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Species and Speciation

Terms in this set (53)

In 1973 the Grants began to observe a yearly migration of large ground finches front heir home island to Daphne Major for a few months in between breeding seasons. Finally in 1982, thirteen colonists stayed on Daphne Major throughout the breeding season, and five of them produced offspring. These colonists represented a new population, allopatric with their migrant source population, because the two populations bred and nested on different islands.

Over twelve years. the Grants caught, weighed, and measured most of the parents and offspring on Daphne Major. When they compared the data with the measurements of the large ground finches in the migrant population, they discovered that the average beak size int he colonist population was much larger.
Two evolutionary process could be responsible for the chafe in beak size:
1. The founder event produced a colonizing population that happened to have particularly large beaks relative to the migrant population (genetic drift)
2. Natural Selection in the new environment could favor alleles associated with larger beaks.

The population of large ground finches is not yet considered a separate species from the migrant population, because some gene flow continues to occur - such as when a new migrant joins the colonist population. Given enough time, however, the populations may continue to diverge. Dispersal and colonization, followed by genetic drift and natural selection, are thought to be responsible for speciation in Galapagos finches and many other island groups.
Even though sympatric populations are not geographically isolated, they may become reproductively isolate by adapting to different ecological niches via disruptive selection.

example: apple maggot flies rely on apples to complete their life cycle - apples are an important part of their niche. Male and female apple maggots usually court and mate on apple fruits. The female then lays a fertilized egg inside the fruit, which will be to food source for the growing larva, After the fruit drops off the tree, the larva burrows into the ground and pupates. The new adult then emerges the following spring, starting the cycle anew.

Phylogenetic trees, estimated from synapomorphies in DNA sequence data, indicate that apple maggot flies are very closely related to hawthorn maggot flies, which are native to North America. These data suggest that apple maggot flies originated from hawthorn maggot flies following the introduction of apples (apple trees were introduced to North America 300 years ago from Europe). Hawthorn flies lay their eggs in hawthorn fruits and apple flies lay their eggs in apples, even though hawthorn trees and apple trees of the grow almost side by side.

Only 6% of matings observed are between apple and hawthorn maggot flies.

-Apple flies respond most strongly to apple scents. Hawthorn flies respond most strongly to hawthorn scents.
-In both types of flies, there is no difference in the response to a mix of both scents or no scent at all.
-Apple flies avoid hawthorn scent, and hawthorn scent avoid apple scent.

-A fly's ability to discriminate scents has a genetic basis - meaning apple flies and hawthorn flies have different alleles associated with attraction to fruit
-Specific odor receptor cells are responsible for the difference in scent response
-Hybrid individuals do not orient to fruit scents as well as their parents.

The upshot is that although apple flies and hawthorn flies live in the same geographical area, pre zygotic reproductive isolation is occurring as a result of natural selection for adaptive to two different niches. Apple flies mate on apples (avoiding hawthorn fruits) and hawthorn flies mate on hawthorn fruits (avoiding apple fruits). Hybrid flies have lower fitness due to their reduced success in finding fruits and thus mates - disruptive selection is occurring.