The Origin of Species

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The Origin of Species
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Key Questions

• How do existing species give rise to new species?
• How do species diversify?
• What does the family tree of species look like?
• Are there any challenges to the idea of evolution?

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How do existing species give rise to new species?

• When populations in an existing species CAN NO
  LONGER give birth to fertile hybrid offspring
  under natural conditions
• Why does this happen? REPRODUCTIVE ISOLATING
  MECHANISMS make it impossible for populations to
  produce viable offspring

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Categories of Reproductive Isolating Mechanisms

• ECOLOGICAL isolation Different populations (of
  the same species) adapt to different
  environmental niches (ex. Hominid speciation)
• SEASONAL isolation Different populations mate
  at different times of the year
• SEXUAL isolation Different populations have
  different courtship behaviors
• MECHANICAL isolation Different populations have
  incompatible organs of reproduction

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• DIFFERENT POLLINATOR isolation In flowering
  plants, different populations attract different
  insects, birds, or bats to facilitate pollination
• GAMETE isolation Different populations have
  different cells of reproduction no fertilization
  
• HYBRID INVIABILITY Different populations can
  mate and become fertile, but the hybrid zygotes
  do not survive
• HYBRID STERILITY Different populations produce
  living hybrids, but they are sterile (ex. mules)

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When does SPECIATION occur?

• When ANY of these reproductive isolating
  mechanisms evolve!
• HOW do these reproductive isolating mechanisms
  evolve?
• Through EVOLUTIONARY PROCESSES of Mutation,
  Natural Selection, Genetic Drift, Gene Flow

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How do species diversify?

• ADAPTIVE RADIATION Spreading out of related
  species into new niches

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When does Adaptive Radiation occur? (3 instances)

• 1. When an environment supports no similar,
  competing species
• Ex. 3mya A small group of finches migrated from
  South or Central America. They radiated into
  different environmental niches and evolved into
  14 different species!

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When does Adaptive Radiation occur?

• 2. When Extensive Extinction wipes out competing
  species in a set of environments
• Ex. 65 mya Mammals survive Cretaceous/Tertiary
  (K/T) extinction of dinosaurs

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When does Adaptive Radiation occur?

• 3. When a new group of a related species is
  adaptively GENERALIZED, it can disperse
  successfully into different niches, displacing
  species already there
• Ex. 40 mya Monkeys more generalized than
  Prosimians larger brains, diurnal, arboreal,
  mixed diet, soradiated to Madagascar the New
  World, displacing most prosimians in Old World

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Continents at 65 mya
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Interpretations of Speciation2 Theories

• 1. DARWINIAN GRADUALISM Evolution occurs in slow
  changes in species over time, so family tree
  of species has few, gracefully diverging
  branches.
• 2. PUNCTUATED EQUILIBRIUM Species tend to remain
  stable, experiencing oscillating selection.
  Evolution occurs in spurts of relatively rapid
  change.

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What does the family tree of species look like?

• A BUSH WITH MANY TWIGS!
• Twigs evolutions experiments, potential new
  species
• Natural Selection Editor of Evolution,
  maintains adaptation of a species to its
  environment
• Ex. Grants finches beaks changed back forth
  as environ. conditions changed

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Evidence for Evolution

• 1. FOSSIL RECORD Fossils and the order in which
  they appear in layers of sedimentary rock
  (strongest evidence)
• 2. BIOGEOGRAPHY Geographical distribution of
  species
• 3. Presence of TRANSITIONAL fossils
• 4. TAXONOMY Classification of life forms

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Evidence for Evolution

• HOMOLOGOUS STRUCTURES Structures that are
  similar because of common ancestry (comparative
  anatomy)
• COMPARATIVE EMBRYOLOGY Study of structures that
  appear during embryonic development
• MOLECULAR BIOLOGY Shared DNA

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Evidence for Scientific Creationism

• FAITH in a literal translation of the Book of
  Genesis in the Bible
• What do you think about Creationism?

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Brief Evolutionary Timetable

• 15 bya Universe forms (Big Bang)
• 12 bya Galaxies form
• 5 bya Solar System forms
• 4.5 bya EARTH forms
• 3.8 bya LIFE on Earth (single-celled
  organisms)
• 543 mya CAMBRIAN EXPLOSION!
• (all major body plans of complex
  multi-cellular organisms evolve!)

