8/19/2014

1

Key Events in Life’s History

O2 accumulates in

atmosphere

(2.7 bya)

Humans

(200,000)

Evidence for Evolution

Chapter 22 Pg. 460 - 468

Evidence From Various Areas

• Paleontology

• Biogeography

• Embryology

• Comparative anatomy

• Molecular biology

• Direct observation

Evolution is consistent with all scientific observations!

Paleontology

• Fossils reveal prehistoric existence of extinct species

– We can study changes in and the formation of species

• Fossil deposits are often found among sediment layers (strata), where the deepest fossils represent the oldest specimens

– Example: fossil oysters removed from successive layers of

sediment show gradual changes in the size of the oyster shell

alternating with rapid changes

in shell size

– Large, rapid changes produced

new species

Fossils

Oldest vertebrate fossils are fish, then amphibians, reptiles, mammals and birds

Consist of mineralized hard body structures

Organic specimens are rare in fossils but found in amber, frozen, tar pits

Incomplete record – many organisms not preserved, fossils destroyed, or not yet found

However, many “missing links” have been found – Lucy, Artie, “fish” with legs

8/19/2014

2

Missing Links?

• Paleontologists have discovered many transitional forms that link older fossils to modern species, such as the transition from Eohippus to the modern horse Equus

• Archaeopteryx is a fossil that links reptiles and birds

• The fossil record also indicates that all the organisms alive today are only a tiny fraction of all the organisms that ever lived – Most life that existed on Earth in

the past went extinct

Determining Age of Fossils

• Radiometric dating and half-life accurately measure the age of fossils

– Prokaryotes are the oldest fossils, and thus thought to be the first organisms to develop on Earth

• The age of many fossils can be determined using C-14 dating

– In this procedure, the natural decay rate of a radioactive isotope of carbon (C-14) is used to determine the age of the fossil

Both used to date fossils and determine age

Relative Dating Radiometric Dating

Uses order of rock strata to determine relative age of fossils

Measure decay of radioactive isotopes present in layers where fossils are found

Half-life: # of years for 50% of original

sample to decay

Geologic Time Scale Eon Era Period Epoch

(longest to shortest)

Present Day: Phanerozoic Eon, Cenozoic Era, Quaternary Period, Holocene Epoch

8/19/2014

3

Major events during each Era Precambrian: microscopic fossils (stromatolites)

Photosynthesis, atmospheric O2

Eukaryotes (endosymbiont theory)

Paleozoic: Cambrian Explosion

Plants invade land, many animals appear

Permian Extinction (-96% species)

Mesozoic: “Age of Reptiles”, dinosaur, plants

Formation of Pangaea supercontinent

Cretaceous Extinction – asteroid off Mexico’s coast

Cenozoic: Primates

Note: All end with major extinction & start with adaptive radiation

Mass extinctions Diversity of life

• Major periods in Earth’s history end with mass extinctions and new ones begin with adaptive radiations

Biogeography • Uses geography to describe the distribution of

species

• Unrelated species in different regions of the world look alike when found in similar environments

• Different rainforests have different species, but they are very similar to each other

• Species are most closely related to those near them

• Species tend to be very similar to the fossils found in their area, with some changes

• This provides strong evidence for the role of natural selection in evolution

Plate Tectonics • Theory of plate tectonics: continents and oceans

rest on giant plates of Earth’s crust that float on top of the hot mantle

• Convection currents in the mantle cause the slow, continuous movement of the plates

– This is continental drift, and it causes mountains to form as plates collide

• The cumulative effect of plate movement over millions of years has changed the flora and fauna of the Earth

– About 250 million years ago, plate movement brought all the previously separated land masses together into one supercontinent called Pangaea

– Ocean basis became deeper, which lowered sea level and rained shallow waters. Many species went extinct.

