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 Evidence Remember: 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 Questions Page 355 in text # 4, 7, 8bcdef Page 351-352, build the cladogram