Final Exam Part 3: Chapter 24

5 Prezygotic Barriers Include:

1. Habitat isolation 2. Behavioral isolation 3. Temporal isolation 4. Mechanical isolation 5. Gametic isolation.

3 Possible Outcomes of Hybrid Zones

1. Reproductive barriers between species may be strengthened, limiting the formation of hybrids. 2. Reproductive barriers between species may weaken, causing two species to fuse into one. 3. A long-term, stable hybrid zone may form.

Species

A Latin word meaning "kind" or "appearance." A group of similar organisms that can mate with each other and produce fertile offspring.

Rapid Speciation

A growing number of studies suggest that speciation can take place rapidly. _____________ produced the wild sunflower, Helianthus anomalous, by hybridization of two other sunflower species.

A common mechanism that produces polyploid individuals occurs when?

A more common mechanism of producing polyploid individuals occurs when two different species mate to produce allopolyploid offspring. Although the hybrids are usually sterile, they may be quite vigorous and propagate asexually.

What is most effective at blocking genetic exchange?

A single barrier may not block all genetic exchange between species, but a combination of several barriers can effectively isolate a species' gene pool.

Autopolyploid

An ______________ is an individual that has more than two chromosome sets, all derived from a single species. For example, a failure of cell division can double a cell's chromosome number from diploid (2n) to tetraploid (4n).

Selective Pressures

Any phenomena which alters the behavior and fitness of living organisms within a given environment.

Reproductive Isolation may arise as a by product of what?

As the isolated population accumulates genetic differences due to natural selection, genetic drift, and sexual selection, reproductive isolation from the ancestral species may arise as a by-product.

How do biologist distiniguish between species?

Biologists compare the morphology or molecular sequences of species to those of other organisms to distinguish groups of individuals that are sufficiently different to be considered separate species. The difficulty is determining the degree of difference required to indicate separate species.

What percentage of plants are more likely to be descenedents of ancestors that formed by polyploid speciation?

Botanists estimate that more than 80% of the plant species alive today are descended from ancestors that formed by polyploid speciation.

Is it possible to tell from fossils when a new species forms?

For fossil species that show gradual change, we cannot tell when a new species forms because information about reproductive isolation does not fossilize. It is likely that speciation in such groups occurs slowly, over millions of years.

External Fertilization

For species with external fertilization, gamete recognition may rely on the presence of specific molecules on the egg's coat, which adhere only to specific molecules on sperm cells of the same species. Such a mechanism prevents fertilization between the sperm and egg of related species of sea urchin.

Gene flow may be reduced, what?

Gene flow may be reduced by such factors as polyploidy, habitat differentiation, and sexual selection.

How significant must a barrier be to limit gene exchange?

How significant a barrier must be to limit gene exchange depends on the ability of organisms to move about. A geologic feature that is only a minor hindrance to one species may be an impassible barrier to another. The valley of the Grand Canyon is a significant barrier for the ground squirrels that have speciated on opposite sides. For birds that can fly across the canyon, the valley is no barrier.

What would occur if a sperm from one species does fertilize the ozum of another species?

If a sperm from one species does fertilize the ovum of another species, postzygotic barriers may prevent the hybrid zygote from developing into a viable, fertile adult.

Mechanical Isolation

In _______ isolation, closely related species may attempt to mate but fail because they are anatomically incompatible and transfer of sperm is not possible. For example, snails whose shells coil in opposite spirals cannot mate with each other.

Temporal Isolation

In ________ isolation, two species that breed during different times of day, different seasons, or different years cannot mix gametes.

Gametic Isolation

In _________ isolation, the gametes of two species do not form a zygote because of incompatibilities preventing fertilization.

Habitat Isolation

In ___________ isolation, two organisms that use different habitats (even in the same geographic area) are unlikely to encounter each other to even attempt mating.

Reduced Hybrid Fertility

In ___________, even if the hybrid offspring are vigorous, the hybrids may be infertile, and the hybrid cannot back breed with either parental species. This infertility may be due to problems in meiosis because of differences in chromosome number or structure.

Hybrid Breakdown

In ___________, first-generation hybrids are viable and fertile. When first-generation hybrids mate with either parent species or with each other, however, the next generation is feeble or sterile.

Reduced Hybrid Viability

In ___________, genetic incompatibility between the two species may abort the development of the hybrid at some embryonic stage or produce frail offspring. Occasional hybrids form by salamanders that belong to the same genus. Most do not complete development, and those that do are frail.

Behavioral Isolation

In ____________ isolation, species use unique and elaborate courtship behaviors to attract mates.

Sympatric Speciation

In ________________, a new species can originate in the geographic amidst of the parent species; occurs in populations that live in the same geographic area.

Autopolyploidy

In a single generation, ______________ can generate reproductive isolation despite sympatry.

Allopatric Sepciation

In allopatric speciation, geographic separation of populations restricts gene flow; it can lead to the origin of species.

