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NATURAL SELECTION Explaining Evolution
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THE ORIGIN OF SPECIES
Darwin published “On the Origin of Species” in 1859
He had spent 20 years amassing evidence and developing his theory of NATURAL SELECTION
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NATURAL SELECTION The way in which nature favours the
reproductive success of some individuals within a population over others
Over time the population changes as advantageous heritable characteristics become more common generation after generation
Evolution is the result of natural selection occurring over many generations
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“SURVIVAL OF THE FITTEST”
A term coined by Herbert Spencer to describe the process of natural selection
The key to natural selection is an individual’s REPRODUCTIVE SUCCESS and its ability to ADAPT
ADAPTATION: a characteristic or feature of a species that makes it well suited for reproductive success and survival
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EVALUATING A THEORY
Darwin’s theory of evolution by means of natural selection is able to: Explain how adaptation can arise in a species
Make predictions about the future evolution of a species
Be a testable scientific theory
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MODERN THEORY OF EVOLUTION
Evolutionary biology has made tremendous advances since Darwin because of the following: RADIOMETRIC DATING MODERN EVOLUTIONARY SYNTHESIS
PALEONTOLOGY
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RADIOMETRIC DATING
Involves the use of radioisotopes to obtain precise estimates of the ages of rocks
RADIOISOTOPE: an atom with an unstable nucleus that is capable of undergoing radioactive decay
HALF-LIFE: the time required for half the quantity of a radioactive substance to undergo decay; the half-life is constant for any isotope
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MODERN EVOLUTIONARY SYNTHESIS
The modern theory of evolution that takes into account all branches of Biology
Involves changes in the GENE POOL of a species over time
GENE POOL: the complete set of all alleles contained within a species or a population
RECALL: genetic mutations provide a continuous supply of new heritable information and variation within a species
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PSEUDOGENES Vestigial genes that no
longer code for functioning proteins
Are found in virtually all species EXAMPLE: Dolphins
have genes that code for smell however they have no need for a sense of smell
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MODERN PALEONTOLOGY
We have made many important discoveries over the last 100 years: Fossils of early human ancestors in
Pakistan Feathered dinosaurs in China
PLATE TECTONICS: the scientific theory that describes the large scale movements and features of Earth’s crust
Explains species distributions
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TYPES OF SELECTION There are FOUR types of
selection:1. DIRECTIONAL2. STABILIZING3. DISRUPTIVE4. SEXUAL
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DIRECTIONAL SELECTION
A selection that favours an increase or decrease in the value of a trait from the current population average
Common in artificial selection where individuals with an enhanced trait are selected
EXAMPLE: A habitat with long flowers will favour hummingbirds with longer bills, and thus create future generations of birds with longer bills
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STABILIZING SELECTION
A selection against individuals exhibiting traits that deviate from the current population average
EXAMPLE: An environment with medium length flowers will select against long billed or short billed hummingbirds
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DISRUPTIVE SELECTION
A selection that favours two or more variations of a trait that differ from the current population average
EXAMPLE: An environment with long and short flowers will select against hummingbirds with medium sized bills
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SEXUAL SELECTION The favouring of
any trait that specifically enhances the mating success of an individual
Results in males and females of a species differing in behaviour and appearance
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SELECTION AT WORK EXAMPLE: Polar bears and white fur
Ability to sneak up on seals on snow covered ice
EXAMPLE: Wolves keen sense of smell Ability to locate and track the
movements of prey EXAMPLE: A human’s large brain
Ability to reason and communicate
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GENETIC DRIFT Changes to allele frequency as a result
of chance Such changes are much more
pronounced in small populations Can result in the allele becoming very
common or disappearing entirely over a number of generations
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GENETIC BOTTLE NECK
A dramatic, often temporary reduction in population size
Usually results in significant genetic drift and a loss of genetic diversity
EXAMPLE: Cheetahs
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HARDY-WEINBERG PRINCIPLE
In large populations in which only random chance is at work, allele frequencies are expected to remain constant from generation to generation
Evolution occurs: If natural selection occurs In a small population If there is a mutation There is immigration or emigration If there is a gaining of new alleles from a
different species
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HUMAN INFLUENCE EXAMPLE: Commercial fishing
The alleles that code for large adult sized cod are being lost
EXAMPLE: Insecticide use Bedbugs are becoming resistant to
pesticides EXAMPLE: Antibiotic and antimicrobial use
Many infectious bacteria (MRSA) are becoming resistant to a variety of antibiotics