How Populations Evolve

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How Populations Evolve

• Chapter 13

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Evolution
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Charles Darwin

• While on the voyage of the HMS Beagle in the
  1830s, Charles Darwin went onshore and collected
  thousands of specimens of fossils and living
  things
• By studying these specimens he observed
• similarities between living and fossil organisms
• the diversity of life in the world including the
  Galápagos Islands, where there was great diversity

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Darwins Theories

• Darwin became convinced that the Earth was old
  and continually changing
• He concluded that living things also change, or
  evolve over generations
• He also stated that living species descended from
  earlier life-forms descent with modification

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Darwins Theories

• Unity among species
• All organisms related through descent from some
  unknown species in the remote past
• As the descendants of the earliest organism
  spread throughout the world over time they
  developed diverse modifications (adaptations)
  that helped them live in diverse environments
• Species that are closely related are very similar
  in appearance

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Darwins Theories
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Darwins theories

• Darwin focused on the causes of the adaptations
  that lead to evolution
• In studying species he observed that
• organisms produce more offspring than the
  environment can support
• organisms vary in many characteristics
• these variations can be inherited
• Because natural resources are limited, a struggle
  for existence insues

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Natural Selection

• Darwin concluded that individuals best suited for
  a particular environment are more likely to
  survive and reproduce than those less well
  adapted this is called natural selection
• Darwin saw natural selection as the basic
  mechanism of evolution
• As a result, the proportion of individuals with
  favorable characteristics increases
• Populations gradually change in response to the
  environment

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Artificial Selection

• Darwin also saw that when humans choose organisms
  with specific characteristics as breeding stock,
  they are performing the role of the environment
• This is called artificial selection
• Example of artificial selection in plants five
  vegetables derived from wild mustard

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Artificial selection over hundreds of years can
produce new breeds of dog
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Natural selection over millions of years can
produce new species
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Darwins Theories

• Two main features
• Diverse forms of life have arisen by descent with
  modification (adaptation) from ancestral species
• The mechanism of modification has been natural
  selection

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• Natural Selection by Adaptation

• Natural Selection in Action

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Key Points about Natural Selection

• Natural selection is more of an editing process
  than a creative mechanism
• Modifies characteristics, doesnt create them
• Contingent on time and place
• Favors characteristics that suit a particular
  time and place in a populations history
• Can occur in a short amount of time
• Preying mantis- over a long period of time
• Insecticide resistance- short period of time

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The Fossil Record
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Fossils

• Fossils are the remains of dead organisms that
  turn to stone by a process called petrification
• Organic material can be trapped in sediment and
  an imprint left
• If an entire organism is trapped in a medium
  where bacteria and fungi cannot decompose the
  organism

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Fossils Provide Evidence for Evolution

• The fossil record shows that organisms have
  appeared in a historical sequence
• Many fossils link early extinct species with
  species living today
• These fossilized hind leg bones link living
  whales with their land-dwelling ancestors

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Other Evidence for Evolution
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Biogeography

• Biogeography- the geographic distribution of
  species
• Species on the Galapagos Islands look more like
  the origin species on the mainland than they do
  other species on similar but distant islands
• Example Finches

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Comparative Anatomy

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

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Comparative Anatomy

• These similarities are variations on a common
  anatomical plan that became adapted to suit
  different functions
• Evolution remodels structures rather than making
  them anew
• Some of these modifications can be imperfect
• Human knee joints and spine

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Comparative Embryology

• The study of structures that appear during the
  development of different organisms
• Closely related organisms often have similar
  stages in their embryonic development
• Example- pharyngeal slits, which all vertebrates
  have in on the sides of their throat in early
  development

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Comparative Embryology
www2.evansville.edu/ evolutionweb/embryos.html
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Molecular Biology

• The study of the molecular basis of genes and
  gene expression
• The similarity of the genetic code is evidence
• Organisms that are closely related have a greater
  similarity in their DNA
• Species that are judged to be closely related by
  other criteria have a greater proportion of the
  DNA and proteins in common than more distantly
  related species

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Molecular Biology
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Population Genetics
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Populations

• A population is a group of individuals of the
  same species living in the same place at the same
  time.
• A population is the smallest unit that can evolve
• Evolution can only happen when the trait, or the
  gene carrying the trait, is perpetuated by mating
  in a population over time
• Population genetics- the study of genetic change
  in populations
• We define a species as a group of populations
  whose individuals have the potential to
  interbreed and produce fertile offspring

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Microevolution

• A gene pool is the total collection of genes in a
  population at any one time
• Consists of all alleles that can be inherited in
  a population
• Microevolution is a change in the relative
  frequencies of alleles in a gene pool over time
• This is evolution on its smallest scale

