When has an organism evolved enough to be called a new species?

When has an organism evolved enough to be called a new species?

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Imagine that we take a population of horses, split them in half and place them in completely different environments. The two species will evolve separate from each other and because the environment is different, the outcome of evolution will be different.

But at what point can you say that these horses have evolved into two different species?

(I do know that they would probably go extinct if we conducted this exact experiment, but this experiment is just to give an example)

I think LuketheDuke's answer is an oversimplification of the biological species concept (possibly resulting from the dictionary having a poor definition). The definition he gives is one of many which are in current use, and is made redundant by many types of organism.

It is important to recognise that because reproduction is not the same process in all organisms, genetic differentiation between individuals occurs in different ways for different groups.

Let's take the definition given in LuketheDuke's answer…

The major subdivision of a genus or subgenus, regarded as the basic category of biological classification, composed of related individuals that resemble one another, are able to breed among themselves, but are not able to breed with members of another species.

Under this definition, lions and tigers (see ligers and tiglons, which are sterile hybrids between the two) would be considered one species, as would donkeys and horses (see mules and hinnys, again sterile hybrids). There are hundreds of other examples of pairs of animal species which can hybridise to produce sterile offspring.

However, these animal hybrids usually only take place with human intervention, by delibrate breeding efforts. Thus we could extend the previous definition to include them…

The major subdivision of a genus or subgenus, regarded as the basic category of biological classification, composed of related individuals that resemble one another, are able to breed among themselves, but do not breed freely with members of another species in the wild.

That last part takes care of the ligers and tiglons. But what if we consider plants? Under the definition I just gave, most grasses (around 11,000 species) would have to be considered as one species. In the wild, most grasses will freely pollinate related species and produce hybrid seed, which germinates. You might then think we could just modify the definition to specify that the offspring must be fertile (i.e. able to reproduce with one another)…

The major subdivision of a genus or subgenus, regarded as the basic category of biological classification, composed of related individuals that resemble one another, are able to breed among themselves, but do not breed freely with members of another species in the wild to produce fertile progeny.

Unfortunately, the situation is still more complicated (we've barely started!). Often wild hybridisation events between plants lead to healthy, fertile offspring. In fact common wheat (Triticum aestivum) is a natural hybrid between three related species of grass. The offspring are able to breed freely with one another.

Perhaps we could account for this by taking into account whether the populations usually interbreed, and whether they form distinct populations…

The major subdivision of a genus or subgenus, regarded as the basic category of biological classification, composed of populations or meta-populations of related individuals that resemble one another, are able to breed among themselves, but do tend not to breed freely with members of another species in the wild to produce fertile progeny.

This accounts for the grasses, but it still leaves a messy area when you have a hybridisation which establishes - until the hybrid population is segregated away from the parent populations it is unclear whether they still count as the same species.

We could probably live with this situation, except for the fact that bacteria refuse to conform to it at all. Bacteria of the same species, or even very different species, can freely transfer genes from one to the other in conjugation, which combined with fission can result in perfectly replicable hybrids. This is such a common occurence that it breaks even the 'tend to' part of the previous definition, and members of a population can be doing this almost constantly, which negates the segregation requirement.

Richard Dawkins had a go at defining around this, by stating that…

two organisms are conspecific if and only if they have the same number of chromosomes and, for each chromosome, both organisms have the same number of nucleotides

This partly gets around the bacterial problem and means that bacteria which result from conjugation are a new species. Unfortunately under this definition we might as well not ever bother trying to classify bacteria as billions of new species would be created every day - something which the medical profession might have something to say about. This definition would also mean that those with genetic diseases like trisomy 21 are not human. The final nail in the coffin of this attempt is that there are many species, including frogs and plants, which are very certainly considered a single species by taxonomists but which have some variety in the presence of small accessory chromosomes, which occur in different combinations between individuals.

Let's consider one last option. We now live in the era of genomics where data about genomes of thousands of organisms is accumulating rapidly. We could try to use that data to build a species definition based upon similarity at the nucleotide level. This is often used for bacteria, by considering organisms with less than 97% nucleotide similarity to be different species.

The major point I've been trying to make, though, is that species is not a natural concept. Humans need to be able to classify organisms in order to be able to structure our knowledge about them and make it accessible to people trying to link ideas together. But the natural world doesn't care about our definitions. Ultimately the species concept is different for different groups of organisms and will continue to change over time as our analytical methods and the requirements of our knowledge change. Note that I've deliberately skipped over many historical species concept ideas.

The direct answer to your horse question is "it depends how you want to define a horse".

The story of the ability to interbreed is even more complicated than the ambiguity posed by horses, donkeys, lions and tigers. In California and Mexico, there are a series of species of lizards which form a geographical 'horseshoe'. Neighboring lizards can interbreed, but species at either end of the horseshoe cannot breed. In other words, lizards at the ends of the horseshoe are clearly of different species, but any pair of geographically close groups of lizards can be argued to be the same species. There's no place to draw the line to say where one species ends, and the next begins.

