Link to Species and Speciation Student Learning Guide handout.

1. Introduction

Darwin’s theory of natural selection explains how adaptations come about. But how do we explain the incredible diversity of life? How can we explain the emergence of over 3,000,000 different species? And, even more fundamentally, what is a species?

2. Species: An Interactive Reading

Even though you might not know some of the terms and definitions in the passages below, you’ll learn best by answering the questions and doing some of your own thinking. Just do your best.

[qwiz qrecord_id=”sciencemusicvideosMeister1961-Species, Interactive Reading”]

[h]Species: An interactive reading

[q]Here are a few different types of dogs.

How many species are shown above?

[c*]1     [c]2     [c]3

[f]That’s correct. The German Shepherd, Golden Retriever, and Poodle shown above are different breeds, or varieties, of the domesticated dog, Canis lupus familiaris. The domesticated dog is itself a subgroup of a larger species that includes both wolves (Canis lupus, with several subspecies), and the domesticated dog, with hundreds of artificial breeds.

So what is a species?

[f]No. The German Shepherd, Golden Retriever, and Poodle shown above are different breeds, or varieties, of the domesticated dog, Canis lupus familiaris. The domesticated dog is itself a subgroup of a larger species that includes both wolves (Canis lupus, with several subspecies), and the domesticated dog, with hundreds of artificial breeds.

So what is a species?

[f]No. The German Shepherd, Golden Retriever, and Poodle shown above are different breeds, or varieties, of the domesticated dog, Canis lupus familiaris. The domesticated dog is itself a subgroup of a larger species that includes both wolves (Canis lupus, with several subspecies), and the domesticated dog, with hundreds of artificial breeds.

So what is a species?

[q labels = “top”]There are several ways of defining the term species. We’re going to start with what’s called the biological species concept, which defines a species as

  • A group of individuals or populations that share a common ___________, and which is ______________ from other such groups.
  • A population that can _______________ to produce ___________ offspring.

[l]fertile

[fx] No, that’s not correct. Please try again.

[f*] Great!

[l]gene pool

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[l]isolated

[fx] No, that’s not correct. Please try again.

[f*] Great!

[l]interbreed

[fx] No, that’s not correct. Please try again.

[f*] Good!

[q]So, with the biological species concept in mind, explain why all breeds of dogs, including the three shown below, are all members of the same species. Write out your answer, and then check it against what you’ll see after clicking the button below.

[c*]Show the answer

[f]All dogs are members of the same species because they make up a single gene pool. All dogs, despite their different appearance, can (and will) breed with one another (and with wolves as well). This dog/wolf gene pool is isolated from the gene pools of other species in the dog family (such as foxes and African wild dogs).

[q]Just to make sure you’ve got it: A group of organisms that can naturally interbreed to produce fertile offspring is a

[hangman]

[c]species

[q]Below are two species of ducks, both of which are found throughout the Northern Hemisphere.

As it relates to mallards and pintails, which of the following would be the most direct consequence of the biological species concept?

[c]Mallards and pintails will nest in different types of habitat.

[f]No. That might be true, but other choices on this list are more directly related to the biological species concept.

[c*]Mallards and pintails will not be able to breed with one another.

[f]Exactly. Based on the biological species concept, each species has a separate gene pool, which means that they won’t be able to successfully interbreed (something we’ll expand upon below).

[c]Some predators will prefer to prey on mallards, while others will prefer to prey on pintails.

[f]No. That might be true, but other choices on this list are more directly related to the biological species concept.

[c]Mallards and pintails will migrate to different breeding areas during the winter.

[f]No. That might be true, but other choices on this list are more directly related to the biological species concept.

[q]Read the passage below, adapted from Wikipedia:

The Bengal cat is a domestic cat breed developed to look like wild jungle cats such as leopards and ocelots. Bengal cats were developed by selective breeding from hybrids of the wild Asian leopard cat with domestic cats. These hybrids were then bred with domestic cats, with the goal of creating a  cat with a highly contrasted and vividly marked coat.

