Chapter 9: Adaptation to Life in Varying Environments

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Chapter 9 Adaptation to Life in Varying
Environments

• Robert E. Ricklefs
• The Economy of Nature, Fifth Edition

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Background - Life in Varying Environments

• The giant red velvet mite lives in one of the
  most forbidding desert environments on earth.
• Its life history represents a series of
  adaptations that optimize survival, growth and
  reproduction while minimizing exposure to
  unsuitable conditions.

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Responses Right and Wrong

• The appropriateness of a response is a function
  of
• the qualities of the environment
• ecological circumstances
• Consider the storage of fat by sparrows
• the right choice if this fat will be needed
• for migration
• to carry the bird through bad weather
• the wrong choice in the absence of such needs,
  because stored fat
• reduces speed and maneuverability
• increases risk of predation

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Adaptation results from natural selection.

• Genotype is the unique genetic constitution of an
  individual.
• Evolution is any change in the genetic makeup of
  a population.
• Natural selection results in evolutionary change
  when genetic factors cause differences in
  fecundity and survival among individuals.
• Fitness is the reproductive success of an
  individual.

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Evolution - An Example

• Evolution of cyanide resistance in citrus scale
• fumigation with cyanide gas was an effective
  control measure early in the 20th century
• as time passed, fumigation became less effective
  as scale evolved genetically based resistance to
  cyanide
• eventually scale regained its former pest status
• Citrus scale had three main ingredients of
  evolution by natural selection
• variation among individuals
• inheritance (genetic basis) of this variation
• differences in fitness related to genetic
  variation

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Evolution guides diversification.

• The diversification of living beings over the
  history of life has been guided primarily by
  natural selection.
• Natural selection is not an external force.
• Natural selection occurs because of differences
  in reproductive success among individuals endowed
  with different form or function within a
  particular environment.

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The phenotype is the expression of the genotype.

• The phenotype is the outward expression of the
  genotype manifested in structure and function
• the genotype is a set of instructions
• the phenotype is the expression of the genotype
  as modified by environmental conditions affecting
  growth and development

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Genes and alleles

• Genes encode proteins
• used as part of an organisms structure
• may function as enzymes or hormones
• Different forms of a particular gene are called
  alleles
• alleles may cause perceptible and measurable
  differences in the phenotype (e.g., eye color)
• defective alleles may cause genetic disorders
• directly sickle-cell anemia, albinism
• indirectly tendencies to develop certain cancers

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Allelic Diversity in Individuals

• Each diploid individual has two copies of each
  allele, one inherited from its mother, the other
  from its father
• a heterozygous individual has two different
  alleles
• a homozygous individual has identical alleles
• alleles may be
• dominant (expressed in heterozygous individual)
• recessive (masked by dominant allele)
• codominant (result in intermediate phenotype in
  heterozygotes)
• most deleterious alleles are recessive

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Phenotypic Plasticity

• Environmentally induced variation in the
  phenotype is referred to as phenotypic
  plasticity.
• The capacity to exhibit phenotypic plasticity may
  itself be an evolved trait.
• It is important that we keep in mind the
  difference between
• plastic responses of individuals
• evolutionary responses by populations

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Each type of organism has an activity space.

• The organism functions best within a narrow range
  of environmental conditions which define its
  activity space.
• activity is equivalent to performance
• activity may be measured as any trait (swimming
  speed, photosynthesis, survival) that influences
  individuals fitness

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Biological activity is related to environmental
conditions.

• For a given environmental factor, we can identify
  various ranges relative to activity
• optimal range (above minimum level required to
  maintain population)
• suitable range (above minimum level required to
  maintain organism)
• marginal range (above minimum level required to
  maintain critical functions)
• unsuitable range (fatal for extended periods)

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Organisms can select microhabitats.

• Within habitats, there are finer-scale variations
  referred to as microhabitats or
  microenvironments
• these represent distinct differences in
  temperature, moisture, salinity, and other
  factors within a particular habitat
• In desert habitats, for example
• shaded ground under shrubs is cooler and moister
  than surrounding areas exposed to direct sunlight
• such differences may vary diurnally or seasonally

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Behavioral Cycles in Lizards

• Lizards can regulate body temperature by diurnal
  behavioral cycles
• lizards do not regulate temperature by generating
  metabolic heat
• by moving about, they select various
  microhabitats
• lizards take advantage of differences in solar
  radiation and temperatures of various surfaces to
  maintain body temperatures within a suitable
  range during a day

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Cactus wrens select microhabitats to optimize
energy budgets.

• The desert habitat offers varied microhabitats,
  ranging from exposed ground in full sun to deep
  shade of trees.
• Cactus wrens of our southwestern deserts take
  advantage of various microhabitats
• in early morning, they forage widely
• as the day becomes progressively warmer, they
  restrict activity to cooler microhabitats
• nests are positioned to shelter from (spring) or
  face (summer) prevailing winds

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Acclimation is a reversible change in structure.

• Acclimation is a shift in the range of
  physiological tolerances of the individual.
• Some examples
• growing thicker fur in winter
• producing smaller leaves in the dry season
• increasing the number of red blood cells at
  higher elevations
• producing enzymes with different temperature
  optima
• producing lipids that remain fluid at different
  temperatures

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Thermal Acclimation in Goldfish

• Goldfish swim most rapidly when
• acclimated at 25oC
• placed in water between 25oC and 30oC
• When acclimated at 5oC, goldfish
• swim most rapidly at 15oC
• sacrifice ability to swim fast at 25oC
• Increased tolerance of one extreme often brings
  reduced tolerance at another.

