ESC 556 Week 5 Part I
What is a population?
 Groups of individuals of
the same species
 Living in a defined area
 Inter-breeding
 Share morphological,
physiological, behavioural
characteristics
Genetic basis of variation
Genotype: Complement of genes that an
individual posses
Phenotype: Observable characteristics
Genotype + Environmental factors
Dominant: Allele that expresses its
phenotypic trait even when heterozygous
with a recessive allele
Recessive: Allele that exerts its effect at only
homozygous condition
Gene: Functional units (protein
subunits, RNA)
Allele: Different forms of a gene
Polymorphic vs monomorphic
Homozygous vs heterozygous
Diploid:Organisms possesing two
sets of chromosomes
Method of Reproduction
1-Asexual Reproduction
 Vegetative
propagation, budding,
parthenogenesis
Disadvantage: Genetic
variation is low:
Mutation
Advantage: Rapid way of
reproduction in stable
environments
Method of Reproduction (Cont.)
2- Sexual Reproduction
 Production of haploid gamets by
meiosis
 Fertilisation: Diploid zygote
High genetic variation:
Segregation of chromosomes
Recombination and linkage
Chance events in fertilisation and genetic
mixing
Advantage in changing environments
(disease, seasonal changes)
Method of Reproduction (Cont.)
 Occasional sexual
reproduction
 Plants, marine
invertebrates, insects
 Rapid reproduction
 Genetic variation
and adaptation to
environment
1- Self-fertilization: Gametes from the same
individuals
 Increased homozygosity
 Genetic diversity decrease
2-Cross-fertilization: gametes from different
individuals
Offsprings different from each other and the
parents
Different combinations for offspring genotypes
Heterozygosity
Method of Reproduction (Cont.)
Heterosis:
 High degree of heterozygosity =
healthier individuals
 High environmental adaptation
 Damaging or lethal alleles
Sickle Cell Anemia
Patterns of Genetic Variation
Gene pool: Various alleles or
phenotypes within a
population
 Differentiation of
populations
Microevolutionary forces
1-Mutation
2-Gene flow
3-Natural selection
4-Genetic drift
Microevolutionary Forces
1-Gene Flow:
 Exchange of genes among populations
because of successful reproduction by
migrants
 May be responsible for a marked
change in gene pool frequencies
2- Mutation
 Homogenizing
 Any kind of nucleotide changes
 New alleles arise
Microevolutionary Forces
3- Natural Selection:
Darwin’s premises:
1-More individuals than can survive
2-Struggle for existence
3-Individuals show variation and
those with advantageous features
have a greater chance of surviving
and reproducing (survival of the
fittest)
4-Advantegous genes will become
more abundant
Fitness: The relative genetic
contribution to future
generations
Microevolutionary Forces
3-Genetic Drift:
Random changes
Not related to any selective process
Biased gamet sampling
Small populations
Founder effect:
 Low number of
individuals = low
genetic diversity
 Isolated populations
(Island, ponds, lakes)
Genetic Bottleneck
Disaster: Small numbers of survivors
Founder of population
Low genetic diversity
High homozygosity
Geneticaly Determined Phenotypic
Variations
Adaptations
Ecotypes: Different growth forms of a
species in different habitats
 Evolve as a result of natural
selection acting on genetically
isolated populations
 When phenotypic variations occur
continuously (along a vertical
gradient on a mountainside)
gradual phenotypic change across a
population = Ecocline.
Larix decidua
Pattern of Population Variation
 the degree of isolation
 the amount of genetic exchange between populations
 the type and distribution of environmental
characteristics
 the degree of natural selection
Geographical Variation
Variation in geographicaly
isolated populations
 Allopatric populations:
Complete geographic
isolation
 Parapatric Population: Some
gene flow
 Sympatric populations:
Populations overlap, major
gene flow
Speciation
 Biological species concept: ‘Species are groups of
actually or potentially interbreeding natural
populations which are reproductively isolated from
other such group”
How species divides into two or more species?
