Transcript Biological Species Concept

The Origin of Species
• Define: Species
– Biological Species Concept
– Ecological Species Concept
• Speciation
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Sympatric Speciation
Allopatric Speciation
Reproductive Barriers
Instantaneous Speciation - Polyploidy
• Role in Speciation
– Genetic Drift
– Adaptive Radiation
• Pace of Evolution
– Gradualism
– Punctuated Equilibrium
I. Define: Species
• What is a species?
– One of the basic units of biological classification and
taxonomic rank
– A group of organisms capable of interbreeding and
producing fertile offspring
• Concept of species must account for two
phenomena:
– Distinctiveness of species that occur together in a
single location or population
– The connection that exists among different
populations within the same species
• Speciation is the process by which new
species arise, either by:
– Transformation of one species into another
– Splitting of one ancestral species into 2
Vermilion Cardinal (Cardinalis phoeniceus)
descendant species
Northern Cardinal,Cardinalis cardinalis
Northern Cardinal female
Bengal tiger, (Panthera tigris tigris)
Siberian tiger (Panthera tigris altaica)
Leopard (Panthera Pardus)
Melanistic morph
• Population - any group of individuals, single
species, occupying a given area at the same time.
– E.g. herd of buffalo, school of fish, flock of birds
• Subspecies - a geographically defined population
or group of populations within a single species
that has distinctive characteristics
– E.g. wolves, coyotes, and dogs
Populations
Biological Species Concept
• Developed by Ernest Mayr in 1942 which
defines a species as: “…groups of actually or
potentially interbreeding natural populations
which are reproductive isolated from other
such groups”
• Therefore:
– Members of a population mate with each other
– AND produce fertile offspring
• Biological species concept defines species as
members of populations that
actually/potentially interbreed in nature not
according to similarity of appearance
– Appearance is helpful in identifying species, it
does not define species.
– Organisms may look different and yet be same
species
– Concept relies on reproductive isolation between
species in nature. Problems arise due to
hybridization.
Western & Eastern Meadowlarks look the same, & ranges
overlap, but they do not interbreed due to differences in their
distinct songs
one might think they are distantly related
species. In fact, they are sisters — of species
Pheidole barbata . They are fulfilling different
roles in same colony
Ecological Species Concept
• An alternative hypothesis that proposes the
distinctions among species are maintained by
Natural Selection.
• With this hypothesis, hybridization has little
effect because alleles introduced into one
species’ gene pool from other species are quickly
eliminated by natural selection.
• This concept also has some exceptions. In reality
some species follow the biological species
concept while others follow the ecological
species concept.
II. Speciation
• Sympatric Speciation – differentiation of
populations within a common geographic
area into new species; some sort of
reproductive barrier has to develop
• Species that occur together:
– Are distinctive entities
– Phenotypically different
– Utilize different parts of the habitat
– Separate behaviors – reproduction, nesting,
migration
• Allopatric Speciation – differentiation of
populations that are geographically isolated;
also known as geographic speciation;
physically isolation creates a reproductive
barrier
Reproductive Barriers
• In nature barriers have developed between
species that cause species to be
reproductively isolated, which means that
population members do not mate with each
or cannot produce fertile offspring.
• These barriers are called reproductive
isolating mechanisms because they prevent
genetic exchange between species.
• There are two broad categories of isolating
mechanisms:
– Pre-zygotic – barrier that prevents the formation
of the zygote (before-zygote)
– Post-zygotic – barrier that prevents the proper
functioning of a zygote after it forms (afterzygote)
Pre-Zygotic
• Geographic Isolation
– Description – species occur in different areas,
which are often separated by a physical barrier,
such as a river or mountain range
– E.g. Grand Canyon – Antelope Squirrels and
Tassel-eared Squirrels
• Ecological Isolation
– Description – Species occur in the same area but
occupy different habitats and rarely encounter
each other
– E.g. Two species of garter snakes in the genus
Theamnophis occur in the same are but one lives
main in water and the other is primarily terrestrial
• Behavioral Isolation
– Description – Species differ in their mating rituals
• Unique visual signals and cues
• Unique call or sounds
• Unique chemical signals – pheromones
– E.g. Birds of Paradise, Blue- Footed Boobies, insect
calls, frog croaks, etc.