Bengal cat

Why isn’t the Bengal cat a species? Write down your answer before clicking the button below.[c*]Show the answer

[f]Like Poodles, or German Shepherds, Bengal cats are a breed, artificially created by humans. Bengal cats will happily interbreed with any other breed of cat, so their gene pool is in no way isolated.

[x]

[restart]

[/qwiz]

3. Talking about Species, Subspecies, Breeds, and Varieties

The interactive reading above used binomial nomenclature (those Latin sounding two part species names like Homo sapiens) and a few other terms to designate groupings (such as breeds or subspecies) that fall below the species level. Let’s master all these terms before going on.

Binomial Nomenclature: Every species has a two part name called a binomial (which you should italicize or underline). For sheep, the binomial is Ovis aries. For the chinook salmon, it’s Oncorhynchus tshawytscha. Each binomial consists of a genus (Ovis or Onchorhynchus in the two preceding examples), and a specific epithet, such as the sapiens part of our species name, Homo sapiens. 

Subspecies: As a species evolves, it can become subdivided into subpopulations. Natural selection and genetic drift can lead these subpopulation to differentiate into subspecies. For example, the wolf, Canis lupus, has 38 subspecies (which you can read more about on Wikipedia). To designate a subspecies, biologists use a trinomial. For example, the Tundra wolf is Canis lupus albus, while the Arabian wolf is Canis lupus arabs. The domesticated dogs we discussed above are also part of that same species, and have the trinomial Canis lupus familiaris. 

As far as we can tell, the dogs don’t care about their breeds.

Breeds and Varieties: Between 10,000 and 15,000 years ago, humans began domesticating a variety of animals (including dogs, sheep, pigs, cattle, horses, chickens, llamas, and silkworms) and plants (including apples, walnuts, maize, peas, and hundreds of others). This domestication has involved artificial selection (or selective breeding) for a variety of traits, which has resulted in the creation of distinct breeds or varieties. The Collie, for example is a breed, or variety, of dog.  A breed is an artificial creation, only maintained through the vigilance of breeders, who restrict the dogs within that breed from mating with dogs from outside that breed. The dogs don’t care: a collie would just as soon mate with a boxer as with another collie.

4. What Keeps Species Apart? Reproductive Barriers Interactive Reading

[qwiz qrecord_id=”sciencemusicvideosMeister1961-Reproductive Barriers, Interactive Reading”]

[h]Reproductive Barriers

[q labels = “top”]The biological species concept is all about ______________ isolation. Essentially, what defines a species is its isolated ___________, one that doesn’t __________ with the gene pool of other species.

[l]gene pool

[fx] No, that’s not correct. Please try again.

[f*] Great!

[l]intermix

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[l]reproductive

[fx] No. Please try again.

[f*] Great!

[q]But what keeps these gene pools separate? As you can see below, Grizzly bears and black bears overlap in much of their range.

What keeps these two species from interbreeding, causing their distinct gene pools to blend into one? Write down your answer before proceeding.

[q labels = “top”]The answer is reproductive barriers. These are ________ (physical, biochemical, ecological, behavioral, etc.) of the organisms within a species that do one of two things:

  • They keep them from __________ with organisms from other __________.
  • If _____________ does occur, it limits the flow of ________ between the two species.

[l]breeding

[fx] No, that’s not correct. Please try again.

[f*] Good!

[l]genes

[fx] No, that’s not correct. Please try again.

[f*] Excellent!

[l]interbreeding

[fx] No. Please try again.

[f*] Great!

[l]species

[fx] No. Please try again.

[f*] Good!

[l]traits

[fx] No, that’s not correct. Please try again.

[f*] Great!

[q labels = “top”]One category of reproductive barrier is called pre-zygotic. Pre-zygotic barriers act before a _________ (a fertilized egg) is formed. These barriers prevent mating, or, if __________ occurs, prevent a zygote from _________.