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Variation in Potential for Acclimation

• The ability to acclimate reflects the typical
  range of conditions experienced
• creosote bush (Larrea) experiences a wide range
  of temperatures and its photosynthetic ability
  acclimates well to both cool and warm
  temperatures
• Atriplex grows under cool conditions and does not
  acclimate well to high temperatures
• Tidestromia grows under hot conditions and does
  not acclimate well to low temperatures

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Developmental responses are irreversible changes.

• Consider the developmental responses of loblolly
  pines grown under different light regimes
• shade-grown seedlings allocate more energy to
  stem and needles
• sun-grown seedlings allocate more energy to root
  systems
• greater proportion of needles in shade-grown
  plants enhances photosynthetic rate per unit
  plant mass, especially under low-light conditions

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Developmental Responses in Grasshoppers

• The grasshopper Gastrimargus africanus can match
  its color to that of its surroundings
• helps avoid detection by predators
• epidermal pigments laid down at each molt respond
  to hormones produced in the brain in response to
  quality and intensity of light
• animals are green in rainy season
• as dry season comes on, animals are brown
• following fires, animals are black

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Where do we find developmental responses?

• As a rule, plants and animals in habitats with
  persistent variation exhibit such responses.
• For plants, spatial heterogeneity creates the
  kind of persistent variation favoring
  developmental responses.

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Migration, Storage, and Dormancy

• When extremes of environment are so adverse as to
  prevent normal activities, organisms
• cannot adapt to such extreme conditions
• or they can adapt, but such adaptations would be
  too costly
• Alternative strategies include migration,
  storage, and dormancy.

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Migration

• Migration is moving to another region where
  conditions are more favorable
• arctic terns make annual migrations of 30,000 km,
  moving from summer in Arctic to summer in
  Antarctic
• monarch butterflies migrate seasonally from
  Mexico and southern US into southern Canada
• African ungulates follow geographic patterns of
  rainfall and fresh vegetation
• migration in locusts represents a developmental
  response at high population densities

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Storage

• Storage is the reliance on resources accumulated
  under more favorable conditions
• desert cacti store water during rainy periods
• plants of infertile habitats store nutrients
  during periods of temporary abundance
• animals of temperate and polar regions store fat
  for periods of severe weather during winter
• some mammals and birds cache food supplies

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Dormancy

• Dormancy is becoming inactive
• tropical and subtropical trees shed leaves during
  seasonal droughts
• mammals undergo hibernation
• some insects enter winter diapause, reducing
  their freezing point and metabolic rate
• other insects enter summer diapause, tolerating
  dessication
• plant seeds and spores of bacteria and fungi
  exhibit effective dormancy mechanisms

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Stimuli for Change

• How do organisms sense impending environmental
  severity?
• proximate factors are cues (such as day length)
  used to assess environmental factors but which do
  not directly affect well-being
• ultimate factors are features of the environment
  (such as food supply) which directly affect
  well-being
• Different populations of the same species may
  respond in dramatically different ways to the
  same cues.

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Animals forage optimally

• Theories of optimal foraging seek explanations
  for decisions that animals make while foraging
• where to forage
• how long to remain in a particular patch
• which types of food to eat
• Optimal foraging theories examine costs and
  benefits to animals of various decisions
• expectation is that animals will select the
  behaviors that yield the greatest benefits

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Central Place Foraging

• When animals are tied to a particular place
  (e.g., a nest with offspring in it) they
  experience tradeoffs associated with distance
  they forage
• increasing foraging range results in
• greater potential for finding food
• greater time, energy costs, risks of travel
• foraging range should maximize amount of food
  returned per unit time

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Are starlings optimal foragers?

• Research with starlings shows that they can
  maximize return rates by selecting intermediate
  foraging times and returning with less than the
  maximum possible amount of food.
• Experimental studies show that starlings do
  forage optimally, adjusting upward their load
  size as round-trip travel time increases, as
  predicted by foraging theory.

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Risk-Sensitive Foraging

• The value of a feeding area is reduced by the
  presence of risks, particularly predation
• predation has been incorporated into foraging
  theory in studies of risk-sensitive foraging
• do animals incorporate risk of predation into
  decision-making?
• experimental studies, in which availability of
  food and predator density were both varied,
  showed that minnows incorporated predation risks
  into their foraging decisions

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Prey Choice

• Foraging decisions include choices concerning
  prey items
• each food item has intrinsic value based on
• nutrient and energy content
• difficulty of handling
• potential danger from toxins
• poor-quality foods may require more handling time
  or take more time to digest, reducing overall
  rate of food intake

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Diet Mixing

• Why do foragers consume a mixed diet?
• different foods may be complementary, each
  providing essential nutrients missing in the
  other
• humans can subsist on rice and beans, but not on
  either of these alone (complementary amino acids)
• grasshoppers foraging on mixed diets grow faster
  than those fed a single food
• birds selectively consume fruits that differ from
  the more abundant background

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Summary 1

• Responses of organisms to their environments have
  evolved in response to selective pressures in
  these environments.
• Organisms have characteristic activity spaces and
  can select appropriate microhabitats.
• Acclimation and developmental responses permit
  organisms to respond to varying environments.

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Summary 2

• When environmental conditions exceed tolerances,
  organisms may migrate, rely on stored materials,
  or become dormant.
• Animals adjust their foraging activities to
  optimize the net capture of resources per unit
  time.
• Foragers also account for risks, and balance
  nutritional needs.