Why Evolution Is True (40 page)
Read Why Evolution Is True Online
Authors: Jerry A. Coyne
28
As I write, a report has just appeared showing that DNA extracted from the bones of Neanderthals contains another light-color form of the gene. It’s likely, then, that some Neanderthals had red hair.
As I write, a report has just appeared showing that DNA extracted from the bones of Neanderthals contains another light-color form of the gene. It’s likely, then, that some Neanderthals had red hair.
29
Different breeds are all considered to fall under the species
Canis lupus familiaris
because they can successfully hybridize. If they occurred only as fossils, their substantial differences would lead us to conclude that there is some genetic barrier preventing them from hybridizing, ergo they must represent different species.
Different breeds are all considered to fall under the species
Canis lupus familiaris
because they can successfully hybridize. If they occurred only as fossils, their substantial differences would lead us to conclude that there is some genetic barrier preventing them from hybridizing, ergo they must represent different species.
30
The insects also adapted to the different chemistry of the plant species, so that each new form of the bug now thrives best on the introduced plant it inhabits rather than the old soapberry bush.
The insects also adapted to the different chemistry of the plant species, so that each new form of the bug now thrives best on the introduced plant it inhabits rather than the old soapberry bush.
31
For descriptions of how blood clotting and the flagellum might have evolved through selection, see Kenneth Miller’s book
Only a Theory,
as well as M. J. Pallen and N. J. Matzke (2006).
For descriptions of how blood clotting and the flagellum might have evolved through selection, see Kenneth Miller’s book
Only a Theory,
as well as M. J. Pallen and N. J. Matzke (2006).
32
To see sage grouse strutting on the lek before females, go to
http://www.you-tube.com/watch?v=qcWx2VbT_j8
.
To see sage grouse strutting on the lek before females, go to
http://www.you-tube.com/watch?v=qcWx2VbT_j8
.
33
The earliest sexually reproducing creature so far identified is a red alga aptly named
Bangiomorpha pubescens.
Two sexes are clearly visible in its fossils from 1.2 billion years ago.
The earliest sexually reproducing creature so far identified is a red alga aptly named
Bangiomorpha pubescens.
Two sexes are clearly visible in its fossils from 1.2 billion years ago.
34
It’s important to remember that we’re talking about the difference between males and females in the
variance
of mating success. In contrast, the average mating success of males and females must be equal, because each offspring must have one father and one mother. In males, this average is attained by a few of them siring most of the offspring while the rest have none. Each female, on the other hand, has roughly the same number of offspring.
It’s important to remember that we’re talking about the difference between males and females in the
variance
of mating success. In contrast, the average mating success of males and females must be equal, because each offspring must have one father and one mother. In males, this average is attained by a few of them siring most of the offspring while the rest have none. Each female, on the other hand, has roughly the same number of offspring.
35
When pressed, creationists explain sexual dimorphisms by resorting to the mysterious whims of the creator. In his book
Darwin on Trial,
intelligent design advocate Phillip Johnson responds to evolutionist Douglas Futuyma’s query: “Do the creation scientists really suppose their Creator saw fit to create a bird that couldn’t reproduce without six feet of bulky feathers that make it easy prey for leopards?” Johnson replies: “I don’t know what creation-scientists may suppose, but it seems to me that the peacock and peahen are just the kind of creatures a whimsical Creator might favor, but that an ’uncaring mechanical process’ like natural selection would never permit to develop.” But a well-understood and
testable
hypothesis like sexual selection surely trumps an untestable appeal to the inscrutable caprices of a creator.
