The Mechanisms of Evolution

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The Mechanisms of Evolution
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The Mechanisms of Evolution

• Charles Darwins Theory of Evolution
• Genetic Variation within Populations
• The HardyWeinberg Equilibrium
• Evolutionary Agents and Their Effects
• The Results of Natural Selection
• Assessing the Costs of Adaptations
• Maintaining Genetic Variation
• Constraints on Evolution
• Cultural Evolution
• Short-Term versus Long-Term Evolution

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Charles Darwins Theory of Evolution

• Darwin was a student at Cambridge University when
  his botany professor recommended him for a
  position as the ships naturalist on the H.M.S.
  Beagle, which was preparing to sail around the
  world.
• Observations made on this trip helped Darwin
  formulate his theory of evolution, which had two
  major components.
• First, species are not immutable, but change, or
  adapt, over time.
• Second, the agent that produces the changes is
  natural selection.

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Figure 23.1 Darwin and the Voyage of the Beagle
(Part 1)
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Figure 23.1 Darwin and the Voyage of the Beagle
(Part 2)
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Charles Darwins Theory of Evolution

• Darwin did not publish his theory of evolution
  immediately he chose to collect more evidence to
  support his ideas.
• Fourteen years after Darwin first made the
  observations, Alfred Russel Wallace came to
  similar conclusions independently.
• On July 1, 1858, Darwins and Wallaces ideas
  were presented to the Linnaean Society of London.
• A year later Darwin published The Origin of
  Species.

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Charles Darwins Theory of Evolution

• Darwin observed that slight variations among
  individuals can significantly affect the chance
  that a given individual will survive and the
  number of offspring it will produce.
• Darwin called this differential reproductive
  success of individuals natural selection.
• It is likely that Darwin used this term because
  he was a pigeon breeder and familiar with
  artificial selection in the breeding of
  domesticated animals.

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Figure 23. Many Types of Pigeons Have Been
Produced by Artificial Selection
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Charles Darwins Theory of Evolution

• Darwin clearly understood a fundamental principle
  of evolutionthat populations, not individuals,
  evolve and become adapted to the environments in
  which they live.
• The term adaptation has two meanings in
  evolutionary biology.
• The first meaning refers to the processes by
  which adaptive traits are acquired.
• The second meaning refers to the traits that
  enhance the survival and reproductive success of
  their bearers.

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Charles Darwins Theory of Evolution

• When Darwin proposed his theory, he had no
  examples of selection operating in nature and
  knew nothing of the mechanisms of heredity.
• The rediscovery of Gregor Mendels publications
  gave rise to the study of population genetics
  which provides a major underpinning for Darwins
  theories.
• Population geneticists apply Mendels laws to
  entire populations.
• Population geneticists study variation within and
  among species in order to understand the
  processes that result in evolutionary changes in
  species through time.

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Genetic Variation within Populations

• For a population to evolve, its members must
  possess heritable, genetic variation, which is
  the raw material on which agents of evolution
  act.
• We observe phenotypes in nature, the physical
  expressions of genes.
• The genetic constitution that governs a trait is
  called its genotype.
• A population evolves when individuals with
  different genotypes survive or reproduce at
  different rates.

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Genetic Variation within Populations

• Genes have different forms called alleles.
• A single individual has only some of the alleles
  found in the population to which it belongs.
• The sum of all the alleles in a population is the
  gene pool.
• The gene pool contains the variation (different
  alleles) that produces the differing phenotypes
  on which agents of evolution act.

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Figure 23.3 A Gene Pool
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Genetic Variation within Populations

• Natural populations possess genetic variation.
• For example, selection for traits in a wild
  mustard has produced many important crop plants.

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Figure 23.4 Many Vegetables from One Species
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Genetic Variation within Populations

• Laboratory experiments also demonstrate the
  genetic variation present in organisms.
• Fruit flies (Drosophila melanogaster) with high
  or low number of bristles on their abdomens were
  selected and bred for 35 generations.
• Numbers of bristles in flies in the two lineages
  then fell well outside the original range of the
  population.

