Five Mechanisms of MicroevolutionFive Mechanisms of Microevolution

1. Genetic drift:1. Genetic drift:

Change in the gene pool of a small Change in the gene pool of a small population due to chance.population due to chance.

• Two examples:Two examples:

a. Bottleneck effecta. Bottleneck effect

b. Founder effectb. Founder effect

a. Bottleneck Effecta. Bottleneck Effect

• Genetic driftGenetic drift (reduction of alleles in a population) resulting from a disasterdisaster that drastically reduces reduces population sizepopulation size.

• Examples:Examples:

1.1. EarthquakesEarthquakes

2.2. Volcano’sVolcano’s

b. Founder Effectb. Founder Effect

• Genetic driftGenetic drift resulting from the colonizationcolonization of a new location by a small number of individuals.

• Results in random changerandom change of the gene pool.

• Example:Example:

1.1. Islands (first Darwin finch)Islands (first Darwin finch)

Five Mechanisms of MicroevolutionFive Mechanisms of Microevolution

2. Gene Flow:2. Gene Flow:

TThe gain or loss of allelesgain or loss of alleles from a population by the movementmovement of individuals or gametes.

• Immigration or emigrationImmigration or emigration.

Five Mechanisms of MicroevolutionFive Mechanisms of Microevolution

3. Mutation:3. Mutation:

Change in an organism’s DNA thatChange in an organism’s DNA thatcreates a new allele.creates a new allele.

4. Non-random mating:4. Non-random mating:

The selection of mates other thanThe selection of mates other thanby chance.by chance.

5. Natural selection:5. Natural selection:

Differential reproduction.Differential reproduction.

Modes of ActionModes of Action

• Natural selectionNatural selection has three modesthree modes of action:

1.1. Stabilizing selectionStabilizing selection

2.2. Directional selectionDirectional selection

3.3. Diversifying selectionDiversifying selection

Number ofIndividuals

Size of individualsSmall Large

1.1. Stabilizing SelectionStabilizing Selection

• ActsActs upon extremesextremes and favorsfavors the intermediateintermediate.

Number ofIndividuals

Size of individualsSmall Large

2.2. Directional SelectionDirectional Selection

• FavorsFavors variants of one extremeone extreme.

Number ofIndividuals

Size of individualsSmall Large

3.3. Diversifying SelectionDiversifying Selection

• FavorsFavors variants of opposite extremesopposite extremes.

Number ofIndividuals

Size of individualsSmall Large

SpeciesSpecies

• A group of populationspopulations whose individualsindividuals have the potential to interbreedinterbreed and produce viableviable offspring.

SpeciationSpeciation

• The evolutionevolution of new species.

Reproductive BarriersReproductive Barriers

• Any mechanismmechanism that impedesimpedes two species from producing fertile and/or viable hybrid fertile and/or viable hybrid offspringoffspring.

• Two barriers:Two barriers:

1.1. Pre-zygotic barriersPre-zygotic barriers

2.2. Post-zygotic barriersPost-zygotic barriers

1.1. Pre-zygotic BarriersPre-zygotic Barriers

a. Temporal isolation:a. Temporal isolation:

Breeding occurs at different times for different species.

b. Habitat isolation:b. Habitat isolation:

Species breed in different habitats.

c. Behavioral isolation:c. Behavioral isolation:

Little or no sexual attraction between species.

1.1. Pre-zygotic BarriersPre-zygotic Barriers

d. Mechanical isolation:d. Mechanical isolation:

Structural differences prevent gamete exchange.

e. Gametic isolation:e. Gametic isolation:

Gametes die before uniting with gametes of other species, or gametes fail to unite.

2.2. Post-zygotic BarriersPost-zygotic Barriers

a. Hybrid inviability:a. Hybrid inviability:

Hybrid zygotes fail to develop or fail to reach sexual maturity.

b. Hybrid sterility:b. Hybrid sterility:

Hybrid fails to produce functional gametes.

c. Hybrid breakdown:c. Hybrid breakdown:

Offspring of hybrids are weak or infertile.

Allopatric SpeciationAllopatric Speciation

• Induced when the ancestralancestral population becomes separatedseparated by a geographical barrier.geographical barrier.

• Example:Example:

Grand Canyon and ground squirrels

Adaptive RadiationAdaptive Radiation

• Emergence of numerous speciesEmergence of numerous species from a common ancestorcommon ancestor introduced to new and diverse environments. Usually happens on islands (Galapagos and Hawaiian)

• Example:Example:

Darwin’s FinchesDarwin’s Finches

Sympatric SpeciationSympatric Speciation

• Result of a radical change in the genome that produces a reproductively isolated sub-reproductively isolated sub-populationpopulation within the parent population (rare).

• Example: Plant evolution - polyploidExample: Plant evolution - polyploid

A species doubles it’s chromosome #chromosome # to become tetraploid.

reproductive sub-population

Parent population

Two types of sympatric speciation:

a. autopolyploidy – when the new set of chromosomes belongs to a single species

b. allopolyploidy – when the new set of chromosome comes from another species

Polyploidy is much more common in plants than animals. Ex. Oats, cotton, potatoes, tobacco and wheat (is allohexaploid)

Interpretations of SpeciationInterpretations of Speciation

• Two theories:Two theories:

1.1. Gradualist Model (Neo-Darwinian):Gradualist Model (Neo-Darwinian):

Slow changes in species overtime.

2.2. Punctuated Equilibrium:Punctuated Equilibrium:

Evolution occurs in spurts of relatively rapid change.

Convergent EvolutionConvergent Evolution

• SpeciesSpecies from different evolutionary branchesevolutionary branches may come to resemble one another if they live in very similar environments.very similar environments.

• Example:Example:

1.1. Ostrich (Africa) and Emu (Australia).Ostrich (Africa) and Emu (Australia).

2.2. Sidewinder (Mojave Desert) andSidewinder (Mojave Desert) and

Horned Viper (Middle East Desert)Horned Viper (Middle East Desert)

CoevolutionCoevolution

• Evolutionary changeEvolutionary change, in which one species act as a selective forceselective force on a secondsecond species, inducing adaptations that in turn act as selective force on the firstfirst species.

• Example:Example:

1.1. Acacia ants and acacia treesAcacia ants and acacia trees

2.2. Humming birds and plants with flowers Humming birds and plants with flowers with long tubeswith long tubes

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