In animals, sympatric speciation may occur because of what?

In animals, sympatric speciation may occur when a subset of the population is reproductively isolated by a switch to a new habitat or food source.

Courtship

In many species, elaborate courtship displays identify potential mates of the correct species and synchronize gonadal maturation.

Sympatric Speciation can result from what?

In plants, sympatric speciation can result from accidents during cell division that result in extra sets of chromosomes, a mutant condition known as polyploidy.

In plants, the most common mechanism of sympatric speciation is what?

In plants, the most common mechanism of sympatric speciation is hybridization between species or errors in cell division that lead to polyploid individuals.

Internal Fertilization

In species with internal fertilization, the environment of the female reproductive tract may not be conducive to the survival of sperm from other species. Process in which eggs are fertilized inside the female's body

Hyrbid Fertile Polyploid

In subsequent generations, various mechanisms may transform a sterile hybrid into a fertile polyploid. These polyploid hybrids are fertile with each other but cannot breed with either parent species. Thus, they represent a new biological species. Such speciation may be rapid enough that scientists have documented several such speciations in historical times.

Intrinsic Reproductive Barriers can prevent what?

Intrinsic reproductive barriers can prevent breeding with the parent even if the populations reestablish contact.

What did laboratory experiments indicate about natural selection?

Laboratory experiments mimicking events in nature indicated that natural selection could produce genetic changes in hybrid populations over short periods of time.

Macroevolution

Macroevolution includes the origin of new groups of organisms, such as mammals or flowering plants, through a series of speciation events. Large-scale evolutionary changes that take place over long periods of time

Evidence of Allopatric Speciation

Many studies provide evidence of allopatric speciation. Regions that are isolated or highly subdivided by geographic barriers tend to have more species than do similar regions that lack these features. Many unique plants and animals are found on the geographically isolated Hawaiian Islands. Laboratory and field experiments provide evidence that reproductive isolation between two populations increases with the distance between them.

Why do members of a popultion resemble each other?

Members of a population often resemble each other because their populations are connected by gene flow, the ongoing exchange of alleles that holds populations together genetically. As a result, they share a common gene pool.

Microevolutionary Mechanisms

Microevolutionary mechanisms include mutation, natural selection, genetic drift, and gene flow.

What would happen once gene flow is interrupted?

Once gene flow is interrupted, the populations must diverge genetically to the extent that they become reproductively isolated. This must occur before another event causes gene flow to resume, thus reversing the speciation process.

What happens once geographic seperation is established?

Once geographic separation is established, the separated gene pools may begin to diverge through a number of mechanisms.

Punctuated Equilibrium

Paleontologists Niles Eldredge and Stephen Jay Gould coined the term ________________ to describe the pattern of apparent stasis punctuated by sudden change.

Increased Reproductive Isolation

Perhaps extensive gene flow from outside the narrow hybrid zone overwhelms selection for increased reproductive isolation inside the hybrid zone. The study of hybrid zones shows how barriers to reproduction between closely related species may change over time.

Postzygotic Barriers

Postzygotic barriers include reduced hybrid viability, reduced hybrid fertility, and hybrid breakdown.

Reproductive Barriers

Prevent successful breeding between organisms.

How can reproductive barriers be categorized?

Reproductive barriers can be categorized as prezygotic or postzygotic, depending on whether they function before or after the formation of zygotes.

When can reproductive barriers occur?

Reproductive barriers can occur before mating, between mating and fertilization, or after fertilization.

Reproductive barriers evolve from what?

Reproductive barriers must evolve between sympatric populations.

What can result from reporductive isolation?

Reproductive isolation can result when genetic factors enable a subpopulation to exploit resources not used by the parent population. One example is the North American maggot fly, Rhagoletis pomonella. The fly's original habitat was native hawthorn trees. About 200 years ago, some populations colonized newly introduced apple trees. Because apples mature more quickly than hawthorn fruit, the apple-feeding flies have been selected for more rapid development and now show temporal isolation from the hawthorn-feeding maggot flies.

Mosquitofish Experiment

Researchers brought together mosquitofish from the two types of ponds. Female mosquitofish prefer to mate with males whose body shape is similar to their own. This preference establishes a barrier to reproduction between mosquitofish from ponds with predators and those from ponds without predators. Thus, as a by-product of selection for avoiding predators, reproductive barriers have started to form in these allopatric populations.

When does speciation begin?

Speciation begins only after gene flow between populations is interrupted, not after a given period of time.

Speciation forms what?

Speciation forms a conceptual bridge between microevolution, changes in allele frequencies within a population, and macroevolution, the broad pattern of evolution over time.

Sympatric Speciation may result from waht?

Sympatric speciation may also result from sexual selection or habitat differentiation, when a subpopulation exploits a habitat or resource not used by the parent population.

Sympatric speciation requires the emergence of what?

Sympatric speciation requires the emergence of a reproductive barrier that isolates a subset of the population without geographic separation from the parent population.