Here are a few types of pre-zygotic barriers. Based on its description, label each one.

  1. _______________: If two species live in different areas, they won’t meet to interbreed. For example, one species of mouse might live on mountain slopes, and a second might live in valleys.
  2. _______________: If one species is active during the day, and a second at night, they won’t meet. Or, if one has a breeding season in the fall, and the second during the spring. they might meet, but they won’t mate.
  3. _______________: Animal mating is usually preceded by complex mating rituals. These include dances and songs, all of which are very specific. If individuals of two species have different mating rituals, they won’t mate. Many species markings (colors, feather patterns) are about letting males and females know that they’re of the same species, which is what starts a mating ritual.
  4. _______________: In animals that have internal fertilization, the sperm has to make it to the egg. If the shape of the male and female organs isn’t right, then mating can’t happen. This is specially important in flowers and insects.
  5. _______________: Once the sperm makes it to the egg, molecular markers on the sperm have to bind with receptors on the egg’s membrane. If these markers and receptors aren’t complementary, the egg won’t open. As a result, the sperm’s nucleus won’t enter, and fertilization won’t take place. In animals and plants that have external fertilization (fish that release egg and sperm into the water, or plants that use wind pollination) this type of barrier is especially important. 

[l]Behavioral

[fx] No. Please try again.

[f*] Good!

[l]forming

[fx] No, that’s not correct. Please try again.

[f*] Excellent!

[l]Gametic

[fx] No, that’s not correct. Please try again.

[f*] Good!

[l]Habitat

[fx] No. Please try again.

[f*] Good!

[l]mating

[fx] No, that’s not correct. Please try again.

[f*] Excellent!

[l]Mechanical

[fx] No, that’s not correct. Please try again.

[f*] Excellent!

[l]Temporal

[fx] No, that’s not correct. Please try again.

[f*] Great!

[l]zygote

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[q labels = “top”]Sometimes, in species that are very closely related, all of the ___________ mechanisms described in the previous card aren’t enough. As a result, members of different species do occasionally mate. Yet the gene pools of these species stay separate. How? This separation is by post-zygotic barriers. Post-zygotic barriers occur ______ zygote formation. In other words, they don’t keep two closely related species from mating, but they prevent a zygote from developing into a fertile, healthy _______.

There are three types of post-zygotic barriers. Based on its description, label each one.

  1. Hybrid __________: Hybrid zygotes don’t develop into adults.
  2. Hybrid __________: The hybrid offspring look healthy, but they can’t produce offspring of their own.
  3. Hybrid __________: The F1s (the first generation) are fine, but when these F1s interbreed, their offspring (the F2s) are weak, sterile, fail to develop, etc.

[l]adult

[fx] No, that’s not correct. Please try again.

[f*] Good!

[l]after

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[l]breakdown

[fx] No. Please try again.

[f*] Excellent!

[l]pre-zygotic

[fx] No, that’s not correct. Please try again.

[f*] Great!

[l]sterility

[fx] No. Please try again.

[f*] Excellent!

[l]inviability

[fx] No. Please try again.

[f*] Good!

[x][restart]

[/qwiz]

5. Reproductive Barriers: Checking Understanding

Identify the reproductive barriers in the examples below.

[qwiz random = “true” qrecord_id=”sciencemusicvideosMeister1961-Reproductive Barrieres, CFU”]

[h]Reproductive Barriers

[i]

[q]Two species might mate and produce offspring, but when the F1s try to produce offspring, their fertilized eggs never develop into adult F2s.

[c]pre-zygotic; gametic

[c]pre-zygotic; temporal

[c]pre-zygotic; mechanical

[c*]post-zygotic; hybrid sterility

[c]post-zygotic; hybrid breakdown.

