Review: Radiometric Dating

If Uranium-239 isotope has a half-life of 80 000 years, how many years would it take for all but 1/8 of the uranium to decay?

100% Ur-239 ½ ¼

1/8

First Half-life Second Half-life Third Half-life

80 000 yrs 80 000 yrs 80 000 yrs

= 240 000 yearsIt takes 240 000 years for Uranium-239 to decay to 1/8 of its original amount (3 half-lives).

Evidence for Evolution II: Biogeography,

Homologous, Analogous and Vestigial Features

Biogeography: the scientific study of the geographic distribution of organisms based on both living species and fossils

Biogeography

Remember Pangaea–Fossils of the same species are found

on entirely different continents – Africa, India, and Antarctica

Fossil Distribution

Break up of Pangaea

• Mesosaurus – fossils found in both South America and Africa

• Lived during the Permian, when Africa and South America were joined

Fossil Distribution

• Similar species are found on different continents• Share a common ancestor that lived when the

continents were connected• When the continents separated, the populations

were isolated and they diverged into different species.

• Example: Different species of elephants are found in Africa and India

Current Distribution of Species

Current Distribution of Species

Indian Rhino African Rhino

Old World Monkeys (Africa/Asia)

New World Monkeys (America’s)

African Lion

American PumaIndian Tiger

Big Cats

Monkeys

Rhinos

• All of these species share common ancestors but are found far from each other today

• By looking at their current distribution and comparing it with the fossil record, we can establish both current and ancient biogeography

The Link with Biogeography

Example: Australian Marsupials• Australia has the largest and most diverse

population of marsupials in the world• Most other continents mostly have placental

mammals• Why is this??

Australian Marsupials• Marsupials evolved before placental mammals• When placental mammals began outcompeting

marsupials on other continents Australia had already separated and was isolated from them

• This allowed marsupials to diversify and evolve on Australia without competition from placental mammals

Homologous Features

“What can be more curious than the hand of a man, formed for grasping, that of a mole for digging, the leg of a horse, the paddle of a porpoise, and the wing of a bat, should all be constructed on the same pattern, and should include the same bones, in the same relative positions?”

-Darwin

Homologous Features• A flipper or a wing has

no use for fingers, so why should they have a five-fingered structure?

• Why should giraffes have the same number of neck bones as humans?

• If these features were built from scratch, there are more efficient ways to do it

• Homologous Feature: A structure with a common evolutionary origin that may serve different functions in modern species

Homologous Features

• In the forearms of different mammalian species, the same features can be seen

Homologous Features in Mammals

Homologous Features in Mammals

• We can use homologous features to “define” groups of organisms

• In fact those hallmark features of mammals are also homologous:– Live-birthed young–Milk-producing

mammary glands– Hair

• For species to share a homologous feature, they must share a common ancestor

All in the Family

• Leaves and cactus spines

Examples of Homology

• Nest building behaviour

Examples of Homology

• Genes that Code for Eyes

Examples of Homology

• Embryos

Examples of Homology

• Embryos

Examples of Homology

• Embryos

Examples of Homology

• Some homologous features indicate common ancestry, but are no longer used

Some Features go out of Style

• Analogous Feature: a structure that performs the same function as another but is not similar in origin or anatomical structure

Analogous Features

• We observe similarities between unrelated species

• Consider dolphins and sharks

Similarities in Unrelated Species

• We can expect to see analogous features between species that fill similar ecological niches

Analogous Features

• Wings

Examples of Analogous Features

Evolution of Flight or Near-Flight

• Jointed Legs

Examples of Analogous Features

• Beaks in squid and birds

Examples of Analogous Features

• Vestigial Feature: A non-functioning, or only marginally functioning, structure that is homologous to a full functioning structure in closely related species

Vestigial Features

• Vestigial features are homologous features– They just aren’t used by that species any more

Vestigial Features

• Dandelion Sex Organs

Examples of Vestigiality

• Hind Legs and Hips in Whales

Examples of Vestigiality

• Wings on Flightless Birds : Kakapo

Examples of Vestigiality

• When a feature is not used, it is more likely to become vestigial in time

• This can only happen if lack of use does not hurt a population’s chance of survival

• Birds can only become flightless if they have no need for wings

Why do Features become Vestigial?

• Birds can only become flightless if they have no need for wings– In the case of the

Kakapo, what might we guess about predators?

• No predators in New Zealand

Why do Features become Vestigial?

Assignment