Microevolution bioh ch 16
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Microevolution – BioH Ch 16. Where did all organisms come from? Why such variety?. Early Beliefs. Supernatural intervention (BC & early AD) “Gods” Naturalism (1300’s) Chain of Being “Small” known world, small number of species World exploration led to many more species. 16.1.

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Microevolution bioh ch 16

Microevolution – BioH Ch 16

Where did all organisms come from?

Why such variety?

Early beliefs
Early Beliefs

  • Supernatural intervention (BC & early AD)

    • “Gods”

  • Naturalism (1300’s)

    • Chain of Being

    • “Small” known world, small number of species

    • World exploration led to

      many more species


More early beliefs
More Early Beliefs

  • Biogeography (1500’s)

    • Organization of

      organisms based on

      geographic distribution

  • Comparative morphology (1700’s)

    • Perfection


Microevolution bioh ch 16

  • Geologic

  • Evidence

  • Evolution theory

    • Modification over time

New theories
“New Theories”

  • Growing evidence

    • Fossil record abrupt changes

    • Discontinuity between sedimentary layers

  • “Catastrophism”

  • Charles Darwin

    • Influenced by geology

    • Gradual, uniform



Darwin s theory of evolution by natural selection
Darwin’s Theory of Evolution by Natural Selection

  • Individual organisms differ; some differences are heritable

  • Organisms produce more offspring than can survive; many that do survive do not reproduce

  • Because more organisms are produced than can survive, they compete for limited resources

  • Each unique organism has advantages & disadvantages in the struggle for existence. “Survival of the fittest”. These organisms pass on those advantageous traits to their offspring. Those that do not have this advantage either die out, or leave fewer offspring.

  • Species alive today descended with modification from ancestral species that lived in the distant past. This process, by which all diverse species evolve from common ancestors, unites all organisms on earth into a single “tree of life”.

Key concepts
Key Concepts

  • Populations evolve, not individuals

    • A group of individuals of same species in same area

    • Two or more different forms of traits (polymorphism)

  • Gene pool

    • All genes in population

    • Different gene forms = alleles

    • Allele frequency


Which alleles end up in which gamete and eventually in which new individual
Which alleles end up in which gamete and eventually in which new individual?

Depends upon five factors:

  • Gene mutation (producing new alleles)

  • Meiosis crossing over(producing new combinations of alleles)

  • Meiosis independent assortment(producing random mix of paternal & maternal genes)

  • Fertilization (producing random combination of parental alleles)

  • Chromosomal mutations(producing changes in chromosome number or chromosome gene positions)

Stable allele frequencies = “genetic equilibrium”

NOT exhibiting evolution

All five factors do not usually happen at the same time

Microevolution bioh ch 16

Population Changes and Microevolution

Changes in combinations of alleles lead to variations in a populations’ phenotypes. (This could be structural, functional and/or behavioral changes).

MICROEVOLUTION refers to small-scale changes in allele frequencies as caused by mutations, gene flow & genetic drift. These all shuffle alleles into, through or out of populations.

Allele mutations can be lethal, neutral or beneficial.

Gene flow involves the natural movement of individuals into, through or out of a population – moving their specific alleles with them.

Genetic drift is the natural, random change in allele frequencies over time caused by chance alone

Natural selection and population change for polygenic traits
Natural Selection and Population Change for Polygenic traits

Natural Selection = an outcome (result) of the differences in survival and reproduction among individuals that show variation in heritable traits.

With time, natural selection can lead to increased fitness (better adaptation to the environment).

Directional Selection

Allele frequencies change in one

direction in response to environmental changes or changes caused by


16.6 16.7

Microevolution bioh ch 16

Peppered moths and the Industrial Revolution

Pre-Industrial Revolution – light moths common on light tree trunks and darks moths were rare

Post-Industrial revolution – moth population depended upon how close the trees were to an industrial site (and soot)

Stabilizing and disruptive selection
Stabilizing and Disruptive Selection

Stabilizing selection – forms of a trait are favored by environment conditions. The trait becomes prominent

Disruptive selection – the intermediate form of the trait is NOT favored. The extreme forms of the trait become prominent


Microevolution bioh ch 16

Selection types

  • A bright red feathered bird is just as successful at mating than a black feathered bird, but any colors in between have a hard time finding mates. = disruptive selection

  • Birds with medium sized beaks are most common in their population while birds with small or large beaks are a rarity. The birds with medium sized beaks are able to get food most efficiently making them able to survive. = stabilizing selection

  • There used to be only small seeds available for the finches to eat, but when only large seeds were available, their beak size changed from small to the large. = directional selection

Other types of selection
Other types of Selection

  • Sexual dimorphism – distinct

    male and female phenotypes

    resulting in selective breeding

  • Balanced polymorphism – where non- identical alleles for a trait remain somewhat constant


Gene flow
Gene Flow

  • Over time, individuals within the same species move about, so that alleles are mixed from population area to population area

    • Immigration

    • Emigration

  • Physical flow of alleles into, through

  • and out of a population = gene flow

  • Helps to keep the separated

  • populations genetically similar

Genetic drift
Genetic Drift

  • Random change in allele frequencies due to chance alone

    • Significant effect in small populations

    • Insignificant effect in very large populations

In small populations, genetic drift may lead to a homozygous condition with the loss of genetic diversity


Genetic drift conditions
Genetic Drift Conditions

  • Bottleneck condition – severe reduction in

  • population size caused by intense selection

  • pressure (contagious disease, hunting,

  • natural disaster)

    • Results in altered allele frequencies

  • Founder effect– a few individuals

  • leave a population for a new location

  • and start a new population

    • Results in altered allele

    • frequencies

  • Inbreeding – non-random breeding causing homozygous conditions