Tag Archives: species concepts

How do new species form?

by Piter Kehoma Boll

A long, long time ago, I wrote two posts here about the definition of species, explaining briefly the most important horizontal and vertical species concepts. So we all agree that species exists, but how they emerge? How one species become two, or how one species become another?

The phenomenon by which it occurs is called speciation. Well, sort of… It all depends on how you define a species, actually (so be certain to have read the posts I mentioned above).

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Model of a lineage splitting into two lineages that evolve independently and eventually become separated species. Extracted from Hawlitschek et al. (2012)*

Speciation is usually defined as the evolution of reproductive isolation, therefore it deals more with the concept of biological species, but also with the ecological concept and certainly needs some insights on the vertical concepts. If two populations are reproductively isolated, it means that the individuals of one of them are unable or unwilling to breed with those of the other. This usually arrives through genetic and ecological differences that lead to differences in behavior, morphology, physiology. And considering that, we can classify reproductive isolation into two groups: pre-zygotic and post-zygotic isolation.

In pre-zygotic isolation, the two species are reproductively isolated because they do not want or cannot mate and produce an zygote. This may happen simply because of different behaviors in which the two species occupy different places in the environment, mate at different times of the year or even because they are not sexually attracted to each other. There are several experiments using fruitflies that demonstrate how this may evolve pretty fast.

In the late 1980s, William R. Rice and George W. Salt separated individuals of Drosophila melanogaster depending on their preference for dark × light and wet × dry environments, allowing them to mate only with other specimens showing the same preferences. After several generations, the individuals of one group were unable to mate with those of other groups because of their strong habitat preferences, making them unlikely to interact. A similar experiment was performed by Diane Dodd using the species Drosophila pseudoobscura, in which one population was raised with starch as food and other with maltose as food. In this case, after several generations the flies showed a strong preference to mate with individuals of the same group and to reject those of the other group.

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Evolution of reproductive isolation in fruit flies of the species Drosophila pseudoobscura after several generations fed with different sugars.

Such speciation events are called ecological speciation and are also well-documented in the widl, especially regarding fish preferring different habitats, such as shallow × deep water or still × running water. Eventually the individuals will diverge into two groups that are ecologically isolated in the same environment and consequently become reproductively isolated as well.

Post-zygotic isolation is generally a more advanced form of isolation that indicates deep genetic divergences. This is more commonly associated with the notion of biological species and is based on the inability of the individuals of the two species to produce viable offspring. They may mate with each other and even produce a zygote, but this will be unable to developed into an embryo or the offspring will be sterile or otherwise unable to survive enough to breed. A classical example is the mule, the hybrid of a mare and a donkey that is usully sterile.

Equus

A mare, Equus ferus caballus (left), a donkey, Equus africanus asinus (right) and a mule (center). Photos by ‘Little Miss Muffit’ (flickr.com/people/42562654@N00)(mare), Adrian Pingstone (donkey) and Dario Urruty (mule).

In both forms of speciation mentioned above, reproductive isolation usually arises from the accumulation of small differences due to natural selection. This may be enhanced by two phenomena, pleiotropy and genetic hitchhiking.

Pleiotropy is the phenomen by which a single gene have influence over more than one phenotypic trait. For example, a gene that influences the shape of a bird’s bill may also make it change its diet and its song. Several human genetic diseases, such as phenylketonuria (PKU), are examples of pleiotropy.

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The frizzled trait in chickens, which makes the feather curl outward, also leads to delayed sexual maturity and decreased metabolism rate. Photo by flickr user Just chaos.*

Genetic hitchhiking, on the other hand, is the phenomenon by which a gene that is naturally selected carries neighbours genes that are in the same DNA chain with it. In fruitflies, for example, a gene that is linked to courtship behavior may be drawn with the gene linked to a digestive enzyme, so that flies that specialize in one kind of sugar have a different courtship behavior than others specialized in another sugar.

That’s all for now. In a future post, I’ll talk about the geographic and genetic variables in species formation.

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References and further reading:

Bolnick, D. I., Snowberg, L. K., Patenia, C., Stutz, W. E., Ingram, T. & Lau, O. L. 2009. Phenotype-dependent native habitat preference
facilitates divergence between parapatric lake and stream stickleback. Evolution, 63(8): 2004-2016.

Hendry, A. P.2009. Ecological speciation! Or the lack thereof? Canadian Journal of Fisheries and Aquatic Sciences, 66: 1383-1398.

Hoskin, C. J. & Higgie, M. 2010. Speciation via species interactions: the divergence of mating traits within species. Ecology Letters, 13: 409-420.

Maan, M. E., Hofker, K. D., van Alphen, J. J. M. & Seehausen, O. 2006. Sensory drive in cichlid speciation. The American Naturalist, 167(6):
947-954.

Nosil, P. 2008. A century of evolution: Ernst Mayr (1904-2005). Ernst Mayr and the integration of geographic and ecological factors in
speciation. Biological Journal of the Linnean Society, 95: 26-46.

