phylogeny. a shared history we’ve talked a lot about common ancestors we’ve seen evidence that...
TRANSCRIPT
Phylogeny
A Shared History• We’ve talked a lot about common ancestors• We’ve seen evidence that species are related to each
other• Today we’ll examine how to demonstrate
evolutionary relationships
Phylogeny• Phylogeny: the
evolutionary relationships of different groups of species or organisms
• Evidence for relationships is based on fossils, morphology, and genetic evidence
Phylogeny• This is an entire field
of evolutionary biology where evolutionary relationships are demonstrated graphically
Phylogeny• For example, birds and
reptiles are closely related
• Phylogeny attempts to map out their evolutionary history
• If we have a common ancestry with this lamprey, phylogeny will illustrate it
Cladistics• Cladistics: a method of showing evolutionary relationships
based on the presence of recently evolved (or derived) traits• Synapomorphy: A derived trait shared by two or more
species or groups– Indicate a closer evolutionary relationship than to organisms that
do not share this trait
Synapomorphy• Basically a homologous structure• Feathers are an example of synapomorphy:– All birds have feathers, but reptiles do not
therefore birds are more closely related to each other than to reptiles
Synapomorphy• Cladistics makes inferences based on
synapomorphies• This can be a challenge: sometimes suspected
synapomorphies turn out to be false– Birds and humans both walk on two legs, but this trait
arose independently (analogous feature)
A Visual Demonstration• Cladograms: are used to show the
evolutionary relationships between groups of organisms
Cladograms• Each branch represents
a different group or species– Branching points
represent the most recent common ancestor of groups
– A dramatic demonstration of how species are related, and how far back
Cladograms
Cladograms
Cladograms
Cladograms
Cladograms
Cladograms
Cladograms
Beauty in Cladograms
Beauty in Cladograms
The Pace of Evolution• How long does it take for a
new species evolve?– How quickly does a species
adapt to changes in its environment?
• Single species can mutate fairly quickly, at the level of single genetic mutations
• On the other hand, the giraffe neck took millions of years to evolve
The Theory of Gradualism• The Theory of Gradualism:
as new species evolve, they appear very similar to the original species and only gradually become more distinctive
• Small changes accumulate gradually over millions of years
Theory of Gradualism• Is there support
for this theory?
• The fossil evidence for both horses and whales illustrates this pattern
The Theory of Punctuated Equilibrium
• The fossil record does not always indicate many intermediate steps
• In some cases the fossil record shows a fairly sudden appearance of new species
The Theory of Punctuated Equilibrium
• The Theory of Punctuated Equilibrium: suggests that most evolutionary changes occur in relatively rapid spurts, followed by long periods of little or no change
The Theory of Punctuated Equilibrium
• Three main assertions: 1) New species evolve rapidly 2) Speciation usually occurs in small isolated populations and leaves few transitional fossils 3) After the initial burst of evolution, additional changes are slow
Comparing Theories• The differences lay partly in time:– Gradualism: small, progressive changes over a
long period of time– Punctuated Equilibrium: abrupt changes occur,
followed by long periods of little change
Comparing Theories• It is accepted that both theories play a role in
evolution– In cases where there is little environmental
change, evolutionary change can happen slowly– When an environment changes drastically, we can
expect rapid evolution
Gaps in the Record• A major source of evidence for
evolution is the fossil record• But we know that fossils form
very rarely, and only in certain conditions– Species with delicate features
are less likely to fossilize• As a result there are significant
gaps in the fossil record
Gaps in the Record• Transitional Form: A fossil
or species intermediate between two other species in a direct line of descent
• In Darwin’s day, the fossil record was very limited, and few transitional forms were known
Missing Links• These gaps in the fossil
record are called “Missing Links”
• At one time, there was no fossil evidence of early birds, representing the earliest days of feathered flight
• Until 1861, when the first fossil was found
Missing Links
Archaeopteryx
Missing Links• One of the greatest mysteries of evolution:• How did we evolve from fish to land animals??
Missing Links• Throughout the years, paleontologists have
searched for the transitional fossil between fish and land animals
• The “Holy Grail” of fossils• This fossil would have to be able to live on
land AND in water• For many years they could only find animals
that lived only in water or only on land
Missing Links• In 2004, a team of researchers digging in
Nunavut, Canada found an extremely interesting fossil
• It had traits of both fish and tetrapods (4 legged land animals)
Fish-Like Traits “Fishapod” Traits
Tetrapod Traits
Gills Limb bones and joints
Rib bones
Scales Wrist joint with radiating fin-toes
Mobile neck
Fins Ear was half fish/half tetrapod
Lungs
Tiktaalik
Tiktaalik
The Importance of EvidenceRemember: a lack of evidence does not mean evidence against a theory• An understanding of evolution still allows
evolutionary biologists to predict transitional forms
Filling in the Gaps• Today we have a much
larger fossil record• Many of these missing
links have been found• Each new fossil
provides new clues about evolution– And often raises more
questions
Homework QuestionsPage 355 in text # 4, 7, 8bcdef
Page 351-352, build the cladogram