How Populations Evolve

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How Populations Evolve
Ch. 13
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Learning Objectives

• Comprehend (describe/predict)
• Evolution theory
• List the evidence for evolution theory
• Comprehend and list the mechanisms of evolution
• Ex natural selection

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All organisms are adapted to the environment in
which they live
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Evolutionary Adaptations

• Evolutionary adaptations are inherited traits
  that enhance an organisms ability to survive and
  reproduce in a particular environment
• Evolution change in genetic inheritance over time

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History of Evolution Contd.

• In the early 1800s Larmark suggested that
  evolution was responsible for the similarity
  between fossils and living organisms
• However, Larmark suggested that organisms evolve
  by using or not using body parts
• Giraffes developed longer necks due to stretching

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History of Evolution Contd.

• After his voyage on the Beagle to the Galapagos,
  and after a period of study, Darwin wrote On the
  Origin of Species by Means of Natural Selection
  (1859)
• referred to evolution as Descent with
  Modification
• He developed the theory of evolution
• D\Charles Darwin\Charles Darwin.mpg

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History of Evolution Contd.

• Darwin postulated that
• the descendents of the earliest organisms spread
  over the continents over millions of years,
  adapting to local environments
• The history of life resembled a tree
• At each fork of the tree, is a common ancestor
• The dog and wolf share a common ancestor

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History of Evolution Contd.

• Evolution is the unifying theme in biology
• An abundance of support through repeated testing
• Over the years, other scientists have added to
  the theory of evolution

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Support Fossils
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Support Fossils

• The fossil record
• The ordered array in which fossils appear within
  layers of sedimentary rocks
• reveals that organisms appeared in sequence

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Support Fossils

• Earliest fossils were prokaryotes
• Appeared in fossil record about 3.5 billion years
  ago

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Chronological Appearance of Vertebrates
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Support Fossils
Fossils link past and present
Whales have forelegs in the form of flippers,
however, they also have vestigial hind-leg and
foot bones that do not extend from their body
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Support Fossils
Basilosaurus is an extinct whale that had
hind-legs
These also are vestigial legs as the ancient
whale was an aquatic animal
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Other Support Biogeography

• Biogeography geographic distribution of species
• Galapagos Islands
• Environment similar to tropical islands
• However, animals resembled species of nearby
  South America
• Galapagos species evolved from South American
  immigrant species

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Other Support Comparative Anatomy

• Comparative Anatomy comparison of body
  structures in different species
• Anatomical similarities among many species give
  signs of common descent

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Other Support Comparative Anatomy

• Ex Our knee and spine were derived from
  structures that originally supported other
  mammals that walked on 4 legs
• If they were designed specifically for bipedal
  organisms, we would expect to have fewer problems
  with these structures
• AHHHH thats why so many people have knee and
  back problems OUCH!

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Other Support Comparative Embryology

• Comparative Embryology study of structures that
  appear during the development of different
  organisms
• Closely related organisms often have similar
  stages in their embryonic development
• Ex Endostyles that appear on sides of the throat
• All vertebrates have these slits during embryonic
  stage of development
• They develop into other features as development
  progresses

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Other Support Molecular Biology

• Molecular Biology study of molecular basis of
  genes and gene expression
• Related individuals have greater similarity in
  their DNA and proteins than do unrelated
  individuals of the same species
• Similar species have a greater proportion of
  their DNA and proteins in common than unrelated
  species

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Other Support Molecular Biology
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Mechanisms of Evolution

• Charles Darwin observed that
• All species had a tendency to overproduce
• Every environment has a limited supply of
  resources
• Individuals of a population vary in their
  characteristics
• These characteristics are passed on from one
  generation to the next

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Mechanisms of Evolution

• Because all environments have limited resources
  and populations tend to be in excess of available
  resources
• What do you think will happen???
• Those with characteristics best suited for a
  particular environment will enable them to
  survive and reproduce
• Darwin called this process Natural selection

