
• However, after an organism dies, the proportion
  of carbon-14 to the total carbon declines as
  carbon-14 decays to nitrogen-14.
• An isotopes half life, the time it takes for 50
  of the original sample to decay, is unaffected by
  temperature, pressure, or other variables.
• The half-life of carbon-14 is 5,730 years.
• Losses of carbon-14 can be translated into
  estimates of absolute time.

7

• Over time, radioactive parent isotopes are
  converted at a steady decay rate to daughter
  isotopes.
• The rate ofconversion isindicated as
  thehalf-life, thetime it takesfor 50 ofthe
  isotopeto decay.

Fig. 25.2
8

• While carbon-14 is useful for dating relatively
  young fossils, radioactive isotopes of other
  elements with longer half lives are used to date
  older fossils.
• While uranium-238 (half life of 4.5 billion
  years) is not present in living organisms to any
  significant level, it is present in volcanic
  rock.
• If a fossil is found sandwiched between two
  layers of volcanic rock, we can deduce that the
  organism lived in the period between the dates in
  which each layer of volcanic rock formed.
• Paleontologists can also use the ratio of two
  isomers of amino acids, the left-handed (L) and
  right-handed (D) forms, in proteins.
• While organisms only synthesize L-amino acids,
  which are incorporated into proteins, over time
  the population of L-amino acids is slowly
  converted, resulting in a mixture of L- and
  D-amino acids.
• If we know the rate at which this chemical
  conversion, called racemization, occurs, we can
  date materials that contain proteins.
• Because racemization is temperature dependent, it
  provides more accurate dates in environments that
  have not changed significantly since the fossils
  formed.

9
Phylogeny has a biogeographical basis in
continental drift

• The history of Earth helps explain the current
  geographical distribution of species.
• For example, the emergence of volcanic islands
  such as the Galapagos, opens new environments for
  founders that reach the outposts, and adaptive
  radiation fills many of the available niches with
  new species.
• In a global scale, continental drift is the major
  geographical factor correlated with the spatial
  distribution of life and evolutionary episodes as
  mass extinctions and adaptive radiations.

10
The continents drift about Earths surface on
plates of crust floating on the hot mantle.
Fig. 25.3a
11

• About 250 million years ago, all the land masses
  were joined into one supercontinent, Pangaea,
  with dramatic impacts on life on land and the
  sea.
• Species that had evolved in isolation now
  competed.
• The total amount of shoreline was reduced and
  shallow seas were drained.
• Interior of the continent was drier and the
  weather more severe.
• The formation of Pangaea surely had tremendous
  environmental impacts that reshaped biological
  diversity by causing extinctions and providing
  new opportunities for taxonomic groups that
  survived the crisis.

12

• A second majorshock to lifeon Earth
  wasinitiated about180 million yearsago, as
  Pangaeabegan to breakup into separatecontinents
  .

Fig. 25.4
13

• Each became a separate evolutionary arena and
  organisms in different biogeographic realms
  diverged.
• Example paleontologists have discovered matching
  fossils of Triassic reptiles in West Africa and
  Brazil, which were continguous during the
  Mesozoic era.
• The great diversity of marsupial mammals in
  Australia that fill so many ecological roles that
  eutherian (placental) mammals do on other
  continents is a product of 50 million years of
  isolation of Australia from other continents.

14
The history of life is punctuated by mass
extinction

• The fossil record reveals long quiescent periods
  punctuated by brief intervals when the turnover
  of species was much more extensive.
• These brief periods of mass extinction were
  followed by extensive diversification of some of
  the groups that escaped extinction.

• A species may become extinct because
• its habitat has been destroyed,
• its environment has changed in an unfavorable
  direction
• evolutionary changes by some other species in its
  community may impact our target species for the
  worse.
• As an example, the evolution by some Cambrian
  animals of hard body parts, such as jaws and
  shells, may have made some organisms lacking hard
  parts more vulnerable to predation and thereby
  more prone to extinction.
• Extinction is inevitable in a changing world.

15

• During crises in the history of life, global
  conditions have changed so rapidly and
  disruptively that a majority of species have been
  swept away.
• The fossil record records five to seven severe
  mass extinctions.

16

• Factors that may have caused the Permian mass
  extinction include
• disturbance to marine and terrestrial habitats
  due to the formation of Pangaea,
• massive volcanic eruptions in Siberia that may
  have released enough carbon dioxide to warm the
  global climate
• changes in ocean circulation that reduced the
  amount of oxygen available to marine organisms.

• The Cretaceous mass extinction (65 million years
  ago) doomed half of the marine species and many
  families of terrestrial plants and animals,
  including nearly all the dinosaur lineages.
• This event defines the boundary between the
  Mesozoic and Cenozoic eras.

17
Taxonomy employs a hierarchical system of
classification

• The Linnean system, first formally proposed by
  Linneaus in Systema naturae in the 18th century,
  has two main characteristics.
• Each species has a two-part name.
• Species are organized hierarchically into broader
  and broader groups of organisms.
• Under the binomial system, each species is
  assigned a two-part latinized name, a binomial.
• The first part, the genus, is the closest group
  to which a species belongs.
• The second part, the specific epithet, refers to
  one species within each genus.
• The first letter of the genus is capitalized and
  both names are italicized and latinized.

18

• A hierachical classification will group species
  into broader taxonomic categories.
• Species that appear to be closely related are
  grouped into the same genus.
• For example, the leopard, Panthera pardus,
  belongs to a genus that includes the African lion
  (Panthera leo) and the tiger (Panthera tigris).
• Biologys taxonomic scheme formalizes our
  tendency to group related objects.

• Genera are grouped into progressively broader
  categories family, order, class, phylum,
  kingdom and domain.

19
(No Transcript)
20
Phylogenetic trees reflect the hierarchical
classification of taxonomic groups nested within
more inclusive groups

• .

Fig. 25.8