Population Ecology 4

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Population Ecology 4
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• 4- Population Ecology
• Human Impacts on Populations Chpt 18
• Population management concepts
• Managing r-selected populations
• Managing k-selected population
• Caughleys six points of harvesting
• Managing habitats
• Managing populations
• Collapse of whale stocks
• Collapse of fish stocks
• Symptoms of over-exploitation
• Restoration

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• 4- Population Ecology
• Human Impacts on Populations Chpt 18
• Humans are the top predator and ultimate
  competitor.
• We are specialized predators who simultaneously
  exploit populations
• while attempting to manage them.

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• Population management concepts
• The standing crop is the biomass present in a
  population at the time it is measured.
• Productivity is the difference between the
  biomass left in the population after harvesting
  at time t and the biomass present in the
  population just before harvesting at some
  subsequent time t 1.
• 2. The objective of regulated exploitation of a
  population is sustained yield the yield per unit
  time being equal to productivity per unit time.

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3. Maximum sustained yield - the level of
sustained yield at which the population declines
if exceeded. 4. Optimum sustained yield- the
level of sustained yield determined by
consideration of other factorssuch as species
interactions, esthetics, habitat needs, land use
problems, etc.as well as maximum sustained
yield.
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• The major factor determining sustained yield is
  the rate of increase (r) difference between
  birth rate and death rate of the exploited
  population.
• Achievement of maximum sustained yield is not
  always an appropriate goal because it fails to
  consider factors such as species interactions,
  esthetics, etc.

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• The management of r-selected species differs from
  that of K-selected species
• The management of r-selected species must
  consider that current population is not
  correlated with future population size and that
• just a few years of reproductive failure can
  result in collapse of a population.

species characterized by scramble competition,
short life spans, high reproductive rates at low
population densities, a large number of offspring
with low survival, and density-independent
population regulation (i.e., regulated by climate
or temperature)
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- Management of K-selected species (long-lived
specialists) must consider factors such as age
structure and fluctuations in fecundity.

• K-strategies exist in environments in which
  mortality relates more to density than to
  unpredictability of conditions
• they are specialists efficient users of a
  particular environment
• but their populations are at or near carrying
  capacity and are resource limited.
• The maximum rate of harvest depends on age
  structure, frequency of harvest, number left
  behind after harvest, fluctuations in the
  environment, and variations in fecundity, as well
  as density of the population to be harvested and
  the rate of harvest needed to stabilize the
  density at that level.

species characterized by density-dependent
population regulation, high competition,
long-lived individuals, a slower growth rate at
low populations, but the ability to maintain that
growth rate at high densities, and low number of
offspring
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Example of managing an r-selected species

• R-selected populations are often difficult to
  manage because the stock can be severely depleted
  unless there is repeated reproduction.
• An example is the Pacific sardine, a species in
  which there is little relationship between
  breeding stock and the subsequent number of
  progeny produced.
• Exploitation of the Pacific sardine population in
  the 1940s and 1950s shifted the age structure to
  younger age classes.
• Prior to exploitation, 77 of the reproduction
  was distributed among the first five years.
• In the fished population, 77 of the reproduction
  occurred in the first two years of life.
• The population approached that of single-stage
  reproduction subject to pronounced oscillations.
• Two consecutive years of reproductive failures
  resulted in a collapse of the population from
  which it has yet to recover.
• Overfishing, environmental changes, and an
  increase in a competing fish (anchovy) made the
  population collapse.

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An example of r- strategy
(a) Simulation of an exploited and an unexploited
population of sardines, both subject to random
environmental variation in reproductive success.
The dashed lines indicate population size at K.
(b) The annual catch of Pacific sardine along the
Pacific Coast of North America.
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• Caughleys (1976) six points applicable to
  harvesting of populations
• A population stable in numbers must be reduced
  below a steady density to obtain a croppable
  surplus.
• There is an appropriate sustained yield for each
  density to which a population is reduced.
• For each level of sustained yield, there are two
  levels of density from which this sustained yield
  can be harvested.
• Maximum sustained yield can be harvested at only
  one density.

