What is a Species?
• Biological species = A population or group
of populations whose members have the
potential to interbreed with one another in
nature and to produce viable, fertile
offspring.
Maintaining Reproductive
Isolation
• Prezygotic barriers impede mating between
species or hinder fertilization of the ova should
members of different species attempt to mate.
• Prezygotic isolating mechanisms
– Habitat isolation (different habitats within the same area)
• Ex: two species of garter snakes occur in the same areas, one
species lives mainly in water and the other is mainly terrestrial.
– Since these two species live primarily in separate habitats, they
seldom come into contact as they are ecologically isolated.
– Temporal isolation (breeding at different times)
• Ex: brown trout and rainbow trout cohabit the same streams,
but brown trout breed in the fall and rainbow trout in the spring
– Behavioral isolation
Maintaining Reproductive
Isolation
• Prezygotic isolating mechanisms
– Behavioral isolation (species-specific signals and elaborate
behavior)
• Ex: male fireflies blinking their lights in a characteristic
pattern, females discriminate among the different signals and
respond only to flashes of their own species.
• Galapagos finches
– Mechanical isolation (anatomical incompatibility)
• Male dragonflies use of special appendages for reproduction.
• Plant’s floral anatomy adapted to a specific pollinator.
– Gametic isolation
• For animals that use internal fertilization, the sperm of one
species may not be able to survive the internal environment of
the female reproductive tract of a different species.
• Molecular gamete recognition
Maintaining Reproductive
Isolation
• Postzygotic isolation = after the formation of a
zygote
– Reduced hybrid viability
• Genetic incompatibility between the two species may abort
development of the hybrid at some embryonic stage.
– Hybrid sterility
• If two species mate and produce hybrid offspring that are
viable, reproductive isolation is inact if the hybrids are sterile
because genes cannot flow from one species’ gene pool to the
other.
– Chromosomes differ in number or structure
– Hybrid breakdown
• First generation of hybrids are viable and fertile, but when those
hybrids mate with one another, the offspring of the next generation
are feeble or sterile.
Speciation
• An essential episode in the origin of a species
occurs when the gene pool of a population is
separated from other populations of the parent
species.
• Two general modes of speciation:
– Allopatric speciation = speciation that occurs due to
geographical isolation
• Mountain ranges, movement of glaciers, formation of land
bridges, formation of lakes, canyons (Grand Canyon)
– Sympatric speciation = formation of a new species
within the range of parent populations.
• Reproductive isolation evolves without geographical isolation
Speciation
• Adaptive radiation
– The evolution of many diversely adapted
species from a common ancestor
– Island chains
• Galapagos Islands
– Finches
– Sea iguana
• Australia
– Marsupials began with the colonization and subsequent
adaptive radiation of a single ancestral species.
Patterns of Evolution
• Divergent evolution
– Two populations which adapt to different environments
accumulate differences in the frequencies of alleles and
genotypes
• Reproductive barriers may evolve
• Convergent evolution
– Describes two unrelated species that share similar traits
– Similarities arise, not from a common ancestor, but
because the each species has independently adapted to
similar ecological conditions or lifestyles
– Analogous traits
• Fins of fish and aquatic mammals
• torpedo-shaped bodies of sharks, porpoises, and penguins
Patterns of Evolution
• Parallel evolution
– Two related species that have made similar
evolutionary changes after their divergence from a
common ancestor
• Marsupial mammals and placental mammals
– Independently evolved similar adaptation when ancestors
encountered comparable environments.
• Coevolution
– Occurs between predator and prey, plants and planteating insects, pollinators and flowering plants,
pathogens and the immune system of animals.
Macroevolution
• Gradualism
– The descent of species from ancestral forms as
branches that gradually diverge with each new
species evolving continuously over long spans
of time.
• Theory behind such a tree is the extrapolation of
microevolutionary processes to the divergence of
species.
• Big changes thus occur due to the accumulation of
many small changes
Macroevolution
• Punctuated equilibrium
– Proposed by Niles Eldredge and Stephen Jay Gould in
1972.
– It depicts species undergoing most of their
morphological modification as they first separate from
the parent species then showing little change as they
produce additional species.
– In this theory gradual change is replaced with long
periods of stasis punctuated with episodes of speciation.