MECHANISMS OF EVOLUTION

CHAPTER 15.2

I. POPULATION GENETICS AND EVOLUTION A. Populations, Not Individuals

Evolve Evolution occurs as a populations genes

and their frequencies change over time Gene pool: all of a populations genes Allelic frequency: the percentage of any

specific allele in a gene pool Genetic equilibrium: when the

frequency of a populations allele remains the same over time

Calculating Allelic Frequency

Phenotype frequency

Allele frequency

First generation

Second generation

White = 0 R’ = 0.25

Pink = 0.5

Red = 0.5

R = 0.75

Phenotype frequency

Allele frequency

White = 0.125

Pink = 0.25

Red = 0.625

R = 0.75

R’ = 0.25

I. POPULATION GENETICS AND EVOLUTION B. Changes in Genetic Equilibrium Population in G.E. is NOT evolving You must disrupt G.E. for a population to

evolve Things that can disrupt G.E.

Mutations Environmental factors

Chemicals or radiation exposure Genetic drift: the alteration of allelic

frequencies by chance events

I. POPULATION GENETICS AND EVOLUTION B. Changes in Genetic Equilibrium Genetic equilibrium also disrupted by

immigration and emigration Which is “into”? Which is “out of”?

Gene flow: the transport of genes by migrating individuals

I. POPULATION GENETICS AND EVOLUTION C. Natural Selection Acts on

Variations 3 DIFFERENT TYPES OF NATURAL

SELECTION Stabilizing Selection Directional Selection Disruptive Selection

I. POPULATION GENETICS AND EVOLUTION C. Natural Selection Acts on

Variations 1. Stabilizing Selection:

Favors the “average” individual in a populationSelection for average size spiders

Normal variation

I. POPULATION GENETICS AND EVOLUTION C. Natural Selection Acts on

Variations 2. Directional Selection:

Favors “one direction or the other direction”

Normal variation

Selection for longer beaks

I. POPULATION GENETICS AND EVOLUTION C. Natural Selection Acts on

Variations 3. Disruptive Selection:

Favors “both directions” Selection for light limpets

Normal variation

Selection for dark limpets

I. POPULATION GENETICS AND EVOLUTION C. Natural Selection Acts on

Variations All of these types of natural selection will

favor one specific trait in a species This selection can lead to the “evolution

of a new species”

II. THE EVOLUTION OF SPECIES Speciation: the evolution of a new species

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 Can be a land barrier or an ocean barrier New species evolve when G.I. occurs

II. THE EVOLUTION OF SPECIES B. Reproductive Isolation can Result in

Speciation Reproductive isolation: occurs when

formerly interbreeding organisms can no longer mate and produce fertile offspring (P.F.O.)

C. A Change in Chromosome Numbers and Speciation

Polyploid: any individual or species with a multiple of the normal set of chromosomes Results from an error in “meiosis”

II. THE EVOLUTION OF SPECIES 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 A. Diversity in New Environments 2 types of evolution associated with being

moved to a new environment Adaptive radiation Divergent evolution

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 A. Diversity in New Environments 2. Divergent Evolution:

Is s type of “adaptive radiation” Is a pattern of evolution in which similar

species diverge and become increasingly distinct/different.

Occurs when a population changes as they adapt to different environments

III. PATTERNS OF EVOLUTION B. Different Species can Look Alike Convergent evolution: a pattern of

evolution where distantly related organisms evolve similar traits/characteristics

Occurs when unrelated species occupy similar environments

Polar bear and a “Snowy Owl” or “White Fox”

TODAYS ASSIGNMENT

Pg. 133 #1-4 Pg. 137 #1-4 Pg. 142 #1-4 Pg. 146 #1-3 Pg. 147 #1-6 Should be a total of 21 questions Turn in to me before you leave

HOMEWORK

Due tomorrow as soon as you walk in the door.

Pg 1 1-25

Pg. 83 1-7

Pg 117 1-10