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(a) A sustained yield model for K-strategists
harvested under three regimes. The 45o line
represents the replacement level of the
population. Where it intercepts the curve,
reproduction balances losses. In the first
regime, the population is harvested down from a
steady state to size NtA. The dashed line a
represents the number that could be harvested
each year to hold the population stable at A. In
the second regime, the population is reduced to
NtB. A number represented by the dashed line
could be harvested each year to hold the
population stable at B. Maximum sustained yield
is at M, where the diagonal line and the curve
have maximum separation.
(b) A parabolic recruitment curve illustrates the
concept in a different fashion. Maximum
sustained yield is approximately K/2, represented
at M. At A the equilibrium is stable at high
density, much above MSY. B is an unstable
equilibrium point, much below MSY.
(
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• If a constant number is harvested from a
  population each year, the population will decline
  from steady density and stabilize at the upper
  population size for which that number is the
  sustained yield.
• If this number exceeds the maximum sustained
  yield, the population declines to extinction.
• If a constant percentage of the population is
  harvested each year (the percentage applying to
  the standing crop of that year), the population
  will decline and stabilize at a level at
  equilibrium with the rate of harvesting.
• This level may be above or below that generating
  maximum sustained yield.

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• Principles of wildlife management
• Manage habitat (vegetation composition, density
  and structure)
• Manage animal populations

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MANAGING HABITAT Refuges, Reserves, and Land
Management Both habitat loss and fragmentation
are consequences of human development. The
impact of such loss of habitat varies with the
geographic range and distribution of a species
and the species niche. Habitat destruction is
particularly injurious to endemic species and
migratory birds. Refuges are used to both
conserve and protect species and to provide
habitat for recovery.
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• Reserves and Refuges - used to both conserve and
  protect species and to provide habitat for
  recovery.
• Reserve and refuge size
• Buffers and management of surrounding lands
• Potential (genetic) isolation of populations
• Overpopulation of refuge
• Need for corridors between reserves and refuges
• Focus on restoration, maintenance and
  preservation of ecosystems.
• Land Management
• Economic incentives and zoning
• Collaboration and coordination among owners
• Differing landowner objectives and practices

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• Few areas escape urban, suburban, and industrial
  expansion.
• As human overtake the land, native
  inhabitantsplants, wildlife, and even soil
  organismsare forced into continuously shrinking
  parcels of habitat, separated from one another by
  a sea of development or extensive areas of
  unsuitable habitat.
• A disturbing trend is the fragmentation of
  forest lands in the mid-Atlantic states into
  smaller patch sizes.
• This is the result of a number of factors
  including subdivision into smaller parcels with
  an increasing number of owners, urban sprawl and
  other development activities.

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Status of forest communities in the United States.
A recent assessment of the status of forest
communities in the United States conducted by the
USDA Forest Service shows that a high proportion
of the nations forests are imperiled or
vulnerable to further critical losses.
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• As human populations and the accompanying demand
  for land increase, they intrude into and compress
  and fragment the natural range of species.
• Species with large geographical ranges can
  withstand a greater degree of habitat loss and
  fragmentation than can those with small
  geographic ranges.
• Such species have the greatest proportion of
  local populations living within the center of
  their range.
• Species with small or restricted geographic
  ranges, especially endemic and ecologically
  specialized species, tend to have a higher
  proportion of local populations living near the
  periphery, where environmental conditions are
  more extreme.
• Once fragmented, these peripheral populations
  face a higher risk of going extinct and their
  range shrinks even further.

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• For migratory species, habitat loss and
  fragmentation must be considered across their
  total range.
• Habitat destruction and fragmentation are not
  confined to terrestrial systems.
• Aquatic habitats also are experiencing similar
  losses.
• Drainage of wetlands for agriculture and
  development eliminate habitat for organisms
  dependent on aquatic habitats.
• Dams isolate and fragment flowing water
  habitats, block movement of fish, impose lakelike
  conditions on free-flowing streams and rivers,
  alter water temperatures, and increase sediment
  loads.

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• Such habitat destruction is a major cause of
  decline in fauna and flora around the world.
• Two out of every three bird species worldwide
  are declining and 11 are
  threatened with extinction.
• 25 of all of the 4400 species of mammals are
  declining because of habitat loss
• 11 are endangered.

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• MANAGING POPULATIONS
• Fixed quota- a certain percentage of the
  population is removed each harvest period based
  on MSY estimates.
• Because populations fluctuate from harvest period
  to harvest period, the MSY will vary from year to
  year. If this fluctuation is not taken into
  account, there will be times of overharvest.
• Combined with environmental changes, overharvest
  has been responsible for the demise of some
  fisheries such as the Pacific sardine.