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Figure 23.5 Artificial Selection Reveals Genetic
Variation
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Genetic Variation within Populations

• The study of the genetic basis of evolution is
  difficult because genotypes do not uniquely
  determine phenotypes.
• Dominance can lead to a particular phenotype
  being expressed by more than one genotype.
• Different phenotypes can also be produced by a
  given genotype, depending on environmental
  conditions encountered during development.

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In-Text Art p. 464
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Genetic Variation within Populations

• A locally interbreeding group within a geographic
  population is called a Mendelian population.
• The relative proportions, or frequencies, of all
  alleles in a population are a measure of that
  populations genetic variation.
• Biologists can estimate allele frequencies for a
  given locus by measuring numbers of alleles in a
  sample of individuals from a population.

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Genetic Variation within Populations

• Measurements of allele frequencies range from 0
  to 1, and the sum of all allele frequencies at a
  locus is 1.
• An alleles frequency (p) is calculated by
  dividing the number of copies of the allele in a
  population by the sum of alleles in the
  population.
• If only two alleles (A and a) for a given locus
  are found among the members of a diploid
  population, they may combine to form three
  different genotypes AA, Aa, and aa.

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Genetic Variation within Populations

• Allele frequencies can be calculated using
  mathematics with the following variables
• NAA the number of individuals that are
  homozygous for the A allele (AA)
• NAa the number of individuals that are
  heterozygous (Aa)
• Naa the number of individuals that are
  homozygous for the a allele (aa)
• Note that NAA NAa Naa N, the total number
  of individuals in a population.

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Genetic Variation within Populations

• The total number of alleles in a population is 2N
  because each individual is diploid (in this case,
  either AA, Aa, or aa).
• p the frequency of allele A.
• q the frequency of allele a.
• For each population, p q 1.

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Figure 23.6 Calculating Allele Frequencies
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Genetic Variation within Populations

• The two populations in this example have the same
  allele frequencies for A and a, but they are
  distributed differently. Therefore, the genotype
  frequencies of the two populations are different.
• Genotype frequency is the number of individuals
  with the genotype divided by the total number of
  individuals in the population.
• The frequencies of different alleles at each
  locus and the frequencies of different genotypes
  in a Mendelian population describe its genetic
  structure.

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The HardyWeinberg Equilibrium

• A population of sexually reproducing organisms in
  which allele and genotype frequencies do not
  change from generation to generation is said to
  be at HardyWeinberg equilibrium.
• Five assumptions must be made in order to meet
  HardyWeinberg equilibrium.
• Mating is random.
• Population size is very large.
• There is no migration between populations.
• There is no mutation.
• Natural selection does not affect the alleles
  under consideration.

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The HardyWeinberg Equilibrium

• If the conditions of the HardyWeinberg
  equilibrium are met, two results follow.
• The frequencies of alleles at a locus will remain
  constant from generation to generation.
• After one generation of random mating, the
  genotype frequencies will not change.
• The second result can be stated in the form of
  the HardyWeinberg equation p2 2pq q2 1.

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Figure 23.7 Calculating HardyWeinberg Genotype
Frequencies (Part 1)
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Figure 23.7 Calculating HardyWeinberg Genotype
Frequencies (Part 2)
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The HardyWeinberg Equilibrium

• The most important message of the HardyWeinberg
  equilibrium is that allele frequencies remain the
  same from generation to generation unless some
  agent acts to change them.
• The equilibrium also shows the distribution of
  genotypes that would be expected for a population
  at genetic equilibrium.
• The HardyWeinberg equilibrium allows scientists
  to determine whether evolutionary agents are
  operating and their identity (as evidenced by the
  pattern of deviation from the equilibrium).

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Evolutionary Agents and Their Effects

• Evolutionary agents cause changes in the allele
  and genotype frequencies in a population.
• These are observed as a deviations from the
  HardyWeinberg equilibrium.
• The known evolutionary agents are mutation, gene
  flow, random genetic drift, nonrandom mating, and
  natural selection.