Turelli, M., Barton, N. H. & Coyne, J. A. 2001. Theory and speciation. TRENDS in Ecology and Evolution, 16(7): 330-343.

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*Creative Commons License
This work is licensed under a Creative Commons Attribution 2.5 Generic License.

**Creative Commons License
This work is licensed under a Creative Commons Attribution 2.0 Generic License.

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What’s a species 2: Vertical species concepts

by Piter Kehoma Boll

Hello, guys!

I finally decided to go on and write the second part of my article about species concepts. You can see the first part here, where I talked about horizontal species concepts. Today I’m going to talk about the other perspective, the vertical species concepts, which are based on lineages, i.e., how species happen through time.

Vertical concepts are hardly used to actually define a species, since they do not represent the current situation of living beings and it’s hard to know the real history of a population to determine its status through all his existence. However these concepts are useful in phylogenetic reconstructions and to understand how new species arrive from others through time.

Well, let’s see the two main vertical species concepts.

1. Cladistic species concept

Proposed by Ridley in 1989, it defines a species as a set of organisms between two speciation events, or between one speciation event and one extinction event. According to this, a species comes to exist when a lineage of organisms is split in two. There are no paraphyletic species in this concept, since when a speciation event occur, the ancestral species becomes extinct, giving rise to two new species.

Cladistic concept: every time a speciation event occur, two new species are created and the ancestral species becomes extinct.

2. Evolutionary species concept

An evolutionary species is defined as a set of organisms from a single lineage that has its own evolutionary tendencies and historical fate. Differently from the cladistic species, the evolutionary species does not necessarily become extinct when another lineage split from it, so being able to be paraphyletic, i.e., if a population is divided in two, the one that continues to have the same general features and the same evolutionary path is considered the same species as the ancestral one.

Evolutionary concept: a species does not necessarily become extinct during a speciation event. Species 1 is paraphyletic after split from species 2.

Since there is no record of the evolutionary history of organisms, there is no way to determine it for any species. Some ideas may be proposed and highly supported by genetic analyses, but we can never know for sure how things really happened, so that vertical concepts cannot be applied practically and are more useful to infer genetic relationships between different populations and so guide their correct management in conservation efforts.

Another point is that by vertical concepts, two organisms are considered separate species as soon as they move on in different lineages, in different populations that will not come in touch again, so that even two cousins would be different species, even though genetically, morphologically and ecologically very similar.

So vertical concepts are more useful to determine phylogeny and help in population genetics, but not to actually define species in any ecosystem, since in this case the situation is characterized by the present status of organisms and so better supported by horizontal approaches.

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References:

Mayden, R. L. 1997. A hierarchy of species concepts: the denoument in the saga of the species problem, in M. F. Claridge, H. A. Dawah and M. R. Wilson (eds.), Species: The units of diversity, London: Chapman and Hall, 381-423

Ridley, M. 2004. Evolution. Blackwell Publishing. ISBN 1-4051-0345-0.

Stamos, D. N. 2002. Species, languages, and the horizontal/vertical distinction. Biology and Phylosophy, 17, 171-198.

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What’s a species 1: Horizontal species concepts

by Piter Kehoma Boll

What’s a species?

Maybe to you that may sound like something too obvious to think about, but actually the concept of species is one of the most intriguing and controversial topics in biology. Sometimes it’s not so easy to distinguish one species from another and that may lead to problems in several fields of biology, including not only systematics, but also ecology, conservation and evolution.

Many times, of course, two individuals can easily be recognized as different species. Nobody would doubt that a giraffe and a banana tree belong to different species, right?

A giraffe and a banana tree, surely not a couple. Pictures by Anna Cervova and Andrew Schmidt.

Now look at these two butterflies. Are they different species?

Two very similar butterflies, Danaus erippus (left) and D. plexippus (right). Photos by Gabriela ruellan* (flickr.com/people/56823778@N00) and David Wagner.

As you can see now, sometimes it’s not so easy to say if a group of individuals forms one or more species.

Trying to solve this problem, many concepts of what’s a species have arisen through the decades. As far as 22 concepts were elaborated, trying to cover all kind of situations related to species differentiation. Those concepts can be divided in two different approaches of the problem: horizontal and vertical. When analyzing species in a horizontal way, we look at them how they are in the present, comparing the populations in the way they look like, behave and are distributed. On the other hand, a vertical approach considers how species happen through time, putting priority on historical and evolutionary aspects.

Here I’ll make a quick review of the three main horizontal species concepts, which are the most practically used as we try to define what a species is.

1. Biological species concept

Probably the best known concept and the most highly applied. It defines a species as a set of organisms where individuals recognize one another and seek each other for mating, so maintaining the intercommunication of their genes. A biological species is isolated from other species by intrinsic or extrinsic features that prevent interbreeding.

