BI113 Gen Bio II

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BI-113 Gen Bio II

• Presentation 6
• Evolution and Speciation

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Greek Philosophers
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Origins of organismal diversity

• Many Greek philosophers believed in the gradual
  evolution of life, but the two most influential
  philosophers, Plato and Aristotle, did not
• Plato - saw variations in plants and animals as
  imperfect versions of Gods idealized form
• Aristotle - recognized increasing structural
  complexity exhibited by organisms this
  increasing complexity became known as the scala
  naturae (the scale of nature) with species
  being fixed in time/space like rungs on a ladder

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Carolus Linnaeus (1707-1778)

• Swedish physician/botanist
• Developed binomial system of naming organisms
  designed to organize the diversity of life ad
  majorem Dei gloriam. Father of TAXONOMY, the
  naming and classification of organisms
• Systema Naturae (1735)

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Jean-Baptiste Lamarck (1744-1829)

• French naturalist
• Two prevailing ideas of evolutionary mechanisms
• Use/disuse - Those physical traits that are used
  become built up, those that are not used
  deteriorate
• Inheritance of acquired characteristics
• giraffes neck story

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Inheritance of acquired traits

• Much of Lamarcks ideas about evolution are
  ridiculed, today, but we must remember that he
  had no knowledge of genetics or inheritance
• Lamarck was vilified in his own time, since
  evolution was an idea that opposed the popular
  notion of divine creation

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Georges Cuvier (1769-1832)

• French anatomist / paleontologist
• Catastrophism explained the discontinuity in the
  fossil record (Biblical events of Gods making)
• Cuvier saw species as fixed entities unchanging

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Two views of species

• Species emigrated to repopulate and colonize
  devastated areas
• God created new species after each global
  catastrophe

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Charles Darwin (1809-1882)

• English naturalist
• At age 22, began the voyage that would help him
  form his ideas on evolutionary mechanisms aboard
  the HMS Beagle
• On the Origin of Species published in 1859
• Evolution by natural selection

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Darwins trip
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Darwins ideas

• Darwin incorporated aspects of geology and
  biology into his theory of evolution
• Earth is very old and constantly changing
• Drew on Huttons/Lyells concept of
  uniformitarionism and the fossil record
• Descent with modification
• Darwin perceived evolution as a slow process
• Life has come about such that organisms have the
  potential to change - this change is, in part,
  due to reproduction (genetic variability that is
  inherent in sexual reproduction)

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Alfred Wallace (1822-1884)

• Contemporary of Darwins - actually encouraged
  Chuck to publish his ideas about evolution by
  natural selection
• Developed many of the same theories as Darwin
  while in the East Indies
• "...every species comes into existence coincident
  in time and space with a preexisting closely
  allied species." (1855)

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How does evolution occur?

• All species have the ability to reproduce
  themselves (reproductive potential)
• Species tend to produce more offspring than can
  be supported
• There are a limited number of resources to
  support members of a population
• As a result, there is competition among
  individuals for these limited resources

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Variability is crucial

• Within any species (population) there is
  variation
• Individuals that are best adapted to a particular
  set of environmental conditions will survive and
  reproduce - succeed
• Those with unsuccessful traits will not reproduce
• Success is defined by the ability to reproduce
  and pass on your genetic material
• Since environmental conditions also change over
  time, the traits that are successful now may NOT
  be successful in the future or given a different
  set of environmental conditions

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Gregor Mendel (1822-1884)

• Austrian monk
• Father of Genetics
• Worked with garden peas in monastery

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Genetic variation

• These concepts have become central to
  evolutionary theory, but were unknown to Darwin
• Mendels concepts about genetic inheritance of
  characteristics (traits) were the piece of the
  puzzle that Darwin lacked to explain the
  mechanism of natural selection

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Natural selection

• One organism cannot evolve or show range of
  genetic variation
• Populations are the functional units of evolution
• Group of populations that can interbreed is a
  species
• Local populations are more likely to be related
  to each other and, thus, be more genetically
  similar
• All individuals in a population contribute to the
  gene pool (all the genes present for species)

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The Basis for Genetic Variation