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Evolutionary Agents and Their Effects

• The origin of genetic variation is mutation. A
  mutation is any change in an organisms DNA.
• Most mutations appear to be random and are
  harmful or neutral to their bearers.
• Some mutations can be advantageous.
• Mutation rates are low one out of a million loci
  is typical.
• Although mutation rates are low, they are
  sufficient to create considerable genetic
  variation.

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Evolutionary Agents and Their Effects

• One condition for HardyWeinberg equilibrium is
  that there is no mutation.
• Although this condition is never met, the rate at
  which mutations arise at single loci is usually
  so low that mutations result in only very small
  deviations from HardyWeinberg expectations.
• If large deviations are found, it is appropriate
  to dismiss mutation as the cause and look for
  evidence of other evolutionary agents.

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Evolutionary Agents and Their Effects

• Gene flow results when individuals migrate to
  another population and breed in their new
  location.
• Immigrants may add new alleles to the gene pool
  of a population, or they may change the
  frequencies of alleles already present if they
  come from a population with different allele
  frequencies.
• No immigration is allowed for a population to be
  in HardyWeinberg equilibrium.

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Evolutionary Agents and Their Effects

• Genetic drift is the random loss of individuals
  and the alleles they possess.
• In very small populations, genetic drift may be
  strong enough to influence the direction of
  change of allele frequencies even when other
  evolutionary agents are pushing the frequencies
  in a different direction.
• Organisms that normally have large populations
  may pass through occasional periods when only a
  small number of individuals survive (a population
  bottleneck).

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Figure 23.8 A Population Bottleneck
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Evolutionary Agents and Their Effects

• During a population bottleneck, genetic variation
  can be reduced by genetic drift.
• Populations in nature pass through bottlenecks
  for numerous reasons for example, predation and
  habitat destruction may reduce the population to
  a very small size, resulting in low genetic
  variation.

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Figure 23.9 A Species with Low Genetic Variation
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Evolutionary Agents and Their Effects

• When a few pioneering individuals colonize a new
  region, the resulting population will not have
  all the alleles found among members of the source
  population.
• The resulting pattern of genetic variation is
  called a founder effect.

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Figure 23.10 A Founder Effect
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Evolutionary Agents and Their Effects

• Nonrandom mating occurs when individuals mate
  either more often with individuals of the same
  genotype or more often with individuals of a
  different genotype.
• The resulting proportions of genotypes in the
  following generation differ from HardyWeinberg
  expectations.
• If individuals mate preferentially with other
  individuals of the same genotype, homozygous
  genotypes are overrepresented and heterozygous
  genotypes are underrepresented in the next
  generation.
• Conversely, individuals may mate preferentially
  with individuals of a different genotype.

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Figure 23.11 Flower Structure Fosters Nonrandom
Mating (Part 1)
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Figure 23.11 Flower Structure Fosters Nonrandom
Mating (Part 2)
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Evolutionary Agents and Their Effects

• Self-fertilization (selfing) is another form of
  nonrandom mating that is common in many
  organisms, especially plants.
• Selfing reduces the frequencies of heterozygous
  individuals below HardyWeinberg expectations and
  increases the frequencies of homozygotes, without
  changing allele frequencies.

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Evolutionary Agents and Their Effects

• For adaptation to occur, individuals that differ
  in heritable traits must survive and reproduce
  with different degrees of success.
• When some individuals contribute more offspring
  to the next generation than others, allele
  frequencies in the population change in a way
  that adapts individuals to the environments that
  influenced their success.
• This process is known as natural selection.

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Evolutionary Agents and Their Effects

• The reproductive contribution of a phenotype to
  subsequent generations relative to the
  contributions of other phenotypes is called its
  fitness.
• The fitness of a phenotype is determined by the
  average rates of survival and reproduction of
  individuals with that phenotype.