[f]No. Pre-zygotic barriers prevent fertilization. If individuals from different species produce offspring, the barrier can’t be pre-zygotic

[f] No. Pre-zygotic barriers prevent fertilization. If individuals from different species produce offspring, the barrier can’t be pre-zygotic

[f] No. Pre-zygotic barriers prevent fertilization. If individuals from different species produce offspring, the barrier can’t be pre-zygotic

[f]That’s right. If the F1s can’t reproduce, then we have hybrid sterility.

[f]No. This barrier is post-zygotic, but the reproductive barrier involves the F1 generation. Hybrid breakdown involves the F2s being unable to reproduce.

[q]Two species of moths have slightly different pheromones (sexually attracting hormones).

[c]pre-zygotic; gametic

[c]pre-zygotic; mechanical

[c*]pre-zygotic; behavioral

[c]post-zygotic; hybrid unviability

[c]post-zygotic; hybrid breakdown

[f]No. In gametic isolation, mating occurs, but the sperm can’t fertilize the egg.

[f]No. In mechanical isolation, mating can’t occur because reproductive parts are structured so that a sperm or pollen can’t reach the egg.

[f]Excellent. This is definitely a pre-zygotic barrier, and, of the choices listed, behavioral is the only reasonable match.

[f]No. Post-zygotic barriers follow fertilization, which in this case never takes place.

[f]No. Post-zygotic barriers follow fertilization, which in this case never takes place.

[q]The mating ritual of one species of lemur seems a little weird for a second species.

[c]pre-zygotic; hybrid breakdown

[c*]pre-zygotic; behavioral

[c]pre-zygotic; habitat

[c]post-zygotic; gametic

[c]post-zygotic; hybrid sterility

[f]No. First of all, hybrid breakdown is post-zygotic. Secondly, what’s at work here is a mechanism that prevents mating (so it has to be pre-zygotic).

[f]Nice job. This is a pre-zygotic, behavioral barrier.

[f]No, but you’re on the right track. This is a pre-zygotic mechanism, but there’s no evidence that these lemurs live in different habitats.

[f]No. Remember that post-zygotic barriers follow mating. Are these lemurs going to mate?

[f]No. Remember that post-zygotic barriers follow mating. Are these lemurs going to mate?

[q]The pollen tube of one species of orchid can’t grow to reach the eggs of a second type of orchid.

[c]pre-zygotic; hybrid sterility

[c]pre-zygotic; habitat

[c]post-zygotic; hybrid inviability

[c*]pre-zygotic; mechanical

[c]post-zygotic; behavior

[f]No. First of all, hybrid sterility is a post-zygotic barrier. Second, this inability of the pollen tube to reach the egg is going to prevent fertilization. That makes it ____-zygotic.

[f]No. You’re right about pre-zygotic, but there’s no evidence that these orchids live in different habitats.

[f]No. This inability of the pollen tube to reach the egg is going to prevent fertilization. That makes it ____-zygotic.

[f]Way to go! This is an example of a mechanical, pre-zygotic reproductive barrier.

[f]No. Differences in behavior that prevent mating are a prezygotic barrier.

[q]One type of small rodent is nocturnal (active at night). The other is diurnal (active during the day).

[c*]pre-zygotic; temporal

[c]pre-zygotic; habitat

[c]pre-zygotic; gametic

[c]post-zygotic; hybrid sterility

[c]post-zygotic; hybrid breakdown

[f]Fabulous! The difference between nocturnal and diurnal lifestyles is a temporal, pre-zygotic barrier.

[f]No. While this is a pre-zygotic barrier, there’s no evidence that these rodents live in different habitats.

[f]No. While this is a pre-zygotic barrier, there’s no evidence that these rodents are experiencing a gametic incompatibility problem that would prevent fertilization.

[f]No. Note that because these rodents are active at different times, they’re unlikely to ever mate. That makes this a _____-zygotic barrier.

[f]No. Note that because these rodents are active at different times, they’re unlikely to ever mate. That makes this a _____-zygotic barrier.

