23. Speciation

Species

Types of Speciation: Allopatric, Sympatric, Peripatric, and Parapatric

Neanderthals disappeared about 40,000 years ago when the modern human population increased. This is an example of what outcome(s) of secondary contact (see Table 24.4)? Predict one way this result might have come about. <IMAGE>

For two populations of organisms to be considered separate biological species, they must be                   .
a. reproductively isolated from each other;
b. unable to produce living offspring;
c. physically very different from each other;
d. A and C are correct;
e. A, B, and C are correct

Speciation: Of Ligers & Men - Crash Course Biology #15

Allopatric & Sympatric Speciation | A-level Biology | OCR, AQA, Edexcel

A revolution in the study of human evolution is under way thanks to the invention of techniques that enable DNA sequencing of not only modern humans, but also ancient humans.

Modern humans (Homo sapiens) and archaic humans called Neanderthals (H. neanderthalensis) shared a common ancestor in Africa but diverged in different geographic areas. When modern humans migrated out of Africa and dispersed around the world starting about 100,000 years ago, they overlapped with Neanderthals in Europe. This scenario is best described as
a. species living in sympatry following allopatric speciation

b. species living in sympatry following sympatric speciation

c. species living in allopatry following allopatric speciation

d. species living in allopatry following sympatric speciation

Sexual selection favors individuals with traits that increase their ability to obtain mates, such as mating calls in crickets. Using this example, propose a scenario where sexual selection could contribute to divergence in sympatric speciation.

What is a Species? Crash Course Zoology #13

Speciation

 Murky, polluted water is causing reduced hybrid viability. 

 No. Reduced hybrid viability would contribute to reinforcement of reproductive barriers rather than fusion of species. 

 Overfishing is causing the species to fuse due to a lack of potential same-species mates for many fish. 

 No. There is no evidence that overfishing is contributing to fusion of species. 

 Extinctions are being caused by the Nile perch, an introduced predator. 

 No. Although the introduction of the Nile perch has led to the extinction of some cichlid species, it does not appear to be contributing to the observed fusion. 

 The viability of cichlid eggs is being reduced by water pollution. 

 No. Unless the pollution was directly selecting for or against hybrid eggs, such a situation would have no bearing on fusion of species in the lake. 

 Females have a difficult time visually selecting males of the same species in the murky, polluted water. 

How can changes in the environment promote or discourage speciation?

In the Lake Victoria cichlids, what appears to be contributing to fusion of different species?

 No. A single gene has been shown to cause a mechanical barrier to reproduction, but there is a better answer. 

 No. At least 26 chromosomal segments have been implicated in the speciation of certain sunflowers, but there is a better answer. 

 No. At least four gene loci are involved in a fruit fly speciation event, but there is a better answer. 

 No. Undoubtedly a speciation event has occurred based on changes in 14 genes, but there is a better answer. 

 There is no set number of genes or loci that produces a new species. Genetic and environmental factors interact. 

Think about how many different causes of reproductive isolation there might be.

How many genes must change in order to form a new species?

What distinguishes a morphospecies?
a. It has distinctive characteristics, such as size, shape, or coloration.
b. It represents a distinct branch in a phylogeny of populations.
c. It is reproductively isolated from other species.
d. It is a fossil from a distinct time in Earth history.

When the ranges of two different species meet, a stable “hybrid zone” occupied by hybrid individuals may form. How is this possible?
a. Two diverged populations are capable of mating and producing viable and fertile offspring.
b. Hybrid individuals are always allopolyploid and are thus unable to mate with either of the original species.
c. Hybrid individuals may have reduced fitness and thus be strongly selected against.
d. One species has a selective advantage, so as hybridization continues, the other species will go extinct.

Short Video: Prezygotic Barriers

 No. This prezygotic mechanism involves incompatible gametes. 

 No. This is an anatomical incompatibility that prevents mating. 