Pangaea = Supercontinent • Formed 250 mya • Continental drift explains

many biogeographic puzzles

Movement of continental plates change geography and climate of Earth Extinctions and speciation

8/19/2014

4

Continental Drift

• Continental drift has changed the distributions of life – Example: marsupials are almost exclusively in Australia

while other continents have eutherians (true placental mammals)

• Fossil evidence suggests that marsupials originated in what is now Asia and reached Australia via South America and Antarctica while the continents were still joined

• When the continents broke apart and moved to different climates, Australia carried both marsupials and eutherians

• In Australia the marsupials diversified, filling every available niche, while true placental mammals, which were not adapted to the Australian environment and climate, went extinct

• On other continents the opposite occurred – only true placental mammals survived and diversified, while marsupials went extinct

Rodents and Biogeography • Rabbits did not exist in Australia until introduced by

humans – a native Australian hare wallaby resembles a rabbit both in structure and habit

– As similar as these two animals appear, they are not that closely related

• The rabbit is a placental mammal – the wallaby is a marsupial mammal

– The fetus of a placental mammal develops in the female uterus and gets nourishment from the mother.

– The fetus of a marsupial leaves the mother's uterus at an early stage of development and completes the remaining development while attached to a teat in the abdominal pouch

• The great similarity of the rabbit and the wallaby is the result of natural selection.

Embryology • Reveals similar stages in development (ontogeny)

among related species, especially early in development before lots of specialization

• The similarities help establish evolutionary relationships (phylogeny).

• Example: tails are found in fish, chicken, pig, and human embryos.

• Example: all vertebrate embryos go through a stage in which they have gill pouches on the sides of their throats. – In fish, they develop into

gills. – In mammals, they develop

into Eustachian tubes in the ears.

Comparative Anatomy The anatomies of related species are similar,

even if they are used differently i.e. bones of humans, cats, whales, and bats are

similar

It seems unlikely a designer would make a bat's wing similar to a whale's fin

Describes two kinds of structures that contribute to the identification of evolutionary relationships among species:

– Homologous structures (homologies)

– Analogous structures (analogies)

Homologous Structures (Homologies)

• Body parts that resemble one another in different species because of a common ancestor

• Anatomy may be modified for survival in specific environments – Homologous structures may LOOK different but will

resemble one another in PATTERN (how they are put together)

– Example: forelimbs of cats, bats, whales, and humans are homologous because they have all evolved from a common ancestral mammal

• In some species, homologous structures have become vestigial (no longer serve any function) – Remnants of limbs in snakes,

hind limbs in whales, and wings of flightless birds

– Vestigial structures show evidence of evolutionary heritage

Analogous Structures (Analogies)

Body parts that resemble one another in different species as a result of independently evolving in similar environments

Example: fins and body shapes of sharks, penguins, and porpoises are analogous because they are

adaptations to swimming – they do not share a common ancestor

8/19/2014

5

Vestigial Organs

Leftover body parts that do not help a species

i.e. whales still have pelvises and remnants of leg bones

These structures serve no purpose in the modern animal

Molecular Biology • Examines the nucleotide and amino acid

sequences of DNA and proteins from different species

• Closely related species share higher percentages of sequences than species distantly related

• All living thing share the same genetic code and, with minor variations, the same basic biochemical pathways

– Includes replication, protein synthesis, respiration, and photosynthesis

• This data strongly favors evolution of different species through modification of ancestral genetic information

– More than 98% of the nucleotide sequences in humans chimps are identical

More on Molecular Biology… • Organisms that have a common ancestor will have

common biochemical pathways – The more closely related the organisms are to each

other, the more similar their biochemistry. – Ex: humans and mice, both mammals, are so closely

related that researchers can test new medicines on mice and extrapolate the results to humans

• Since all aerobic (requires O2) organisms contain cells that carry out aerobic cell respiration, they all contain the polypeptide cytochrome c. – A comparison of the amino acid

sequence of cytochrome c among different organisms shows which are and are not closely related • Cytochrome C in human cells is

almost identical to the chimps’ and gorilla’s, but differs from that of a pig.

Evolution of new forms results from changes in DNA or regulation of developmental genes

Heterochrony: evolutionary change in rate of developmental events

Paedomorphosis: adult

retains juvenile structures

in ancestral species

• Homeotic genes: master regulatory genes determine location and organization of body parts; e.g. Hox genes

Hox gene expression

and limb development.

Evolution of Hox genes

changes the insect body plan.

8/19/2014

6

Direct Observation There are some direct observations of

evolutionary change in our lifetime

Examples

Insect populations can rapidly become resistant to pesticides such as DDT

Evolution of drug-resistant viruses and antibiotic- resistant bacteria