Ch 24 (1)

Darwin wrote " Both in space and time, we seem to be brought somewhat near to that great fact- that __- the first appearance of new beings on this Earth.

the "__" that captivated Darwin is __, the process by which one species splits into two or more species.

Why did speciation fascinate Darwin?

What does speciation explain?

When one species splits, the species that result share many characteristics because they are __ from this common ancestral species?

Speciation also forms a conceptual bridge between __, changes over time in allele frequencies in a population, and __, the broad pattern of evolution over long time spans.

An example of __ change is the origin of new groups of organisms, such as mammals or flowering plants, through a series of speciation events.

The word __ is Latin for kind or appearance. In daily life we commonly distinguish between various kinds of organisms- dogs and cats, for instance- from differences in their appearance.

Are organisms truly divided into the discrete units we call species, or is this classification an arbitrary attempt to impose order on the natural world?
- To answer this, biologists compare not only the __ (body form) of different groups of organisms but also less obvious differences in physiology, biochem, and DNA sequences. The results generally confirm that morphologically distinct species are indeed discrete groups, with many differences in addition to morphological ones.

The primary definition of species is referred to as the __. According to this concept, a __ is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring- but do not produce viable, fertible offspring with members of other such groups.

Thus, the members of a _- are united by being reproductively compatible, at least potentially.

What hlds the gene pool of a species together, causing its members to resemble each other more than they resemble other species?
-- To answer this question, we need to reconsider __, the transfer of alleles between populations

Members of a species often resemble each other because their populations are connected by __. Populations near one another exchange alleles relatively often. But for populations separated by long distances, Scoot Edwards showed that a low level of gene flow occurred between even widely separated populations. SImilar results have been found for other animal species, as well as for various fungi and plants.

Such results illustrate that __ has the potential to hold teh __ of a species together, so long as it is not outweighed by effects of __ or __ (either of which can result in populations diverging.)

Because biological species are defined in terms of __, the formation of a new species hinges on __ - the existence of biuological factors (barriers) that impede members of two species from producing viable, fertime offspring. Such barriers block gene flow between teh species and limit the formation of __, offspring that result from an interspecific mating. Although a single barrier may not prevent all gene flow, a combination of several barriers can effectively isolate a species' gene pool.

Barriers between species can be classified according to whether they contribute to __ before or after fertilization. __ ("before the zygote") block fertilizationf rom occurring. Such barriers typically act in one of three ways: __, _, __

If a sperm cell from one species overcomes __ and fertilizes an ovum from another species, a variety of __ ("after the zyogte") may contribute to reproductive isolation after the __ zygote is formed. Or problems after birth may cause __ to be infertile or may decrease their chance of surviving long enough to reproduce.

One strength of the __is that it dirrects our attention to how speciation occurs: __. However, the number of species to which this concept can be usefully applied is limited.

There is, for ex, no way to evaluate the __ of fossils. The __ also does not apply to organisms that reprduce asexually all or most of the time like prokaryotes.

Furthermore, in the __, species are designated by __ of gene flow. There are, however, many pairs of species that are morphologically and ecologically distinct, and yet gene flow occurs between them. __ can cause such species to remain distinct despite gene flow. This observation has led some researchers to argue that biological species concept overemphasizes __ and downplays the role of __. Because of the limitations to the __, alternative species concepts are useful in certain situations.

True or False:
While the biological species concept emphasizes the separateness of species from one another due to reproductive barriers, several other deifinitions emphasize the unity within a species.
True False

THe __ characterizes a species by body shape and other structural features. It has several advantages.
- it can be applied to asexual and sexual organisms, and it can be useful even without info on the exxtent of gene flow. - In practice, this is how scientists distinguish most species.
One disadvantage is that this definiteion relies on subjective criteria: researchers ay disagree on which strutural features distinguish a species.

The __ view a species in terms of its __, the sum of how members of hte species interact with the nonliving and living paarts of their environment. Unlike the biological species concept, this concept can accommodate asexual as well as sexual species; it also emphasizes the role of disruptive natural selection as organisms adapt to different environmental conditions.

The __ defiens a species as teh smallest group of individuals that share a common ancestor, forming one branch on the tree of life. Biologists trace the __ history of a species by comparing its characteristics such as morphology or molecular sequences, with those of ther organisms. Such analyses can distinguish groups of individuals that are sufficiently different to be considered separate species. Of course, the difficulty with this species concept is determining the degree of difference required to indicate separate species.

True or False:
In addition to those, less than 20 species definitions have been proposed. The usefulness of each depends on the situation and the research questions being asked.
True False