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Mechanisms of Evolution

• Artificial Selection the selective breeding of
  plants and animals
• Provides sound evidence for Natural Selection
• In artificial selection, we select successful
  features, whereas in natural selection, nature
  (environment) selects successful features

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Artificial Selection
German shepherd
Yorkshire terrier
English springerspaniel
Mini-dachshund
Golden retriever
Hundreds tothousands of yearsof
breeding(artificial selection)
Ancestral dog
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Natural Selection
African wilddog
Coyote
Fox
Wolf
Jackal
Thousands tomillions of yearsof natural
selection
Ancestral canine
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Darwin Summary

• Evolution is genetic modification over time
• Descent with modification from a common ancestor
• The mechanism of modification is natural
  selection which occurs over enormous spans of
  time

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Natural Selection in Action
Camouflage of mantids in different environments
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Natural Selection in Action
Chromosome with geneconferring resistanceto
insecticide
Insecticideapplication
Additionalapplications of thesame insecticide
willbe less effective, andthe frequency
ofresistant insects inthe populationwill grow
Survivor
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Populations are the unit of evolution

• Population a group of individuals of the same
  species

Cypress trees
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Evolution Populations

• Measure evolution as a change in the prevalence
  of certain heritable traits in a population over
  generations
• Natural selection favored insects w/ insecticide
  resistance, those insects reproduced, over time
  the trait for insecticide resistance became more
  prevalent
• While Darwin understood that populations evolve,
  he did not understand the genetic basis of
  population change

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Evolution Populations

• Today we know that genetic traits are carried by
  genes on chromosomes and that mutations may
  produce new traits
• Population genetics science of genetic change in
  populations
• Developed in 1920s
• Modern synthesis theory of evolution that
  incorporates genetics
• Developed in 1940s
• Population genetics combined with the theory of
  natural selection

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Evolution Populations

• Central to modern synthesis is the relationship
  between populations and species
• Species a group of populations whose individuals
  are able to interbreed and produce fertile
  offspring

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Evolution Populations

• If a population is isolated from other
  populations, the individuals of the population
  will be more closely related

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Microevolution

• Microevolution a change in a populations gene
  pool over a succession of generations
• Occurs over relatively brief period of time (not
  millions of years)
• Gene pool total collection of genes in a
  population at any give time
• Reservoir from which members of next generation
  derive their genes

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Objectives

• Comprehend and define
• Microevolution
• Gene pool
• Hardy-Weinberg
• Adaptive Change
• Variation
• Neutral Traits
• Darwinian Fitness
• Outcomes of natural selection

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Microevolution

• Gene pool contains alleles of all individuals in
  a population
• Each gene generally has 2 forms or 2 alleles

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Microevolution
Chromosome with geneconferring resistanceto
insecticide
Insecticideapplication
Gene pool changes as most alleles that do not
confer insecticide resistance are removed
Survivor
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Gene pool of nonevolving populations

• Hardy-Weinberg equilibrium states that the gene
  pool remains constant unless acted upon by other
  agents
• Shuffling of genes during sexual reproduction
  does not alter the proportion of different
  alleles in a population
• Occurs only under very specific conditions
• Population is very large and isolated (no
  migration)
• Mutations do not alter gene pool
• Mating is random
• Natural selection does not occur

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Potential causes of microevolution

• Microevolution a change in a populations gene
  pool over a succession of generations
• Deviations from conditions that cause
  Hardy-Weinberg can cause microevolution
• Genetic drift change in the gene pool of a small
  population due to chance
• Ex flip a coin 10 times

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Causes of Microevolution

• Bottleneck effect genetic drift that results
  from event that drastically reduces population
  size

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Causes of Microevolution

• Founder effect random change in gene pool that
  occurs in a small colony of a population
• A type of genetic drift
• Over time, random changes in allele frequencies
  will continue until population is large enough
  that genetic drift is minimal

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Causes of Microevolution

• Gene flow occurs when fertile individuals move
  into or out of a population
• ALSO occurs when gametes are transferred between
  populations
• Plants (sperm in pollen)
• Tends to reduce genetic differences between
  populations
• Ex humans

