The Origin of Species
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?
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
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
• 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)
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
How do species diversify?
ADAPTIVE RADIATION =
Spreading out of related species
into new niches
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!
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
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,
so…radiated to Madagascar & the
New World, displacing most
prosimians in Old World
Continents at 65 mya
Interpretations of Speciation
2 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.
What does the “family tree” of
species look like?
A BUSH WITH MANY TWIGS!
Twigs = evolution’s 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
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
Evidence for Evolution
5. HOMOLOGOUS STRUCTURES: Structures
that are similar because of common
ancestry (comparative anatomy)
6. COMPARATIVE EMBRYOLOGY: Study of
structures that appear during embryonic
development
7. MOLECULAR BIOLOGY: Shared DNA
Evidence for Scientific
Creationism
• FAITH in a literal translation of
the Book of Genesis in the
Bible
• What do you think about
Creationism?
Brief Evolutionary Timetable
15 bya
12 bya
5 bya
4.5 bya
3.8 bya
Universe forms (“Big Bang”)
Galaxies form
Solar System forms
EARTH forms
LIFE on Earth (single-celled
organisms)
543 mya CAMBRIAN EXPLOSION!
(all major body plans of
complex multi-cellular
organisms evolve!)
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
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
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
Anomalocaris
•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
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
Aysheaia
•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
Cambrian Explosion (cont.)
Hallucigenia
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
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!
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
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
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
Chimpanzees!
to be continued…
Dettwyler, Chs. 7 & 8
1. 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?