How is a species defined according to the biological species concept quizlet?

Recommended textbook solutions

Hole's Human Anatomy and Physiology

13th EditionDavid N. Shier, Jackie L. Butler, Ricki Lewis

1,402 solutions

Human Resource Management

15th EditionJohn David Jackson, Patricia Meglich, Robert Mathis, Sean Valentine

249 solutions

Biology

1st EditionKenneth R. Miller, Levine

2,591 solutions

Clinical Reasoning Cases in Nursing

7th EditionJulie S Snyder, Mariann M Harding

2,512 solutions

Recommended textbook solutions

Human Resource Management

15th EditionJohn David Jackson, Patricia Meglich, Robert Mathis, Sean Valentine

249 solutions

Hole's Human Anatomy and Physiology

13th EditionDavid N. Shier, Jackie L. Butler, Ricki Lewis

1,402 solutions

Hole's Human Anatomy and Physiology

15th EditionDavid Shier, Jackie Butler, Ricki Lewis

1,950 solutions

Clinical Reasoning Cases in Nursing

7th EditionJulie S Snyder, Mariann M Harding

2,512 solutions

In the 1940s, biologists incorporated reproduction and genetics into the question of what constitutes a species. According to the biological species concept, a species is a population, or a group of populations, whose members can interbreed and produce fertile offspring.

i. Strength: The biological species definition does not rely on physical appearance, so it is much less subjective than Linnaeus's observations. Under the system of Linnaeus, it would be impossible to determine whether two similar-looking butterflies belong to different species. Using the biological species concept, however, we can say that they belong to one species if the two groups can produce fertile offspring together (pg 130).

i. Weaknesses: Nevertheless, the biological species concept raises several difficulties. First, it cannot apply to asexually reproducing organisms, such as bacteria, archaea, and many fungi and protists. Second, it is impossible to apply the biological species definition to extinct organisms. Third, some types of organisms have the potential to interbreed in captivity, but they do not do so in nature. Further, reproductive isolation is not always absolute. Closely related species of plants, for example, may occasionally produce fertile offspring together, even though their gene pools mostly remain separate. As a result, the biological species concept does not provide a perfect way to determine the "boundaries" of each species. DNA sequence analysis has helped to fill in some of these gaps. Biologists working with bacteria and archaea, for example, use a stretch of DNA that encodes ribosomal RNA to define species. If the DNA sequences of two specimens are more than 97% identical, they are considered to be the same species. These genetic sequences, however, still present some ambiguity because they cannot reveal whether genetically similar organisms currently share a gene pool. Despite these difficulties, reproductive isolation is the most common criterion used to define species. The rest of this chapter therefore uses the biological species concept to describe how speciation occurs.

a. morphological species concepts- A more subjective classification. Organisms are classified in the same species if they appear identical by morphological (anatomical) criteria. This is used when species do not reproduce sexually, some are known only from fossils. This definition is the working definition used by biologists that cannot, or should not, use the "Biological Species Concept."

a. Swedish botanist Carolus Linnaeus (1707-1778) was not the first to ponder what constitutes a species, but his contributions last to this day. Linnaeus defined species as "all examples of creatures that were alike in minute detail of body structure." Linnaeus also devised a hierarchial system for classifying species, as described in section 14.6. His classifications organized life's diversity and helped scientists communicate with one another. His system did not, however, consider the role of evolutionary relationships. Linnaeus thought that each species was created separately and could not change. Therefore, species could not appear or disappear, nor were they related to one another. Charles Darwin (1809-1882) finally connected species diversity to evolution. He predicted that classifications would come to resemble genealogies, or extended "family trees." As the theory of evolution by natural selection became widely accepted, scientists no longer viewed classifications merely as ways to organize life. They considered them to be hypotheses about life's evolutionary history.

i. Weaknesses: Relies on physical appearance, so it is subjective and based on human interpretation to define species rather than allowing natural mechanisms to determine species. It would be impossible to determine whether two similar-looking butterflies belong to different species. Two species can be morphologically very similar, but can be genetically distinct.

i. Strengths: Long history of use, easy to use and intuitive, can use in place of the biological species concept. Used when species do not reproduce sexually, some are known only from fossils. Used whenever biologists cannot or should not use the biological species concept. More useful for asexually reproducing organisms such as bacteria. Morphology can be readily observed, in many cases without handling or harming the organisms. It is relatively easy to communicate with a whole range of people about morphology.