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The Burgess Shale Site about 550 mya

• An "avalanche" of fine mud sliding down from the
  submerged reef top carried off any animals living
  in the shallow reef waters above
• The hard parts of all these animals caught in
  the mudslide were preserved as fossils, like the
  process at any other Cambrian site
• However, here the fine mud also penetrated and
  filled all available spaces within the animals,
  thus preserving the shapes and locations of all
  the soft parts. This is a rare event and has made
  these fossils extremely valuable

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Cambrian Explosion

• Three definite body segments a head with two
  prominent tentacles, an unsegmented trunk with
  stubby side fins, and a flattened tail
• Fins and tail suggest this was an active swimmer
  (also suggested by its rare appearance in the
  Burgess Shale formation)

Amiskwia
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Cambrian Explosion (cont.)

• Fearsome-looking beast is the largest known
  Burgess Shale animal. Some related specimens
  found in China reach a length of six feet!
• Giant limbs in front, which resemble shrimp
  tails, were used to capture and hold its prey
• Mouth on the undersurface of the head had a
  squared ring of sharp teeth that could close in
  like nippers to crack the exoskeleton of
  arthropods or other prey

Anomalocaris
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Cambrian Explosion (cont.)

• Unusual assembly of spines and grasping arms at
  the head end. Its mouth lies in the center of
  that ring of six finger-like projections
• Thought to have been a parasite living on sponges
  since it is commonly found in association with
  their remains
• Presumably, the spiny parts at its head were
  designed for grasping and feeding on its prey

Aysheaia
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Cambrian Explosion (cont.)

• Hallucigenia

• Even today, scientists can't be sure which end is
  the head!
• When originally discovered at the Burgess Shale
  site in Canada, the Hallucigenia fossils were
  squashed flat within the shale layer (like every
  other Burgess Shale fossil) with two sets of
  "spines" appearing to stick out in one direction
  and one set of "tentacles" in the other

Hallucigenia
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Cambrian Explosion (cont.)

• Opabinia

Opabinia is thought to have lived in the soft
sediment on the seabed, although it presumably
could have swum after prey using its side lobes.
On the bottom, the proboscis could have plunged
into sand burrows after worms. Sizes ranged up to
3 inches, plus that unique, amazing 1 inch
proboscis!
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Cambrian Explosion (cont.)
Pikaia

• Earliest known representative of the phylum to
  which we ourselves belong
• Averaging about 1 1/2 inches in length, Pikaia
  swam above the seafloor using its body and an
  expanded tail fin
• Note the characteristic muscle blocks lying along
  the centrally important feature, the notochord

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Brief Evolutionary Timetable

• 425 mya Fish evolve, Plants Animals colonize
  land
• 400 mya Insects evolve
• 350 mya Reptiles evolve
• 250 mya MASS EXTINCTION (volcanic eruptions in
  Siberia?, 95 marine land species
  extinct!)
• 256 mya Mammal-like Reptiles evolve
• 235 mya Dinosaurs evolve

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Brief Evolutionary Timetable

• 220 mya True Mammals evolve
• 150 mya Small Dinosaurs w/feathers evolve,
  ancestors of Birds
• 100 mya Flowering Plants evolve
• 65 mya CRETACEOUS/TERTIARY (K/T) EXTINCTION (6
  mile asteroid crashes through Earth in the
  MX Yucatan, dinosaurs extinct!)
• 55 mya PRIMATES evolve

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Chimpanzees!

• to be continued

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Dettwyler, Chs. 7 8

• How did Dettwyler collect information concerning
  traditional beliefs about infant feeding? Do you
  think group interviews of this kind provide
  biased data? How does this compare to the older
  ethnographic practice of relying on one or two
  key informants for information about cultural
  beliefs?
• 2. Compare the advantages and disadvantages of
  life in Mali and in the US for children with Down
  syndrome. What advantages do children with Down
  syndrome have in the US? What advantages do they
  have in Mali? What advantages do pregnant women
  have in the US? In Mali?