I think that this lies at the heart of the question… the concept of a species is an artificial construct. By and large, there are large enough separations of space and time that the lines are easy to draw, but during the split, the lines are in no way clear.

The biological definition of a species on is as follows;

The major subdivision of a genus or subgenus, regarded as the basic category of biological classification, composed of related individuals that resemble one another, are able to breed among themselves, but are not able to breed with members of another species.

So what we take from this is that a species, by definition, are those individuals/organisms that can successfully breed together. It is worth pointing out that some members of closely related species can breed together (e.g. a male horse and a female donkey can produce a 'mule'), but these animals are not fertile, so this is not considered 'successful' breeding, and the two organisms (in this case the horse and the donkey) are of different (if closely related) species.

In your example, the populations of horses would be said to have sufficiently diverged from one-another when they can no longer successfully breed.

A species is a group of individual organisms that interbreed and produce fertile, viable offspring. According to this definition, one species is distinguished from another when, in nature, it is not possible for matings between individuals from each species to produce fertile offspring.

Members of the same species share both external and internal characteristics, which develop from their DNA. The closer relationship two organisms share, the more DNA they have in common, just like people and their families. People’s DNA is likely to be more like their father or mother’s DNA than their cousin or grandparent’s DNA. Organisms of the same species have the highest level of DNA alignment and therefore share characteristics and behaviors that lead to successful reproduction.

Species’ appearance can be misleading in suggesting an ability or inability to mate. For example, even though domestic dogs (Canis lupus familiaris) display phenotypic differences, such as size, build, and coat, most dogs can interbreed and produce viable puppies that can mature and sexually reproduce ([link]).

In other cases, individuals may appear similar although they are not members of the same species. For example, even though bald eagles (Haliaeetus leucocephalus) and African fish eagles (Haliaeetus vocifer) are both birds and eagles, each belongs to a separate species group ([link]). If humans were to artificially intervene and fertilize the egg of a bald eagle with the sperm of an African fish eagle and a chick did hatch, that offspring, called a hybrid (a cross between two species), would probably be infertile—unable to successfully reproduce after it reached maturity. Different species may have different genes that are active in development therefore, it may not be possible to develop a viable offspring with two different sets of directions. Thus, even though hybridization may take place, the two species still remain separate.

Populations of species share a gene pool: a collection of all the variants of genes in the species. Again, the basis to any changes in a group or population of organisms must be genetic for this is the only way to share and pass on traits. When variations occur within a species, they can only be passed to the next generation along two main pathways: asexual reproduction or sexual reproduction. The change will be passed on asexually simply if the reproducing cell possesses the changed trait. For the changed trait to be passed on by sexual reproduction, a gamete, such as a sperm or egg cell, must possess the changed trait. In other words, sexually-reproducing organisms can experience several genetic changes in their body cells, but if these changes do not occur in a sperm or egg cell, the changed trait will never reach the next generation. Only heritable traits can evolve. Therefore, reproduction plays a paramount role for genetic change to take root in a population or species. In short, organisms must be able to reproduce with each other to pass new traits to offspring.

What is a Native Species? (with pictures)

A native species is an organism that is living in an area for entirely natural reasons, with no human intervention involved. This may be because the organism evolved in that environment, or it may have been brought there by natural causes. Wind can spread plant seeds widely for example, and species may be carried by animals or birds individuals may also migrate seeking food or territory. By contrast, a non-native species has been introduced, intentionally or accidentally, by humans and may become invasive, taking over the natural environment and choking out native species. Invasive species often develop quickly and spread aggressively, making it difficult for natives to compete.

Over time, a native species usually evolves to perfectly fit in the environment where it has settled. Subspecies that have refined themselves to take advantage of subtle variations in the environment can also develop, and some of these may eventually evolve into new species. This increase in specialization, however, can render an organism less able to cope with changes in its environment

Endemic and Indigenous Species

Two types of native species are recognized: endemic species are found only in the area concerned, while indigenous species are also found naturally in other areas. Endemic organisms are particularly vulnerable to extinction as they may be confined to a very small region and to particular habitats within that region. They may have evolved in a very specialized way, to adapt to an unusual and uncommon environment, and may only be able to survive in a very limited range of conditions.

Populations of indigenous organisms tend to be more robust, as they are more widespread. They have usually evolved to thrive in habitats that are quite commonplace or to be able to adapt to a wide range of conditions. Sometimes, they can recolonize areas from which they have disappeared, and in some cases, they have been reintroduced by humans.

One reason for an organism to be native to a region is because it evolved there. This is seen most commonly on islands, where comparative isolation allows unique species to develop over extended periods of time. Islands consequently tend to have many endemic species, and their distinctive ecologies are very vulnerable. When island environments are disrupted, the native species found nowhere else may quickly become extinct.

The Spreading of Indigenous Species

Indigenous species may have evolved in a habitat where they are found, or they may have arrived there, by natural means, from elsewhere. As organisms have evolved to fit an evolutionary niche, they will spread to other areas where that niche remains unfilled, if they have the means to do so. Transportation of plants and seedlings can occur on wind currents, on the bodies and in the bellies of animals in their natural range, and as a result of putting out shoots and runners. A plant species, for example, can therefore end up covering a very large territory.

