1. 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.

2. 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
Mechanical isolation
Gametic isolation

3. 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

4. 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.

5. 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

6. 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.

7. Patterns of Evolution Divergent evolution Convergent 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
Eyes of squid and vertebrates

8. Patterns of Evolution Parallel evolution Coevolution
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 plant-eating insects, pollinators and flowering plants, pathogens and the immune system of animals.

9. 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

10. 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.