a. Prezygotic (pg 131-132)- Prevent the formation of a zygote, or fertilized egg; collectively, they are the most common way for one gene pool to become isolated from another.
i. Examples:
1. habitat isolation- different environments; ladybugs feed on different plants
2. temporal isolation- active or fertile at different times; field crickets mature at different rates
3. behavioral isolation- different courtship activties; frog mating calls differ
4. mechanical isolation- mating organs or pollinators incompatible; sage species use different pollinators
5. gametic isolation- gametes cannot unite; sea urchin gametes are incompatible
ii. Mechanisms of prezygotic reproductive isolation affect the ability of two species to combine gametes and form a zygote. As you can see in figure 14.3, two of the prezygotic barriers keep members of two different species from ever encountering each other. The populations may live in different places (habitat isolation) or be active at different times of the day or year (temporal isolation).
iii. Even if individuals from two species do encounter each other, prezygotic barriers may still prevent breeding. The two species may use such different mating rituals that they are not attracted to each other (behavioral isolation). For example, mate selection in many birds is based on intricate courtship dances. Any variation in the ritual from one group to another could prevent them from mating.
iv. Prezygotic barriers may apply even if the members of two species attempt to mate. In animals, the male and female parts of two related species may not match; in plants, the insect that pollinates one species may be unable to fit into the flower of another. Both scenarios illustrate a reproductive barrier called mechanical isolation, which prevents male and female gametes from meeting even if mating does occur.
v. The final prezygotic barrier is gametic isolation. If a sperm cannot fertilize an egg cell, then no reproduction will occur. For example, many marine organisms, such as sea urchins, simply release sperm and egg cells into the water. These gametes display unique surface molecules that enable an egg to recognize sperm of the same species. In the absence of a "match," fertilization will not occur, and the gene pools will remain separate.

a. Act after fertilization and reduce the fitness of a hybrid offspring. (A hybrid, in this context, is the offspring of individuals from two different species.) Figure 14.3 summarizes the reproductive barriers; the rest of this section describes them in detail.
i. Postzygotic reproductive isolation examples:
1. Hybrid inviability- hybrid offspring fail to reach maturity. Hybrid eucalyptus seeds and seedlings are not viable.
2. Hybrid infertility (sterility)- hybrid offspring unable to reproduce. Lion-tiger cross (liger) is infertile.
3. Hybrid breakdown- second-generation hybrid offspring have reduced fitness. Offspring of hybrid mosquitoes have abnormal genitalia.
ii. Individuals of two different species may produce a hybrid zygote. Even then, three types of postzygotic barriers may keep the species separate by selecting against the hybrid offspring.
iii. One type of postzygotic reproductive barrier is hybrid inviability. In this case, a hybrid embryo dies before reaching reproductive maturity, typically because the genes of its parents are incompatible. Alternatively, the hybrid offspring may develop to adulthood but be unable to produce offspring of its own (hybrid infertility). The most familiar example of this postzygotic barrier is a mule, which is the hybrid offspring of a female horse and a male donkey. Meiosis does not occur in the mule's germ cells because the two parents contribute different numbers of chromosomes. As a result, the mule cannot produce gametes. Similarly, a liger is the hybrid offspring of a male lion and a female tiger. Like mules, ligers are usually sterile.
iv. Some species produce hybrid offspring that are fertile. When the hybrids reproduce, however, their offspring may have abnormalities that reduce their fitness. Some second-generation hybrid offspring of two mosquito species, for example, have abnormal genitalia that make mating difficult. This last type of postzygotic reproductive barrier is called hybrid breakdown.