Animals spread naturally as their populations increase and they range further in search of food and territory. Native animals may follow seasonal migrational patterns or may relocate their populations periodically in response to various pressures, natural curiosity, or changing landscapes. Some animals travel vast distances in search of new territory certain bird and insect species are particularly notable for their lengthy migrations.

Threats to Native Species

Native species face a variety of threats to their survival, and some are critically endangered as a result of human activities. A number of animals and birds have been lost in the last few centuries as they have been hunted to extinction. In the present day, destruction of habitats is a major threat, as more land is built over or used for agriculture.

The Protection of Native Species

Recognition of the importance of native species and unique environments has led to the formation of a number of organizations that promote native plants and animals. The International Union for Conservation of Nature (IUCN) maintains lists of endangered and critically endangered species and campaigns to protect their interests. The World Wide Fund for Nature (WWF), formerly known as the World Wildlife Fund, works to help preserve the habitats of native animals, plants, and other organisms, and runs adoption campaigns for endangered animals. At a more local level, there are many organizations that encourage people to use native plants in their gardens, participate in eradication campaigns for invasive organisms, and educate people about the risks to native animal species posed by imported livestock and pets.

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a AllThingsNature researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a AllThingsNature researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Primitive Microbe Offers Model For Evolution Of Animals

December 18, 2001 -- A microorganism whose evolutionary roots can be traced to the era of the first multicellular animals may provide a glimpse of how single-celled organisms made a critical evolutionary leap.

In analyzing the single-celled choanoflagellates, scientists discovered that the organisms have a type of molecular sensor usually found in multicellular animals. This is the first time that such a sensor, called a receptor tyrosine kinase, has been found in a single-celled organism, said Sean B. Carroll, a Howard Hughes Medical Institute investigator at the University of Wisconsin, Madison. Carroll and Wisconsin colleague Nicole King reported their findings in the December 18, 2001, edition of the Proceedings of the National Academy of Sciences.

Choanoflagellates are a group of about 150 species of single-celled protists, which use a whip-like flagellum to swim and draw in food. Surrounding this flagellum is a circle of closely packed, finger-like microvilli that filter food from the water. Scientists have long suspected that choanoflagellates might represent modern examples of what the ancestors of multicellular animals, or metazoans, looked like.

And the circumstantial evidence supporting that notion was compelling -- choanoflagellates are nearly identical to cells called choanocytes in sponges that also carry out food-gathering and some species of choanoflagellates tend to form colonies.

"The existing scientific literature, however, has been conflicting or ambiguous about whether these protists are the closest living relatives of animals without actually being animals," said Carroll. "So, Nicole King proposed that we explore protein sequences that hadn't been examined before, and which might provide unambiguous support for the relationship between choanoflagellates and animals."

The researchers first compared genes in one species of choanoflagellate, Monosiga brevicollis, to four animal genes that express proteins that are highly conserved throughout the animal kingdom. These structural proteins -- called elongation factor 2, alpha-tubulin, beta-tubulin and actin -- are widely used as molecular markers to explore relationships among species.

"When we compared the sequences of the choanoflagellate and animal genes, we got a much clearer statistical signal than we expected that they were related," said Carroll. The comparisons constituted the strongest sequence-based support yet for the hypothesis of the kinship between choanoflagellates and metazoans, he said. Confident that they had established a kinship between the organisms, the researchers next surveyed the choanoflagellate genome for animal-related genes.

"It was something of a shotgun approach, but we tuned our search for genes for a few specific types of molecules that had not been found outside of the metazoans," said Carroll. The search concentrated on molecules involved in cell adhesion and cell signaling, which single-cell organisms would not be expected to have, said Carroll.

"Among several hundred common gene sequences we obtained, out popped this receptor tyrosine kinase, a molecule that has never before been found outside of metazoans," Carroll said.

Receptor tyrosine kinases are molecular sensors that nestle in the cell membrane. When an external chemical plugs into the receptor, like a key into a lock, a signaling pathway is activated inside the cell. The discovery of the receptor tyrosine kinase, called MBRTK1, is important because it implies that the choanoflagellates had evolved some of the machinery necessary to interact with one another like animal cells, said Carroll.

Further analysis of the MBRTK1 protein and comparison of its structure with kinases in other organisms could yield important evolutionary insights. "We'd like to know if this protein might be a founding member of this class of molecules -- a common ancestor that may have appeared on the eve of animal evolution," said Carroll. Also, he said, the scientists hope to trace the signaling pathway activated by MBRTK1, to understand what effect the external signal produces in the choanoflagellate.

"In general, these discoveries have made us confident that we've picked the right organism to understand what happened on the eve of animal evolution," said Carroll. "Thus, we believe we can discover in this organism more elements of the genetic toolkit that animals first used to build animals."

