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Cladistics & Populations
5.4
How do scientists show how different
species are connected?
• Clade = a group of organisms
that have evolved from a
common ancestor
• Cladistics – (greek for “branch”)
is a method of classifying species
of organisms into groups (clades)
which consist of an ancestor
organism and all its descendants
• A cladogram is a branched diagram which shows how
organisms are related.
• At each branch, or "Y”, characteristics of evolutionary origin
are used to separate one group from another.
The traits which tie the clades
together are called shared
derived characters
Cladograms show most probable
sequence of divergence in clades
Traits can be analogous or homologous
Which two organisms are more similar: gecko
and parrot or the crocodile and parrot?
What shared derived character sets them apart?
Cladograms can also be used for
organisms very closely related
Evidence from cladistics helped
reclassify figwort family
• Less than half of the
original species
remain in the Figwort
family; now only the
36th largest among
angiosperms
• Reclassification was
helpful since old
Figwort family was
too large and
dissimilar to be a
helpful grouping
Populations
• A population is a group of organisms of the same
species that can mate and produce fertile offspring.
• It is important to study populations (and population
sizes) for many reasons:
– To monitor endangered species
– To monitor environmental health
– To estimate demands for natural resources
– To monitor change in areas over time
– To make informed policy decisions
– To monitor anthropogenic (human) effects on/ damage
to the environment due to increasing human population
Population Size
• As you have learned, populations of organisms change (allele
frequencies) over LONG periods of time through natural
selection (evolution)
• What about short-term? How are population sizes affected
over time?
• There are four main factors that affect the SIZE of a
population (at any given time):
– 1. Natality: (new members due to reproduction – births)
• Increases population size
– 2. Mortality: deaths
• Decreases population size
– 3. Immigration: members moving in
from other locations
• Increases population size
– 4. Emigration: members moving out (leaving to go to other
locations)
• Decreases population size
Population Growth
• Typically, growing populations follow a sigmoid (“s-shaped”)
growth curve
• Stage 1: The
Exponential Phase
– Number of individuals
in a population increases
at a faster and faster rate
(natality rate exceeds
mortality rate) due to:
• Plentiful resources (food,
space, light, water)
• Little/ no competition
• Favorable environmental (abiotic) conditions – ex: temperature, dissolved
oxygen levels
• Little/ no predation
• Little/ no disease
Note: Growth begins slowly due to limited numbers of reproducing
individuals (may be widely dispersed as well)
• Stage 2:
The Transitional Phase
– The population is still growing, but at a slower and slower rate
(population growth rate slows down as natality rate decreases and
mortality rate increases) due to:
• Increased competition for resources (more members in population with
same amount of resources)
• Predators moving in to area (more food for them with a larger population)
• Diseases spread more readily within the population (more members in
same amount of space = closer quarters = better chances of sharing/
spreading disease to others)
• Stage 3: The Plateau (Stationary) Phase
– No more NET growth – the number of individuals in the
population has stabilized (balanced) due to:
• Limited resources: Less space (for seeds/ plants/ animals etc.), less
food (limited food supplies = fewer offspring)
• Increased mortality rates (increased predators/ disease)
• Number of births + number of immigrations = Number of deaths +
number of emigrations (may have small fluctuations in population
size up/ down, but no NET growth)
• The population has reached the carrying capacity (K) of the
environment!
Carrying Capacity (K)
• The maximum number of individuals an
environment/ habitat can support is called its
carrying capacity (K)
• The carrying capacity (K) of a habitat is determined
by its limiting factors (limits population increase),
which can be density-dependent or not. These
factors include:
Density-dependent factors
Density-independent factors
Predation/ parasites/ diseases
Climate/ weather conditions
Availability of shelter/ space/ mates
Availability of light/ CO2/ O2
Nutrient availability (water, food etc.)
Environmental change
Accumulation of wastes
Natural disasters