i. Examples:
1. habitat isolation- different environments; ladybugs feed on different plants
2. temporal isolation- active or fertile at different times; field crickets mature at different rates
3. behavioral isolation- different courtship activties; frog mating calls differ
4. mechanical isolation- mating organs or pollinators incompatible; sage species use different pollinators
5. gametic isolation- gametes cannot unite; sea urchin gametes are incompatible
ii. Mechanisms of prezygotic reproductive isolation affect the ability of two species to combine gametes and form a zygote. As you can see in figure 14.3, two of the prezygotic barriers keep members of two different species from ever encountering each other. The populations may live in different places (habitat isolation) or be active at different times of the day or year (temporal isolation).
iii. Even if individuals from two species do encounter each other, prezygotic barriers may still prevent breeding. The two species may use such different mating rituals that they are not attracted to each other (behavioral isolation). For example, mate selection in many birds is based on intricate courtship dances. Any variation in the ritual from one group to another could prevent them from mating.
iv. Prezygotic barriers may apply even if the members of two species attempt to mate. In animals, the male and female parts of two related species may not match; in plants, the insect that pollinates one species may be unable to fit into the flower of another. Both scenarios illustrate a reproductive barrier called mechanical isolation, which prevents male and female gametes from meeting even if mating does occur.
v. The final prezygotic barrier is gametic isolation. If a sperm cannot fertilize an egg cell, then no reproduction will occur. For example, many marine organisms, such as sea urchins, simply release sperm and egg cells into the water. These gametes display unique surface molecules that enable an egg to recognize sperm of the same species. In the absence of a "match," fertilization will not occur, and the gene pools will remain separate.

i. Postzygotic reproductive isolation examples:
1. Hybrid inviability- hybrid offspring fail to reach maturity. Hybrid eucalyptus seeds and seedlings are not viable.
2. Hybrid infertility (sterility)- hybrid offspring unable to reproduce. Lion-tiger cross (liger) is infertile.
3. Hybrid breakdown- second-generation hybrid offspring have reduced fitness. Offspring of hybrid mosquitoes have abnormal genitalia.
ii. Individuals of two different species may produce a hybrid zygote. Even then, three types of postzygotic barriers may keep the species separate by selecting against the hybrid offspring.
iii. One type of postzygotic reproductive barrier is hybrid inviability. In this case, a hybrid embryo dies before reaching reproductive maturity, typically because the genes of its parents are incompatible. Alternatively, the hybrid offspring may develop to adulthood but be unable to produce offspring of its own (hybrid infertility). The most familiar example of this postzygotic barrier is a mule, which is the hybrid offspring of a female horse and a male donkey. Meiosis does not occur in the mule's germ cells because the two parents contribute different numbers of chromosomes. As a result, the mule cannot produce gametes. Similarly, a liger is the hybrid offspring of a male lion and a female tiger. Like mules, ligers are usually sterile.
iv. Some species produce hybrid offspring that are fertile. When the hybrids reproduce, however, their offspring may have abnormalities that reduce their fitness. Some second-generation hybrid offspring of two mosquito species, for example, have abnormal genitalia that make mating difficult. This last type of postzygotic reproductive barrier is called hybrid breakdown.

14.2 Reproductive Barriers Cause Species to Diverge.
· In keeping with the biological species concept, a new species forms when one portion of a population can no longer breed and produce fertile offspring with the rest of the population. That is, the separate groups no longer share a gene pool, and each begins to follow its own, independent evolutionary path.
· One portion of a population can become reproductively isolated in many ways, because successful reproduction requires so many complex events. Any interruption in courtship, fertilization, embryo formation, or offspring development can be a reproductive barrier.
· Biologists divide the many mechanisms of reproductive isolation into two broad groups: prezygotic and postzygotic. Prezygotic reproductive barreirs prevent the formation or a zygote, or a fertilized egg; collectively, they are the most common way for one gene pool to become isolated from another. Postzygotic reproductive barriers, which act after fertilization, reduce the fitness of a hybrid offspring. (A hybrid, in this context, is the offspring of individuals from two different species.) Figure 14.3 summarizes the reproductive barriers; the rest of this section describes them in detail.

Pg. 133
14. 3 Spatial Patterns Define Two Types of Speciation
Reproductive barriers keep related species apart, but how do these barriers arise in the first place? More specifically, how could two populations of the same species evolve along different pathways, eventually yielding two species?
The most obvious way is to physically separate the populations so that they do not exchange genes. Eventually, the genetic differences between the populations would give rise to one or more reproductive barriers. Yet speciation can also occur between populations that have physical contact with each other. Biologists recognize these different circumstances by dividing the geographic setting of speciation into two categories: allopatric and sympatric.