According to Carroll, the studies on choanoflagellates promises to be an important part of his laboratory&rsquos ongoing studies of animal evolution. The theme of this research is also reflected in Carroll&rsquos new book, From DNA to Diversity: Molecular Genetics and the Evolution of Animal Design, which was written with Jennifer Grenier and Scott Weatherbee (Blackwell Science Publications, 2001).

When does an organism become a new species?

During genetics in biology I was taught that a species is a "group of organisms capable of interbreeding and producing fertile offspring". If an organism were to evolve and mutate into a new species, it seems it wouldn't be able to reproduce with its original population unless, two organisms from the same species had the same mutations and could interbreed.

There is a saying, "a species is what a competent taxonomist says it is." This may not sound very satisfying, but it is the literal truth because, as polyphyletic_79 says, the species is a human construct. The usual textbook definition of species only applies to sexually reproducing organisms, and only imperfectly even with them. Related populations diverge from each other in gradual steps over time, and where to draw the line is arbitrary. Many species are known from only a few specimens, and there are many new species being described every year. The actual biology, which populations are genetically isolated from each other, is just not known. Taxonomic specialists in each field try to identify populations that are different enough to warrant recognition at the species level, and figure out what the correct name for that "species" is using the type system. "Types" are individual specimens that are tied to the species name when it is first described. When a species has been given more than one name, the correct name is generally the oldest. The taxonomist relies of morphological, and increasingly DNA evidence to decide which populations deserve recognition. DNA may solve all taxonomic issues some day, but not yet. DNA phylogenies are often based on only a few genes, and how much divergence is necessary to recognize a "species" may not be clear. And depending on the type of organism and the way it was preserved, many older type specimens have degraded DNA that can't even be characterized by current methods.


Although the process of describing biotic diversity has been ongoing in some sense for centuries (since at least the time of Aristotle and Theophrastus), it is only with the increasing threat and reality of its loss in the last few decades that a real focus on the concept of biodiversity has come to the fore. While biotic diversity can be valued and assessed at various levels, including that of the individual organism and the genetic locus, the key level remains the species ( Wilson 1988). A recent consideration of different ways of assessing biodiversity concluded that species richness, while not perfect, is the best metric ( Maclaurin and Sterelny 2008). This does not mean that other levels of biodiversity, such as gene diversity within species, do not exist or are not important, but just that the key level of focus is the species. Species are inextricably linked to the notion of biodiversity because for perhaps most biologists and even for the public at large, they are viewed as the fundamental units of natural biotic diversity. The idea of species as basic phenotypically distinguishable groups in nature is common and has a long history. Species, or something closely approximating them (assemblages of individuals that share a recognizable similarity among themselves and difference from other such groups), are the basic units of folk taxonomies ( Atran 1990). The units that the classical authors discussed, those of the medieval herbalists, and of flora and fauna writers in the post-Renaissance era, approximate many of the units that we still recognize as species. Phenotypic distinguishability was thus the first species criterion. However, we do not advocate following such a tradition just because it has always been that way.

We argue rather for the crucial importance of role (and its manifestation as phenotype) because of its inherent relevance to biodiversity. The critical value of biodiversity lies in the myriad roles (in the sense of Simpson 1951, 1961) that organisms exhibit that make them part of complex biotic systems. This diversity is a direct result of the different morphological, chemical, and behavioral properties that organisms display. We view role broadly as the ways in which individuals interact with their environment and the total complement of expressed properties (beyond genotype) that they exhibit it is an organism’s correspondence to the concept of ecological niche sensu Hutchinson (1957 an |$n$| -dimensional hypervolume composed of all biotic and abiotic organismal interactions).

We assert that role is a necessary part of the species concept and that Simpson was right to include it as part of the definition of the ESC. Although Wiley and Mayden (2000) interpreted Simpson’s use of role to mean no more than “individuality,” Simpson (1961, p. 154) explicitly described roles as “definable by their equivalence to niches” and further stated that “morphological resemblances and differences (as reflected in populations, not individuals) are related to roles if they are adaptive in nature [emphasis in original].” This is a clear connection between the ecological part that species play and their definition. Hull (1965) felt that Simpson did not provide sufficient criteria to circumscribe role, but this is an operational criticism rather than a conceptual one—that is, Hull did not object to the idea of role but to Simpson’s characterization of it. Van Valen (1976), in an explicit refinement of Simpson’s (1961) concept that is known as the former’s Ecological Species Concept, described a species as “a lineage (or a closely related set of lineages) which occupies an adaptive zone minimally different from that of any other lineage in its range and which evolves separately from all lineages outside its range” (p. 154). However, Van Valen did not fully develop this concept he called it “a vehicle for conceptual revision, not a standing monolith.” Levin’s (2000) ecogenetic concept is also similar to this view in that ecological function is part of his specification. Later in his career even Mayr (1988) came to view role as critical with his emended definition of species as “a reproductive community of populations (reproductively isolated from others) that occupies a specific niche in nature.”