Birds of Paradise
• Temporal Isolation
– Description – Species reproduce in different
seasons or at different times of the day
– E.g. Field Crickets – mature different time of year
Spotted Skunk – mate different times
• Mechanical Isolation
– Description – structural differences between species
prevent mating
– E.g. Different species of snail, Bradybaena, have shells
that spiral in opposite directions which interferes with
the alignment of their sex organs. Also seen in many
plant species.
• Prevention of Gamete Formation
– Description – gametes of one species function
poorly with the gametes of another species or
within the reproductive tract of another species.
– E.g. Egg and sperm are not compatible – Seen in
lots of aquatic animals that release sperm and egg
into the water in order to become fertilized.
Post Zygotic
• Hybrid Inviability
– Description – hybrid embryos do not develop
properly and/or hybrid adults do not survive in
nature
• E.g. hybrid embryos between sheep and goats
die early in development. Hybrid seeds
between different species of plants never
germinate into a new plant.
• Hybrid Infertility
– Description – Hybrid adults are sterile or have reduced
fertility
• E.g. mule, liger, zorse, beefalo, cama
• How do ancestral species become reproductively
isolated?
• Speciation is a continuous process
– 2 populations present but only have partially
reproductive isolation with no isolating mechanism
yet. These populations will interbreed.
– Hypothetically - hybrids between these two
populations have decreased fertility and are not well
adapted. What would happen to these hybrids?
– Selection is favoring the parental populations and
begins to prevent hybridization.
– The 2 populations eventually becomes reproductively
isolated so that no genetic exchange occurs creating
two different species
• Natural selection reinforces incomplete isolating
mechanism unit it is complete
Instantaneous Speciation
• This type of speciation can occur by polyploidy
which creates an individual that is now
reproductively isolated from the other member
of the species.
• Polyploidy – individuals with more than 2 sets of
chromosomes.
• Seen in plants which have 4 sets of chromosomes
(4n = tetraploids) that survive but cannot be
fertilized by the original diploid (2n) individuals.
• Allopolyploid – when two species hybridize
which results in an offspring with 1
chromosome from each species and cannot be
fertilized by the original 2 species (infertile).
– Can reproduce asexually
– Fertile if chromosomes double (become
polyploidy)
• Tetraploids can interbreed creating a new
species.
– Frequently seen in Plants
– Sometimes in insects, fish and salamanders
III. Role of Speciation
Genetic Drift
• Random fluctuations in allele frequencies over
time by chance
• Random changes may cause reproductive
isolation, which results in new species.
• Genetic drift in small populations can occur
due to:
• Founder Effect
– Loss of genetic variation that occurs when a new
population is established by a small number of
individuals
– E.g. Common in island populations - Finches
Isolated religious communities - Amish
• Bottleneck Effect
– Population’s number vastly decreases
– Due to natural disaster
– E.g. Populations that survives after a forest fire or
earthquake
Adaptive Radiation
• The evolution of several divergent forms from a primitive
and unspecialized ancestor
• Can also result when a new trait, called a key innovation,
evolves within a species allowing it to use resources or
other aspects of the environment that were previously
inaccessible.
• Related species formed that adapted to different parts of a
new environment often with decreased competition and
many resources.
• Requires both speciation and adaptation to a new
environment.
• E.g. Finches
IV. Pace of Evolution
• Gradualism – accumulation of small changes
over a long period of time. Evolution is a long
gradual process. (older hypothesis)
• Punctuated Equilibrium – long period of no
change, followed by rapid change. More
recent hypothesis – “bursts of speciation”
• * NOTE: Evolution may include BOTH models
of change.