[q]When two types of frogs mate, the eggs develop into tadpoles, but don’t develop into adult frogs.

[c]pre-zygotic; behavioral

[c]pre-zygotic; temporal

[c]pre-zygotic; hybrid sterility

[c]post-zygotic; gametic

[c*]post-zygotic; hybrid inviability

[f]No. Note that these frogs are mating, and producing tadpoles. That means that the barrier is ____-zygotic.

[f]No. Note that these frogs are mating, and producing tadpoles. That means that the barrier is ____-zygotic.

[f]No. There are two problems with this choice. First, the barrier is post-zygotic. Second, hybrid sterility involves hybrids that grow to be healthy adults (like mules) but which can’t reproduce. Here, the hybrid offspring don’t develop past the tadpole stage.

[f]No. You’re right about “post-zygotic,” but “gametic” is a pre-zygotic barrier in which sperm can’t fertilize eggs to produce zygotes. Here fertilization is occurring, and zygotes are developing until the tadpole stage.

[f]Excellent. These frogs are fertilizing each other’s eggs, but the zygotes are only developing to the tadpole stage. That means that the barrier is post-zygotic hybrid inviability.

[q]One type of bird is ready to mate in the fall. A second is ready to mate in the spring.

[c]pre-zygotic; hybrid breakdown

[c*]pre-zygotic; temporal

[c]pre-zygotic; habitat

[c]post-zygotic; hybrid sterility

[c]post-zygotic; gametic

[f]No. Hybrid breakdown is a post-zygotic barrier, not pre-zygotic one.

[f]Good job. The difference in mating season is an example of a temporal, pre-zygotic reproductive barrier.

[f]No. You’re right about the barrier being pre-zygotic, but there’s no evidence that this was based on habitat.

[f]No. These birds have different mating seasons. Does that make this barrier pre- or post-zygotic?

[f]No. First, these birds have different mating seasons. Does that make this barrier pre- or post-zygotic? Second, gametic barriers are pre-zygotic (since they prevent formation of zygotes.)

[q]Male fiddler crabs wave their large claws to attract females. Each species has a slightly different wave.

[c]pre-zygotic; temporal

[c]pre-zygotic; hybrid breakdown

[c*]pre-zygotic; behavioral

[c]post-zygotic; hybrid sterility

[c]post-zygotic; habitat

[f]No. While the barrier is pre-zygotic, there’s no evidence about there being any time-based barriers between the different fiddler crab species.

[f]No. You’re right about the barrier being pre-zygotic. But hybrid-breakdown (the inviability of F offspring) is a post-zygotic barrier.

[f]Terrific. The specificity of the claw-wave used to attract mates is a behavioral, prezygotic reproductive barrier.

[f]No. Think about this behavior…is it a pre or a post-zygotic reproductive barrier?

[f]No. Think about this behavior…is it a pre or a post-zygotic reproductive barrier?

[q]Two species of mice are mated in the lab and produce fertile hybrid offspring, but the offspring of the hybrids (the F2s) are sterile

[c]pre-zygotic; hybrid breakdown

[c]pre-zygotic; habitat

[c]pre-zygotic; behavior

[c*]post-zygotic; hybrid breakdown

[c]post-zygotic; hybrid sterility

[f]No. Note that the mice are able to produce offspring. Does that make the barrier pre-zygotic or post-zygotic?

[f]No. Note that the mice are able to produce offspring. Does that make the barrier pre-zygotic or post-zygotic?

[f]No. Note that the mice are able to produce offspring. Does that make the barrier pre-zygotic or post-zygotic?

[f]Excellent. In hybrid breakdown, the F1 hybrid offspring are able to reproduce, but their offspring (the F2s) are a genetic dead end.

[f]No. If this were hybrid sterility, then the hybrid offspring would be infertile.