Reproductive barriers arise through allopatric speciation and sympatric speciation

A. Allopatric Speciation Reflects a Geographic Barrier
· In allopatric speciation, a new species forms when a geographic barrier physicially separates a population into two groups that cannot interbreed. (Allo- means "other," and patria means "fatherland"). The barrier may be a river, desert, glacier, mountain range, large body of water, dam, farm, or city. Rising sea levels may also trap populations on isolated islands.
· If separate parts of a population cannot contact each other, migration between them stops. Meanwhile, mutations and the other forces of microevolution continue to alter allele frequencies in each group. The result may be one or more reproductive barriers. When the descendants of the original two populations can no longer interbreed, one species has branched into two.
· Island groups offer ideal opportunities for allopatric speciation. For example, 11 subspecies of a tortoise occupy the Galápagos islands. According to DNA analysis, a few newcomers from the South American mainland first colonized either San Cristóbal or Española (Hood Island) a couple of million years ago. As the population grew, the tortoises migrated to nearby islands, where they encountered new habitats that selected for different adaptations, especially in shell shape. Dry islands with sparse vegetation selected for notched shells that enable tortoises to reach for higher food sources. The Hood Island tortoise in figure 14.5 illustrates this characteristic "saddleback" shape. On islands with lush, low-growing vegetation, the tortoises have domed shells; the tortoise from Santa Cruz Island is an example.
· Although many of the subspecies look distinctly different from one another, the tortoises can interbreed. They are not yet separate species, but the genetic similarities among tortoises on each island suggest that migration from island to island was historically rare. The Galápagos tortoises illustrate an ongoing process of allopatric speciation.
· In addition to island groups, isolated springs also offer opportunities for allopatric speciation. The Devil's Hole pupfish, which inhabits a warm spring near Death Valley, California, provides one example. The spring was isolated from other bodies of water about 50,000 years ago, preventing genetic exchange between fish trapped in the spring and those in the original population. Generation after generation, different alleles have accumulated in each pupfish population. Since the time that the spring became isolated, the gene pool has shifted enough that a Devil's Hole pupfish cannot mate with fish from nearby springs. It has become a distinct species.

B. Sympatric Speciation Occurs in a Shared Habitat
· In sympatric speciation, a new species arises while living in the same physical area as ancestral species (sym- means "together"). Among evolutionary biologists, the idea of sympatric speciation can be controversial. After all, how can a new species arise in the midst of an existing population?
· Often sympatric speciation reflects the fact that a habitat that appears uniform actually consists of many microenvironments. Fishes called cichlids, for example, have diversified within African lakes. Figure 14.7 shows two cichlids in Cameroon's tiny Lake Ejagham. This 18-meter-deep lake has distinct ecological zones. Its bottom is muddy near the center, whereas leaves and twigs cover the sandy bottom near the shore. The two types of fish belong to the same species, but the larger ones consume insects near the shore, whereas the smaller ones eat tiny floating prey in deeper waters. The fish breed where they eat, so the two forms typically remain isolated. As genetic differences between the sub-populations continue to accumulate, sympatric speciation may occur.
· In plants, a common mechanism of sympatric speciation is polyploidy, which occurs when the number of sets of chromosomes increases. Nearly half of all flowering plant species are natural polyploids, as are about 95% of ferns. Moreover, many major crops, including wheat, corn, sugarcane, potatoes, and coffee, are derived from polyploid plants.
· Polyploidy sometimes arises when gametes from two different species fuse. Cotton plants provide an example. An Old World species of wild cotton has 26 large chromosomes, whereas one from Central and South America has 26 small chromosomes. The two species interbred, forming a diploid hybrid with 26 chromosomes (13 large and 13 small). This hybrid was sterile. But eventually the chromosome number doubled. The resulting cotton plant is a fertile polyploid with 52 chromosomes (26 large and 26 small); this new polyploid species formed sympatrically in the midst of its ancestors. Farmers around the world cultivate this species to harvest cotton for cloth.