In other words, a biological species is a set of organisms able to interbreed and have fertile offspring. A classic example of two biological species is the horse Equus ferus caballus and the donkey Equus africanus asinus which can breed to produce a mule, but that’s sterile.

A mare, Equus ferus caballus (left), a donkey, Equus africanus asinus (right) and a mule (center). Photos by ‘Little Miss Muffit’* (flickr.com/people/42562654@N00)(mare), Adrian Pingstone (donkey) and Dario Urruty (mule).

Even though this concept applies well to most plants and animals, it doesn’t fit to bacteria and other microorganisms that reproduce only asexually.  Despite that, some plant species can easily form fertile hybrids between distinct species. Orchids are champions on that, with lots of hybrids species arising from crossing species of the genus Cattleya.

Cattleya forbesii (left), C. guttata (right) and their hybrid, Cattleya x dayana (center). That’s a naturally occuring fertile hybrid. Photos by Dr. Volkmar Rudolf (C. forbesii), Wikimedia Commons’ User Orchi (commons.wikimedia.org/wiki/User:Orchi)(C. guttata) and Adilson A. Filho (www.flickr.com/people/adilsfilho/)(C. x dayana).

2. Ecological Species Concept

An ecological species is a set of organisms belonging to a single or closely related lineages that basically occupy the same niche in an ecosystem, i.e., have the same habitat and the same habits and needs for physical resources and conditions to survive.

Due to the fact that different species use ecological resources differently, they use to become divergent in aspect, behavior and location, so leading to isolating from one another. Maybe they would be able to interbreed, but it doesn’t use to happen because of their different locations or time of mating.

Two species of giant roundworms, Ascaris lumbricoides and A. suum, are very closely related and similar in shape, but the first is a parasite of humans and the latter a parasite of pigs, so that they are isolated from each other for using different habitats.

Pictures of Ascaris lumbricoides and A. suum. Almost identical, but living in different hosts. Pictures by the Centers for Disease Control and Prevention, U.S. federal government and by nematodes.org, respectively.

Another example are the grizzly bear Ursus arctos and the polar bear Ursus maritimus. Even though living in different habitats and having different behaviors, including the fact that the grizzly tends to mate on land while the polar mates in the water, several hybrids have been reported, including wild ones, and they are fertile, so that by the biological concept, they would belong to a single species, even though by ecological aspects they are quite different ones.

Taxidermied “prizzly bear”, a polar-grizzly bear hybrid, at Rothschild Museum, Tring, England. Picture by Sarah Hartwell*, extracted from wikipedia.

3. Phenetic species concept

This concept defines basically that a species is a set of organisms that look similar enough, i.e., similarity is the primary criterion for defining a species. Different from the two previous concepts, this one simply considers that species exists, but doesn’t justify how they came to be so.

Despite of its apparent inaccuracy to define natural species, it’s actually the primary method used to differentiate species. When a new species is described, it is usually so defined by comparing it to species already known, highlighting morphological and behavioral aspects.

Two beautiful species of macaw, the blue-and-yellow macaw Ara ararauna and the scarlet macaw Ara macao, are quite different in coloration, so considered different species. Their habitats overlap in nature, but they do not produce hybrids except in captivity, and those are not fertile.

Blue-and-yellow macaw, Ara ararauna (left), Scarlet macaw, Ara macao (right) and their sterile hybrid Catalina macaw (center). Photos by Wikimedia Commons’ user Fiorellino* (A. ararauna), Matthew Romack* (www.flickr.com/people/stoichiometry/) (A. macao) and Wikimedia Common’s user Arkansas Lad* (catalina macaw).

However, this concept doesn’t work many times. A typical example is when considering two fruit fly species, Drosophila persimilis and D. pseudoobscura. They look almost identical to each other, but when put together, they never breed, so indicating that they actually do not belong to the same species.

Two different species that look almost identical, Drosophila persimilis (left) and D. pseudoobscura (right). Photos by BIO Photography Group*, Biodiversity Institute of Ontario, extracted from boldsystems.org

I hope that now you are starting to see why is not so easy to tell where one species begins and the other ends. Perhaps there is no such thing as a species after all, at least not like we think about it.

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* These images are licensed under Creative Commons license, hold by their respective owners.

References:

Mayden, R. L. 1997. A hierarchy of species concepts: the denoument in the saga of the species problem, in M. F. Claridge, H. A. Dawah and M. R. Wilson (eds.), Species: The units of diversity, London: Chapman and Hall, 381-423

Ridley, M. 2004. Evolution. Blackwell Publishing. ISBN 1-4051-0345-0.

Smith, D., Lushai, G., & Allen, J. 2005. A classification of Danaus butterflies (Lepidoptera: Nymphalidae) based upon data from morphology and DNA Zoological Journal of the Linnean Society, 144 (2), 191-212 DOI: 10.1111/j.1096-3642.2005.00169.x

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