[q]The salamanders Ambystoma tigrinum and A. maculatum live in the same areas. A. tigrinum breeds in Feb-March, A. maculatum breeds in April-May.

[c*]pre-zygotic; temporal

[c]pre-zygotic; hybrid breakdown

[c]pre-zygotic; habitat

[c]post-zygotic; hybrid sterility

[c]post-zygotic; gametic

[f]Good job. The difference in mating season is an example of a temporal, pre-zygotic reproductive barrier.

[f]No. Hybrid breakdown is a post-zygotic barrier, not pre-zygotic one.

[f]No. You’re right about the barrier being pre-zygotic, but there’s not evidence that this was based on habitat.

[f]No. These birds have different mating seasons. Does that make this barrier pre- or post-zygotic?

[f]No. First, these birds have different mating seasons. Does that make this barrier pre- or post-zygotic? Second, gametic barriers are pre-zygotic (since they prevent formation of zygotes.)

[q]When fruit flies of two particular species are crossed in the lab, their offspring are unable to produce eggs and sperm.

[c]pre-zygotic; hybrid breakdown

[c]pre-zygotic; habitat

[c]pre-zygotic; behavior

[c]post-zygotic; hybrid breakdown

[c*]post-zygotic; hybrid sterility

[f]No. Note that the mice are able to produce offspring. Does that make the barrier pre-zygotic or post-zygotic?

[f]No. Note that the mice are able to produce offspring. Does that make the barrier pre-zygotic or post-zygotic?

[f]No. Note that the mice are able to produce offspring. Does that make the barrier pre-zygotic or post-zygotic?

[f]No. In hybrid breakdown, the hybrid offspring are able to reproduce, but their offspring are a genetic dead end. Compare that to what’s described above, and see if you can find a better answer.

[f]Excellent. This is an example of hybrid sterility, a post-zygotic reproductive barrier.

[q]Blackjack oak grows in dry woodlands, and scrub oak grows in dry, rocky, open areas. Oaks are wind pollinated, but pollen from one species rarely lands on the flowers of the other species.

[c]pre-zygotic; gametic

[c]pre-zygotic; temporal

[c]post-zygotic; hybrid breakdown

[c]post-zygotic; hybrid sterility

[c*]pre-zygotic; habitat

[f]No. Gametic barriers involve the sperm of one species not being able to fertilize the eggs of another, and there’s no evidence of gametic incompatibility above. Look at the description, and see if you can identify which barrier is at work.

[f]No. Temporal barriers involve time: time of day, season of the year, etc. There’s no evidence of temporal incompatibility above. Look at the description, and see if you can identify which barrier is at work.

[f]No. Post-zygotic barriers follow successful fertilization. Do you see any evidence of fertilization above?

[f]No. Post-zygotic barriers follow successful fertilization. Do you see any evidence of fertilization above?

[f]Nice job. It looks like what’s keeping these two oaks separate is the different habitats where each one is found.

[x][restart]

[/qwiz]

6. The Biological Species Concept: Limitations and Alternatives

6a. Limits to the Biological Species Concept

[qwiz]

[h]The Life Cycle of Sexually Reproducing Organisms: A Review

[q]

In what follows, you’ll see that there are circumstances where the biological species concept – a population of organisms that can interbreed to produce fertile offspring – can’t be applied. That’s because interbreeding assumes that the organisms in question have a life cycle that involves sexual reproduction, such as the human life cycle shown at left.

Since you’ve probably already studied meiosis, use this as an opportunity to identify the numbered parts in the diagram. Click “show the answer” when you’re ready.

[c*]Show the answer

[f]At 1a and 1b you have sexually mature adults. In ovaries (2a) and testes (2b) germ-line cells undergo meiosis (3) to produce haploid sperm (4b) and eggs (4a). Fertilization (5) produces a zygote (6), which develops (7) into the next generation of mature adults (1a and 1b)

[/qwiz]

The three domains. Click on the image to see a larger version of this same image in a new tab.