When pressed, creationists explain sexual dimorphisms by resorting to the mysterious whims of the creator. In his book
Darwin on Trial,
intelligent design advocate Phillip Johnson responds to evolutionist Douglas Futuyma’s query: “Do the creation scientists really suppose their Creator saw fit to create a bird that couldn’t reproduce without six feet of bulky feathers that make it easy prey for leopards?” Johnson replies: “I don’t know what creation-scientists may suppose, but it seems to me that the peacock and peahen are just the kind of creatures a whimsical Creator might favor, but that an ’uncaring mechanical process’ like natural selection would never permit to develop.” But a well-understood and
testable
hypothesis like sexual selection surely trumps an untestable appeal to the inscrutable caprices of a creator.
36
You may ask why, if females have a preference for unexpressed traits, those traits never evolve in males? One explanation is simply that the right mutations didn’t occur. Another is that the right mutations
did
occur, but reduced the male’s survival more than it enhanced his ability to attract mates.
You may ask why, if females have a preference for unexpressed traits, those traits never evolve in males? One explanation is simply that the right mutations didn’t occur. Another is that the right mutations
did
occur, but reduced the male’s survival more than it enhanced his ability to attract mates.
37
You might object that this concordance shows only that all human brains are neurologically wired to divide up what is really a continuum of birds at the same arbitrary points. But this objection loses force when you remember that
the birds themselves
recognize the same clusters. When it comes time to reproduce, a male robin courts only female robins, not female sparrows, starlings, or crows. Birds, like other animals, are good at recognizing different species!
You might object that this concordance shows only that all human brains are neurologically wired to divide up what is really a continuum of birds at the same arbitrary points. But this objection loses force when you remember that
the birds themselves
recognize the same clusters. When it comes time to reproduce, a male robin courts only female robins, not female sparrows, starlings, or crows. Birds, like other animals, are good at recognizing different species!
38
For example, if 99 percent of all species produced went extinct, we still need a speciation rate of only one new species arising per hundred million years to produce 100 million living species.
For example, if 99 percent of all species produced went extinct, we still need a speciation rate of only one new species arising per hundred million years to produce 100 million living species.
39
For a lucid presentation of how science reconstructs ancient events in geology, biology, and astronomy, see C. Turney. 2006.
Bones, Rocks and Stars: The Science of When Things Happened.
Macmillan, New York.
For a lucid presentation of how science reconstructs ancient events in geology, biology, and astronomy, see C. Turney. 2006.
Bones, Rocks and Stars: The Science of When Things Happened.
Macmillan, New York.
40
Here’s a more detailed description of how a new allopolyploid species arises. Bear with me, for although understanding the process isn’t hard, it requires keeping track of a few numbers. Every species, except for bacteria and viruses, carries two copies of each chromosome. We humans, for example, have forty-six chromosomes, comprising twenty-two pairs, or
homologs,
plus the two sex chromosomes: XX in females and XY in males. One member of each chromo- some pair is inherited through the father, the other through the mother. When individuals of a species make gametes (sperm and eggs in animals, pollen and eggs in plants), the homologs get separated from one another, and only one member of each pair goes into a sperm, egg, or pollen grain. But before that, the homologs must line up and pair with each other so that they can be properly divided. If the chromosomes can’t pair up properly, the individual can’t produce gametes and is sterile.
Here’s a more detailed description of how a new allopolyploid species arises. Bear with me, for although understanding the process isn’t hard, it requires keeping track of a few numbers. Every species, except for bacteria and viruses, carries two copies of each chromosome. We humans, for example, have forty-six chromosomes, comprising twenty-two pairs, or
homologs,
plus the two sex chromosomes: XX in females and XY in males. One member of each chromo- some pair is inherited through the father, the other through the mother. When individuals of a species make gametes (sperm and eggs in animals, pollen and eggs in plants), the homologs get separated from one another, and only one member of each pair goes into a sperm, egg, or pollen grain. But before that, the homologs must line up and pair with each other so that they can be properly divided. If the chromosomes can’t pair up properly, the individual can’t produce gametes and is sterile.