Although at first the notion of role may seem elusive, the idea is no vaguer than that of population and, therefore, lineage both ideas can be difficult to apply empirically. Diversity in expressed organismal properties is specifically due to diversity in their phenotypes rather than genotypes per se, since many genotypic changes are not expected to lead to expressed changes. Synonymous third base position changes in coding DNA are expected to yield no difference in amino acid sequence, for example, but situations exist in which such a change could lead to alternative splicing and thus have a phenotypic effect. Although the traditional view has been that phenotypic change is the direct result of underlying genotypic change, we now know that not all phenotypic change can be attributed directly to genotypic change our ever-increasing knowledge of epigenetic determination of phenotype falsifies an exclusive correspondence (e.g., Cortijo et al. 2014). Beyond even epigenetic manipulation of the genome, extra-genomic determinants of organismal properties have been described (cf. Freudenstein et al. 2003 Bonduriansky and Day 2009 Danchin et al. 2011). Whatever their basis, as long as such attributes are heritable by some mechanism, they may affect organismal properties and thus species role. Hence, we are invoking a very broad “extended phenotype” ( Dawkins 1982) as the raw material for role determination and when we refer to phenotype from this point forward we mean it in this broadest sense.

Pointing to phenotype as the basis for role and to phenotypic difference as critical in species distinguishability raises the empirical question of how much phenotypic difference is required to shift role. This question is similar to one that might be asked of a purely lineage-based approach—how does one know when one has a distinct lineage? How distinct does the lineage have to be? These questions reflect the epistemological challenge of applying such concepts. The answer is that one needs enough evidence (of lineage or role) to build a persuasive case for a particular real-world instance. In practice, we often do not know the ecological effect of particular character changes. Therefore, we suggest that any fixed change in expressed organismal properties provides evidence for a hypothesis of role shift. Ultimately it is the task of the investigator to identify phenotypic changes that actually shift roles. Species circumscribed in this way, or in any other way, always remain hypotheses subject to further test.

Convergent Evolution vs. Divergent Evolution: A Critical Comparison

Of the several confusions that persist in the field of evolutionary biology, one is that about convergent and divergent evolution. What exactly is the difference between the two?

Of the several confusions that persist in the field of evolutionary biology, one is that about convergent and divergent evolution. What exactly is the difference between the two?

I do not believe any concept in science has ever given rise to as many controversies and controversial debates as evolution has. But even with so many controversies, “Nothing in biology makes sense except in the light of evolution” (Theodosius Dobzhansky). When we think of evolution, the first name to strike even someone who is not from a biological sciences background would be Charles Darwin. Not a word can be written on evolution without mentioning the name of this pioneering evolutionary biologist. His work On the Origin of Species has been the topic of much discussion ever since it was first published.

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Of the many concepts that were elaborated in this book (like survival of the fittest) one was Darwin’s Principle of Divergence. The other term – Convergent Evolution – is often regarded as the opposite of Divergence. However, convergent evolution was only a play of words that evolutionary biologists came up with to explain one of their many observations it has got nothing to do with divergence as such. Here is a brief account of convergent evolution vs. divergent evolution. However, for the ease of understanding, I will first explain divergent evolution and then move onto convergent evolution.

Divergent Evolution

Imagine evolution of life to be a process like the flowing of a river. As it leaves the mountain, it will take its due course and meander about. If a boulder were to obstruct its flow into a crevice, it would take a turn around the crevice. If however a big enough boulder were to stand in between its path, the river would split into two and give rise to two new smaller rivers, each of which would meet its separate fates as it flowed down the mountain.

This is exactly what divergent evolution is all about. The boulder represents ‘natural selection’. Natural selection is more of an outcome of natural pressure that organisms find themselves subjected to (typically pressure of competition, pressure of meet the mating preferences of the opposite sex, etc.) rather than the actual pressure. It is the result rather than the process. However, the term ‘natural selection’ is now interchangeably used as cause and effect.

So, divergent evolution is the creation of new species through accumulation of many small ‘changes’ that have originated as a result of the natural selective pressures. Essentially the two new species created will diverge from each other as they further evolve. This is an important statement (as will be evident when we take a look at convergent evolution vs. divergent evolution). Three main triggers of divergent evolution have been identified –

  1. To Overcome Competition – Two individuals belonging to the same species pose greater competition for each other than 2 individuals belonging to different species (for the reason that individuals of the same species would all have the same requirements of food, resources, mates etc.) If the competition gets too tough, divergence is the result.
  2. Adaptation to Micro-Niches – Not all monkeys in the US live together in one single territory. They are scattered all over. So those in Florida may adapt to a tropical climate while those in, say South Dakota, would adapt to a continental type of climate. This could create two different monkey species.
  3. Neutral Evolution – Sometimes the changes that occur at the level of the genes cannot be attributed to a specific trigger, these are called neutral mutations. Evolution also takes place in this way. If the accumulative neutral mutations are significant enough to affect the species, they may give rise to a new species.

One thing to be borne in mind however, is the fact that the ‘boulder’ has to be ‘big enough‘. Competition between just two monkeys for one apple on a tree is not enough to create a new species. But if there are two big groups of monkeys, all competing for the apples of a single tree, and if there is also a banana tree nearby, then one of the groups may discover bananas and evolve into a new species of banana-eating monkeys. (This is just a hypothetical example – do not take it literally! Take home the essence!)