The life cycle above applies not only to humans, but to salmon, horseshoe crabs, orchids, and sphagnum moss. However, for many organisms, the sexual life cycle is not how reproduction works. Consider, for example, the E. coli bacteria that live in your gut (and the guts of most mammals). When these bacteria reproduce, there’s no meiosis and fertilization. There’s just binary fission (replication of the single circular chromosome, followed by splitting of the parent cell into two daughter cells: click here for a review). And while genes are exchanged between bacteria through processes like transformation and conjugation, bacteria don’t have sex in any way that parallels sexual reproduction in eukaryotes.

What’s true of E. coli is true of all of Domain Bacteria, and it’s true of Domain Archaea as well. In other words, for two of the three domains of life, the biological species concept doesn’t work.

Even in eukaryotes, the biological species can’t always be applied. The animal shown on the left below is a rotifer. Rotifers are tiny freshwater animals (less than a millimeter in size). An entire class of rotifers (the Bdelloidea) consist of over 450 species that are exclusively asexual. In these species, females produce daughters through parthenogenesis, a process by which unfertilized eggs can develop into new individuals without meiosis, recombination, or combining of haploid genomes through fertilization. It’s estimated these species have been asexual for the past 25 million years (source: Wikipedia). On the right below is the desert whiptail lizard, a species that consists exclusively of females that also reproduces parthenogenetically (click here to read more about these lizards in a new tab). As with all the organisms in domain Bacteria and Archaea, there’s no breeding that happens in these species, so the interbreeding criterion of the biological species can’t be applied.

Two exclusively asexual animal species

A rotifer

Desert Whiptail Lizard

Here’s another limit for the biological species concept. Look at the two triceratops skeletons below. Are they representatives of two species, or one? Obviously, there’s no way to know if they could have interbred. That, of course, is also true of all of the thousands of species that we only know about through their fossil remains.

Two species or one?

When the biological species concept can’t be applied, biologists use a few other species concepts. We’ll look at three of them.

6b. Morphological species

Morphology is the study of form. A morphological species is a group of organisms that have a shared phenotype. If you’ve ever gone birding, or used a field guide to identify the wildflowers in your local area, then you’ve used a morphological species approach. Based on morphology, for example, the two triceratops species are thought to belong to two different species: Triceratops prorsus (left), and Triceratops horridus (right).

6c. Phylogenetic species

from Wikipedia

Phylogeny is “the evolution of a genetically related group of organisms.” (Merriam Webster). A phylogenetic species is the tip of a distinct branch on a phylogenetic tree (UC Berkeley, Evolution). Such species can be identified by shared homologous features (which can be anatomical or molecular). DNA sequencing, for example, could be used as evidence to group the rotifers described above into the same species. Or, to think about this more generally, the species at the tip of lineage “A” in the phylogenetic tree at left would be distinct from other rotifer species that have a different genetic, molecular, and morphological profile (“b” and “c”).

6d. Chronospecies

Let’s embark on a thought experiment. If you went back in time 10,000 years, you’d find yourself in a world where agriculture was a relatively new thing. But if you could get a set of clothes from the villagers in the village where your time machine landed, and grew your hair to look like theirs, along with adopting whatever style of jewelry or other adornment they might have been into, you’d fit right in. If you decided to ditch your time machine and stay, you could find a mate. After all, these villagers would be, like you, members of Homo sapiens. You could have children, and contribute to their gene pool.

Now, reset your time machine. Go back 1,000,000 years. Fitting in with these bipedal hominin ancestors would be tougher. You’d be morphologically different. Underlying these morphological differences would be genetic differences. The people you’d encounter might not even be people.

You probably get where I’m going. At a certain point, you wouldn’t be able to interbreed with members of your own ancestral population. At some point, you’d have to say that your ancestors were members of a different species than the one that you belong to. But there’s no firm line. There’s just evolution over time, with the morphological and genetic differences that accompany it.