 No. Fertilization occurs in hybrid breakdown, but the second-generation hybrid is not viable. 

 No. This involves hybrid individuals producing fewer viable offspring. 

The answer is a type of prezygotic barrier.

Two species of water lilies in the same pond do not interbreed because one blooms at night and the other during the day. The reproductive barrier between them is an example of __________.

 No. Living in the same area does not make them the same species. 

 No. Although very similar, they are different enough in appearance that they were once classified as two species. 

 No. There is no evidence pointing to that possibility. 

 No. They should never have been considered two species because they can interbreed and produce fertile offspring. 

 successfully interbreed and produce fertile offspring 

Consider the different species concepts.

Bird guides once listed the myrtle warbler and Audubon's warbler as distinct species that lived side by side in parts of their ranges. However, recent books show them as eastern and western forms of a single species, the yellow-rumped warbler. Apparently, the myrtle warbler and Audubon's warbler __________.

 No. If they are geographically isolated from each other, it is unlikely that they arose by sympatric speciation. 

 No. Gene flow would be more likely to prevent speciation. 

 No. Polyploidy among animals is extremely rare. Also, molecular studies done on the Gal&#225;pagos finches have not indicated that polyploidy was a factor in their speciation. 

 No. Hybridization tends to increase gene flow between previously isolated populations. 

How does geographic isolation lead to speciation?

The Gal&#225;pagos finches collected by Darwin are thought to have descended from a very small parent population. Different species of finches that are confined to only one or two islands most likely arose by __________.

No. The two gametes would be n = 2 and n = 4. They would combine for a total of 6, which after doubling would be 12.

 None of the listed responses is correct. 

 No. One of the other choices is correct. 

Think about the gametes that will be produced by each of the parental species.

If a new species of plant is to be produced by means of allopolyploidy from two parental species that are 2n = 4 and 2n = 8, how many chromosomes would you expect in the somatic cells of the new species?

Define Species

 No. This is isolation due to the inability of gametes to unite. 

 No. This is due to structural differences preventing mating or pollen transfer. 

 No. In this case, many hybrid zygotes fail to develop. 

 No. A geographic barrier is a physical barrier preventing interbreeding. 

Review the forms of reproductive isolation.

There are two groups of pine trees that appear to be very similar phenotypically and genotypically. However, one releases pollen in January, when the female structures of that group are receptive, and one in March. What kind of reproductive barrier is this?

Bird guides once listed the myrtle warbler and Audubon’s warbler as distinct species. Recently, these birds have been classified as eastern and western forms of a single species, the yellow-rumped warbler. Which of the following pieces of evidence, if true, would be cause for this reclassification?
a. The two forms interbreed often in nature, and their offspring survive and reproduce well.
b. The two forms live in similar habitats and have similar food requirements.
c. The two forms have many genes in common.
d. The two forms are very similar in appearance.

 One species of frog mates in April, but another mates in May. 

 No. Because mating does not take place, this is a prezygotic barrier, specifically, temporal isolation. 

 The sperm of a marine worm can only fertilize eggs of the same species. 

 No. Because a zygote is not produced, this is a prezygotic barrier, specifically, gametic incompatibility. 

 One species of flower grows in forested areas, another in meadows. 

 No. This is a prezygotic barrier, specifically, ecological (spatial) isolation, and thus no mating takes place. 

 Two pheasant species perform different courtship dances. 

 No. This is an example of behavioral isolation, which is a prezygotic barrier. 

 Two fruit flies of different species produce sterile offspring. 

Consider which of these requires the formation of a zygote but one that doesn't work all that well. Review the forms of reproductive isolation.

