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Population Ecology 4


- Management of K-selected species* (long-lived specialists) must consider factors such as age structure and fluctuations in fecundity. K-strategies exist in ... – PowerPoint PPT presentation

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Title: Population Ecology 4

Population Ecology 4
  • 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

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

  • 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.

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
  • The major factor determining sustained yield is
    the rate of increase (r) difference between
    birth rate and death rate of the exploited
  • Achievement of maximum sustained yield is not
    always an appropriate goal because it fails to
    consider factors such as species interactions,
    esthetics, etc.

  • 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)
- 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
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.

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.
  • 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
  • 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.

(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.
  • 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.

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

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.
  • Reserves and Refuges - used to both conserve and
    protect species and to provide habitat for
  • 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

  • Few areas escape urban, suburban, and industrial
  • 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.

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.
  • 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.

  • 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
  • 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

  • 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.

  • 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.

  • 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
  • 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.

  • 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
  • e.g. Fishing mortality can be additive to
    natural mortality
  • growth rates and recruitment to the population
    are both affected by population density

  • 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.

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.
  • 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
  • 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.

  • 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
  • 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.

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.
  • Fisheries follow the same pattern, as
    exemplified by the collapse of the North Atlantic
  • 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.

  • 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
  • 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
  • As fish catches declined, even bigger factory
    ships were used.

  • 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.

  • 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.

  • 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
  • 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.

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.
  • Symptoms of overexploitation
  • -Exploiters experience decreased catch per unit
  • -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
  • -The species fails to increase its numbers
    rapidly after harvest, due to a change in
    productivity relative to age and age-specific

  • 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.

  • 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
  • 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.

  • 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.

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.
  • Humans also introduce pollutants into ecosystems.
  • Many of these pollutants, especially pesticides,
    impact organisms found at the base of the aquatic
    food pyramid.

  • 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.

  • 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
  • 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.

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.
  • 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.

  • Captive propagation, however, has its problems
  • 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
  • For this reason, it is important to consider
    opportunities for reintroduction when feasible.

  • 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.

  • 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.

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.
There will now be a short intermission
  • Population ecology (continued) Chpt 19
  • Population genetics
  • Genetic variation
  • Selection
  • Inbreeding

  • 4 - Population Genetics Chpt 19
  • A significant amount of genetic variation results
    from gene recombination during sexual
  • 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.

  • 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

  • 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
  • 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.

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.
  • 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

  • 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.

  • 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
  • Such species are presumed to occur in STABLE
    habitats and are often of LIMITED MOBILITY.

  • 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
  • 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,
    possesses the gene.

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.
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.
  • 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.

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.
  • 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

  • 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

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.
  • 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
  • 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

  • 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.
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.
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
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
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.
  • Neighborhood size - the effective breeding size
    of a population whose individuals are more or
    less continuously distributed over a LARGE AREA,
    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

  • 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.

  • The minimum viable population must be large
    enough to cope with
  • random series of ENVIRONMENTAL CHANGES, and
  • random changes in ALLELE FREQUENCIES or GENETIC

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.
  • 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
  • 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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