EvolutionEdited by L. Bridge

May 2015

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.1

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Earth’s Age

Earth formsEarliest life forms

The Concept of Evolution• In biology evolution is simply defined as biological changes

that have occurred in living organisms since the beginning of life.

• Evolution is “descent with modification”, which is possible because of the changeability/malleability of the DNA code.– “Modifications” = random mutations can constantly change

genotypes/phenotypes from generation to generation = novel variations of traits in the population

• Evolution assumes a certain relatedness (“common ancestry”) between organisms.

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Evolution “for dummies 9:58

http://www.youtube.com/watch?v=SeTssvexa9s

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Phylogeny: the study of the evolutionary history of groups of organisms

Plant evolution(cladogram)Like a “family tree”

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Primates

Fossils as Evidence of Evolution– Hard body parts are preserved

in most cases

– Often embedded in sedimentary rock

– Deposited in layers called strata• Each stratum is older than the one

above and younger than the one below

– “Transitional fossils”• Especially significant finds;

represent evolutionary links between groups 6

However fossilization is a relatively uncommon occurrence, usually requiring hard body parts and death near a site where sediments are being deposited, the fossil record only provides sparse and intermittent information about the evolution of life.

trilobite

Transitional Fossilsexample: Archaeopteryx

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Vertebrate groups

New Fossils Of Extremely Primitive 4-Legged Creatures Close The Gap Between Fish And Land Animals

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ScienceDaily (June 27, 2008) — New exquisitely preserved fossils from Latvia cast light on a key event in our own evolutionary history, when our ancestors left the water and ventured onto land. Swedish researcher Per Ahlberg from Uppsala University and colleagues have reconstructed parts of the animal and explain the transformation in the new issue of Nature.

Tiktaalik

Geological Evidence of Evolution

• Geological Timescale– History of Earth is divided into eras, then periods, and then

epochs– Based on dating of fossil evidence

– Relative dating method – determines the relative order of fossils because the fossil-containing sedimentary rocks occur in layers. Top layers are younger and thus the fossils in them are younger. Thus it is a matter of “What came first, second, third”

– Absolute dating method - radioactive carbon dating (radiometric dating): Makes use of radioactive elements that decay into other more stable elements according to a strict timetable.

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Radioisotope Dating• Ex: Radioactive 14C (In Organic Matter) changes into 14N (1/2

of 14C will change into 14N in 5,730 yrs)

• Assuming that organic matter always begins with the same amount of 14C.

• Goal is to compare the 14C radioactivity of the fossil to that of a modern sample of organic matter. The amount of radiation left in the fossil can be converted to the age of the fossil.

• Other similar methods are used to date rocks that are thought to be billions of years old.

ex: Starting with 8g of 14C, down to 1g; how much time has elapsed?ans: 8 4 2 1 = 5730 + 5730 + 5730 = 17,190 yrs

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Parent Daughter Change in…

Carbon-14 Nitrogen-14 5730 years

Uranium-235 Lead-206 4,470 million yrs

Potassium-40 Argon-40 1,208 million yrs

Thorium-232 Lead-208 14,010 million yrs

Rubidium-87 Strontium-87 48,800 million yrs

Commonly used radioisotopes for dating:

Geological/Fossil Evidence of Evolution

• Background extinctions

• “Mass Extinctions”– Large numbers of species become extinct in a short period of time

• Remaining species may spread out and fill habitats left vacant

– Five Major Extinctions have occurred

– It is proposed that many mass extinctions have resulted from

extra-terrestrial events, volcanism, atmospheric fluctuations, global

warming, cooling (ice ages), sea levels, etc.• However, a current SIXTH one is in progress due to human

encroachment

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The last major mass extinction• The K-T Event

– 65 million yrs ago– Killed off the dinosaurs, among others– Marked the end of the Cretaceous period,

beginning of Tertiary• Clay from that period is high in iridium, an

element in meteorites• Proposed that meteorites hit Earth and dust

filled the atmosphere– Blocked sunlight, plants died– One reason why we don’t see some of the

evolutionary ancestors today is because of mass extinctions. 15

Biogeographical Evidence of EvolutionBiogeography is the study of the geographic

distribution of species throughout the world– The Earth has six biogeographical regions

• Each has its own distinctive mix of species– Barriers prevented evolving species from migrating to other

regions

– Continental Drift-• The positions of continents and oceans has shifted

through time• The distribution of fossils and existing species allows us

to determine approximate timeline• Example: oldest camel fossils 45-40 mya in N. America!

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Continental Drift

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Distribution of Large Cats

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Evidence of Evolution: Anatomical Evidence

• Common descent offers explanation for anatomical similarities

• Homologous Structures– Same underlying

structure, adapted for different functions, indicating a common ancestor• Ex: human arm and

whale forelimb20

Analogous Structures

Same basic function but different origins.

Underlying anatomy is different, thus unlikely to have evolved from common ancestor

ex: wing of bird and wing of an insect

Not a good indicator of relatedness

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Anatomical Evidence: Vestigial Structures

Anatomical structures fully functional in one group and reduced, nonfunctional in anotherEx: Modern whales have a pelvic girdle and hind leg bones

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Probable origin of appendix

Gastric caecum in herbivores

Evidence of evolution: Embryological Development

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Biochemical Evidence of Evolution

– All organisms use same basic biochemical molecules• DNA coding: A,T,C,G• Amino acid sequence of proteins

– Many developmental genes are shared (homeotic)

– Degree of similarity between DNA base sequences (and amino acid sequences of similar protein) indicates the degree of relatedness

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• Canis lupus familiaris• Canis lupus lupus• Canis lupus dingo• Vulpes vulpes• Canis rufus

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Compare a homologous (shared) gene sequence.

This can be used to quantify similarities/differences. Provides verifiable data.

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…Or can use a shared protein sequence for comparison

Muscle myosin sequence

The Molecular Clock

• Based on the molecular clock hypothesis (MCH), this relates the amount of time since two species diverged to the number of molecular differences measured between the species' DNA or protein sequences or proteins.

• It is sometimes called a “gene clock” or “evolutionary clock”.

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The Molecular Clock