Species and Populations
Biology/ Env S 204
Spring 2009
Species and Populations-Introduction
Species concepts = how species are
defined
Population biology = how individuals are
distributed, how they vary
Species and Populations-Introduction
1) Most measures of biodiversity rely heavily
on the number of species in an area;
2) Our ability to estimate abundance or
ecological roles depends on our ability to
identify species;
3) Assignment of conservation status and
priorities depends directly on accurate
knowledge of species and populations; &
4) Practical use of biodiversity depends on
ability to identify species and understand
their biology.
Biological Species Concept
Species are groups of
interbreeding (or potentially
interbreeding) natural populations
that are reproductively isolated
from other such groups.
Biological Species Concept
• Emphasis on the process of
reproductive isolation as the basis of
the concept
• Have to have some kind of mechanism
that prevents reproduction (in other
words, that promotes reproductive
isolation)
Biological Species Concept
Two types of isolating mechanisms:
1) Prevent mating altogether between
members of different species
or
2) Prevent fertile offspring if mating
occurs.
BSC—Isolating mechanisms to
prevent mating
• Temporal separation of mating activities
(day vs. night or season of flowering)
• Specific mating behaviors (displays in birds)
• Recognition systems (bird calls, firefly
patterns)
• Physical barriers (lock and key genitalia in
many insects)
• Niche separation among closely related
species (e.g., flycatchers)
BSC—Isolating mechanisms to
prevent offspring
• Molecular recognition systems (e.g.,
pollen-stigma incompatibility)
• Failure of cell division (mitosis) if
cross-fertilization occurs
• Infertility of offspring:
female horse X male donkey
mule (usually sterile)
BSC-Isolating mechanisms
May evolve for other reasons
(e.g., better use of resources
driving temporal separation or
niche separation) but they end up
producing a barrier to
reproduction between two
populations that results in
speciation.
Problems with the BSC
• Difficult to apply in groups in which
sexual reproduction occurs rarely or
not at all (e.g., bacteria, many plants
with asexual reproduction)
• Groups in which reproductive barriers
are non-existent among closely
related species (e.g., many plants)
Problems with the BSC—cont’d.
• Difficult to observe the limits of
reproduction (e.g., over large
geographic areas)
• Can’t apply over time (e.g., fossils)
• Impossible to determine for all
species (limited funding)
One alternative to the BSC
Morphospecies = recognition of species
based on discontinuities in physical
characteristics (e.g., size, coloration,
shape, hairiness, etc.)
Good working definition
Species are groups of organisms
that differ in one or more
characteristics and do not
interbreed extensively in nature
even if they occur together.
What’s in a name?
•
•
•
•
Dusky seaside sparrow (U.S.A.)
Pocket gophers (U.S.A.)
Tuatara (New Zealand)
Kemp’s Ridley sea turtle
All discussed by John Avise in “A Rose
is a Rose is a Rose”
What’s in a name?
Taxonomy may not reflect true
genetic relationships, that is,
variation among populations may
be disguised or overemphasized.
What’s in a name?
• Florida panther (Puma concolor coryi)
classified as a subspecies of the puma
(Puma concolor)
• Small population size created genetic
abnormalities
• Texas pumas (another subspecies)
were imported in the 1990’s in a
breeding program
What’s in a name?
• Should the Florida panther continue to
be classified as a separate
subspecies?
• If the classification changes, the
Florida panther could lose its federal
protection
Species Exist as Populations
• Study of species cannot be
separated from study of
populations
• Evolutionary processes operate at
level of populations
• Genetic diversity is partitioned
into populations
Species Exist as Populations
• Individuals of a species carry genetic
diversity from one generation to the
next
• Individuals exist in populations and
interbreed within those populations (or
between populations of the same
species)
• If reproduction between populations is
cut off, speciation may result
Species Exist as Populations
• Conservation of a species means
protection of viable populations
• Viable populations have enough genetic
diversity and breeding individuals to
survive over time
Species Exist as Populations
Population = a group of individuals of
a given species in a particular
geographic area; expect that all
individuals in a population will have the
same biotic and abiotic requirements
(that is, occupy the same niche)
Species Exist as Populations
Can measure:
-size (# of individuals)
-density (# indiv. per unit area or volume)
-dispersion (spacing of individuals)
-sex ratio (proportion of males, females)
-age distribution
-birth/death rates
-genetic diversity
Biotic Potential
Biotic potential = rate at which a
population will increase if there are no
limits of any kind; a theoretical
concept
Example: 1 bacterium dividing every 20
minutes—after 36 hours the Earth
would be covered by a layer 1 foot
deep
Biotic potential
But there are always limiting
factors—populations do not achieve
their biotic potential although some
approach it.
Described in the 19th century by
Malthus and others; important
concept in the development of
Darwin’s ideas on natural selection.
Carrying Capacity
Carrying capacity = the maximum stable
population size that a particular
environment can support over a
relatively long period of time;
property of the environment (habitat),
not the organism
Opportunistic vs. Equilibrial
Species
Opportunistic
(r-selected)
Equilibrial
(K-selected)
Spectrum of life history characteristics
Opportunistic Species
• Tend to have periods of growth that
approach their maximum biotic potential
• Usually short maturation times and short
lifespans, therefore short generation times
(reproduce at an early age)
• Many offspring produced per reproductive
event but
• Often the number of reproductive events is
limited to one or a few
Opportunistic Species
• Mortality (death) of offspring is often
great
• The above features are correlated
with small size
• These species often take advantage
of short-term ecological opportunities
(e.g., disturbances)
• Examples include garden weeds,
desert annuals, many insects
Equilibrial Species
• May exhibit a growth period that is
relatively fast, but mostly do not approach
their biotic potential
• Long maturation times and long lifespans
therefore long generation times (reproduce
at a later age)
• Relatively few offspring per reproductive
event but
• Can have several to many events per
lifespan
Equilibrial Species
• Correlated with large size
• Tend to maintain a stable population
size at or near the carrying capacity
(K) of their habitat
• Slow maturation and long parental
care of offspring
• Sequoias, oaks, whales, many large
terrestrial vertebrates
Opportunistic vs. Equilibrial
Species
Opportunistic
(r-selected)
Equilibrial
(K-selected)
Spectrum of life history characteristics
Many species exhibit intermediate strategies or
may even exhibit different strategies at different
times in the same population or at different stages
in their life cycle.