Origins of Life on Earth4.7-4.8 Billion Year History

• Evidence from chemical analysis and measurements of radioactive elements in primitive rocks and fossils.

• Life developed over two main phases:– Chemical evolution (took about 1 billion years)

• Organic molecules, proteins, polymers, and chemical reactions to form first “protocells”

– Biological evolution (3.7 billion years)• From single celled prokaryotic bacteria to eukaryotic creatures

to eukaryotic multicellular organisms (diversification of species)

Summary of Evolution of Life

Formationof the

earth’searly

crust andatmosphere

Small organic

moleculesform inthe seas

Large organic

molecules(biopolymers)

form inthe seas

First protocells

form inthe seas

Single-cellprokaryotes

form inthe seas

Single-celleukaryotes

form inthe seas

Variety ofmulticellular

organismsform, firstin the seas and lateron land

Chemical Evolution(1 billion years)

Biological Evolution(3.7 billion years)

DNA

Watson and Crick and their model of DNA

www.chem.ucsb.edu/~kalju/chem110L/public/tutorial/images/WatsonCrick.jpgen.wikipedia.org/wiki/DNA

DNA replication

• The double-helix structure of DNA was discovered in 1953.

• This showed how genetic information is transferred from one cell to another almost without error.

MUTATIONS

• Changes in the structure of the DNA

• Adds genetic diversity to the population

• May or may not be adaptive– Depends on the environment!

Mutation

Mutant fruitfly

• Mutations are rare and often have damaging effects. •Mutations may be caused by radiation, viruses, or carcinogens.

• However, occasional mutations or copying errors can and do occur when DNA is replicated.

•Consequently organisms have special enzymes whose job it is to repair faulty DNA.

Rates of mutation

• Measured by phenotypic effects in humans:– Rate of 10-6 to 10-5 per gamete per generation

• Total number of genes?– Estimates range from about 30,000 to over

100,000!– Nearly everyone is a mutant!

Rates of mutation

• Mutation rate of the HIV–AIDS virus:– One error every 104 to 105 base pairs

• Size of the HIV–AIDS genome:– About 104 to 105 base pairs

• So, about one mutation per replication!

Rates of mutation

• Rates of mutation generally high• Leads to a high load of deleterious (harmful)

mutations

Types of mutations

• Point mutations– Base-pair substitutions– Caused by chance errors during synthesis or repair

of DNA– Leads to new alleles (may or may not change

phenotypes)

Types of mutations

• Gene duplication– Result of unequal crossing over during meiosis– Leads to redundant genes• Which may mutate freely• And may thus gain new functions

Types of mutations

• Chromosome duplication– Caused by errors in meiosis (mitosis in plants)– Common in plants• Leads to polyploidy• Can lead to new species of plants

– Due to inability to interbreed

Transitions are more common than transversions becauseDNA repair enzymes can recognize wrong insertion representing a a transition better than a transversion

Variation

•Some mutations will persist and increase genetic variation within a population.

• Variants of a particular gene are known as alleles. For example, the one of the genes for hair colour comprises brown/blonde alleles.

Natural Selection

en.wikipedia.org/wiki/Image:Mutation_and_selection_diagram.svg

• Mutant alleles spread through a population by sexual reproduction.

• If an allele exerts a harmful effect, it will reduce the ability of the individual to reproduce and the allele will probably be removed from the population.

• In contrast, mutants with favorable effects are preferentially passed on

Selection of dark gene

When faced with a change in environmental condition, a population of a species can get MAD:

•MIGRATE to a more favorable location• ALREADY be adapted •DIE

Natural selection can only act on inherited alleles already present in the population.

Reproductive capacity may limit a population’s ability to adapt

If you reproduce quickly (insects, bacteria) then your population can adapt to changes in a short time.

If you reproduce slowly (elephants, tigers, corals) then it takes thousands or millions of years to adapt through natural selection

What’s Evolution?The change in a POPULATION’S genetic makeup (gene pool) over time (successive generations)

Those with selective advantages (i.e., adaptations), survive and reproduce.

All species descended from earlier ancestor species

4 major mechanisms that drive evolution:

• Natural Selection• Mutation• Gene Flow• Genetic Drift

Three types of Natural Selection• Directional

– Allele frequencies shift to favor individuals at one extreme of the normal range• Only one side of the distribution

reproduce• Population looks different over

time• Stabilizing

– Favors individuals with an average genetic makeup• Only the middle reproduce• Population looks more similar over

time (elim. extremes)• Disruptive (aka Diversifying)

– Environmental conditions favor individuals at both ends of the genetic spectrum • Population split into two groups

Microevolution

Small genetic changes in a population such as the spread of a mutation or the change in the frequency of a single allele due to selection (changes to gene pool)

Four Processes cause Microevolution

Mutation (random changes in DNA—ultimate source of new alleles)

Exposure to mutagens or random mistakes in copyingRandom/unpredictable relatively rare

Natural Selection (more fit = more offspring)Gene flow (movement of genes between pop’s)Genetic drift (change in gene pool due to random/chance events)

Ex: Peppered Moth

http://en.wikipedia.org/wiki/Image:Biston.betularia.7200.jpgen.wikipedia.org/wiki/Image:Biston.betularia.f.carbonaria.7209.jpgen.wikipedia.org/wiki/J._B._S._Haldane

• The Peppered Moth is an example of Natural Selection in action discovered by Haldane

• During the Industrial Revolution the trees on which the moth rested became soot-covered.

• This selected against the allele for pale colour in the population (which were poorly camouflaged from predators) and selected for the dark colour allele.

Microevolution

www.puppy-training-solutions.com/image-files/dog-breed-information.jpg

• The dog is another example of how selection can change the frequency of alleles in a population.

• Dogs have been artificially selected for certain characteristics for many years, and different breeds have different alleles. • All breeds of dog belong to the same species, Canis lupus (the wolf) so this is an example of Microevolution as no new species has resulted.

Dogs are wolves

Macroevolution

Long term, large scale evolutionary changes through which new species are formed and others are lost through extinction.

• Macroevolution is the cumulative result of a series of microevolutionary events– Typically seen in fossil record– Nobody around to see the small, gene pool

changes over time.

Macroevolution

www.ingala.gov.ec/galapagosislands/images/stories/ingala_images/galapagos_take_a_tour/small_pics/galapagos_map_2.jpg

Galapagos finches

• However, if two populations of a species become isolated from one another for tens of thousands of years, genetic difference may become marked.

• If the two populations can no-longer interbreed, new species are born. This is called Macroevolution.

• Darwin’s Galapagos finches are an example of this process in action.

COEVOLUTION: Interaction Biodiversity

• Species so tightly connected, that the evolutionary history of one affects the other and vice versa.