That’s what a chronospecies is: Read this passage (slightly adapted from Wikipedia) and study the accompanying diagram.

wikipedia

A chronospecies is a species that gradually evolves over time from an extinct ancestral form on an evolutionary time scale. This succession of alterations eventually produces a population (such as species B in the diagram at right) which is physically, morphologically, and/or genetically distinct from the original ancestors (species A). Throughout this change, there is only one species in the lineage at any point in time, as opposed to cases where divergent evolution produces contemporary species with a common ancestor (wikipedia).

 

7. What is a Species: Checking Understanding

For now, that’ll do it in terms of:

  1. Understanding the biological species concept.
  2. Differentiating species from breed, subspecies, or variety.
  3. Understanding binomial nomenclature.
  4. Distinguishing between pre-zygotic and post-zygotic reproductive barriers.
  5. Examining the limitations of the biological species concept, and looking at some alternatives (the morphological species concept, the phylogenetic species concept, and chronospecies).

In the next tutorial, we’ll look at how speciation occurs. But before moving onto that, let’s review all of the concepts explored above.

[qwiz random = “true” qrecord_id=”sciencemusicvideosMeister1961-What is a Species, CFU”]

[h]What is a species? Checking Understanding

[i]

[q]A defining feature of a species is that it’s a population that’s genetically __________ from other species.

[hangman]

[c*]isolated

[q]In the biological species concept, all the reproductively active members of a species can interbreed to produce _________ offspring.

[hangman]

[c*]fertile

[q]Collies, dachshunds, and Kerry Blue Terriers aren’t species: they’re domesticated ____________.

[hangman]

[c*]breeds

[q]Every species has a two part name called a __________.

[hangman]

[c*]binomial

[q]A natural, genetically distinct and often geographically identifiable sub-population of a species.

[hangman]

[c*]subspecies

[q]For a domesticated species, this means the same as breed. In nature, this is synonymous with subspecies.

[hangman]

[c*]variety

[q]These barriers help keep closely related species from interbreeding.

[hangman]

[c*]reproductive

[q]Reproductive barriers prevent the flow of ___________ between species.

[hangman]

[c*]genes

[q]This kind of barrier includes a bird’s song, or the courtship dance of Drosophila.

[hangman]

[c*]behavioral

[q]Barriers that prevent formation of an embryo (note: skip the “-” in your answer).

[hangman]

[c*]prezygotic

[q]This kind of barrier could involve seasons, or moments in the lunar cycle, or time of day.

[hangman]

[c*]temporal

[q]A mule is a healthy and valuable animal, but it can’t reproduce. This is an example of hybrid

[hangman]

[c*]sterility

[q]The reproductive barrier that’s analogous to the lock and key model of enzyme/substrate interaction.

[hangman]

[c*]mechanical

[q]Two finch species live on the same island. One feeds on cacti, while one prefers areas with seed producing shrubs. The reproductive barrier is about their

[hangman]

[c*]habitat

[q]When the first generation of hybrids is healthy, but the second is sterile or inviable, the barrier is hybrid

[hangman]

[c*]breakdown

[q]The biological species can only work with species that reproduce

[hangman]

[c*]sexually

[q]In the three domain system, this is the only domain where the biological species concept applies.

[hangman]

[c*]eukaryotes

[q]With _______ populations like dinosaurs, you can’t use the biological species concept.

[hangman]

[c*]extinct

[q]This species concept sees a species as the tip of a branch in an evolutionary tree.

[hangman]

[c*]phylogenetic

[q]This species concept is based on appearance, or phenotype.

[hangman]

[c*]morphological

[q]The binomial for the Galapagos ground finch is Geospiza fortis. Geospiza indicates this species’

[hangman]

[c*]genus

[q]A species grouping that emerges as a result of evolutionary changes over long spans of time.

[hangman]

[c*]chronospecies

[x][restart]

[/qwiz]

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