WEIT 2012 Chap7
THE ORIGIN OF SPECIES
A species ? What's that ?!
First of all, we will define a species as a group of population whose members have the potential to reproduce with one individual of the same group in nature, and produce fertile offspring (this can be illustrated by the famous byword that says that cats don't generate dogs ! Meaning that cats, as they reproduce, their offspring will inherit half of each of their parents genetic information. Thus when both parents are cats, generating dogs is IMPOSSIBLE !)
Of course, the most immediate way to define species is by looking at them. We tend to say that beings who look the same and share a similar morphology are part of the same species, for example we define cats as small, four-legged tailed animals who meow and have varying colors of fur; this is called MSC (Morphological Species Concept). However, a woman coming from Japan can marry a French man and they can have children together even if visually the two parents don't look the same. And as mentioned previously, defining species by their appearance is possible, but there exists some animals such as the eastern meadowlark and the western one (two species of birds living in North America) which are extremely similar in appearance, however they do not interbreed (which means reproduce with one another)! And conversely, sexually dimorphic species, species whose male and female look different, but still reproduce together, such as the painted bunding (see appendix 1). Why MSC is wrong is explained simply : while an african man and a bolivian woman may look very different, they are extremely similar inside : they both have two kidneys, lungs, arms, muscles in the same place, etc...
To sum up the definition of species we'll say that species are a group of reproductively isolated individuals; meaning these individuals that can exchange genes only between individuals of the same group and have offspring that are fertile, this is called BSC, or Biological Species Concept and is the only valid way to differentiate species.
How do species appear ?
After defining what are species, we must explain how they appeared. They appeared thanks to a process called speciation. The process of speciation is mainly due to two kind of barriers : the reproductive barriers and the geographic barriers. It is now evident that cats won't interbreed with dogs or cows with horses, but what prevents closely related such as human beings and primates like the bonobos (with whom we share almost 99% of our genetic data) from interbreeding ? Is it just the geography or is there something else more complex ? This is what we'll be discussing now.
What's allopatric speciation ?
Geographic barriers are natural elements such as mountains or seas which split one or several species in different places. For example, a population of a certain species of plant could be split in two if an island is divided in two because of erosion, or if a mountain appeared because of an earthquake. Geographic barriers affect speciation because they separate a population.
Then, because the populations live in different environments, they will develop unique characteristics. These characteristics won't be shared between the populations, since they are not in the same place, and sooner or later reproductive barriers will kick in and they will be too different from one another to exchange genes, even if they encounter each other again.
For example, imagine that a population of a species of flowering plant would suddenly be geographically isolated in two by a mountain; one side would have lots of insects and the other hummingbirds. On the former, plants would evolve to have traits that are best suited to transmit pollen via insects, such as large petals so insects can land, little nectar, because insects do not require a lot and pink petals, bees' favorite color. While on the latter, they would evolve to have traits that are best suited to transmit pollen via birds, such as small petals, because hummingbirds do not land, lots of nectar to reward birds and red petals, birds' favourite color. Imagine then, that the two populations were now together again in the same area : no matter how long you wait, the populations would not mix again because of reproductive barriers that had a huge impact on these plants, since they have been subjected to evolution. Their genes would not mix either, because their pollen should be transported by different species : birds for one, and bees for the others. They are, from that moment on, reproductively isolated.
This also means that man does not come from monkeys in the way we usually mean. Current monkeys are not our ancestors, but rather we share a common ancestor, who then went to form separated species (men and monkeys).
Below are some examples of reproductive barriers.
Reproductive barriers
Ecological isolation
Some species live in the same area, however they do not interbreed because they don't they live in the same habitat. For example tigers and lions (who can mate and produce ligres) in India until about 150 years ago had habitats which overlapped for thousands of square kilometers; in spite of this no traces of interbreeding are to be found. In fact, lions tend to live on open grassland which is an easy place for hunting, whereas tigers prefer to live in forests since they are solitary creatures. This perfectly illustrates the fact that because of their differences of living places, closely related species can be prevented from mating.