1-Splitting
2-Budding
Speciation (Cont.)
1-Splitting
 involves the gradual
separation of one species
into two different species
 the loss of the original
form
2-Budding
 separation of a new
species from the original
one which continues
unchanged.
Isolating mechanisms
Pre-zygotic (pre-mating)
1-Habitat isolation: geographical
2-Temporal isolation: Seasonal
3-Behavioural Isolation:
Inappropriate physiology or
behaviour
4-Mechanical isolation:
Incompatibility between
reproductive organs (Flower
orientation, tube length)
5-Gametic isolation: gametes are
incompatible
Isolating Mechanisms (Cont.)
B) Post-zygotic (post-mating)
1-Hybrid inviability: Hybrid dies
early embryonic stages, or
before maturity
2-Hybrid sterility: Viable hybrids
are sterile (mule)
3-Hybrid breakdown:
Hybrid: Viable and fertile
Offspring: Low viability or fertility
Population Growth
Properties of population:
 Size
 Density
 Dispersion
Niche: Particular role and particular place of
population in its ecosystem.
Population structure: Age and distribution of
members, size of population
Population Size and Density
Population size:
• Directly related to a
population’s potential for
survival
• Small populations are more
susceptible to disturbances
and negative genetic factors
Population density: The number of
individuals of a species per unit area or
volume
 Important in intraspecific and
interspecific competition
 Management of a species involves the
regulation of population density.
 increase density: agricultural species
and threatened or endangered species,
 reduce density in the control of pest
species and disease organisms.
Biomass: the total mass of individuals
Population Dispersion
 Reflects patterns of settlement,
survival, immigration and
emigration
1-Random distribution:
 rare in nature
 Indicates few or no environmental
factors influencing the population
distribution
2-Clumped distribution:
 Found in gregarious species, in
species where individuals settle
close to their parents
 Where habitats and/or resources
are concentrated in particular areas
Population Dispersion (Cont.)
3- Gradient distribution:
 Associated with environmental
gradients (tidal shores, in estuaries
and around point sources of
pollution)
4-Regular distribution:
 Unusual
 Occur in some plants where
competition for resources is intense
 Animals where there are social
interactions involving territoriality
Size and Age Structure
 Size of individuals related to ages: smaller individuals
being younger.
Age structure: the proportion of individuals in each age
group making up its age distribution
 Determined by the rates of birth and death
 If both rates are high, young age groups dominate
 If both are low, there is a relatively even distribution of age
groups.
Population Growth
Birth rates > death rates = Populations will increase in
size
 The rate of population increase:
r = (birth rate + immigration) - (death rate + emigration)
 Rate of demographic events, determined by
 life-history traits of the species
 the impact of environmental factors
 The number of animals in a population at any given
time:
Nnow = (Nstart + number born + number immigrated)(number died + number emigrated)
Population Growth (Cont.)
 Life Table
 Survivorship Curve
Population Growth (Cont.)
 If the conditions are optimal: exponential
growth
Populations may grow rapidly for short
periods, not very long:
 Resource shortages
 Environmental limitations
Carrying capacity: Maximum number of
individuals that environment can support
 The availability of resources
 Predation
 Disease
 Any social interactions
Population Growth (Cont.)
 Logistic Growth Equation:
K: carrying capacity;
dN/dt:The rate of change of size of the population;
N: the number of individuals;
r : the difference between the average per capita birth rate and the average
per capita death rate.
 When conditions are optimal for the population: r has its highest value
called r max’
 Population growth stops when N = K
(K - N) = 0 and dN/dt=0
Natality and Mortality
Natality (Birth): Number of
organisms born per female per unit
time Production of new individuals
by birth, hatching, germination or
fission.
 depends on the type of organism.
Fecundity: The number of offspring
per parent
 inversely related to
1- the amount of resources
2- after care provided by the parent
Mortality (Death):
 Age, size and sometimes sex.