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The Results of Natural Selection

• Most characters are influenced by alleles at more
  than one locus and are more likely to show
  quantitative rather then qualitative variation.
• For example, the size of individuals in a
  population is influenced by genes at many loci,
  and distribution of sizes is likely to be a
  bell-shaped curve.
• Natural selection can act on characters with
  quantitative variation in three ways
• Stabilizing selection
• Directional selection
• Disruptive selection

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The Results of Natural Selection

• Stabilizing selection preserves the
  characteristics of a population by favoring
  average individuals.
• Stabilizing selection occurs when the extremes of
  a population contribute relatively fewer
  offspring than the average members to the next
  generation.
• Stabilizing selection operates on human birth
  weight. Babies that are born lighter or heavier
  than the population mean die at higher rates than
  babies whose weights are close to the mean.

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Figure 23.12 Natural Selection Can Operate on
Quantitative Variation in Several Ways (Part 1)
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Figure 23.13 Human Birth Weight Is Influenced by
Stabilizing Selection
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The Results of Natural Selection

• Directional selection changes the characteristics
  of a population by favoring individuals that vary
  in one direction from the mean of the population.
• Directional selection occurs when one extreme of
  a population contributes more offspring to the
  next generation.
• Directional selection produced resistance to
  tetrodotoxin (TTX) in garter snakes.

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Figure 23.12 Natural Selection Can Operate on
Quantitative Variation in Several Ways (Part 2)
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Figure 23.14 Resistance to TTX Is Associated
with the Presence of Newts
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The Results of Natural Selection

• Disruptive selection changes the characteristics
  of a population by favoring individuals that vary
  in both directions from the mean of the
  population.
• Disruptive selection occurs when individuals at
  both extremes of a population are simultaneously
  favored.
• The bill sizes of black-bellied seedcrackers
  provide an example of disruptive selection.

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Figure 23.12 Natural Selection Can Operate on
Quantitative Variation in Several Ways (Part 3)
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Figure 23.15 Disruptive Selection Results in a
Bimodal Distribution
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The Results of Natural Selection

• Sexual selection was Darwins explanation for the
  evolution of apparently useless but conspicuous
  traits in males of many species, such as bright
  colors, long tails, horns, antlers, and elaborate
  courtship displays.
• He hypothesized that these traits either improved
  the ability of their bearers to compete for
  access to members of the other sex (intrasexual
  selection) or made them more attractive to the
  other sex (intersexual selection).
• Sexual selection may result in sexually dimorphic
  species.

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The Results of Natural Selection

• In widowbirds, males with longer tails attract
  significantly more females than do males with
  shorter tails.
• Females may prefer males with longer tails
  because the ability to grow and maintain such a
  structure may indicate that the male is vigorous
  and healthy.

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Figure 23.16 The Longer the Tail, the Better the
Male
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The Results of Natural Selection

• The hypothesis that having well-developed
  ornamental traits signals vigor and health has
  been tested experimentally.
• Zebra finch bills are bright red because of
  carotenoids in their diet.
• Carotenoids are antioxidants and part of the
  immune system. Males in good health will have
  brighter bills because they need to allocate
  fewer carotenoids to immune function.
• Zebra finch males were fed diets with and without
  carotenoids. The diet with carotenoids enhanced
  immune function.

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Figure 23.17 Bright Bills Signal Good Health
(Part 1)
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Figure 23.17 Bright Bills Signal Good Health
(Part 2)
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Assessing the Costs of Adaptations

• Adaptations generally impose costs as well as
  benefits.
• Determining the costs and benefits of a
  particular adaptation is difficult because
  individuals differ not only in the degree to
  which they possess the adaptation, but also in
  many other ways.
• Recombinant DNA techniques allow investigators to
  compare individuals that differ only in the
  genetically based adaptation of interest.

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Assessing the Costs of Adaptations

• In plants, plasmids can be used to transfer
  specific alleles to experimental individuals.
• Plasmid transfer techniques have been used to
  measure the cost associated with the resistance
  to an herbicide conferred by a single allele in
  Arabidopsis thaliana.
• Plants with the resistance allele produce 34
  percent fewer seeds than nonresistant plants,
  indicating a high cost for resistance to the
  herbicide.