Behavorial isolation
Species can differ in their mating rituals : birds and many other species (e.g. frogs) use sounds to attract mates, meaning that for one species there is one particular sound which is recognized by the other members of the same species. The courtship rituals' sounds don't have any influence on individuals which don't belong to the same group.
Another kind of mating ritual mechanism relies on the detection of chemical signals called pheromones, which are particular smells. Some species emit pheromones when they are ready to breed in order to attract the partner. The point is, pheromones can differ in chemical compounds or in amount of chemical compounds used, resulting in a prevention of interbreeding with the "wrong" partner.
These differences in mating rituals are a good way to avoid species, even if they are closely related, from reproducing together.
Temporal isolation
Sometimes what prevents a species from interbreeding with another one is not necessarily the habitat or the way they are attracted to each other, but rather the season or the time of the day they reproduce. Many organisms, mainly vegetals, have very different reproducing periods. For example, some closely related trees which could produce viable hybrids rarely interbreed because some bloom in spring and the other ones in summer, and some frogs can also be subject of this particular kind of mating prevention since their peak breeding time doesn't occur at the same time.
Mechanical isolation
The shape of organism can also be a barrier to reproduction. For obvious reason a member of a species which is big cannot breed with a little one, and the copulatory organs may also have an impact on the mating process. For example, in many insects and other arthropod groups there exists such a big variety in sexual organs, particularly the male ones, that it is the only way that we can distinguish the species.
Prevention of gamete fusion
Gametes of one species can simply fail to fertilize gametes of another species. The reason why it may not work is not only because of incompatible gametes, but the reproductive tract and organs can simply not match.
What's sympatric speciation ?
Sometimes speciation can occur without any reproductive barrier, even though it is a rare event, there exists few ways of having this happen.
Allopolyploid speciation
Sometimes, species, a diploid one for example, could have a problem during meiosis resulting in a gamete with 4 chromosomes, the invidual, with 4 chromosomes gametes, could mate with a triploid species(three pairs of chromosomes) creating an hybrid with 7 chromosomes which would be usually infertile (see hybrids section), but if this hybrid mate with the previous triploid species, the pairing of chromosomes would work and result in a new pentaploid species. See appendix #4 for an illustration of this example.
Autopolyploid speciation
Autopolyploid speciation, as allopolyploid speciation, begins with a meiotic error, for example, a triploid species (2n = 6)produces gametes with 6 chromosomes. The gametes produced could self-fertilize and create a tetraploid species (4n = 12) which may be viable and self-fertile, but which would not be able to mate and produce fertile with triploid one. See appendix #3 for an illustration of this example.
This type speciation occurs most of the time with plants, a fourth of the species of fowering plants are estimated to come from polyploidy. This event could also occur with some animals, such as insects, fishes, salamanders, however it's much more rare than with plants.
What about hybrids ? (Postzygotic isolations)
It happens that species from a different family group mate and most of the time, these hybrids which are produced die either at the stage of embryo, or before attaining the adult age, and if they survive up to adult age, most adult hybrids are sterile.
A big part of the hybrids are created with the help of the human being. Indeed, this kind of mating rarely occurs in nature because of all the reproductive barriers that exist, as discussed before.
One of the most well-known hybrid created by humans is the mule, which is an offspring of a male donkey and a female horse, this offspring survives most of the time to adult age, but it is infertile and can't mate; it is however very strong and resilient.
In fact, natural hybrids are very rare (except for the plant example above); most hybrids were created by man because of their usefulness, since hybrids tend to inherit the traits from both parent species, like bananas or wheat which has six sets of chromosomes. Some hybrids are also created due to the movement of humans around the world, closely related species meet each other again and form hybrids.
Hybrids are mostly human creations. Since men can have control over certain individuals of closely related species, they can cross-breed two individuals from different species (such as zebra and horse) in order to obtain the traits of both parents, or for science. For example a zorse (zebra and horse) looks just between a horse and a zebra : it has got both zebra and horse fur and they are precisely separated.
Sources
Why Evolution Is True, Jerry A. Coyne, Penguin Books, 2009
Biology Ninth Edition, Raven,Johnson, Mason, Losos and Singer, McGraw-Hill, 2011
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