This failure to pair is the basis of allopolyploid speciation. Suppose, for example, that a plant species (let’s be imaginative and call it A) has six chromosomes, three pairs of homologs. Suppose further that it has a relative, species B, with ten chromosomes (five pairs). A hybrid between the two species will have eight chromosomes, getting three from species A and five from species B (remember that the gametes of each species carry only half of its chromosomes). This hybrid may be viable and vigorous, but when it tries to form pollen or eggs, it runs into trouble. Five chromosomes from one species try to pair with three from the other, creating a mess. Gamete formation is aborted, and the hybrid is sterile.
But suppose that somehow the hybrid could simply duplicate
all
of its chromosomes, raising the number from eight to sixteen. This new super-hybrid will be able to undergo proper chromosome pairing: each of the six chromosomes from species A will find its homolog, and likewise the ten chromosomes from species B. Because pairing occurs properly, the super-hybrid will be fertile, producing pollen or eggs carrying eight chromosomes. The super-hybrid is technically known as a
allopolyploid,
from the Greek for “different” and “many-fold.” In its sixteen chromosomes, it carries the complete genetic material of both parental species, A and B. We would expect it to look somewhat like an intermediate between the two parents. And its new combination of traits might enable it to live in a novel ecological niche.
all
of its chromosomes, raising the number from eight to sixteen. This new super-hybrid will be able to undergo proper chromosome pairing: each of the six chromosomes from species A will find its homolog, and likewise the ten chromosomes from species B. Because pairing occurs properly, the super-hybrid will be fertile, producing pollen or eggs carrying eight chromosomes. The super-hybrid is technically known as a
allopolyploid,
from the Greek for “different” and “many-fold.” In its sixteen chromosomes, it carries the complete genetic material of both parental species, A and B. We would expect it to look somewhat like an intermediate between the two parents. And its new combination of traits might enable it to live in a novel ecological niche.
The AB polyploid is not only fertile, but will produce offspring if it is fertilized by another similar polyploid. Each parent contributes eight chromosomes to the seed, which will grow into another sixteen-chromosome AB plant, just like its parents. A group of such polyploids makes up a self-perpetuating, interbreeding population.
And it will also be a new species. Why? Because the AB polyploid is reproductively isolated from both parental species. When they hybridize with either species A or species B, the offspring are sterile. Suppose it hybridizes with species A. The polyploid will produce gametes having eight chromosomes, three originally from species A and five from species B. These will fuse with the gametes from species A, which contain three chromosomes. The plant arising from this union will have eleven chromosomes. And it will be sterile, for while each A chromosome has a pairing partner, none of the B chromosomes do. A similar situation arises when the AB polyploid mates with species B: the offspring will have thirteen chromosomes, and the five A chromosomes can’t pair during gamete formation.
The new polyploid, then, produces only sterile hybrids when it mates with either of the two species that gave rise to it. Yet when the polyploids mate with each other, the offspring will be fertile, having all sixteen chromosomes of their parents. In other words, the polyploids form an interbreeding group that is reproductively isolated from other groups—and that’s just what defines a distinct biological species. And this species has arisen without geographical isolation—that’s necessary because if two species are to form hybrids, they must live in the same place.
How does the polyploid species form in the first place? We needn’t go into the messy details here except to say that it involves the formation of a hybrid between the two parental species followed by a series of steps in which those hybrids produce rare pollen or eggs carrying double sets of chromosomes (these are called
unreduced gametes).
Fusion of these gametes produces a polyploid individual in only two generations. And all of these steps have been documented in both the greenhouse and in nature.
unreduced gametes).
Fusion of these gametes produces a polyploid individual in only two generations. And all of these steps have been documented in both the greenhouse and in nature.