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• MANAGING POPULATIONS
• harvest effort- the number removed is manipulated
  by controlling hunting effort the number of
  hunters in the field, hunting season length, and
  size of the bag limit.
• The reverse approach is used to increase the
  kill.
• In general, such a rule-of-thumb approach has
  been more successful in managing exploitable
  populations that the fixed-quota approach.
• The permit system is a special variant of the
  harvest- effort approach.

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• MANAGING POPULATIONS
• Dynamic pool model- individuals removed replace
  those lost via density-related natural mortality,
  and no more.
• It assumes a constant natural mortality rate
  that is independent of density and is same for
  all age classes. Growth rates are age-specific
  but unrelated to density.
• The flaws in such an approach should be obvious
  based on previous discussions of population
  dynamics.
• e.g. Fishing mortality can be additive to
  natural mortality
• growth rates and recruitment to the population
  are both affected by population density

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• In practice, the dynamic pool model translates
  fishing mortality into fishing effort, based on
• type of equipment such as size-selective
  gill-nets that sort out age (size) classes,
• efficiency of equipment, and
• seasonal nature of exploitation.
• Few dynamic pool models have been developed, let
  alone put into practice.
• A general weakness of the model is the inability
  to estimate natural mortality accurately.

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All three of the previous management models are
based on the logistic growth model and have
numerous limitations. They often fail to
incorporate critical information about the
managed population such as life history in
relation to size and age classes, sex ratios,
survivorship, environmental uncertainty, etc.
Problems with exploitation of animals
frequently arise from failure to consider the
role of exploited species in the ecosystem and
the drive for economic gain without consideration
of ecological consequences.
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• Theoretically, with conservative exploitation on
  lower economic and biological scales, the
  resource could still be exploited.
• BUT because of the failure of harvest
  regulations, nationally and internationally, and
  the common nature of the resource, exploitation
  efforts increase, even in the face of declining
  stocks.
• Instead of reduced harvest effort, it is
  increased by technological improvements at
  finding and harvesting the remaining resources to
  a point that it collapses.
• That is the story of the whaling industry that
  led to the sharp decline and the near extinction
  of various species of whales.

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• The following figure shows the catches of
  various whales in the Southern Hemisphere between
  1910 and 1977, when declining stocks in the
  Northern Hemisphere forced whalers to seek more
  productive grounds.
• The precipitous decline in blue whales (blue
  line) began before 1940.
• After World War II the fin whale (red line) bore
  the heaviest exploitation, but for a while blue
  whale take increased.
• The increase in harvesting intensity on the blue
  whale points out a truism in resource
  exploitation. A high harvesting effort on a
  declining stock can continue if some alternative
  resource is abundant enough to (economically)
  support that effort the alternate take
  subsidizes the primary take on the declining
  stock.
• i.e. the stock of fin whale supported the
  incidental harvesting of blue whales.
• Then the story was repeated for the fin whale.
• As that stock declined, whalers turned to the
  smaller sei (light green) and minke (dark green)
  whales in an effort to maintain their investment
  in boats and equipment.
• Finally, the whaling industry collapsed, but
  multiple species of whales had been brought close
  to extinction.

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Catches of whales in the Southern Hemisphere,
1910-1977, the focal point of whaling after
stocks in the Northern Hemisphere had been
depleted. Note the virtual cessation of whaling
during World War II, 1941-1945.
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• Fisheries follow the same pattern, as
  exemplified by the collapse of the North Atlantic
  cod.
• Since the 1600s the North Atlantic cod fishery
  was an important source of food for western
  Europe in the form of salted cod. Settlers of
  eastern Canada and New England found rich lode of
  cod off the coasts of New England, Nova Scotia,
  and Newfoundland.
• Early exploitation of the resource by fisherman
  using small ships gave way to larger schooners
  that enabled them to reach the fisheries of the
  Grand Banks. Improved equipment, such as the
  introduction of long trawl lines with baited
  hooks, increased the catches enormously.
• The belief even in the scientific community at
  the time was that fishing stocks could not be
  depleted and that natural checks would occur
  before overfishing took place.
• As the North Sea stocks became depleted, the
  fishing industry moved to the North Atlantic.

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• Two factors that helped spell the end of the
  North Atlantic cod fishery
• One was the development by Clarence Birdseye in
  1924 of several methods of freezing cod fillets
  and sticks that could be shipped to the market as
  fresh.
• Large corporate fishing fleets, many government
  subsidized and dominated by factory ships that
  caught, processed, and froze fish without having
  to return to shore, displaced smaller fishing
  vessels.
• As fish catches declined, even bigger factory
  ships were used.