Which of the following is an example of a postzygotic reproductive barrier?

 isolated species changing over a few thousand generations represent graduated equilibrium, not punctuated equilibrium 

 No. A few thousand generations is a very short period in geological time, so this could appear as a punctuated equilibrium. 

 polyploidy is not a mechanism of punctuated equilibrium 

 No. Polyploidy represents one of the main mechanisms of punctuated equilibrium, especially in plants. 

 fossils of organisms that existed during periods of stasis should be as abundant as those of organisms that existed during periods of rapid change 

 No. In the model of punctuated equilibrium, periods of stasis are much more common than are periods of rapid change. 

 the tempo of evolution consists of abrupt episodes of speciation among long periods of equilibrium 

Think about the patterns observed in the fossil record.

According to the punctuated equilibrium model of evolution, __________.

Males of different species of the fruit fly Drosophila that live in the same parts of the Hawaiian Islands have different elaborate courtship rituals. These rituals involve fighting other males and making stylized movements that attract females. What type of reproductive isolation does this represent?
a. habitat isolation
b. temporal isolation
c. behavioral isolation
d. gametic isolation

A revolution in the study of human evolution is under way thanks to the invention of techniques that enable DNA sequencing of not only modern humans, but also ancient humans.

Before the application of DNA sequencing to fossils, which species concept was most useful for distinguishing human fossils? What is one disadvantage of this approach?

 A catastrophic event must cause geographic separation of two populations. 

 No. Geographic separation is not always necessary for speciation (remember sympatric speciation), and it may be gradual, not catastrophic. 

 No. A hybrid zone could not be established until gene flow had already been interrupted for some length of time. In addition, a hybrid zone might tend to slow or stop speciation. 

 The populations must become reproductively isolated. 

 No. Although reproductive isolation is necessary for speciation according to the biological species model, reproductive isolation is unlikely to occur until gene flow is interrupted and the populations have diverged genetically to some extent. 

 One of the populations must become polyploid. 

 No. Though polyploidy can stop gene flow almost instantaneously, it is not very common, nor is it necessary for speciation to occur. 

 Gene flow between populations must be interrupted. 

What is the basic element that holds the gene pool of a species together?

What is the first thing that must happen in order for speciation to occur?

A revolution in the study of human evolution is under way thanks to the invention of techniques that enable DNA sequencing of not only modern humans, but also ancient humans.

Human populations today are not considered separate species under the biological species, morphospecies, and phylogenetic concepts. Explain what kind of evidence supports this statement.

According to the most accepted scientific hypothesis about the origin of two new species from a single common ancestor, most new species arise when                   .
a. many mutations occur;
b. populations of the ancestral species are isolated from one another;
c. there is no natural selection;
d. a supernatural creator decides that two new species would be preferable to the old one;
e. the ancestral species decides to evolve

Which of the following describes vicariance?

a. Small populations coalesce into one large population.
b. A population is fragmented into isolated subpopulations.
c. Individuals colonize a novel habitat.
d. Individuals disperse and found a new population.

 The island birds all have red feathers, but the mainland birds have green feathers. 

 No. Populations with different appearances are not necessarily different species. 

 The preferred food of the island birds is quite different from the diet of mainland birds. 

 No. Though two such populations might be considered different species under the ecological species concept, the biological species concept requires reproductive isolation. 

 Individuals from the different populations frequently direct courtship behavior toward members of the other population. 

 No. This suggests that speciation did not occur. 

 Hybrid offspring of matings between individuals from the two populations do not look like either parent. 

 No. This is not evidence of speciation. 

 Individuals from the different populations sometimes mate with each other, but all of the resulting eggs are sterile. 

Consider the definition of the biological species concept.

Imagine that part of a population of South American finches is blown by a storm onto an island far offshore and manages to survive and reproduce there for a period of 10,000 years. After that period, a climate change results in lower sea levels and the reconnection of the island with the mainland. Members of the formerly isolated island finch population can now interact freely with members of the original mainland population. Which of the following observations would, by itself, lead you to conclude unequivocally that the island finch population had evolved into a distinct species, according to the biological species concept?

What is a species?

True or false? Speciation is a slow process. Justify your answer.

 No. Genetic drift can result in reproductive isolation of allopatric populations, increasing the likelihood of speciation. 

