Download Evolution/ Natural selection

Evolution and Natural Selection
Pre – Darwin “Origin of species”
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Earth relatively young
(thousands of years) ; this hypothesis was being replaced
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In the 1700’s and early 1800’s, geological evidence suggested that the earth
was not young, but quite old, and that it had undergone considerable change
over its history. Massive geologic formations, such as the Grand Canyon, were
seen as the result of slow geologic processes
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There were a limited number of fossils found and most did not
appear dramatically different from current species; species were
thought not to change between generations and the number of
species on the earth was constant.
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Organisms were thought to be perfectly adapted to their
environment.
Post- Darwin “Origin of species”
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Species are related by descent
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Adaptation to the environment is the result of the interplay of
random variation and natural selection
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The number of species was not constant and species changed
over time.
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Descent with modification
Conditions necessary for decent with
modification
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Variation within a population
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The variation is heritable
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The reproductive success depends on the available
variation.
Evolution theory
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Variation exists among individuals within a species
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Organisms produce more offspring than the environment can support
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Competition exists among individuals
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The organisms whose variation best fit them to the environment are the ones
who are most likely to survive, reproduce, and pass those desirable variations
to the next generation
Gene Variation
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Macroevolution - Evolutionary change on a grand scale,
encompassing novel designs, evolutionary trends and episodic
mass extinction.
Microevolution - Differential survival and reproduction due to
natural selection. Gradually alters population to include more
individuals with advantageous characteristics.
Gene Variation
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Darwin’s explanation of evolution:
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Adaptation by natural selection is responsible for evolutionary
changes within a species(microevolution), and accumulation of
these changes leads to development of new species
(macroevolution).
Evidences for evolution:
fossil record
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Fossils are preserved traces of once-living organisms
created when organisms become buried in sediment and
calcium in hard surfaces mineralizes.
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Often provide evidence of successive evolution.
Fossilization process limits available fossils
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Requires burial, sedimentary rock, deposition of minerals
replacing hard parts of an organism.
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Soft parts preserved in impressions or casts in soft sediment
Dating the fossil record – isotopic dating
Timelines
Molecular Record
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Evolutionary theory allows evolutionary change involves
substitution of new versions of old genes.
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New alleles arise by mutation and come to predominate through favorable
selection.
Molecular Clocks
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The longer the time since divergence, the greater the number of differences
in nucleotide sequence of cytochrome C.
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Changes accumulate at constant rate.
Molecular Record
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Phylogenetic Trees
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Evolutionary history of a gene can be mapped as a phylogenetic
tree.
Mechanisms of evolution
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Genetic drift
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Mutation
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Random fluctuations in the allele frequencies within a population
A random change in the coding of an allele or gene
Natural selection (including sexual selection)
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the differential reproduction of genotypes caused by factors in the
environment.
Anatomical Record – products of evolutionary process
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Homologous Structures - Structure and function have
diverged between body parts of different animals, but are
derived from same part present in a common ancestor.
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Analogous Structures - Features resemble each other as a
result of parallel evolution in separate lineages.
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Forelimbs of Vertebrates
Flippers of penguins and dolphins
Vestigal Organs - Organs no longer of use.
Products of evolution
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Adaptation – organisms are not perfectly adapted ( have
to make due with your genetics)
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Examples: panda thumb
Hardy-Weinberg Rule
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From 1920’s onward, scientists began formulating theory of how
alternative gene forms (alleles) behave in a population, and how
changes in gene frequencies lead to evolutionary change.
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1908 Hardy and Weinberg pointed out in the absence of forces, in a large
population with random mating, allelic frequencies remain constant.
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Hardy Weinberg equilibrium
Hardy-Weinberg individuals do not evolve
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Assumptions:
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Large population size
Random mating
No mutation
No immigration
Absence of natural selection
Change in Allelic Frequencies
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Mutation
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Error in replication of a nucleotide sequence in DNA (Very slow
rate).
Migration
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Movement of individuals from one population to another
(Dependent on strength of selective forces).
Change in Allelic Frequencies
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Genetic Drift
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Change in allelic frequencies due to random events.
Founder Effect - Population started by few individuals and thus a
restricted gene pool (Rare genes may become common).
„ Bottleneck Effect - Gene pool becomes very small, usually due to small
population size.
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Change in Allelic Frequencies
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Nonrandom Mating
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Individuals with certain genotypes mate with more or less
commonly than expected on a random basis.
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Inbreeding - Mating with relatives.
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Increases homozygosity
Change in Allelic Frequencies
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Selection
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Some individuals leave behind more progeny than others.
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Artificial - Breeder selects desired characteristics.
Natural - Environment determines adapted characteristics.
Forms of Selection
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Disruptive
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Stabilizing
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Selection acts to eliminate intermediate phenotypes.
Selection acts to eliminate both extremes from an array of phenotypes.
Directional
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Selection acts to eliminate one extreme from an array of phenotypes.
Three Forms of Selection
Sickle-Cell Anemnia
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Hereditary disease affecting hemoglobin molecules.
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Sickle-Cell homozygosity frequently leads to a reduced life span.
Heterozygosity causes enough hemoglobin to be produced to
keep red blood cells healthy.
Very common in Africa.
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Stabilizing selection as heterozygosity infers less susceptibility to
malaria.
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One of leading causes of death in Africa.
Stabilizing Selection in Sickle-Cell
Peppered Moths and Industrial Melanism
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Until the mid nineteenth century, Peppered Moths ,Biston betularia,
had predominately light-colored wings.
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Subsequently, dark individuals became predominant.
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Industrial smog helped turn lichens on tree trunks dark.
Contrasting colors between trunk color and moth color led to differential predation by
birds.
Peppered Moths and Industrial Melanism
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Second half of the twentieth century saw widespread
implementation of pollution controls, thus trends reversed
and light colored moths again dominated.
But, caution must be taken, as the selective agent could be
some factor other than wing coloration.
Industrial Melanism
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Example of directional selection
Species Concept
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A species is generally defined as a group of organisms
unlike other such groups and does not integrate
extensively with other groups in nature.
Species Formation
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Local populations adapt to the specific circumstances each
faces.
When they become different enough, the populations
become ecological races.
Natural selection reinforces differences through isolating
mechanisms.
Two races become incapable of interbreeding and are
considered two separate species.
Prezygotic Isolating Mechanisms
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Prevent formation of Zygote:
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Geographic Isolation
Ecological Isolation
Behavioral Isolation
Temporal Isolation
Mechanical Isolation
Prevention of Gamete Fusion
Geographical and Ecological Isolation In
Oaks
Postzygotic Isolating Mechanisms
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Prevent zygotes from developing into normal, functional
offspring.
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Improper development
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Reduced fertility or sterility.