Evidence for Evolution

Evidence for Evolution

• Darwin argued that living things have been evolving on Earth for millions of years. Evidence for this process could be found:– In the geographical distribution of living

species– In the homologous structures of living

organisms– In the similarities in early development– In the fossil record

includes

Evidence of Evolution

Physical remains of organisms

Common ancestral species

Similar genes Similar genes

which is composed of which indicates which implies which implies

The fossil recordGeographic

distribution of living species

Homologous body structures

Similaritiesin early

development

Evidence of Evolution

Geographic Distribution of Living Species

• Similar animals in different locations are the product of different lines of evolutionary descent

• Because some animals on different continents live under similar ecological conditions, they are exposed to similar pressures of natural selection. Because of this, different animals evolve certain features in common even though each descended from different ancestors.

Geographic Distribution of Living Species

Beaver

NORTH AMERICA

Muskrat

Capybara SOUTH AMERICA

Coypu

Beaver

Muskrat

Beaver andMuskrat

Coypu

Capybara

Coypu andCapybara

Homologous Structures of Living Species

• Similarities among bones of different vertebrates, such as reptiles, birds, and mammals. Some limbs are arms, others are wings, legs, or flippers

• Limbs differ greatly in form and function, yet are made from the same bones

• Each of these limbs has adapted in ways that enable organisms to survive in different environments

• Structures that have different mature forms, but develop from the same embryonic tissues are homologous structures

Homologous Body Structures

Turtle Alligator Bird Mammals

Typical primitive fish

Homologous Structures of Living Species (continued)

• Not all homologous structures serve important functions. These organs are reduced in size and are traces of homologous organs of other species = vestigial organs.– They probably served a purpose a long time ago– Ex. Whales have a pelvis (hip bone), but obviously don’t have legs = at

one point, they probably walked– Ex. Humans have a tail bone, but obviously no tail = at one point, they

probably had a tail– Ex. Humans have an appendix (part of the digestive tract) that doesn’t

do anything = at one point, it probably did

Similarities in Early Development

• Early stages (embryos) of many animals with backbones are so similar they can be hard to tell apart

• Embryonic cells develop in similar order = homologous structures

Fossil Record and Evidence

• Fossil – remains of ancient life• Different layers of rock form at different times in

Earth’s history• Fossils that had formed in the different layers of

rock were evidence of gradual change over time• Fossil record – provides evidence about the

history of life on Earth. It also shows how different groups of organisms have changed over time

• 99% of all species that ever lived on Earth have become extinct

How Fossils Form

• Not all organisms that die become fossils– 1) Organism dies and is buried in sediment– 2) Soft tissues of the organism decay– 3) Sediment becomes new rock

Water carries small rock particles to lakes and seas.

Dead organisms are buried by layers of sediment, which forms new rock.

The preserved remains may later be discovered and studied.

Interpreting Fossil Evidence

• Paleontologists determine the age of fossils using two techniques– Relative dating– Radioactive dating (aka absolute dating)

Relative Dating• Average age of fossil is determined by

comparing its placement with that of fossils in other layers of rock

• Rock layers form in order by age – oldest layers on the bottom and more recent layers on top. What does this mean about the fossils found there?

• Index fossils used to compare the relative ages of fossils. Index fossils are organisms that are easily recognized, existed for a short period, and had a wide geographic range

• ****Relative dating provides NO information about a fossil’s exact age in years****

A trilobite – an index fossil – it lived everywhere about 540 mya

Radioactive Dating (aka Absolute Dating)

• ****Used to assign exact ages to rocks and fossils****• Radioactive elements in rocks decay at a steady rate

measured in a unit called half-life• Half-life – length of time required for half of the

radioactive atoms in a sample to decay• In radioactive dating, scientists calculate the age of a

sample based on the amount of remaining radioactive isotopes it contains

• Different radioactive elements have different half-lives and therefore provide natural clocks that “tick” at different, but constant rates

Relative Dating

Can determine

Is performed by

Drawbacks

Absolute Dating

Comparing Relative and Absolute Dating of Fossils

Imprecision and limitations of age data

Difficulty of radioassay laboratory methods

Comparing depth of a fossil’s source stratum to the position of a reference fossil or rock

Determining the relative amounts of a radioactive isotope and nonradioactive isotope in a specimen

Age of fossil with respect to another rock or fossil (that is, older or younger)

Age of a fossil in years