The principles of evolution are rooted in population genetics

Population Genetics and Evolution

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Population Genetics and Population Genetics and EvolutionEvolution

A A populationpopulation consists consists of all the individuals of of all the individuals of a species that live a species that live together in one place at together in one place at one time andone time and can can successfully breedsuccessfully breed..

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Population characteristicsPopulation characteristics1.1. Population sizePopulation size~ ~ numbernumber of of

individuals in a populationindividuals in a population2.2. DensityDensity ~~ number of number of

individuals per unit of areaindividuals per unit of area3.3. DispersionDispersion~~ pattern of spacing pattern of spacing

••randomrandom ~ unpredictable, ~ unpredictable, patternless spacing (a) patternless spacing (a)

••clumpedclumped ~~ patchy patchy spacingspacing (b) (b)

••uniformuniform ~ even spacing (c)~ even spacing (c)

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Factors that affect PopulationsFactors that affect Populations BirthrateBirthrate (natality)~ (natality)~ number of offspring number of offspring

producedproduced

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Factors that affect PopulationsFactors that affect Populations Death rateDeath rate (mortality)-how many people (mortality)-how many people

diedie

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Factors that affectFactors that affect PopulationsPopulations

Age structureAge structure~ relative number of ~ relative number of individuals of each ageindividuals of each age

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Factors that affect PopulationsFactors that affect Populations

Survivorship Survivorship curvecurve~ plot of ~ plot of numbers still numbers still alive at each alive at each ageage

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Carrying capacityCarrying capacity is maximum number of is maximum number of individuals a habitat can supportindividuals a habitat can support

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Population limiting Population limiting factorsfactors

Density-Density-dependent dependent factorsfactors CompetitionCompetition PredationPredation stress/crowding stress/crowding waste accumulationwaste accumulation

Density-Density-independentindependent factorsfactors••weather/climateweather/climate••periodic disturbancesperiodic disturbances

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Population “strategies”Population “strategies”r-selectedr-selected opportunisticopportunistic Short lifespanShort lifespan ManyMany small offspring small offspring reproduces oncereproduces once nono parental care parental care High death rateHigh death rate

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Population “strategies”Population “strategies” K-selected K-selected LongLong lifespan lifespan Few large offspring Few large offspring several several

reproductions late reproductions late in life in life

extensive parental extensive parental carecare

LowLow death rate death rate

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Population Growth ModelsPopulation Growth Models ExponentialExponential model model (blue)(blue) ((JJ-curve)-curve) r-selected r-selected

LogisticLogistic model model (red)(red) ((SS-curve)-curve) K-selected speciesK-selected species

Changes in genetic equilibriumAnother mechanism that disrupts a population’s genetic equilibrium is genetic drift— the alteration of allelic frequencies by chance events.

Changes in genetic equilibrium

• The movement of genes by migration is called gene flow.•When an individual leaves a population, its genes are lost from the gene pool. •When individuals enter a population, their genes are added to the pool.

Natural selection acts on variations• Some variations increase or decrease an organism’s chance of survival in an environment. •There are three different types of natural selection that act on variation:• stabilizing selection• directional selection• disruptive selection

• Stabilizing selection is a natural selection that favors average individuals in a population.

Selection for average size spiders

Normal variation

Natural selection acts on variations

Natural selection acts on variations• Directional selection occurs when natural selection favors one of the extreme variations of a trait.

Normal variation

Selection for longer beaks

Natural selection acts on variations• In disruptive selection, individuals with either extreme of a trait’s variation are selected for.

Selection for light limpets

Normal variation

Selection for dark limpets

Natural selection acts on variations

• Natural selection can alter the genetic equilibrium of a population’s gene pool over time. •Significant changes in the gene pool could lead to the evolution of a new species over time.

• In nature, physical barriers can break large populations into smaller ones.• Geographic isolation occurs whenever a physical barrier divides a population. •A new species can evolve when a population has been geographically isolated.

The Evolution of Species

• When geographic isolation divides a population of tree frogs, the individuals no longer mate across populations.

•Tree frogs are a single population.

The Evolution of Species

• The formation of a river may divide the frogs into two populations.

The Evolution of Species

• Over time, the divided populations may become two species that may no longer interbreed, even if reunited.

• As populations become increasingly distinct, reproductive isolation can arise.

• Reproductive isolation occurs when formerly interbreeding organisms can no longer mate and produce fertile offspring.

Reproductive isolation can result in speciation

• Reproductive Isolation can occur when there are changes in the species:•Genetic material (DNA)•Behavior

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The Western meadowlark (left) and the Eastern meadowlark (right) appear The Western meadowlark (left) and the Eastern meadowlark (right) appear to be identical, and their ranges overlap, but their distinct songs prevent to be identical, and their ranges overlap, but their distinct songs prevent interbreedinginterbreeding

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