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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.

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Species and Populations

Biology/ Env S 204

Spring 2009

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Species and Populations--Introduction

Species concepts = how species are defined

Population biology = how individuals are distributed, how they vary

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Species and Populations--Introduction

  • 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.

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Biological Species Concept

Species are groups of interbreeding (or potentially interbreeding) natural populations that are reproductively isolated from other such groups.

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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)

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Biological Species Concept

Two types of isolating mechanisms:

1) Prevent mating altogether between members of different species


2) Prevent fertile offspring if mating occurs.

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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)

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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)

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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.

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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)

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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)

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One alternative to the BSC

Morphospecies = recognition of species based on discontinuities in physical characteristics (e.g., size, coloration, shape, hairiness, etc.)

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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.

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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”

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What’s in a name?

Taxonomy may not reflect true genetic relationships, that is, variation among populations may be disguised or overemphasized.

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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

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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

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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

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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

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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

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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)

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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

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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

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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.

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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

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Opportunistic vs. Equilibrial Species





Spectrum of life history characteristics

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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

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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

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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

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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

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Opportunistic vs. Equilibrial Species





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.