Ch 24 (2)

__ can occur in two main ways, depending on how gene flow is interrupted between populations of the existing species.

In __, gene flow is interrupted when a population is divided into geographically isolated subpopulations. __ can also occur without geologic remodeling, such as when individuals colonize a remote area and their descendants become geographically isolated from the parent population.

True or False:
Birds, mt. lions, and coyotes can cross rivers and canyons. Nor do such barriers hinder the windblown pollen of pine trees or the seeds of many flowering plants. In contrast, small rodents may find a wide river or a deep canyon a formidable barrier.
True False

Once geographic seperation as occurred, the spearated __ may diverge through other mechanisms. Different __ arise, __ acts on the separated organisms, and __ alters allele freqeuncies.

__ may then arise as a byproduct of selection or drift having ccaused the populations to diverge genetically.

The gene pools of highly isolated populations experience very little __ and hence are particularly likely to undergo __.

Many studies provide evidence that speciation can occur in allopatric populatiosn, such as __ and __.

The importance of __ is alaso suggested by the fact that regions that are highly subdivided by geographic barriers typically have more species than do regions with fewer barriers.

Lab and field tests also provide evidence that __ between two populations generally increases as the distance between them increases.

Although __ prevents interbreeding between allopatric populations, separation itself is not a biological barrier to reproduction. Biological reproductive barriers are intrinsic to the organisms themselves. Hence these barriers can prevent interbreeding when memebers of dif. pops come into contact with one another.

In __, speciation occurs in populations that live in teh same geographic area?

How can reproductive barriers form between sympatric populations while their members remain in contact with each other?
- Although such contact (and the ongoing __ that results) makes __ less common than __, __ can occur if __ is reduced by such factors as __, __, and __.

A species may originate from an accident during cell division that results in extra sets of chromosomes, a condition called __. There are two distinct forms. An __ is an individual that has more than two chromosome sets that are all derived from a single species.

A second form of __ can occur when two different species interbreed and produce hybrid offspring. Most hybrids are sterile because the set of chromosomes from one species cannot pair during meiosis with the set of chromosomes from the other species. However, an infertile hybrid may be able to propagate itself asexually (as many plants can do.) In subsequent generations, various mechanisms can change a sterile hybrid into a fertile polyploid called an __, which are fertile when mating with each other but cannot interbreed with either parent species; thus, they represent a new biological species.

Although it occurs in animals, __ is more common in plants. Many important agricultural croops- oats, cotton, potatoes, tobacco, and wheat- are __. The wheat for bread is an __ (six sets of chromosomes, two sets from each of three different species.)

__ can also occur when genetic factors enable a subpopulation to exploit a habitat or resource not used by the parent population.
-- SOmetimes, __ operating on certain alleles provides a __ to reproduction, further limiting gene flow. Altogether, alothough the two populations are still classified as subspecies rather than separate species, __ appears to be well under way.

Sympatric speciation can also be driven by __. Clues to how this can occur have been found in fish from one f Earth's hot spots of animal peciation, East Africa's Lake Victoria. (ex. of cichlids on p. 497)

What are the two main modes by which new species form?

in __, a new species forms in geographic isolation from its parent population. Geographic isolation severely restricts gene flow. As a rsult, other reproductive barriers from the ancestral species may arise as a byproduct of genetic changes that occur within the isolated population. Many different processes can produce such genetic changes, inculdign __ under dif. envi. conditions, __, and __. Once formed, intrincsic reproductive barriers that arise in __ populations can prevent interbreeding witht he parent population even if the popoulations come back into contact.

__, in contrast, requires the emergence of a reproductive barrier that isolates a subset of a population from the remainder of the population in the same area. Though rarer than __, __ can occur when gene flow to and from the isolated subpopulation is blocked. This can occur as a result of __, a condition in which an organism has extra sets of chromosomes. __ also can occur when a subset of a population becomes reproductively isolated because of __ that results from a switch to a habitat or food source not used by the parent population. Finally, it can result from __.