Mechanisms of evolution
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Mechanisms of Evolution. Chapter 16 and Chapter 17. How Does Evolution Work?. Individual organisms cannot evolve. Populations of a particular species evolve. Natural selection acts on the range of phenotypes in a population.

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Mechanisms of Evolution

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Mechanisms of Evolution

Chapter 16 and Chapter 17

How Does Evolution Work?

  • Individual organisms cannot evolve. Populations of a particular species evolve.

  • Natural selection acts on the range of phenotypes in a population.

  • Microevolution occurs as the frequency of alleles in a population changes.

Evolution-What Happens?

  • Macroevolution or Evolution occurs when there is a change in allele frequency which produces a new species.


  • Gene pool: All alleles of the population’s genes.

  • Allelic frequency: % of a specific allele in the gene pool.

  • Example: Approximately 75% have dominant allele for tongue rolling. 25% non-rolling

  • Genetic Equilibrium: This exists when the frequency of alleles remains the same over generations. The population is not evolving.

When Does Evolution Occurs?

  • Evolution results when there are Forces that change allelic frequencies.

  • Forces that cause Evolution:

    1. Geneflow: Transport of genes by migrating individuals.

    2. Nonrandom Mating: Mating based on preferences

    Example: a female may choose a mate based on male size, color, or ability to gather food.

Forces of Evolution Continued

3. Mutation: Change in DNA

4. Genetic Drift: chance event changes allelic frequencies – Greatly affect small populations such as the animals of the Galapagos Islands or Amish.

Causes of Genetic Drift

  • Mating over a long time period in a small population.

  • Little movement of males or females into or out of the population.

3 Types of Natural Selection

  • Stabilizing selection – favors average individuals

  • Directional selection – favors one of the extreme variations of a trait

  • Disruptive selection – favors individuals with both extremes of a trait (eliminates intermediate phenotypes)

What is a Species?

  • A population or group of populations whose members have the ability to breed with one another and produce fertile offspring

Evolution of Species (Speciation)

  • Significant changes in the gene pool can lead to evolution of a new species over time.

  • Speciation occurs when members of similar populations no longer interbreed to produce fertile offspring within their natural environment.

Artificial Speciation

Diane Dodd’s fruit fly lab, 1989

Why Don’t the Populations Interbreed?

  • 1. Geographic isolation – physical barrier divides a population.

  • 2. Reproductive isolation – formerly interbreeding organisms can no longer mate to produce offspring..

  • 3. Change in niche -- Change in food source. Example Finches

1. Geographic Isolation

A physical barrier that separates a population into groups.

Can be

  • Mountains or Rivers

  • Islands with water in between Darwin’s 13 finches on Galapagos

  • Valleys caused by lava flow

  • Roads/Highways

1. Geographical Isolation

2. Reproductive Isolation

  • Prevents closely related species from interbreeding

    • Timing

    • Behavior

    • Habitat


Similar species have different breeding seasons

Eastern Spotted Western Spotted

Skunk Skunk


Similar species may have different courtship or mating behaviors.

Ex: Eastern & Western meadowlarks almost identical in color shape and habitat, but difference in courtship rituals differ different species


  • Species remain reproductively isolated because they are adapted to different habitats.

    Ex: Stickleback fish one is a bottom feeder, one spends time in the top open layers of lakes in British Columbia, Canada

Patterns of Evolution

  • Divergent Evolution – evolutionwhere species diverge or become less and less alike as they adapt to different environments.

  • Adaptive Radiation-ancestral species evolves into an array of species to fit diverse habitats. This is a type of divergent evolution

  • Both the wooly mammoth, which occupied parts of North America, and the elephant, still found in Asia and Africa are presumed to have evolved from a common ancestor.

  • Their geographical isolation and environmental selection pressures caused further evolution of the species.

  • Each, in its own location, occupies(d) a similar niche.

Patterns of EvolutionContinued

2. Convergent Evolution– Unrelated species occupy similar environments in different parts of the world.

Similar pressures of natural selection lead to similar adaptations.

Example of Convergent


A Hummingbird Moth

A Humming Bird




Speciation can occur quickly or slowly

  • Gradualism – idea that species originate through a gradual accumulation of adaptations.

  • Punctuated equilibrium – hypothesis that speciation occurs relatively quickly, in rapid bursts, with long periods of genetic equilibrium in between.


  • Gradual changes in species over time

  • Evidence of many intermediate forms in fossil records

Punctuated Equilibrium

  • Scientists found remains of intermediate forms

  • Also saw that populations remained the same over large periods of time then suddenly changed

The End

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