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Causes of Microevolution

• Mutation random change in an organisms DNA that
  may create a new allele
• Rare event 1 per gene locus per 105 or 106
  gametes
• In a large population mutation does not have much
  of an affect
• However, over time, mutation is vital to
  evolution as it is the ONLY force that creates
  new alleles

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Adaptive change

• Adaptive change results when natural selection
  upsets genetic equilibrium
• All individuals in a population are not equal in
  their ability to reproduce as stated in
  Hardy-Weinberg
• Natural selection results in the accumulation and
  maintenance of traits that adapt a population to
  its environment

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Variation is extensive in populations

• Phenotypic (observable features) changes may be
  environmental or genetic in origin
• ONLY genetic changes result in evolutionary
  adaptation
• Populations can exhibit polymorphism 2 or more
  morphs present in a population
• Morph 2 or more forms of a phenotypic
  characteristic

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Variation is extensive in populations

• 2 morphs of California King Snake

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Variation is extensive in populations

• Geographic variation variation in an inherited
  characteristic between populations
• Most species exhibit this
• Plants may change in size with increasing
  latitude
• How do inherited variations arise???

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Mutation Sexual Recombination

• Mutation sexual recombination generate
  variation
• Mutation can create new alleles
• Rare that a mutation improves the adaptation of a
  bearer to its environment
• In microorganisms, mutation generates genetic
  variation rapidly (why??)
• For organisms with longer life spans, sexual
  recombination provides genetic variation from one
  generation to the next

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How does natural selection affect variation?

• What prevents natural selection from removing the
  variation in a population?
• Why arent only the good alleles in a
  population?
• Most eukaryotes are diploid (2 sets of
  chromosomes)
• Recessive alleles are subject to natural
  selection ONLY if it affects the phenotype
• Only if 2 copies are present

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How does natural selection affect variation?

• Recessive alleles are hidden from natural
  selection in heterozygotes
• This allows large numbers of recessive alleles to
  remain in population
• Why is this advantageous???

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Not all genetic variation is subject to natural
selection

• If traits do not have an impact on reproductive
  success, they may not be subject to natural
  selection
• These are known as neutral traits
• Variation in a heritable characteristic that
  provides no apparent selective advantage for some
  individuals over others
• Finger prints
• Is it certain that these traits will always be
  neutral?

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Survival of the Fittest

• Misleading phrase as reproductive success is more
  subtle
• Darwinian fitness the contribution an individual
  makes to the gene pool of the next generation
  relative to the contribution of other individuals
• Production of fertile offspring contributes to
  fitness

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3 general outcomes of natural selection
Originalpopulation
Frequency ofindividuals
Phenotypes (fur color)
Originalpopulation
Evolvedpopulation
Stabilizing selection
Directional selection
Diversifying selection
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3 general outcomes of natural selection

• Stabilizing selection favors intermediate
  variants
• Typical in relatively stable environments
• Probably very common in most populations
• Directional selection shifts the overall makeup
  of the population by acting against individuals
  at 1 of the phenotypic extremes
• Most common during periods of environmental
  change
• When species migrate to a new habitat w/
  different environmental conditions

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3 outcomes of natural selection

• Diversifying (disruptive) selection occurs when
  environmental conditions are varied in a way that
  favors individuals at both extremes of a
  phenotypic range.
• Can lead to balanced polymorphism
• California King snake

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Sexual selection

• Leads to development of secondary sexual traits
• Sexual dimorphism

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No Perfect Organisms

• Organisms are a product of their past
• Evolution does not scrap ancestral anatomy
• Adaptations are often compromises
• Each organism must do different things
• Not all evolution is adaptive
• Chance affects genetic structure of populations
  too
• Selection can only edit existing variations
• Favors only fittest variations from the
  phenotypes that are available

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Antibiotic Resistant Bacteria
D\Antibiotics\Antibiotics.mpg