Tetraploid

The ___________ can reproduce with itself (self-pollination) or with other tetraploids. The tetraploid cannot mate with diploids from the original population because of abnormal meiosis by the triploid hybrid offspring.

How does gene flow affect species?

The absence of gene flow plays a key role in formation of new species, as well as keeping them apart once their potential to interbreed has been reduced.

How does the biological species concept define a species?

The biological species concept defines a species is defined as a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring, but cannot produce viable, fertile offspring with other such groups.

Ecological Species Concept

The ecological species concept defines a species in terms of its ecological niche, the sum of how members of the species interact with the nonliving and living parts of their environment. For example, two species of amphibian may be similar in appearance but differ in their tolerance of dry conditions.

What does the ecological species concept emphasize?

The ecological species concept emphasizes the role of disruptive natural selection as organisms adapt to different environmental conditions. This concept also accommodates asexual and sexual species.

The formation of new species hinges on what?

The formation of a new species hinges on the development of reproductive isolation. Because the distinction between biological species depends on reproductive incompatibility, the formation of a new species hinges on the development of reproductive isolation, the existence of biological barriers that prevent members of two species from producing viable, fertile hybrids. Such barriers block gene flow, limiting the formation of interspecies hybrids.

How long does it take a new species to form?

The fossil record and molecular data provide information about the timing of speciation in specific taxa. In the fossil record, many species of marine invertebrates appear as new forms rather suddenly (in geologic terms), persist essentially unchanged, and then disappear from the fossil record.

Morphological Species Advantages and Disadvantages

The morphological species concept has certain advantages. It can be applied to asexual and sexual species, and it can be useful even without information about the extent of gene flow. However, this definition relies on subjective criteria, and researchers may disagree about which structural features identify a species.

Morphological Species Concept

The morphological species concept, the oldest and still most practical, defines a species by body shape and other structural features.

Parent Species, and Hyrbid Species Number of Chromosomes

The parent species and hybrid all have the same number of chromosomes (2n = 34).

The Phylogenetic Species Concept

The phylogenetic species concept defines a species as the smallest group of individuals that shares a common ancestor and forms one branch on the tree of life.

Speciation

The process by which one species splits into two or more species—is at the focal point of evolutionary theory because the appearance of new species is the source of biological diversity.

Fusion of Hybridizing Species

The process of speciation may reverse, causing the two hybridizing species to fuse into one. This may be happening among the cichlids of Lake Victoria. The increasingly murky and polluted waters of the lake have reduced the ability of females to distinguish males of their own species, reducing sexual selection and increasing the frequency and success of hybridization. The same thing may be happening with polar and grizzly bears. As the ice-pack habitat of polar bears disappears, polar bears spend more time on land, where they encounter - and mate with - grizzly bears.

Are reproductive barriers intrinisc or due to geographic separation?

The reproductive barriers are intrinsic to the organisms, not due to simple geographic separation.

What does the study of hrbrid zones show?

The study of hybrid zones shows how barriers to reproduction between closely related species may change over time.

How are species differentiated?

Traditionally, morphological differences have been used to distinguish species. Today, differences in physiology, biochemistry, behavior, and DNA sequences are also used to differentiate species.

Hybrid Zone

When allopatric species renew contact with one another, a hybrid zone may form. A hybrid zone is a region in which members of different species meet and mate, producing hybrid offspring. Some hybrid zones form narrow bands. Many hybrid zones are stable. In more than 20 years of studying the Bombina hybrid zone, scientists have found no evidence for reinforcement or declining numbers of hybrids.

Reinforcement

When hybrids are less fit than members of parental species, natural selection strengthens prezygotic reproductive barriers, reducing the formation of unfit hybrids. This process of _____________ strengthens reproductive barriers, especially between sympatric species.

What happens when the barreirs to reproduction in the hybrid zone are weak?

When the barriers to reproduction in the hybrid zone are weak, gene flow may weaken the reproductive barriers between species.

Sympatric Speciation Can be Driven by what?

__________ can also be driven by sexual selection. Sympatric speciation is one mechanism that has been proposed for the explosive adaptive radiation of cichlid fishes in now polluted Lake Victoria in East Africa.

Polyploid Speciation

___________ speciation occasionally occurs in animals. However, polyploidy is far more common in plants.

Habitat Differentiation

____________ may lead to sympatric speciation in animals.

Geologic Processes

_____________ can fragment a population into two or more isolated populations. Mountain ranges, glaciers, land bridges, or splintering of lakes may divide one population into isolated groups.

Geographic Isolation restricts what?

_______________ isolation severely restricts gene flow.

Prezygotic Barriers

_________________ block fertilization between species in one of three ways: by impeding members of different species from attempting to mate, by preventing an attempted mating from being completed successfully, or by hindering fertilization if mating is completed successfully.

Reproductive Isolation

__________________ prevents populations belonging to different species from interbreeding, even when their ranges overlap.