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• Use of sonar, helicopters, communication between
  ships, and other equipment efficiencies allowed
  fishermen to locate and focus on an area of fish,
  clean it out, and move on to another.
• Large powerful ships allowed the use of
  miles-long drift nets and huge otter trawls
  specially equipped with chains to stir up the
  bottom and drive fish into the nets.
• Few fish escape, and the bottom is virtually
  cleared of all benthic invertebrate life.
• As a result, the fishing industry in New England
  and eastern Canada has virtually collapsed.

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• The ultimate collapse causes were threefold
• (1) an overestimation of abundance and
  underestimation of fishery mortality
• (2) ability to find and catch fish at low levels
  of abundance,
• an increased effort related to overcapacity in
  the fleet, and
• economic incentive to maintain high catch and
• (3) increased discarding and nonreporting of
  small fish as population declined and fishing
  mortality increased.

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• As the major species declined, especially cod
  and haddock, the fishing industry turned mainly
  to smaller bony oil fish once considered as trash
  and stock and species higher up the food chain
  such as shark and swordfishwith disastrous
  outcomes.
• Size of swordfish, taken on lines 50 km long
  baited with thousands of hooks, has dropped from
  120 to 30 kg in the past 20 years.
• The breeding population of swordfish and sharks
  has been reduced by one-half off the southeastern
  coast of the United States.

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Landings of Atlantic cod and haddock from Georges
Bank for the period 1893-1996. Note the increase
in fishing intensity for both species in 1960
through 1980, followed by collapse.
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• Symptoms of overexploitation
• -Exploiters experience decreased catch per unit
  effort
• -Exploiters experience a decreasing catch of one
  species relative to the catch of related species
• -The population has a decreasing proportion of
  pregnant females, due both to sparse populations
  and to a high proportion of young non-reproducing
  animals
• -The species fails to increase its numbers
  rapidly after harvest, due to a change in
  productivity relative to age and age-specific
  survival

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• Another problem is that traditional population
  management, especially by fisheries, considers
  stocks of individual species as single biological
  units rather than components of a larger
  ecological system.
• Each stock is managed to bring in a maximum
  economic return, overlooking the need to leave
  behind a certain portion to continue its
  ecological role in the ecosystem i.e., as
  predator or prey.
• This attitude encourages a tremendous discard
  problem, euphemistically called, bypass.
• The ecological effects of bypass in fisheries
  can be enormous. Because much of the bypass
  consists of juvenile and undersized fish of
  commercial species, the practice can seriously
  affect the future of those fisheries.
• The removal or reduction of other species can
  interfere with predator-prey interactions in the
  sea, the dynamics of interspecific competition,
  and intraguild predation.

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• A similar economic attitude also prevails in the
  management of some game species.
• In too many instances, biologists have
  emphasized the increase of recreational
  opportunities for hunters over the welfare of the
  species.
• For example, rather than restrict hunting of
  moose, the approach is to kill wolves to reduce
  natural predatory losses.
• The reluctance to reduce seasons or to more
  tightly restrict bag limits relates in part to
  the economics (and politics) of hunting.
• Problem - Most wildlife programs directly or
  indirectly depend on the revenue generated from
  the sale of hunting licenses.

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• Management of exploitable populations depends
  too much on crisis management.
• No steps are taken to rescue a species until its
  population has fallen so low that the species
  becomes endangered.
• Then it is protected and expensive recovery
  programs are initiated with the hope that
  populations will recover.

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Wildlife restoration techniques include
protecting small populations so their size can
increase and the transplantation of wild and
captive populations into suitable habitat.
Problems associated with restoration include
rapid population growth until the species becomes
a pest and.. the need to assure genetic
diversity in reintroduced populations.
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• Humans also introduce pollutants into ecosystems.
  
• Many of these pollutants, especially pesticides,
  impact organisms found at the base of the aquatic
  food pyramid.

