BISC 130 - Chapters 18/19: Evolution

Hypothesis- A suggested explanation for an observation which one can test.

• Little or no supporting evidence

  • The reason for additional observations/experiments.

• Scientific Theory - a tested and confirmed explanation for observations or phenomenon.

  • Based on reproducible experimentation and/or observation

• Broad in scope

  • "How" and "Why"

• Scientific Law

  • Based on reproducible experimentation and observation

  • Narrow in scope

    • "What"

  • *NOT a "better-supported" theory.

• Evolution: The change in the genetic makeup of a population (group of individuals) over time - generations

  • Population - all the individuals within a single species living in a specific area

• Evolution occurs at a population level

  • Individuals do not evolve.

• Evolution is change in allele frequency

  • Certain alleles become more or less common

• Evolution occurs over multiple generations

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Chapter 19 (Evolution) continued.

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Observations:

1. All organisms reproduce exponentially.

   1. To create more of themselves

1. Population sizes tend to remain stable

1. Resources are limited

   • Ex: Food, space, sunlight, water

   • Inference: In nature, there is a struggle for survival (to get limited resources) and only a portion of offspring will survive and reproduce (not everyone).

1. Variation exists within populations

   • Phenotypic and genotypic variation.

     • There are different alleles in different individuals

       • Mutations can create new alleles

1. Variations can affect evolutionary fitness

   1. Through adaptations

      1. Defined by selective pressures

         1. Any force/factor that causes one allele or phenotype to be better than the other.

            1. Temperature, predators, etc.

1. Much of this variation is heritable

   1. Passed to offspring.

• Inference: The struggle for survival depends on traits and alleles that can be passed down to offspring.

Natural Selection

• The few individuals that survive and reproduce will be those with traits and alleles that best help them survive and reproduce. Thus, their offspring will have these traits too.

  • Over a few generations, this leads to an increase in allele frequency in this population of favorable alleles

    • Evolution

    • Natural Selection cuases Evolution

      • Specfically Adaptive Evolution - Over time, an increase in the frequency of beneficial alleles and a decrease in deleterious in a population due to N. selection.

Factors other than natural selection that result in evolution.

• Genetic Drift

  • Effects are stronger in smaller populations

• Bottleneck Effect

  • A natural disaster/catastrophic event dramatically, randomly reduces population size

    • Rare alleles may become more common, less common, or lost completely (OR none of these things at all).

• Founder Effect

  • A small group of randomly selected individuals start a new population in a new region.

    • Rare alleles may become more common, less common, or lost completely.

• Gene Flow

  • Can introduce novel alleles to a population

• Mutations

  • Can create new alleles

• Nonrandom mating

  • Mating that happens at random - tree spreading its acorns, sponges or other underwater organisms releasing sperm, etc.

• Sexual selection

• If an allele helps an organism secure a mate, it will be selected for.

• Can lead to sexual dimorphism - phenotypic difference between a population's males and females.

The Theory of Evolution

• Evolution occurs, and is driven by natural selection, genetic drift, and other forces mentioned above.

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• If changes are observed, then evolutionary forces are at work

Evolution Shapes Populations

• Stabilizing selection

  • Ex: Baby birth weight

• Directional selection

  • One extreme is favorted

  • Ex: Havalina looking for cacti with less spines

• Diversifying Selection

  • Either extreme is favorable, but the average is bad.

  • Ex: Finches and their diverse beak sizes - small and big ones.

• Microevoution

• Macroevolution

  • Broader scale evolutionary changes that scientists see over huge time periods.

• Species

  • A group of individuals that reproduce to produce FERTILE offpsring.

  • Interbreeding produces sterile offspring.

• Speciation

  • Formation of a new species

• For this to occur, populations of the same species must evolve until their genetic differences are so great they can no longer interbreed.

• 2 mechanisms of speciation

  1. Allopatric Speciation

     1. Speciation that occurs via geographic barriers.

     2. Reproductive Isolation

        1. No gene flow

        2. Can be due to a physical change in the environment

        3. Can be due to a dispersal of new members to a new area

• Mutations and genetic drift leads to differences that, over time, result in speciation.

1. Sympatric Speciation

   • Food sources act as selective pressure

     • Multiple different adaptations arise

       • Differences accumulate

         • Eventually results in speciation

   • Sympatric Speciation can lead to Adaptive Radiation

• Fossils

  • Fossil record: all fossils discovered to date

• Fossilization is a rare event

  • Most organisms that have lived and died have not been fossilized.

• The age of fossils can be determined

• Examining fossils of closely related species can reveal evolutionary histores of modern living organisms

  • Can account for all diversity of life on Earth.

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