Parapatric speciation is a type of speciation that occurs when an extreme change in habitat occurs and separates a species from members of its groups. This does not have to do with geographic barriers, but a difference in climates or conditions. In this case, the salinity of the soil on one side separated the flowers from mating with their own species, thus delineating them.

Allopatric speciation is a separation of species due to geographic location. For example, the speciation of Darwin's finches is allopatric speciation—the finches eventually lost the ability to interbreed due to being on separate islands. 

Sympatric speciation is when a species is still in the same geographic location, but becomes reproductively isolated. This usually happens through a mutation during reproduction when the offspring recieves twice the number of normal chromosomes. This individual possesses a quality called tetraploidy, and cannot reproduce with other diploids. 

Peripatric is similar to allopatric speciation in that the individuals are separated by geographic barriers. However, the main difference is that the group that is broken off is much smaller and may possess a certain quality or trait that, due to the small group, is made a more predominant quality or trait.

The biological fitness of an organism is dependent on its ability to survive and reproduce in a given environment. If different traits or alleles increase the fitness of an organism, those alleles will consequently increase in the gene pool, and that trait will increase in the population. This is how natural selection affects a population.

There is inherent trade-off in biological fitness. A trait that increases ability to survive, but makes an individual sterile, decreases fitness because the organism cannot produce offspring to carry on the trait. Similarly, if a trait increases the ability to reproduce, but makes it harder to the organism to survive, it may die before being able to produce offspring. Both survival and reproduction are essential to defining the fitness of an organism.

Biological fitness in the evolutionary sense is only related to fitness in terms to reproduction. Because the primary goal of all organisms is to reproduce, or to pass their DNA onto offspring, fitness is defined as the ability to reproduce and create viable offspring.

"Favorable" traits, such as intelligence, size, or strength, may increase the ability of an individual to survive and reproduce, thus increasing biological fitness, but cannot be used to directly define the fitness of the individual.

The term "fitness" in evolutionary biology means the ability of an organism to pass on its genetic material to its offspring. Biological or "Darwinian" fitness is being able to live long enough to reproduce and keep the population or species alive. Most students confuse biological fitness with physical fitness because that is the context most often associated with the word.

Biological or Darwinian fitness is defined based on the specimen's ability to reproduce and generate viable offspring. Essentially, the fitness of the individual is based on its ability to pass genetic information on to the next generation, as opposed to any physical characteristic or trait.

Measuring the number of offspring who contribute to the gene pool is the best way to determine how genetically fit the iguana is. No matter how strong, large, old, or free of predation an animal is, if it cannot reproduce, it is not considered fit.

All of the examples given provide an evolutionary advantage. A white rabbit in a snow covered environment has camouflage, which protects it from its predators. The same is true with the black moth living in a  in a soot-covered industrial area. A cheeta that can run fastest has the greatest chance of catching prey and feeding himself/herself and his/her offspring. The same is true for a bird that can crack nuts in an area where nuts are the main source of food. 

The term biological fitness refers to reproductive success and is different than physical fitness. Since the most spotted male fathered the most cubs that survived to adulthood to reproduce themselves, he would be considered the most biologically fit. It is also important to note the inclusion of the "survived to adulthood" aspect since reproductive success is dependent on an organism's offspring being able to reproduce and contribute to the gene pool as well. For example, if the most spotted male had fathered a litter that initially had nine cubs, but only one of them survived to adulthood to have cubs of its own, he would no longer be considered the most biologically fit.

Genetic drift can be defined as changes in allele frequencies in a population caused by random events or chance. Large populations are capable of retaining fairly stable allele frequencies, thereby maintaining a small degree of genetic drift.  

In contrast, in a small population, either losing the alleles of a single individual or an over-contribution by a single individual in reproduction can significantly change the frequency of alleles within the population. This could eventually lead to the loss of alleles from the population.

Co-evolution is defined as a change of two or more species in close association with one another. Co-evolution is most common with mutualistic relationships, in which both species gain benefit. Such evolution can result in symbiotic relationships, in which the organisms depend on one another for survival. For example, bees make honey from flowers and flowers are pollinated by bees. Neither species can survive without the other.

Parasitism, commensalism, and predator-prey relationships can also demonstrate co-evolution, as one species develops defenses and the other develops offensive traits.

The endosymbotic theory describes the evolution of eukaryotic cells from prokaryotes. Small independent organisms were engulfed by larger ones; both were prokaryotic and unicellular. This engulfment benefited both organisms, and the genes that promoted this engulfment were selected for. Over a long time, eukaryotic cells evolved with many membrane-bound organelles, each with their own specific structure.