· In plants, a common mechanism of sympatric speciation is polyploidy, which occurs when the number of sets of chromosomes increases. Nearly half of all flowering plant species are natural polyploids, as are about 95% of ferns. Moreover, many major crops, including wheat, corn, sugarcane, potatoes, and coffee, are derived from polyploid plants.
· Polyploidy sometimes arises when gametes from two different species fuse. Cotton plants provide an example. An Old World species of wild cotton has 26 large chromosomes, whereas one from Central and South America has 26 small chromosomes. The two species interbred, forming a diploid hybrid with 26 chromosomes (13 large and 13 small). This hybrid was sterile. But eventually the chromosome number doubled. The resulting cotton plant is a fertile polyploid with 52 chromosomes (26 large and 26 small); this new polyploid species formed sympatrically in the midst of its ancestors. Farmers around the world cultivate this species to harvest cotton for cloth.

Natural selection driving a population in two different directions at once. Two or more extreme phenotypes are favored over any intermediate phenotype. Therefore, disruptive selection favors polymorphism, the occurrence of different forms in a population of the same species. For example, British land snails (Cepaea nemoralis) are found in low-vegetation areas (grass fields and hedgerows) and in forests. In low-vegetation areas, thrushes feed mainly on snails with dark shells that lack light bands; in forest areas, they feed mainly on snails with light banded shells. Therefore, these two distinctly different phenotypes, each adapted to its own environment, are found in this population. Over time, and with enough disruptive selection, a population can be completely divided. When this happens, the two populations can become diverse enough to form separate species.

a. Disruptive selection is usually seen in high-density populations. In these populations resources become scarcer, and competition for the resources increases. This intraspecific competition can cause differences between organisms to have a more profound effect on each organism's survival. Selective pressures that might not have factored into a low-density population can take effect, and the resulting disruptive selection can drive a population apart. In doing so, the populations are often pushed to different niches, lowering the competition between them. This leads to sympatric speciation, or speciation that occurs while populations occupy the same area.

In punctuated equilibrium, relatively brief bursts of rapid speciation interrupt long periods of little change. Fossils of diverse animals such as bryozoans, mollusks, and mammals all reveal many examples of rapid evolution followed by periods of stability.

i. Evolution seems to happen "suddenly" in punctuated equilibrium, with few transitional fossils documenting the evolution of one species into another. What accounts for the missing transitional forms? One explanation is that the fossil record is incomplete, for reasons explored in chapter 13. Another is that the predicted "missing links" may have been too rare to leave many fossils. After all, periods of rapid speciation would mean that few examples of any given transitional form ever existed, so we are unlikely to find their remains.

ii. The punctuated equilibrium model fits well with the concept of allopatric speciation. Consider the population of desert pupfish that became isolated from its ancestral population long ago. If the climate changed and the spring containing new species rejoined it "old" spring, the fossil record might show that a new fish species suddenly appeared with its ancestors—after all, 50,000 years is a blink of an eye in geologic time. Afterward, unless the environment changed again, a period of stability would ensue.

iii. A rapid bout of speciation may also occur when some members of a population inherit a key adaptation that gives them an advantage. For example, after flowering plants appeared 144 million years ago, they diversified rapidly into the hundreds of thousands of species that now inhabit Earth. The new adaptation—the flower—apparently unleashed an entirely new set of options for reproduction, promoting rapid diversification.

iv. Rapid speciation may also occur when some members of a population inherit adaptations that enable them to survive a major environmental change. After the poorly suited organisms perish, the survivors diversify as they exploit the new resources in the changed environment. Mammals, for example, underwent an enormous burst of speciation when dinosaur extinctions opened up many new habitats about 65 million years ago.

Darwin envisioned one species gradually transforming into another through a series of intermediate stages. The pace as he saw it was slow, although not necessarily constant. This idea, which became known as gradualism, held that evolution proceeds in small, incremental changes. If the gradualism model is correct, then "slow and steady" evolutionary change should be evident in the fossil record. Microscopic protists such as foraminiferans and diatoms, for example, have evolved gradually. Vast populations of these asexual organisms span the oceans, leaving a rich fossil record in sediments. Since isolated populations rarely form in this uniform environment, it is unsurprising that speciation has been gradual. Much of the fossil record, however, supports a different pattern (punctuated equilibrium).

Carolus Linnaeus, the biologist introduced at the start of the chapter,
made a lasting contribution to systematics. First, he was the first investigator to give every species a two-word name. Each name combines the broader classification genus (plural: genera) with a second word that designates the species. The scientific name for humans, for example, is homo sapiens. Second, he grouped similar genera into a nested hierarchy of orders, classes, and kingdoms.