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Hardy-Weinberg Equilibrium

• Hardy-Weinberg equilibrium states that the
  shuffling of genes during sexual reproduction
  does not alter the proportions of different
  alleles in a gene pool
• No matter how many times an allele is shuffled
  into different combinations by fertilization, the
  frequency of the allele in the gene pool will
  remain constant unless acted on by other agents
• We can use these principles to estimate the
  frequency of an allele in a population

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Hardy-Weinberg Equilibrium
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Hardy-Weinberg Equation

• Because of Hardy-Weinberg we can estimate the
  frequency of alleles in a population
• Each individual carries two alleles and if we
  count the alleles then we can estimate
• P2 2PQ Q2 1
• Where P and Q are the two alleles present
• Consider the gene pool of a population of 500
  boobies
• 320 (64) are homozygous dominant (WW)
• 160 (32) are heterozygous (Ww)
• 20 (4) are homozygous recessive (ww)

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Hardy-Weinberg Equation

• Frequency of dominant allele (W) 80 p
• 80 of alleles in the booby population are W
• Frequency of recessive allele (w) 20 q
• 20 of alleles in the booby population are w
• Frequency of all three genotypes must be 100 or
  1.0
• p2 2pq q2 100 1.0
• homozygous dominant heterozygous homozygous
  recessive 100

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Allele Frequency and the Population

• Public health scientists use the Hardy-Weinberg
  equation to estimate frequencies of
  disease-causing alleles in the human population
• Five conditions have to be met in order for a
  population to be at equilibrium
• The population is very large
• The population is isolated
• Mutations do not alter the gene pool
• Mating is random
• All individuals are equal in reproductive success
• These conditions are not often met, but the HW
  equation is a good estimator

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Causes of Microevolution

• Deviations from HW can cause changes in the gene
  pool, or microevolution
• Two main causes of microevolution are genetic
  drift and natural selection
• Genetic drift is a change in a gene pool due to
  chance
• The smaller the population the greater the chance
  of genetic drift
• Example is tossing a coin
• Genetic drift can be caused by the bottleneck
  effect or by the founder effect

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Genetic Drift

• Bottleneck effect- genetic drift resulting from
  an event that drastically reduces population size
• Produce a population that is unlikely to have the
  same genetic makeup as the original population

Greater Prairie Chicken
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Founder Effect

• Founder effect- colonization of a new location
  by a small number of people
• Explains high frequency of certain disorders
  among certain populations that were established
  by a small group of people

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Gene Flow

• Gene flow can change a gene pool due to the
  movement of genes into or out of a population
• Reduces genetic variation between populations
• Mutation changes alleles
• Essential to evolution because it is the only
  force that introduces a new allele
• Natural selection leads to differential
  reproductive success

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Variations and Natural Selection
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Variation in Natural Populations

• Phenotypic variation may be environmental or
  genetic in origin
• But only genetic changes result in evolutionary
  adaptation
• Many populations exhibit polymorphism and
  geographic variation

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Genetic Basis for Variation

• If a mutant allele improves the adaptation of a
  species it increases its reproductive success
• Sexual recombination can do the same thing

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Not all variations are subject to natural
selection

• Some variations may be neutral, providing no
  apparent advantage or disadvantage
• Example human fingerprints

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Endangered Species and Genetic Variation

• Low genetic variability may reduce the capacity
  of endangered species to survive as humans
  continue to alter the environment
• Studies have shown that cheetah populations
  exhibit extreme genetic uniformity
• Thus they may have a reduced capacity to adapt
  to environmental challenges

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The Reality of Darwinian Fitness

• Survival of the fittest is not a violent
  take-over of the population, but rather a passive
  process
• An individuals Darwinian fitness is the
  contribution it makes to the gene pool of the
  next generation relative to the contribution made
  by other individuals
• Production of fertile offspring is the only score
  that counts in natural selection

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The Possible Outcomes of Natural Selection

• Stabilizing selection- favors intermediate
  variations
• Prevails most of the time
• Directional selection- shifts the direction
  acting against individuals at one extreme of the
  spectrum
• Most common during periods of environmental
  change
• Diversifying selection happens when
  environmental conditions are varied and favor
  both extremes
• Can lead to balanced polymorphism

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Outcomes of Natural Selection
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Natural Selection Doesnt Make Perfect Organisms

• This is due to
• historical constraints
• Evolution modifies organisms, it doesnt make new
  ones
• adaptive compromises
• Sometimes things that work one way make things
  awkward in the other
• chance events
• Not all evolution is adaptive, sometimes chance
  has a part
• availability of variations
• Selection favors only the best from a pool of
  variations