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• The wild turkey is a good example of restoration
  of a species from the brink of extinction - as
  shown in the following graph of the growth of the
  wild turkey population following restoration.
• Originally the turkeys range included all or
  parts of the 39 states and extended into Ontario,
  Mexico, and Central America.
• By the mid-1800s the species had been eliminated
  from the northeastern United States, and by 1900
  from the Midwestern states.
• In 1949, only small populations of eastern wild
  turkey survived on about 12 of their original
  range. Most existed in more remote areas of the
  Appalachian Mountains.
• Basic to restoration was live-trapping social
  groups of turkeys from the wild and their release
  into empty habitat.
• Maturing forests that improved turkey habitat,
  continued intensive studies of the bird, and the
  wild turkeys unforeseen ability to adapt to
  habitats previously thought unsuitable aided
  large-scale restoration efforts.
• Today the wild turkey population is about 3
  million birds.

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• Restoration of some populations of animals
  relies on introduction of individuals from
  captive breeding programs.
• Introduction requires pre-release and
  post-release conditioning, including the
  acquisition of food, shelter finding, interaction
  with conspecifics, and fear and avoidance of
  humans.
• Problems with reintroductions of captive-bred
  individuals include high costs, logistical
  difficulties, shortage of habitat, and the
  questionable ability of the individuals to adapt
  to the wild.

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Growth of the wild turkey population in the
United States following restoration (through
1964), as derived from hunting harvest data.
Note the sharp increase in growth after 1962.
Turkey populations are still increasing.
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• Both wild and captive-bred individuals may be
  translocated to build up the numbers of
  individuals already present and to introduce new
  genetic material into populations.
• Such translocations must be done carefully. Not
  only is there danger of introducing disease, but
  the new individuals must be integrated into the
  social and breeding structure of the native
  population to achieve the desired results.
• Just as important is the genetic background of
  the transplanted animals to ensure they are
  adapted to their new environment. If not, these
  individuals can weaken the resident stock.

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• Captive propagation, however, has its problems
  including
• small population size,
• potential inbreeding,
• incompatibility of captive individuals relative
  to mating, and
• lack of social interaction.
• Captive propagation programs cannot be carried
  out indefinitely.
• After a number of generations, depending on
  population size, the captive stock will begin to
  show signs of inbreeding depression and
  domestication.
• For this reason, it is important to consider
  opportunities for reintroduction when feasible.

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• Successful restoration, then, involves a number
  of facets
• Scientific studies of the species biology,
  ecology, and behavior provide the data needed to
  successfully manage the growing populations.
• Protection through strict law enforcement,
  adequate financing, and needed public concern and
  cooperation to aid population expansion.
• Protected reserves, large areas of suitable
  habitat, and the adaptability of the species
  further ensure the species recovery.

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• Humans are responsible for intentional and
  unintentional introduction of nonnative species.
  
• The lack of biological controls allows these
  species to flourish in suitable habitats where
  they interact with native species through
• competition
• predation
• introduction of their own parasites.

49
A pest is any species that humans consider
undesirable therefore, the designation as a
pest varies with time and culture. Pests are
managed using chemical, biological, genetic,
mechanical, and cultural controls, and through
integrated pest management.
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There will now be a short intermission
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• Population ecology (continued) Chpt 19
• Population genetics
• Genetic variation
• Selection
• Inbreeding

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• 4 - Population Genetics Chpt 19
• A significant amount of genetic variation results
  from gene recombination during sexual
  reproduction.
• Although new alleles (sections of DNA leading to
  expressed characteristics) arise only from
  MUTATIONS, much of the observable variation among
  population members results primarily from the
  RECOMBINATION of EXISTING genetic information.

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• Genetic variation
• There are two types of variation
• CONTINUOUS - a variation in a character that can
  be placed along a RANGE of values (e.g. height)
• Characters subject to continuous variation can be
  measured and the measurements for a members of a
  population tabulated as a frequency distribution
  and arranged graphically as a histogram.
• DISCONTINUOUS - variations in a specific
  character or sets of characters that separate
  individuals into DESCRETE CATEGARIES, such as
  male or female

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• Genotypes and phenotypes
• Genotype - the sum of the hereditary information
  carried by the individual (in the DNA)
• Phenotype - the EXTERNAL or OBSERVABLE
  expression of the genotype, often resulting from
  the interaction of the genotype and the
  environment
• Phenotypic plasticity - the ability of a
  genotype to give rise to a range of phenotypic
  expression (e.g. different external forms) under
  different environmental conditions.