The name that consists of two parts use Latin grammatical forms, although they can be based on words from other languages. The first part is the generic name, which identifies the genus, and the second part is the specific name or specific epithet which identifies the species within the genus. In modern usage, the first letter of the first part of the name, the genus, is always capitalized in writing, while that of the second part is not, even when derived from a proper noun such as the name of a person or place. Similarly, both parts are italicized when a binomial name occurs in normal text, or underlined in handwriting (underline the genus and species name separately!). Always separate the two parts of the scientific name.

Before this, scientists used a complex polynomial system to ascribe a name to an organism where an organism could have a name with three, four, five, or more parts, so it was cumbersome.

The genus name reflects relatedness—it indicates the close relatives of the organism. (For example, the domestic dog, fox, coyote, and wolf are among the members of the genus, Canis. This reflects their close ancestry.

The species name is indicative of a group of organisms that can interbreed and produce viable, fertile offspring.

a. Carolus Linnaeus- Carolus Linnaeus grouped similar genera into a nested hierarchy of orders, classes, and kingdoms. Although scientists now use additional categories, Linnaeus's idea is the basis of the taxonomic hierarchy used today. The three domains—Archaea, bacteria, and eukarya—are the most inclusive levels. Each domain is divided into kingdoms, which in turn are divided into phyla, then classes, orders, families, genera, and species. A taxon (plural: taxa) is a group at any rank; that is, domain Eukarya is a taxon, as is the order Liliales and the species Aloe Vera. The more features two organisms have in common, the more taxonomic levels they share. A human, a squid, and a fly are all members of the animal kingdom, but their many differences place them in separate phyla. A human, rat, and pig are more closely related—all belong to the same kingdom, phylum, and class (Mammalia). A human, orangutan, and chimpanzee are even more closely related, sharing the same kingdom, phylum, class, order, and family (Order Primates, Family Hominidae). Figure 1.11 shows the full classification for humans.

The goal of modern classification systems is to reflect this shared evolutionary history. Systematics, the study of classification, therefore incorporates two interrelated specialties: taxonomy and phylogenetics. Taxonomy is the science of describing, naming, and classifying species: phylogenetics is the study of evolutionary relationships among species. This section describes how biologists apply the evidence for evolution to the monumental task of organizing life's diversity into groups.

Biologists illustrate life's diversity in the form of phylogenetic (evolutionary) trees, which depict relationships based on descent from shared ancestors. Multiple lines of evidence are used to construct these trees. Anatomical features of fossils and existing organisms are useful, as are behaviors, physiological adaptations, and molecular sequences.

In the past, systematists constructed phylogenetic tree diagrams by comparing as many characteristics as possible among species. Those organisms with the most characteristics in common would be neighbors on the tree's branches. Basing a tree entirely on similarities, however, can be misleading. As just one example, many types of cave animals are eyeless and lack pigments. But these resemblances do not mean that the species that occupy caves are closely related to one another; instead, they are the product of convergent evolution. If the goal of a classification system is to group related organisms together, then attending only to similarities might lead to an incorrect classification.

A cladistics approach solves this problem. Widely adopted beginning in the 1990s, cladistics is a phylogenetic system that defines groups by distinguishing between ancestral and derived characters. Ancestral characters are inherited attributes that resemble those of the ancestor of a group; an organism with derived characters has features that are different from those found in the group's ancestor.

In making a diagram such as figure 14.14., how do researchers know which characters are ancestral and which are derived? They choose an outgroup consisting of comparator organisms that are not part of the group being studied. For example, in a cladistic analysis of mammals that give birth to live young, an appropriate outgroup might be monotremes. Features that are present in all mammals, such as mammary glands and hair, are assumed to be ancestral features. For placental mammals, derived features would include the placenta and other characteristics that do not appear in monotremes or marsupials.

-A species goes extinct when all of its members have died. The change that wipes out a species may be habitat loss, new predators, or new diseases. Extinction may also be a matter of bad luck: Sometimes no individual of a species survives a volcanic eruption or asteroid impact.