• Genes
• Segments of DNA that are located at a particular
  place on the chromosomes
• Each gene is a unit of heredity that codes for a
  particular amino acid sequence, which codes for a
  particular protein and thus a specific trait
• One gene-one protein
• These proteins are often enzymes that are
  responsible for metabolic function, catalytic
  activity, or the expression of cellular events

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Genetic inheritance

• Alleles
• Are variations in genes that code for the same
  protein
• Population Genetics
• The determination of allele frequencies in
  populations
• Natural selection acts on the phenotype, but
  selects particular genotypes that are responsible
  for that phenotype

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Genetic inheritance

• As a result of fertilization, most organisms
  receive one allele from a maternal source and one
  from a paternal source
• For any given gene, alleles may be described as
  being
• Homozygous
• the same allelic form, AA or aa
• Heterozygous
• different alleles, Aa

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Hardy-Weinberg Equilibrium

• Godfrey Hardy and Wilhelm Weinberg (1908)
• Defined a population in equilibrium as one in
  which allele frequencies and the distribution of
  genotypes remain constant with successive
  generations
• p2 2pq q2 1.0
• where p is frequency of A and q is frequency of a
• It does not evolve

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H-W Assumptions

• Requires that several conditions be met
• NO mutation
• NO gene flow between populations that is, there
  must be no net migration of alleles into a
  population (immigration) or out of the population
  (emigration)
• Population must be very large (theoretically
  infinite)
• Mating must be random, not assortative
• There must be no natural selection i.e., all
  genotypes must be equally adaptive and reproduce
  equally well

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Under these conditions,

• Allele frequencies within a population will
  remain constant indefinitely
• If one or more of these conditions are violated,
  then allele frequencies will change
• Evolution will occur
• Hardy-Weinberg conditions are almost never met,
  so what good is the principle?
• It is beneficial for looking at the process of
  evolution

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Factors of evolutionary change

• Mutation
• Gene flow
• Nonrandom mating
• Genetic drift
• Selection

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Mutations

• The ultimate source of genetic variability
• Changes in DNA sequence by deletions, shifts,
  transpositions, substitutions, etc
• Mutations are rare
• 1 in 10,000 to 1,000,000 genes per individual per
  generation
• Not important in changing gene frequencies, but
  are the source for new alleles on which natural
  selection can act

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Mutations are random

• Not goal-directed
• Most are deleterious and never passed on to
  succeeding generations
• selected against
• Whether a mutation is helpful or harmful depends
  on the environmental conditions present when the
  mutation arises

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Gene flow

• Movement of individuals (and thus their genes)
  between populations of a given species
• Migration
• Individuals (and their genes) tend to move from
  one population to another
• Immigration
• movement of alleles INTO a population
• Emigration
• movement of alleles OUT of a population

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Nonrandom mating

• Mating is seldom random
• Tendency to mate with members of a species that
  are geographically close
• Localized populations
• Harem mating
• Assortative mating
• Sexual selection
• Often dictated by the female (mate choice)

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Population size

• Small populations are subject to random changes
  in allele frequencies (much more so than large
  populations)
• This has become an important consideration for
  conservation biologists
• Many species with reduced genetic variability as
  the result of small population size
• Florida panther, California condor, Houston toad

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Genetic drift

• Changes in allele frequencies that are the
  consequence of small population size
• Bottleneck effects
• Reduction in a populations genetic variability
  due to dramatic decrease in numbers (often
  through catastrophic events)
• Worse for small populations
• Loss of genetic variability
• Ladybird beetle example

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20 of population with unique characteristic
Small population (N5)
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Intermediate population size (N25)
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Selection (Natural and otherwise)

• All genotypic traits are not equally adaptive
• Some traits have more relevance to survivorship
  and the production of viable offspring
• Acts on the phenotype, but reflects the
  underlying genotype
• All phenotypic characters have a genotypic basis
• Physiological/metabolic
• Behavioral
• Morphological

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Types of selection

• Stabilizing
• Selection for median trait
• Leads to homogeneity
• Directional
• Selection toward one extreme
• Disruptive
• Selection toward both extremes (away from
  median), can create new species in situ

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Types of Selection
DIRECTIONAL
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Types of Selection
STABILIZING
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Types of Selection
DISRUPTIVE