phylogeny. a shared history we’ve talked a lot about common ancestors we’ve seen evidence that...

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