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Figure 23.18 Producing and Maintaining
Resistance Is Costly (Part 1)
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Figure 23.18 Producing and Maintaining
Resistance Is Costly (Part 2)
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Assessing the Costs of Adaptations

• In polygynous species, such as deer, lions, and
  baboons, one male controls reproductive access to
  many females.
• Polygynous species tend to be sexually dimorphic,
  with males that are generally much larger than
  females and that generally bear weapons.
• There are costs to the males for this sexual
  dimorphism, including higher parasite loads and
  higher mortality rates.

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Figure 23.19 Sexually Selected Traits Impose
Costs
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Maintaining Genetic Variation

• Genetic drift, stabilizing selection, and
  directional selection all tend to reduce genetic
  variation within an animal population.
• However, most species have considerable genetic
  variation.

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Maintaining Genetic Variation

• When organisms reproduce sexually, existing
  genetic variation is amplified.
• Random assortment of chromosomes during meiosis,
  crossing over, and the cellular component of each
  gamete contribute to the diversity of offspring.
• Sexual recombination does not alter the frequency
  of alleles rather, it generates new combinations
  of alleles on which natural selection can act.
• It expands variation in a trait influenced by
  alleles at many loci by creating new genotypes.

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Maintaining Genetic Variation

• An allele that does not affect the fitness of an
  organism is called a neutral allele.
• Neutral alleles tend to accumulate in a
  population of organisms over time, resulting in
  genetic variation.
• Most variation in neutral alleles cannot be
  observed without the aid of molecular biology
  techniques.

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Maintaining Genetic Variation

• A polymorphism is the coexistence of two or more
  alleles at a locus at frequencies greater than
  mutations can produce.
• A polymorphism may be maintained when the fitness
  of a genotype (or phenotype) varies with its
  frequency relative to that of other genotypes (or
  phenotypes).
• This process is know as frequency-dependent
  selection.
• Fish with right- and left-mouthed individuals in
  Lake Tanganyika are an example of
  frequency-dependent selection in action.

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Figure 23.20 A Stable Polymorphism
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Maintaining Genetic Variation

• Subpopulations vary genetically because they are
  subjected to different selective pressures in
  different environments.
• Plant populations can vary geographically in the
  chemicals they synthesize to defend themselves
  from herbivores.
• Clover containing cyanide in Europe is an example
  of this phenomenon.

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Figure 23.21 Geographic Variation in Poisonous
Clovers
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Constraints on Evolution

• Thus far, it has been implied that sufficient
  genetic variation always exists for the evolution
  of favored traits this is not always true.
• Evolution is limited by a serious constraint
  Evolutionary changes must be based on
  modifications of previously existing traits.
• For example, skates and rays evolved from sharks
  with somewhat flattened bodies. They lie on
  their bellies on the sea bottom.
• Plaice and flounder descended from laterally
  flattened fish, and therefore lie on their sides.

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Figure 23.22 Two Solutions to a Single Problem
(Part 1)
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Figure 23.22 Two Solutions to a Single Problem
(Part 2)
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Cultural Evolution

• Cultural evolution is a means of acquiring new
  traits by learning them from other individuals.
• Cultural evolution is most highly developed in
  humans, but is seen in other animals including
  birds and apes.
• The only requirement for traits to evolve via
  cultural evolution is that individuals have the
  ability to learn them.
• Birds will copy the songs of other individuals,
  resulting in the evolution of song dialects.
• Apes use a number of learned behaviors, including
  specialized feeding techniques and alternative
  forms of social signals.

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Figure 23.23 Orangutans Have Culturally
Transmitted Behaviors
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Short-Term versus Long-Term Evolution

• Short-term changes in allele frequencies within
  populations can be observed directly and
  exemplify actual evolutionary processes in
  action.
• However, they do not allow scientists to predict
  (or postdict, because they have already
  happened) long-term evolutionary changes.
• Patterns of evolutionary change can be strongly
  influenced by events that occur so infrequently
  or so slowly that they are unlikely to be
  observed during short-term studies.
• Also, the ways in which evolutionary agents act
  may change with time.