Mechanisms of EvolutionMechanisms of Evolution

BiologyBiology

Mr. SolisMr. Solis

Populations, Not Individuals EvolvePopulations, Not Individuals Evolve

An organism cannot evolve a new phenotype, An organism cannot evolve a new phenotype, but rather natural selection acts on the range of but rather natural selection acts on the range of phenotypes in a population.phenotypes in a population.Evolution occurs as a populations genes and Evolution occurs as a populations genes and their frequencies change over time.their frequencies change over time.Gene Pool- All the alleles of a populations Gene Pool- All the alleles of a populations genes.genes.Allelic Frequency- the percentage of any specific Allelic Frequency- the percentage of any specific allele in a gene pool.allele in a gene pool.Genetic Equilibrium- frequency of alleles Genetic Equilibrium- frequency of alleles remains the same over generations.remains the same over generations.

Changes in genetic equilibriumChanges in genetic equilibrium

A population in genetic equilibrium is not A population in genetic equilibrium is not evolving.evolving.

Anything that affects the genes in gene Anything that affects the genes in gene pool and affects the frequency will cause pool and affects the frequency will cause the population to evolve.the population to evolve.

Genetic Drift- the alteration of allelic Genetic Drift- the alteration of allelic frequencies by chance events.frequencies by chance events.

Natural Selection acts on variationsNatural Selection acts on variations

Some variations increase or decrease an Some variations increase or decrease an organisms chance of survival in an organisms chance of survival in an environment.environment.

Three types of natural selection that act on Three types of natural selection that act on variations.variations.– StabilizingStabilizing– Directional Directional – DisruptiveDisruptive

StabilizingStabilizing

Natural selection that Natural selection that favors average favors average individuals in a individuals in a population.population.Example: organisms that Example: organisms that range from small, range from small, medium and large.medium and large.The large ones are easily The large ones are easily seen and eaten, the small seen and eaten, the small have difficulty finding have difficulty finding food. Therefore the food. Therefore the average size has a average size has a selective advantage.selective advantage.

Directional SelectionDirectional Selection

Natural selection favors Natural selection favors one of the extreme one of the extreme variations.variations.Example an insect Example an insect invades an area invades an area populated with long beak populated with long beak woodpeckers and short woodpeckers and short beak woodpeckers. The beak woodpeckers. The insect lives deep within insect lives deep within the bark only allowing the bark only allowing birds with long beaks to birds with long beaks to reach them. Therefore reach them. Therefore having the selective having the selective advantage.advantage.

Disruptive SelectionDisruptive Selection

Favors both extreme Favors both extreme variations of a trait.variations of a trait.Example: When you Example: When you have a light colored, have a light colored, medium colored and dark medium colored and dark colored organism and the colored organism and the environment contains environment contains surroundings of light and surroundings of light and dark color areas. This dark color areas. This allows the dark and light allows the dark and light colored organisms to colored organisms to survive but the medium survive but the medium color gets eliminated.color gets eliminated.

The Evolution of SpeciesThe Evolution of Species

Speciation: occurs Speciation: occurs when members of when members of similar populations no similar populations no longer interbreed to longer interbreed to produce fertile produce fertile offspring with in their offspring with in their natural environment.natural environment.

Physical Barriers can prevent Physical Barriers can prevent interbreedinginterbreeding

Geographic Isolation: Geographic Isolation: occurs when a physical occurs when a physical barrier divides a barrier divides a population.population.Example: A single Example: A single species of a frog may species of a frog may live in the forest, within live in the forest, within time a river forms time a river forms creating a new trait. creating a new trait. Eventually this new trait Eventually this new trait will procreate and will procreate and create a sub population create a sub population of its own. Eventually of its own. Eventually natural selection will natural selection will occur and these frogs occur and these frogs will never inter breed will never inter breed with each other.with each other.

Reproductive IsolationReproductive Isolation

Occurs when formerly interbreeding organisms Occurs when formerly interbreeding organisms can no longer mate and produce fertile offspring.can no longer mate and produce fertile offspring.

There are two types of Reproductive Isolation.There are two types of Reproductive Isolation.

– When the genetic material is so different that When the genetic material is so different that fertilization will not occur.fertilization will not occur.

– When reproductive behaviors are different.When reproductive behaviors are different.

Speciation RatesSpeciation Rates

Gradualism- species Gradualism- species originate through a originate through a gradual change in gradual change in adaptation.adaptation.

Punctuated Equilibrium: Punctuated Equilibrium: speciation happens speciation happens quickly, in a rapid burst, quickly, in a rapid burst, with long periods of with long periods of genetic equilibrium in genetic equilibrium in between.between.

Patterns of EvolutionPatterns of EvolutionAdaptive radiation: Adaptive radiation: when an ancestral when an ancestral species evolves into species evolves into an array of species to an array of species to fit a number of fit a number of diverse habitats.diverse habitats.Divergent Evolution: Divergent Evolution: When one species When one species evolves to two evolves to two subpopulations and subpopulations and eventually become eventually become two different species.two different species.Convergent Convergent Evolution: A pattern Evolution: A pattern of evolution in which of evolution in which distantly related distantly related organisms evolve organisms evolve similar traits.similar traits.