41
As an example of autopolyploidy, let’s assume that members of a plant species have fourteen chromosomes, or seven pairs. An individual might occasionally produce unreduced gametes containing all fourteen chromosomes instead of seven. If this gamete fused with a normal, seven-chromosome gamete from another individual of the same species, we would get a semisterile plant having twenty-one chromosomes: it’s mostly sterile because during gamete formation, three homologous chromosomes try to pair instead of the normal two, and this doesn’t work well. But if this individual again produces a few unreduced twenty-one-chromosome gametes that fuse with normal gametes from the same species, we get a twenty-eight-chromosome autopolyploid individual. It carries two complete copies of the parental genome. A population of such individuals can be considered a new species, for they can interbreed with other similar autopolyploids but will produce largely sterile twenty-one-chromosome individuals when they try to mate with the parental species. This autopolyploid species has exactly the same genes as members of the single parental species, but in quadruple rather than double dose.
As an example of autopolyploidy, let’s assume that members of a plant species have fourteen chromosomes, or seven pairs. An individual might occasionally produce unreduced gametes containing all fourteen chromosomes instead of seven. If this gamete fused with a normal, seven-chromosome gamete from another individual of the same species, we would get a semisterile plant having twenty-one chromosomes: it’s mostly sterile because during gamete formation, three homologous chromosomes try to pair instead of the normal two, and this doesn’t work well. But if this individual again produces a few unreduced twenty-one-chromosome gametes that fuse with normal gametes from the same species, we get a twenty-eight-chromosome autopolyploid individual. It carries two complete copies of the parental genome. A population of such individuals can be considered a new species, for they can interbreed with other similar autopolyploids but will produce largely sterile twenty-one-chromosome individuals when they try to mate with the parental species. This autopolyploid species has exactly the same genes as members of the single parental species, but in quadruple rather than double dose.
Since a newly formed autopolyploid has the same genes as its parental species, it often resembles it closely. Members of the new species can sometimes be identified only by counting their chromosomes under the microscope and seeing that they have twice as many chromosomes as individuals of the parental species. Because they resemble their parents, many autopolyploid species surely exist in nature that haven’t yet been identified.
42
Although cases of nonpolyploid speciation occurring in “real time” are rare, there is at least one that seems plausible. This involves two groups of mosquitoes in London, which are usually named as subspecies but show substantial reproductive isolation.
Culex pipiens pipiens
is one of the most common urban mosquitoes. Its most frequent victims are birds, and, as in many species of mosquitoes, females lay eggs only after they’ve had a blood meal. During winter, males die but females enter a hibernation-like state called “diapause.” When mating,
pipiens
form large swarms in which males and females copulate en masse.
Although cases of nonpolyploid speciation occurring in “real time” are rare, there is at least one that seems plausible. This involves two groups of mosquitoes in London, which are usually named as subspecies but show substantial reproductive isolation.
Culex pipiens pipiens
is one of the most common urban mosquitoes. Its most frequent victims are birds, and, as in many species of mosquitoes, females lay eggs only after they’ve had a blood meal. During winter, males die but females enter a hibernation-like state called “diapause.” When mating,
pipiens
form large swarms in which males and females copulate en masse.
Fifty feet below, within the tunnels of the London Underground, lives a closely related subspecies:
Culex pipiens molestus,
so called because it prefers to bite mammals, especially ones that ride the Tube. (It became a real annoyance during the Blitz of World War II, when thousands of Londoners were forced to sleep in Underground stations during air raids.) Besides preying on rats and humans,
molestus
doesn’t need a blood meal to lay eggs, and, as one might expect for inhabitants of mild-temperature tunnels, prefers to mate in confined spaces and doesn’t diapause during winter.
Culex pipiens molestus,
so called because it prefers to bite mammals, especially ones that ride the Tube. (It became a real annoyance during the Blitz of World War II, when thousands of Londoners were forced to sleep in Underground stations during air raids.) Besides preying on rats and humans,
molestus
doesn’t need a blood meal to lay eggs, and, as one might expect for inhabitants of mild-temperature tunnels, prefers to mate in confined spaces and doesn’t diapause during winter.