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Phenotypic plasticity in the growth of leaves of
the yellow water-buttercup. The submerged leaves
are divided into threadlike segments. The
floating leaves are much broader and less divided.
56

• Mutation
• mutation- an inheritable CHANGE of genetic
  material in the gene or chromosome.
• macromutations- chromosomal mutations that
  result from a change in the NUMBER of chromosomes
  or a change in the STRUCTURE of the chromosome.
• micromutation- alterations in the DNA sequence
  of one or a few nucleotides in a gene.
• Most mutations are DELETERIOUS

57

• Natural Selection
• The underlying determinant of fitness is the
  individuals genotype.
• Because great genetic variation exists among
  population members, natural selection has been
  visualized as a process that acts on the
  FREQUENCY of occurrence of genotypes over time.

58

• Stabilizing selection - in stable habitats, the
  average expression of genotype/phenotype prevails
  and the frequency distribution of phenotypes
  appears as a normal (bell-shaped) distribution
  (more individuals with the mean of the range, and
  few extreme/ exceptional animals)
• Stabilizing selection is undoubtedly the more
  common selective type on an ongoing basis and it
  occurs in all species
• however, examples are fewer because
  NO NET PHENOTYPIC CHANGE is
  occurring.
• Such species are presumed to occur in STABLE
  habitats and are often of LIMITED MOBILITY.

59

• Directional selection - typically dominance
  shifts from an AVERAGE phenotype to an EXTREME
  phenotype in a frequency distribution.
• Disruptive selection - occurs in patchy habitats
  and results in selection for EXTREMES of
  phenotypic expression and a BIMODAL frequency
  distribution.
• Group selection increases the frequencies of
  genes that BENEFIT A GROUP of individuals (the
  population) RATHER THAN SINGLE individuals that
  carry the gene.
• In fact, if the genes arent selectively neutral,
  they can be DETRIMENTAL TO AN INDIVIDUAL that
  possesses the gene.

60
Group selection can be considered in terms of the
development of ALTRUISTIC behaviors, particularly
among closely related, social species (KIN
SELECTION). During sexual reproduction 50 of
the DNA comes from the mother, 50 from the
father. Therefore sacrificing yourself (loss of
100 of parents genes) causes no loss of fitness
if the action leads to the survival of at least
two offspring (50 50 of parents genes) or
four nieces/nephews/cousins (25 25 25
25) Thus through group selection, individuals
can acquire additional fitness by elevating the
fitness of close relatives.
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Three modes of selection. Stabilizing selection
favors organisms with values close to the
population mean. Directional selection accounts
for most of the change observed in evolution.
Disruptive selection increases frequencies in the
extremes. The curves represent the frequency of
organisms with a certain range of values. Upward
arrows indicate favorable selection, downward
arrows adverse selection. Left and right arrows
indicate evolutionary change. Shaded areas
represent the phenotypes being eliminated.
62

• Inbreeding
• One consequence of existing in small, isolated
  populations is inbreeding.
• In most instances, inbreeding is DETRIMENTAL to
  the population.
• If deleterious recessive alleles are present in
  the population, inbreeding will increase the
  expression of those alleles by maximizing levels
  of homozygosity.
• Inbreeding results from NONRANDOM mating in a
  small population, i.e., the probability is higher
  that an individual will mate with a relative than
  a nonrelative.

63
The percentage of homozygous offspring from
systematic matings with different degrees of
inbreeding. Note how rapidly homozygosity
declines as the relationship of offspring becomes
further removed from the original parent stock.
64

• But inbreeding isnt all bad
• Studies by Bateson showed that Japanese Quail
  prefer mating with first cousins (if separated
  from birth)
• This may maximize the amount of an individuals
  genes in the next generation
• e.g. by mating with a genetically similar cousin
  (sharing many of your genes) you increase the
  chances that your genes will be in your
  offspring increasing fitness

65

• Consequences of inbreeding
• inbreeding depression- when rare recessive
  deleterious genes become expressed in a
  homozygous state they may cause
• premature death,
• decreased mating success,
• decreased fertility,
• decreased fecundity,
• small body size,
• loss of vigor,
• reduction in growth,
• Reduced pollen and seed fertility in plants,
• and various meiotic abnormalities such as poor
  chromosomal pairing (can lead to developmental
  abnormalities)

66
partial dominance hypothesis - inbreeding is the
expression of deleterious genes normally masked
in the heterozygous condition. overdominance
hypothesis- it is believed that HETEROZYGOTES
possess SUPERIOR FITNESS therefore, inbreeding
results in a steady decline in fitness because of
loss of heterozygosity.
67

• Genetic drift (chance fluctuation in allele
  frequencies in small populations as a result of
  random sampling) is another consequence of mating
  among individuals in a small population.
• Genetic drift can be attributed to a POPULATION
  BOTTLENECK (caused by a severe reduction in
  population numbers that reduces the effective
  size)
• or a FOUNDER EFFECT (the gene pool of the
  emigrant or introduced population carries only a
  sample/selection of genes from the parent
  population, so it is subject to random drift).
• Genetic drift is one mechanism that can lead to
  SPECIATION, particularly drift in a population
  started by a small number of immigrants (founder
  effect).

