CHAPTER 15.2

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Individuals in nature differ from one another.
Organisms in nature produce more offspring than can survive, and many of those who do not survive do not reproduce.
Because more organisms produce than can survive, each species must struggle for resources.
Each organism is unique, each has advantages and disadvantages in the struggle for existence.
Individuals best suited for the environment survive and reproduce at the most successful rate
Species change over time
Species alive today descended with modification from species that lived in the past

I. POPULATION GENETICS AND
EVOLUTION
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A. Populations, Not Individuals Evolve
Evolution occurs as a populations genes and their frequencies change over time
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Gene pool: all of a populations genes
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Allelic frequency: the percentage of any specific allele (different gene forms) in a gene pool
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Genetic equilibrium: describes the condition of an allele (different gene forms) or genotype in a gene pool (such as a population) where the frequency (number of occurrences of a repeating event per unit of time) does not change from generation to generation.

First generation
Second generation
Phenotype frequency
White = 0
Pink = 0.5
Red = 0.5
Allele frequency
R’ = 0.25
R = 0.75
Phenotype frequency
Allele frequency
White = 0.125
Pink = 0.25
Red = 0.625
R = 0.75
R’ = 0.25

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What are X-
Men?
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If you can have any super power what would it be and why?

I. POPULATION GENETICS AND
EVOLUTION
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B. Changes in Genetic Equilibrium (G.E.)
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Population in G.E. is NOT evolving
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You must disrupt G.E. for a population to evolve
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Things that can disrupt G.E.
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Mutations (any change or random error in a DNA sequence)
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Environmental factors
 Chemicals or radiation exposure
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Genetic drift: the alteration of allelic frequencies by chance events

I. POPULATION GENETICS AND
EVOLUTION
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B. Changes in Genetic Equilibrium
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Genetic equilibrium can also be disrupted by immigration and emigration
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Which is “into”? Which is “out of”?
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Gene flow: the transport of genes by migrating individuals

I. POPULATION GENETICS AND
EVOLUTION
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C. Natural Selection Acts on Variations
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3 DIFFERENT TYPES OF NATURAL SELECTION
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Stabilizing Selection
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Directional Selection
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Disruptive Selection

I. POPULATION GENETICS AND
EVOLUTION
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C. Natural Selection Acts on Variations
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1. Stabilizing Selection:
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Favors the “average” individual in a population
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Larger spiders get eaten, smaller spiders cant find food.
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Normal spiders have the best chance at survivial
Normal variation

I. POPULATION GENETICS AND
EVOLUTION
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C. Natural Selection Acts on Variations
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2. Directional Selection:
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Favors “one direction or the other direction”
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Longer beaks have a better chance of survival if food is deep inside the tree
Selection for longer beaks

I. POPULATION GENETICS AND
EVOLUTION
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C. Natural Selection Acts on Variations
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3. Disruptive Selection:
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Favors “both directions”
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Both extremely light and extremely dark colored bugs can survive.
Normal variation

I. POPULATION GENETICS AND
EVOLUTION
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C. Natural Selection Acts on Variations
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All of these types of natural selection will favor one specific trait in a species.
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This selection can lead to the “evolution of a new species”

II. THE EVOLUTION OF SPECIES
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Speciation: the evolution of a new species
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Occurs when members of a similar population can no longer interbreed and produce fertile offspring
A. Physical Barriers that Can Prevent
Interbreeding
Geographic isolation: occurs whenever a physical barrier separates a species
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Can be a land barrier or an ocean barrier
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New species evolve when G.I. occurs

II. THE EVOLUTION OF SPECIES
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B. Reproductive Isolation can Result in Speciation
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Reproductive isolation: occurs when formerly interbreeding organisms can no longer mate and produce fertile offspring (P.F.O.)
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C. A Change in Chromosome Numbers and
Speciation

II. THE EVOLUTION OF SPECIES
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D. Speciation Rates
Gradualism: is the idea that species originate through a “gradual” change over time.
Punctuated equilibrium: argues that speciation occurs relatively quickly, in rapid bursts, with long periods of genetic equilibrium in between.
 due to: environmental changes and introduction of new species into an environment

III. PATTERNS OF EVOLUTION
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A. Diversity in New Environments
2 types of evolution associated with being moved to a new environment
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Adaptive radiation
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Divergent evolution

Patterns of Evolution - Adaptive
Radiation
1.
Adaptive Radiation:
When ancestral species evolve into an array of species to fit a number of diverse habitats.
Darwin’s study on
“finches” *(a type of bird; he compared the shapes of their beaks)*

III. PATTERNS OF EVOLUTION –
Divergent Evolution
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A. Diversity in New
Environments
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2. Divergent Evolution:
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Is s type of “adaptive radiation”
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Is a pattern of evolution in which similar species diverge and become increasingly distinct/different.
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Occurs when a population changes as they adapt to different environments

III. PATTERNS OF EVOLUTION
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B. Different Species can Look Alike
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Convergent evolution: a pattern of evolution where distantly related organisms evolve similar traits/characteristics
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Occurs when unrelated species occupy similar environments
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Polar bear and a “Snowy Owl” or “White Fox”