Convergent Evolution

Convergent evolution has nothing to do with divergent evolution. It is a totally different concept. First get this into your head, or you are going to remain in an illusion even by the end of the article! I will explain the concept using the same analogy of the flowing river. Now consider there are two different rivers instead of one. Suppose both encounter a big enough boulder, and both the boulders are similar enough. Consequentially they would both split into two smaller independent rivers.

This is exactly what convergent evolution is all about. Both the rivers encountered a ‘boulder’, hence they were both destined to similar fates – to split. Convergent evolution is when the selective pressure on two unrelated species is same to such an extent that it produces the same adaptations in the two species i.e. the courses of their evolution converge to a single fate. This may be a little difficult to comprehend, because when we think of evolution, we are usually considering the evolution of one species. Hence we often tend to think that convergent evolution is when two species merge into one. But this is one of the major misconceptions in evolution.

One point of consideration here is the fact if the two rivers are on the same mountain or two different mountains – i.e. how related or unrelated are the two species we are talking about. Birds belong to ‘aves’, while bats are in fact ‘mammals’. Yet they have both adapted to fly. Here the two organisms are not related at all. On the other hand, penguins and ostriches are both flightless birds, both of which have evolved to walk on the ground. But in spite of both being birds, they have evolved very differently with respect to other features. However, both can be regarded as instances of convergent evolution.

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The main point to be borne in mind here is that the ‘boulder’ has to be ‘similar enough‘. Using the same example of monkeys, apples and bananas – if you want bears competing for apples to also evolve into banana-eating bears, they should be posed with the same amount of competition as the monkeys only then will two different species (monkeys and bears) evolve towards the same fate (going bananas!).

Difference between Convergent and Divergent Evolution

Divergence makes sense only in the light of ‘gradualism‘. The essence of evolution is that it is slow, gradual. If evolution were to take place in leaps and bounds, there could arise a situation where divergence would in fact lead to convergence! How? Let me explain. Suppose a macro-mutation took place in a species to suddenly give rise to a new species – divergence has occurred. However, this divergence is so drastic, that the two species now have very little in common. This means that they are no more under ‘similar’ selective pressures. Now we have two new species, each of which will evolve on their own, and under quite different selective pressures. Now is it not possible that by mere chance the two species will independently arrive at the same fate?

Confusing? Let’s use the above example. Say the group of monkeys competing for apples ‘macro-mutated’. So now we have one orange-eating species, and one apple eating species. Now, can it not happen that the orange eating species eventually diverged into banana-eating species, just the way the apple-eating species did? If this were to happen, one of the two species would in fact be driven to extinction. Then what was the whole point of nature investing so much energy in creating a new species in the first place? Hence for ‘speciation’ (creation of a new species) to sustain, it is essential that evolution be gradual. Gradualism is what ensures that two new species will diverge and not converge.

A lot of concepts of evolution that were formulated in the subsequent years were in fact the offshoots of the concepts Darwin elaborated in his book. If not directly related to, they have been deduced from concepts in the book, sometimes only as a word play (like convergent and divergent evolution). If you haven’t yet read the book, I would highly recommend you grab yourself a copy of On the Origin of Species. It is amazing to see how much a man can do with only a pair of eyes and a keen, observant and thoughtful mind. With the possibility of sounding redundant, I will still say – such was the genius of Darwin, that “Nothing in biology makes sense except in the light of evolution“.

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Age doesn't matter: New genes are as essential as ancient ones

New genes that have evolved in species as little as one million years ago – a virtual blink in evolutionary history – can be just as essential for life as ancient genes, startling new research has discovered.

Evolutionary biologists have long proposed that the genes most important to life are ancient and conserved, handed down from species to species as the "bread and butter" of biology. New genes that arise as species split off from their ancestors were thought to serve less critical roles – the "vinegar" that adds flavor to the core genes.

But when nearly 200 new genes in the fruit fly species Drosophila melanogaster were individually silenced in laboratory experiments at the University of Chicago, more than 30 percent of the knockdowns were found to kill the fly. The study, published December 17 in Science, suggests that new genes are equally important for the successful development and survival of an organism as older genes.

"A new gene is as essential as any other gene the importance of a gene is independent of its age," said Manyuan Long, PhD, Professor of Ecology & Evolution and senior author of the paper. "New genes are no longer just vinegar, they are now equally likely to be butter and bread. We were shocked."

The study used technology called RNA interference to permanently block the transcription of each targeted gene into its functional product from the beginning of a fly's life. Of the 195 young genes tested, 59 were lethal (30 percent), causing the fly to die during its development. When the same method was applied to a sample of older genes, a statistically similar figure was found: 86 of 245 genes (35 percent) were lethal when silenced.

Because the young genes tested only appeared between 1 and 35 million years ago, the data suggests that new genes with new functions can become an essential part of a species' biology much faster than previously thought. A new gene may become indispensable by forming interactions with older genes that control important functions, said Sidi Chen, University of Chicago graduate student and first author of the study.