 No. Because mutations are random, when they occur in isolated allopatric populations they increase the likelihood of speciation occurring. 

 No. Natural selection can result in reproductive isolation of allopatric populations, increasing the likelihood of speciation. 

 All of the listed responses are correct. 

 No. One of the mechanisms reduces the likelihood of speciation. 

How does speciation occur?

Which of the following evolutionary mechanisms does not contribute to the process of allopatric speciation?

All of the following statements support the hypothesis that humans cannot be classified into biological races except:
a. There is more genetic diversity within a racial group than average differences between racial groups;
b. Alleles that are common in one population in a racial group may be uncommon in other populations of the same race;
c. Geneticists can use particular SNP alleles to identify the ancestral group(s) of any individual human;
d. There are no alleles found in all members of a given racial group;
e. There is genetic evidence of mixing among human populations occurring thousands of years ago until the present.

 No. With reduced hybrid viability, fertilization occurs, but the resulting hybrid is weak and unlikely to survive to reproduce. 

 No. With hybrid sterility, fertilization occurs, but the resulting hybrid is sterile. 

 No. With gametic isolation, mating occurs, but fertilization does not occur because the gametes are not compatible. 

 No. With hybrid breakdown, fertilization occurs, but the second-generation hybrid is not viable. 

Consider which of these is a barrier that prevents gametes from meeting each other. Review the forms of reproductive isolation.

Which of the following reproductive barriers actually prevents individuals of closely related species from copulating successfully?

Three-spine sticklebacks are small fish that originated in the ocean and continue to exist there, but then some took up residence in hundreds of streams and freshwater lakes in the Northern Hemisphere. Predict whether you would expect to find different species in lakes today.

Which of the following is an example of a prefertilization barrier to reproduction?
a. A female mammal is unable to carry a hybrid offspring to term;
b. Hybrid plants produce only sterile pollen;
c. A hybrid between two bird species cannot perform a mating display;
d. A male fly of one species performs a “wing-waving” display that does not convince a female of another species to mate with him;
e. A hybrid embryo is not able to complete development

Biological Species

 No. It usually takes many generations for speciation to occur by geographic isolation. 

 in a very large population spread over a large area 

 No. It normally takes many generations for sexually reproducing organisms to speciate. 

 No. Allopatric speciation occurs by geographic isolation, and it takes several generations for speciation to occur in this manner. 

 No. This most likely will not lead to new species arising in a single generation. 

 if a change in chromosome number creates a reproductive barrier 

Think about some of the mechanisms of sympatric speciation.

A new species can arise in a single generation __________.

Allopatric and sympatric speciation | Biology | Khan Academy

All over the world, natural habitats are being fragmented into tiny islands by suburbs, ranches, farms, and roads. If asked to join a science panel to speak to Congress, could you explain how this fragmentation process could lead to extinction? How it could lead to speciation?

Sympatric Speciation and Hybrid Zones

Animation: Polyploid Plants

Hybrid Zones & Speciation

 No. Hybridization would mark the beginning of gene flow between two formerly isolated populations. 

 No. Sexual reproduction is one reproductive strategy. The cessation of gene flow might indicate a barrier of some kind to sexual reproduction. 

 No. Sexual selection can occur only among members of a population where gene flow is occurring. 

No. Fusion refers to two or more populations reestablishing gene flow and forming one species.

How does gene flow hold a population together?

When gene flow between two populations ceases, the potential for __________ exists.

 No. This is a specific pattern of speciation, not a condition that leads to it. 

 No. However, rapid speciation often follows mass extinctions. 

 Interbreeding among neighboring populations 

Consider the biological species concept.

Which of the following conditions is necessary for speciation to occur?

 prevent a hybrid zygote from developing into a viable, fertile adult 

 No. This would be an action of a postzygotic barrier. 

 are necessary in order for speciation to occur 

 No. Prezygotic barriers are a component of reproductive isolation, which is only one aspect of speciation under the biological species concept.  However, reproductive isolation can also occur due to postzygotic barriers. 

