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?
1. 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
8 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
Categories of Reproductive Isolating
Mechanisms (cont.):

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. Most 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
2. 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 (cont.)?
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
(cont.)?
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 b/c larger brains,
diurnal, arboreal, mixed diet,
so…radiated to the New World,
displacing most prosimians in Old World
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” (not author) of
Evolution, maintains adaptation of a species to
its environment
Ex. Grants’ finches beaks changed back and forth
as environmental 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: DNA and proteins
(amino acids)
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
543 mya
Universe forms (“Big Bang”)
Galaxies form
Solar System forms
EARTH forms
LIFE on Earth (single-celled
organisms)
CAMBRIAN EXPLOSION!
(all major body plans of complex
multicellular 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 (cont.)
425 mya
400 mya
350 mya
250 mya
256 mya
235 mya
Fish evolve, Plants & Animals
colonize land
Insects evolve
Reptiles evolve
MASS EXTINCTION (volcanic
eruptions in Siberia?, 95% marine
& land species extinct!)
Mammal-like Reptiles evolve
Dinosaurs evolve
Brief Evolutionary Timetable (cont.)
220 mya
150 mya
100 mya
65 mya
55 mya
True Mammals evolve
Small Dinosaurs w/feathers evolve,
ancestors of Birds
Flowering Plants evolve
CRETACEOUS/TERTIARY (K/T)
EXTINCTION (6 mile asteriod
crashes through Earth in the MX
Yucatan, dinosaurs extinct!)
PRIMATES evolve
Common Chimpanzee