"New genes come in and quickly interact with older genes, and if that interaction is favorable by helping the organism survive or reproduce better, it is favored by natural selection and stays in the genome," Chen said. "After a while, it becomes essential, and the organism literally cannot live without the gene any more. It's something like love: You fall in love with someone and then you cannot live without them."

The indispensable nature of new genes also questions long-held beliefs about the shared features of development across different species. In 1866, German zoologist Ernst Haeckel famously hypothesized that "ontogeny recapitulates phylogeny" after observing that the early steps of development are shared by animals as different as fly and man.

Biologists subsequently predicted and confirmed that the same ancient, essential genes would be the conductors of this early development in all species. This principle enabled the use of model organisms, including flies, mice, and rats, to be used for research on the mechanisms of human disease.

Intriguingly, in the new study, deleting many of the new genes causes flies to die during middle or late stages of development, while older genes were lethal during early development. So while ancient genes essential for the early steps of development are shared, newer genes unique to each species may take over the later developmental stages that make each species unique. For example, many new genes in the study were found to be involved with metamorphosis, the mid-life stage that drastically transforms the body plan in animals.

"This may change the way we view the developmental program," Long said. "Each species has a different species-specific developmental program shaped by natural selection, and we can no longer say that from Drosophila to humans the development of different organisms is just encoded by the same genetic program. The story is much more complicated than what we used to believe."

As such, a full understanding of biological diversity may require a new focus on genes unique to each organism.

"I think it has important implications on human health," Chen said. "Animal models have proven to be very useful and important for dissecting human disease. But if our intuition is correct, some important health information for humans will reside in the unique parts of the human genome."

The newfound importance of young genes and unique developmental programs may have a dramatic impact on the field, Long said. The discovery will also inspire new research directions examining how quickly new genes can become essential and their exact role in species-specific development.

"Biologists have long assumed, quite reasonably, that ancient genes have survived natural selection because they are essential to life and that new genes are generally less critical to an organism's development," said Irene Eckstrand, PhD, who manages Dr. Long's and other evolutionary biology grants at the National Institutes of Health. "This important study suggests that this assumption is flawed, unlocking new questions that could lead to a deeper understanding of evolutionary processes and their impact on human health."

Conditions that Favor Speciation

Now that we have addressed several mechanisms that keep species from interbreeding, we will focus on some theories about how new species arise (speciation). Essentially, gene flow between closely related populations must be interrupted. This can happen in one of two ways. Allopatric speciation occurs when populations become physically isolated due to some sort of geographical barrier. Sympatric speciation occurs when populations become genetically isolated, even though their ranges overlap.

As suggested, allopatric speciation involves some sort of geographical isolation that physically blocks migration of individuals (or gene flow) between populations. Geographical isolation may arise as a result of changes in water flow, volcanic uprisings, canyon formations, or other landmass changes. A good example of allopatric speciation involves two species of antelope squirrels whose populations are separated by the Grand Canyon. Presumably they evolved from once-interbreeding populations that were isolated by the formation of the canyon.

Adaptive radiation refers to the relatively rapid evolution of many new species from a single common ancestor into diverse habitats. Adaptive radiation is commonly observed on island chains where new opportunities exist for immigrant species. Examples include the speciation of Darwin's finches in the Galapagos chain of islands, the diversification of honeycreepers in the Hawaiian islands, and the radiation of cichlid fish around the globe.

New Species--Keep On Counting!

Just Found After Possibly Millions of Years On Earth: A shaggy, red-bearded monkey that swings among treetops in the Brazilian rain forest. A wild ox with pointed horns that bounds nimbly through Vietnam's mountain underbrush. A tiny fish that nibbles slime from coral reefs.

No, not freaks of nature. Rather, new species that scientists have never--ever--laid eyes on before. From Vietnam to the African Congo, scientists are now sighting about 13,000 new species every year. Like detectives combing for hidden clues, scientists may be gathering new evidence to the unsolved mystery of life's diversity, and doing so at the fastest rate in history.

After more than 200 years of counting and cataloguing, scientists have identified roughly 1.4 million species on Earth. But their best collective guess is that anywhere from 4 to 40 million species of animals, plants, fungi, bacteria, and other organisms may still lurk undiscovered in remote spots like New Zealand forests or the savage Himalayan mountains. Their guess is actually based on a mix of scientific formulas and prediction. Most large mammals, for instance, are believed to have been discovered while many smaller animals--tiny mammals, beetles, and mites--are still thought to exist in the wild, unknown.

What is a species, anyway? Taxonomists (scientists who categorize all life on Earth) define a species as a group of living organisms--bacteria, insects, or fish, for example--that breeds only with others identical to itself. All members of a species share the same general appearance and behavior. Currently more than 4,000 mammals (warm-blooded, milk-drinking vertebrates), for example, have been identified by taxonomists (at least 88 of those have already gone extinct, or died off). But the final mammal count may double before the species counting game is over. "We may think we know what lives on the face of the Earth, but we really don't," says Niles Eldredge, a curator at the American Museum of Natural History in New York City.