-No matter what the external trigger of an extinction, the root cause is always the same: Species die out if evolution fails to meet the pace of environmental change. Any species will eventually vanish if its gene pool does not contain the "right" alleles necessary to sustain the population; genetic diversity is therefore essential in a changing environment.

-Biologists distinguish between two different types of extinction events. The background extinction rate results form the steady, gradual loss of a species due to normal evolutionary processes. Paleontologists have used the fossil record to calculate that the background rate is roughly 0.1 to 1.0 extinctions per year per million species. Most extinctions overall have occurred as part of this more-or-less constant background rate.

-Earth has also witnessed several periods of mass extinctions, when a great number of species disappeared in a relatively short time. The geologic timescale in figure 13.2 shows five major mass extinction events over the past 500 million years (red lines indicate mass extinctions). These events have had a great influence on Earth's history because they have periodically opened vast new habitats for surviving species to diversify.

-Paleontologists study clues in Earth's sediments to understand the events that lead to mass extinctions. For example, the impact theory suggests that meteorites or comets have crashed to Earth, producing huge debris clouds that blocked sunlight and triggered extinctions in a deadly chain reaction. Without sunlight, plants died; animals likewise perished without food and shelter. A meteor impact 65 million years ago apparently doomed nearly all of the dinosaurs; evidence includes thin layers of earth that are rich in iridium, an element rare on Earth but common in meteorites.

-Movements of Earth's crust may also explain some mass extinctions. The crust is divided into many pieces, called tectonic plates. During Earth's history, these plates have drifted apart and come back together. Climates changed as continents moved toward or away from the poles, and colliding continents caused shallow coastal areas packed with life to disappear. Mountain ranges grew, destroying some habitats and creating new ones.

-Many biologists warn that we are in the midst of a sixth mass extinction—this one caused by human actions. Ecologists estimate that the extinction rate is now about 20 to 200 extinctions per million species per year. Habitat loss and habitat fragmentation, pollution, introduced species, and overharvesting combine to imperil many species (see chapter 20). This chapter's Why We Care box, on page 264, lists a few of the many vertebrate species that have recently become extinct, but the problem extends throughout all kingdoms of life. The loss of so many species is likely to severely disrupt the ecosystems we rely on.

Many biologists warn that we are in the midst of a sixth mass extinction—this one caused by human actions. Ecologists estimate that the extinction rate is now about 20 to 200 extinctions per million species per year. Habitat loss and habitat fragmentation, pollution, introduced species, and overharvesting combine to imperil many species (see chapter 20). This chapter's Why We Care box, on page 264, lists a few of the many vertebrate species that have recently become extinct, but the problem extends throughout all kingdoms of life. The loss of so many species is likely to severely disrupt the ecosystems we rely on.

Species extinctions have occurred throughout life's long history. They continue today, often accelerated by human activities. Overharvesting contributes to species extinctions, as does habitat loss to agriculture, urbanization, damning, or pollution. Introduced plants and animals can deplete native species by competing with or preying on them.

D. Many Traditional Groups Are Not Clades

Contemporary scientists using a cladistics approach typically assign names only to clades; incomplete clades or groups that combine portions of multiple clades are not named. Many familiar groups of species, however, are not clades.
For example, according to the traditional Linnaean classification system, class Reptilia includes turtles, lizards, snakes, crocodiles, and the extinct dinosaurs, but it excludes birds. The cladogram in figure 14.15, however, places birds in the same clade with the reptiles based on their many shared derived characters. Most biologists therefore now consider birds to be reptiles, so they make a distinction between birds and nonavian reptiles.
Nor does the kingdom Protista form a clade. Protists include mostly single-celled eukaryotes that do not fit into any of the three eukaryotic kingdoms (plants, fungi, and animals). Yet all three of these groups share a common eukaryotic ancestor with the protists. Biologists are currently struggling to divide kingdom Protista into clades, an immense task.
As yet another example, a group consisting of endothermic (formerly called "warm-blooded") animals includes only birds and mammals. This group is not a clade because it excludes the most recent common ancestor of birds and mammals, which was an ectotherm (formerly called "cold-blooded).

How is a species defined according to the biological species concept?

What is the biological definition of a species quizlet?

How is a species defined according to the biological species concept click or tap a choice to answer the question?

How is a species defined according to the biological species concept Inquizitive?