 The life-history characteristics
• competition for resources
• adverse environmental
conditions
• predator– prey
relationships.
• Environmental factors
Immigration and Emigration
 Rate of immigration = Rate of emmigration
Population Regulation
1-Density Dependent Factors
 Competition for resources
 Predation
 Parasitisation
 Disease
2-Density-independent factors
 Disturbance factors: floods, hurricanes, drought,
fire and environmental extremes.
 Increase the mortality rate, and decrease the
reproductive rate
R and K Strategies
K-strategist: Populations of slow-growing organisms tend to
be limited in number by the environment’s carrying
capacity (K)
 produce only few, offspring, parental support
 live in stable, predictable habitats.
R Strategist:
 very rapid growth little or no parental care, often
followed by sudden and large declines in population
size
 tend to live in unpredictable and rapidly fluctuating
environments and are called opportunistic species.
 a high intrinsic rate of increase (r)
Species Interactions
 Intraspecific interactions: between members of the
same species
 Interspecific Interactions: between species
1-Competition
Both species may suffer from their interaction for
usually limited resources (space, food and
water)
 increases with density with reduction in the
quantity of a resource,
 Exploitation competition: When the users
significantly reduce the resources
 Interference competition: One organism
prevents other individuals from using the
resource
1-Competition (Cont.)
Beneficial Consequences:
1- Short-term fluctuations in
abundance and distribution of
a species
2-Long-term evolutionary
adaptations providing
improved competitive ability
within its particular niche
The negative effects:
1-reduced growth or fecundity
2-exclusion from a habitat
3-mortality
Intraspecific competition may be severe It is
particularly strong
Competitive exclusion: when two directly
competing species within a simple, uniform,
closed system interact, one species will
eventually completely exclude the other:
In patchy environments where each species can
find local conditions that favour its survival
and propagation, a species may persist even
when its population densities are reduced by
competition in some parts of its distribution.
2-Predation
 Predator– prey
interactions: the
interactions
between food
resources and their
consumers
 One species
consumes the other,
benefiting one
species only
 To control
predator– prey
interactions
optimal situation is
to reduce, but not
eliminate, the prey
species (the pest).
2-Predation (Cont.)
Benefits of predation:
1- Maintenance of community diversity
2-Long-term evolutionary change
prey species evolving a rich variety of
responses:
spines, hairs and bristles
toxic or noxious chemicals
mimicry
behavioural modifications
Rabbits and myxomatosis
Rabbits and myxomatosis (Cont.)
 Consequences: Rapid and massive (up to 90 per
cent) reduction in the rabbit populations
 Studies over a long period of time:
1-The virulence of the virus declined markedly over
the next decade.
2-The populations became more resistant to the
virus.
3-Selection for resistant strains in the wild
populations was inevitable because of the lethal
nature of the infection.
Mimicry
 Crypsis: Development of cryptic
body forms
 Catalepsis: Use of a frozen
posture, playing dead)
 Some animals mimic other
animals
Three main strategies:
1-The model to be mimicked
2- The mimic
3-Predator/prey that is to be
fooled
Mimicry (Cont.)
Mullerian mimicry: unpalatable animals
evolve to look the same as other
unpalatable species:
 Reinforces the relationship between the
design and the unpalatability feature
 Provides a group defence
 Aposematic Species: give warnings
(coloration (intense colours), patterns
(black and yellow stripes in insects),
smells (skunks) or sounds (rattlesnake
rattles)
Mimicry (Cont.)
hoverfly
Batesian mimicry:
Mimicry by a palatable
species of an
unpalatable or
dangerous species.
bee
Mimicry (Cont.)
Aggressive mimicry: Mimicry not
used to escape predation but used
to facilitate predation by the
mimic.
 Mimicking of the background
(crypsis): rocky substrates
 Mimicking of flowers