<P> Budding speciation has been proposed as a particular form of sympatric speciation, whereby small groups of individuals become progressively more isolated from the ancestral stock by breeding preferentially with one another . This type of speciation would be driven by the conjunction of various advantages of inbreeding such as the expression of advantageous recessive phenotypes, reducing the recombination load, and reducing the cost of sex </P> <P> The hawthorn fly (Rhagoletis pomonella), also known as the apple maggot fly, appears to be undergoing sympatric speciation . Different populations of hawthorn fly feed on different fruits . A distinct population emerged in North America in the 19th century some time after apples, a non-native species, were introduced . This apple - feeding population normally feeds only on apples and not on the historically preferred fruit of hawthorns . The current hawthorn feeding population does not normally feed on apples . Some evidence, such as that six out of thirteen allozyme loci are different, that hawthorn flies mature later in the season and take longer to mature than apple flies; and that there is little evidence of interbreeding (researchers have documented a 4 - 6% hybridization rate) suggests that sympatric speciation is occurring . </P> <P> Reinforcement, sometimes referred to as the Wallace effect, is the process by which natural selection increases reproductive isolation . It may occur after two populations of the same species are separated and then come back into contact . If their reproductive isolation was complete, then they will have already developed into two separate incompatible species . If their reproductive isolation is incomplete, then further mating between the populations will produce hybrids, which may or may not be fertile . If the hybrids are infertile, or fertile but less fit than their ancestors, then there will be further reproductive isolation and speciation has essentially occurred (e.g., as in horses and donkeys). </P> <P> The reasoning behind this is that if the parents of the hybrid offspring each have naturally selected traits for their own certain environments, the hybrid offspring will bear traits from both, therefore would not fit either ecological niche as well as either parent . The low fitness of the hybrids would cause selection to favor assortative mating, which would control hybridization . This is sometimes called the Wallace effect after the evolutionary biologist Alfred Russel Wallace who suggested in the late 19th century that it might be an important factor in speciation . Conversely, if the hybrid offspring are more fit than their ancestors, then the populations will merge back into the same species within the area they are in contact . </P>

Define species what are the important factors leading to the rise of a new species