Why are scientists suddenly hot on the trail of new species? Groups such as the World Wildlife Fund and Conservation International have rushed crack teams of biologists to collect information about remote regions--before bulldozers and chain saws deforest them, mine them for minerals, or turn them into grazing lands. A campaign called the rapid assessment program (RAP) targets isolated habitats where a rich diversity of species may have evolved beyond the eyes of man.

Two years ago, for instance, scientists scouring mountain jungles on the border of Vietnam and Laos came across 18 animal skulls, the remains of a deer-like species they couldn't identify. The skulls had antlers that looked unlike any found on deer species known to science. Where did the deer come from? Native forest-dwelling hunters informed the scientists of an uncounted population of "mystery" deer alive and hidden amid high mountain peaks. "This is a lost world, a place we haven't ventured into in hundreds of years," says Vietnamese biologist Pham Mong Giao, who worked on the project.

The distinctive antlers found on the skulls perfectly matched the antlers of the live deer photographed in the mountains. Presto! Scientists had come upon a new deer species, now named the Truong Son muntjac. It's one of three large mammals discovered in Vietnam in the last seven years.

Other "hot spots" ripe for discovery of new species include the Philippines, Madagascar, and the Andes Mountains and Amazon River Basin in South America. Endemic species (creatures found nowhere else), such as the black-headed sagui dwarf recently found in the Amazon rain forest, are like poster kids for environmentalists.

"Biologists are using new species to highlight the urgency of deforestation," says Leeanne Alonso, an entomologist (insect scientist) who leads RAP expeditions. So far, RAP teams have led to the creation of six protected areas totaling millions of acres in five countries.

One tool scientists use to identify new or distinct species is DNA-testing. DNA is the genetic code that transmits traits from one generation to the next. In fact, about two-thirds of all "new" species can only be identified by biologists conducting DNA tests.

For example, ornithologists (bird scientists) used to lump the fluffy brown screech owls found separately both in the Comoros Islands and in Madagascar into a single species called Madagascar scops owl. After all, the two sets of birds, found years ago, looked identical. Nonetheless, scientists recently tested the owls' DNA. Guess what? One island's owl has markedly different DNA from its neighbor. They'll now be catalogued as two species: Madagascar scops owl and Anjouan scops owl.

Briefly, here's how scientists use DNA tests: The DNA molecule is shaped like a twisted rope ladder it's often called a double helix. The "rungs" of the ladder consist of four chemical compounds called base pairs. If the genetic codes spelled out in the base pairs don't match closely enough between two animals that may look like twins, scientists conclude they've found a new or distinct species.

South America's four-eyed opossums, for example, are now divided into as many as four different species. "Fifty years ago, scientists took things that were different and threw them together," Niles Eldredge says. "Today the trend is to split them back up."

Biologists in search of new species are racing against time. Their mad dash to identify new species and protect them may not come fast enough to save their discoveries from extinction. Many species--like the saola, a wild ox found seven years ago on the border of Laos and Vietnam--are threatened as soon as they're discovered because their habitats are disappearing. "When you clearcut a forest, you don't know what's in there," says Alonso. "One hundred species can be wiped out in a day."

The natural extinction rate that existed for millions of years before humans evolved claimed about two species per year. Today, 1,000 species vanish each year, according to the World Conservation Union.

"We have reason to be worried," says ornithologist Tom Schulenberg of Chicago's Field Museum.

PIRANHA * FOUND: 1996 * WHERE: Bolivia * SIZE: 22 centimeters * FAST FACT: Young piranhas eat floating fruits and seeds. Adults use razor-sharp teeth to devour fish flesh.

SAOLA * FOUND 1992 * WHERE: Laos and Vietnam * SIZE: 1 meter * FAST FACT: May be missing link that reveals how buffalo, cattle, and spiral-horned antelope evolved.

ZOG-ZOG * FOUND: 1997 * WHERE: Brazil * SIZE: 40 centimeters long, from head to tail * WEIGHT: 1 kilogram * FAST FACT: Couples sing a throaty duet. One of four new monkey species found in a single year in the Amazon region.

PLAND HOOPER * FOUND: 1995 * WHERE: Ecuador * SIZE * 2.5 centimeters * FAST FACT: Sucks sap from leaves and stores it in its upturned snout

SANGHA FOREST ROBIN * FOUNDl 1996 * WHERE: The Central African Republic * SIZE: 8 centimeters * FAST FACT: Ornithologists examined 300 specimens from 89 areas at seven museums before declaring this robin a new species.

AXELROD: FOUND: 1988 * WHERE: Pacific Ocean near New Guinea * SIZE 5 centimeters * FAST FACT: This slime scraper spends its whole life inside coral-reef crevices

BLACK-HEADED SAGUE DWARF * FOUND: 1996 * WHERE: Brazil * WEIGHT: 187 grams * FAST FACT THE average adult measures just 10.1 centimeters, making it the second-smallest monkey ever found.

Watch the video: Formation of New Species by Speciation. Evolution. Biology. FuseSchool (August 2022).