 No. Donkeys and horses are capable of mating, but their offspring (mules and hinnies) are almost always sterile. 

 No. They only occur in sexual organisms. 

 prevent fertilization of gametes from members of closely related species 

Review the different forms of reproductive isolation and think about the prefix "pre-."

Prezygotic barriers __________.

What Makes a Species a Species?

 Yes. After several generations, the reproductive barrier (behavioral isolation) between the starch- and maltose-fed populations was complete. 

 No. Some mating between the starch flies and maltose flies did occur. 

 Yes. After several generations, the reproductive barrier (mechanical isolation) between the starch- and maltose-fed populations was complete. 

 No. Some mating between the starch flies and maltose flies did occur. Also, this experiment was an example of behavioral isolation. 

 No. After several generations, the formation of a reproductive barrier (mechanical isolation) was evident but not absolute. 

 No. This experiment was an example of behavioral isolation. 

 No. Even after several generations, there was no evidence of a reproductive barrier forming between the populations. 

 No. The starch flies were more apt to mate with other starch flies, and the maltose flies were more apt to mate with other maltose flies, although some interbreeding still did occur. 

 No. After several generations, the formation of a reproductive barrier (behavioral isolation) was evident but not absolute. 

Review Dodd's fruit fly experiments on divergence of allopatric populations.

According to the experiment of Diane Dodd, can adaptive divergence of allopatric fruit fly populations lead to reproductive isolation?

Which of the following factors would not contribute to allopatric speciation?
a. The separated population is small, and genetic drift occurs.
b. The isolated population is exposed to different selection pressures than the ancestral population.
c. Different mutations begin to distinguish the gene pools of the separated populations.
d. Gene flow between the two populations is extensive.

 No. There is an easier way to arrive at a conclusion. In addition, many of the speciation events in Lake Victoria are too recent to have left fossil evidence. 

 Compare DNA sequences because the DNA of members of the same species should be identical. 

 No. There is a more direct way to arrive at a conclusion. 

 Do studies on comparative anatomy, especially looking for homologous structures. 

 No. There is an easier way to arrive at a conclusion. 

 No. One of the other choices will suffice. 

 Observe the fish in their natural environment for possible instances of interbreeding leading to several generations of fertile offspring. 

Review the definition of a biological species.

Lake Victoria, in Africa, is home to a group of related fishes known as cichlids. Many of these fishes are similar in appearance but have different feeding habits. What is the best method for scientists to determine conclusively whether the fish are members of a population that has a lot of variation or members of entirely different species?

Speciation: An Illustrated Introduction

 No. Although reduced hybrid viability would tend to strengthen reproductive barriers, there is a better answer. 

 No. Although hybrid breakdown would tend to strengthen reproductive barriers, there is a better answer. 

 No. Although reduced hybrid fertility would tend to strengthen reproductive barriers, there is a better answer. 

 No. Although sexual selection has been shown to strengthen reproductive barriers in some cases, there is a better answer. 

What causes the possible outcomes in a hybrid zone?

In a hybrid zone, which of the following would tend to lead to reinforcement?

 Whale populations of the same species located on opposite sides of the Atlantic Ocean 

 No. Genetic mixing may occur between the two due to the mobility of whales through the ocean. 

 Mountain lions in the canyons of Wyoming and in the nearby canyons of Utah 

 No. These represent one population because mountain lions recognize neither political barriers nor most topographical barriers. 

 Fruit flies on bananas and fruit flies on oranges 

 No. This certainly does not represent a strong geographic barrier. 

 Bacteria in a hospital and bacteria in a nursery 

 No. Although the bacteria don't travel, the people who carry them do. 

 Pine trees in Alaska and pine trees on the island of Madagascar 

Allopatric speciation requires geographic isolation.