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• Population fluctuations

A population faced with an environmental
catastrophe or overexploitation enters a
bottleneck in which the surviving population
consists of only a sample of the total gene pool.
If the population makes a complete recovery, it
may lack the genetic diversity found in the
original population. If the population remains
small, as in captive or isolated situations, it
is subject to random genetic drift.
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Genetic diversity in founder populations of
various sizes. Founder populations of 10 may
hold 90 of the genetic diversity found in the
parent population. Populations of 50 may
contain a nearly 100 of the parent population
genetic diversity. Genetic diversity can be
maintained only if the population expands.
These facts are important in the introduction
of a species into vacant or new habitat.
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Genetic drift (continued) In determining the
amount of genetic drift that may occur, the
actual size of the local population may not be
important rather, the actual number of
individuals contributing to reproduction,
contributing gametes to the next generation
(effective population size Ne) is the crucial
parameter. Effective population size is
influenced by the demographic characteristics of
the population, especially the SEX RATIO and
numbers of NON-BREEDING MALES, the NEIGHBOURHOOD
SIZE of a population, based on the number
breeding individuals per unit area and the amount
of DISPERSION from a birthplace as an index of
gene flow, and MATING SYSTEM (polygamous vs
monogamous).
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Unequal sex ratios
The depression of effective population size Ne
due to disparity in the sex ratio (m/f) of
reproducing animals. Note that as the ratio
widens with fewer males relative to females, as
it may be in polygamous species, especially where
males are hunted, the effective population size
drops dramatically.
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• Neighborhood size - the effective breeding size
  of a population whose individuals are more or
  less continuously distributed over a LARGE AREA,
  which is LARGER THAN THE DISPERSAL DISTANCE of
  the species (the neighborhood area).
• It depends on the NUMBER OF BREEDING individuals
  per unit area and the amount of DISPERSION
  between an individuals birthplace and the
  birthplace of its offspring.
• The effect is the development social groups of
  parents and their offspring created by RESTRICTED
  DISPERSAL and MATING patterns.
• The result, even within large areas of habitat,
  is an overall MOSAIC of SUBPOPULATIONS restricted
  by dispersal distance, which may promote a degree
  of INBREEDING.

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• The minimum viable population density is the
  minimum size of a population needed for the
  population to PERSIST through time. This is an
  important consideration in PROTECTION RECOVERY
  of endangered species.
• Important considerations in establishing minimum
  viable population levels include
• number of individuals,
• age structure,
• sex ratio, and
• compensation of losses in genetic variability
  because of DEATH and DISPERSION with gains in
  genetic information through MUTATIONS.

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• The minimum viable population must be large
  enough to cope with
• CHANCE VARIATIONS in individual BIRTHS DEATHS,
  
• random series of ENVIRONMENTAL CHANGES, and
• random changes in ALLELE FREQUENCIES or GENETIC
  DRIFT.

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Loss of genetic diversity as measured by
heterozygosity due to random genetic drift for
various effective population sizes, based on a
rate of decline in heterozygosity of 1/2Ne x 100
per generation. Note how rapidly genetic
diversity declines when Ne is small. Even with
an Ne (effective population size) of 500, some
loss of genetic diversity takes place.
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• The 50/500 rule a minimum of 500 INDIVIDUALS is
  needed to retard the effects of genetic drift and
  an absolute minimum Ne of 50 is needed to avoid
  inbreeding depression.
• Relying on variations in mutation rates is risky
  because we do not know the reliability of those
  rates and many mutations are deleterious.
• We rarely know the effective size of the
  population
• The formulas for estimating Minimum Viable
  Population (MVP) are appropriate only for
  populations with discrete generations.
• The 50/500 rule is ONLY A GUIDE to the size of
  population that should be maintained.

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