The difference in the way these two subspecies mate leads to strong sexual isolation between the forms in both nature and the laboratory. That, coupled with the substantial genetic divergence between the forms, indicates that they are on their way to becoming different species. Indeed, some entomologists already classify them this way—as
Culex pipiens
and
Culex molestus.
Since construction of the Underground was not begun until the 1860s, and many of the lines are less than a hundred years old, this “speciation” event may have occurred within recent memory. The reason the story is not airtight, though, is that there is a similar pair of species in New York: one above ground and the other in the subway tunnels. It is possible that both pairs of species are representatives of a similar and longer-diverged pair that lives elsewhere in the world, each of which migrated to its respective habitat in London and New York. What we need to attack this problem, and don’t yet have, is a good DNA-BASED family tree of these mosquitoes.
Culex pipiens
and
Culex molestus.
Since construction of the Underground was not begun until the 1860s, and many of the lines are less than a hundred years old, this “speciation” event may have occurred within recent memory. The reason the story is not airtight, though, is that there is a similar pair of species in New York: one above ground and the other in the subway tunnels. It is possible that both pairs of species are representatives of a similar and longer-diverged pair that lives elsewhere in the world, each of which migrated to its respective habitat in London and New York. What we need to attack this problem, and don’t yet have, is a good DNA-BASED family tree of these mosquitoes.
43
This group used to be called
hominids,
but that term is now reserved for all modern and extinct great apes, including humans, chimpanzees, gorillas, orangutans, and all of their ancestors.
This group used to be called
hominids,
but that term is now reserved for all modern and extinct great apes, including humans, chimpanzees, gorillas, orangutans, and all of their ancestors.
44
A sidelight on the competitive nature of paleoanthropology is the number of people sharing credit for the discovery, preparation, and description of
Sahelanthropus:
the paper announcing it has thirty-eight authors—all for a single skull!
A sidelight on the competitive nature of paleoanthropology is the number of people sharing credit for the discovery, preparation, and description of
Sahelanthropus:
the paper announcing it has thirty-eight authors—all for a single skull!
46
See
http://www.pbs.org/wgbh/evolution/library/07/1/1_071_03.html
for a video clip of the footprints and how they were made.
See
http://www.pbs.org/wgbh/evolution/library/07/1/1_071_03.html
for a video clip of the footprints and how they were made.
47
Note that this would actually be the second time that the human lineage had migrated out of Africa, the first being the spread of
Homo erectus.
Note that this would actually be the second time that the human lineage had migrated out of Africa, the first being the spread of
Homo erectus.
48
See
http://www.tallcorigins.org/faqs/homs/compare.html
for a discussion of how creationists treat the human fossil record.
See
http://www.tallcorigins.org/faqs/homs/compare.html
for a discussion of how creationists treat the human fossil record.
49
Unlike most primates, human females show no visible signs when ovulating. (The genitals of female baboons, for example, swell up and turn red when they’re fertile.) There are more than a dozen theories about why human females evolved to conceal their periods of fertility. The most famous is that this is a female strategy to keep their mates around for sustenance and child care. If a man doesn’t know when his wife is fertile, and wants to father children, he should hang around and copulate with her frequently.
Unlike most primates, human females show no visible signs when ovulating. (The genitals of female baboons, for example, swell up and turn red when they’re fertile.) There are more than a dozen theories about why human females evolved to conceal their periods of fertility. The most famous is that this is a female strategy to keep their mates around for sustenance and child care. If a man doesn’t know when his wife is fertile, and wants to father children, he should hang around and copulate with her frequently.
50
The idea that
FOXP2
is a language gene comes from observing that it has evolved extremely fast in the human lineage, that mutant forms of the gene affect people’s ability to produce and comprehend speech, and that similar mutations in mice make the babies unable to squeak.
The idea that
FOXP2
is a language gene comes from observing that it has evolved extremely fast in the human lineage, that mutant forms of the gene affect people’s ability to produce and comprehend speech, and that similar mutations in mice make the babies unable to squeak.
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