Microevolution – BioH Ch 16

Where did all organisms come from?

Why such variety? 1

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

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More Early Beliefs• Biogeography (1500’s)

– Organization of

organisms based on

geographic distribution

• Comparative morphology (1700’s)– Perfection

16.13

•Geologic Evidence

•Evolution theoryModification over time

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“New Theories”

• Growing evidence– Fossil record abrupt changes– Discontinuity between sedimentary layers

• “Catastrophism”

• Charles Darwin– Influenced by geology– Gradual, uniform

changes

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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”.

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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

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

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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)

All five factors do not usually happen at the same time

Stable allele frequencies = “genetic equilibrium” NOT exhibiting evolution

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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

Population Changes and Microevolution

Natural Selection and Population Change for Polygenic traits

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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 mutations

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Peppered moths and the Industrial Revolution

Pre-Industrial Revolution – light moths common on light tree trunks and darks moths were rarePost-Industrial revolution – moth population depended upon how close the trees were to an industrial site (and soot)

Stabilizing and Disruptive Selection

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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

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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

Sexual dimorphism – distinct

male and female phenotypes

resulting in selective breeding

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

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Gene Flow

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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

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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

16.11

Genetic Drift Conditions

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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