microevolution – bioh ch 16 where did all organisms come from? why such variety? 1
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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
16.1 2
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
4
“New Theories”
• Growing evidence– Fossil record abrupt changes– Discontinuity between sedimentary layers
• “Catastrophism”
• Charles Darwin– Influenced by geology– Gradual, uniform
changes
16.2 5
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”.
6
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
716.4
Which alleles end up in which gamete and eventually in which new individual?
8
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
9
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
10
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
16.6 16.7
11
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
12
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
16.8
13
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
16.914
Gene Flow
15
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
16
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
17
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
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