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 another 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) 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 were species, we must explain how they appeared, this is 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 is just the geography or is there something else more complex ? This is what we'll be discussing now.

Prezygotic reproductive isolating mechanisms and their impact on mating

Geographical isolation

Sometimes species are separated by physical barriers such as mountains or oceans. This isolation prevents them from mating/reproducing even though they could produce fertile hybrids ; this isolation can also make a group of individuals evolve in two different species. This is called allopolyploid speciation and will be discussed later on.

Ecological isolation

Some species live in the same area, but however they do not interbreed because they don't they live in the same habitat. For example tigers and lions in India until about 150 years ago had habitats which overlapped for thousands of kilometers square; despite of this fact no traces of interbreeding were to be found. Actually lions tended to live on open grassland which was easy place for hunting whereas tigers prefered to live in forests since they are solitary creatures. This perfectly illustrates the fact that because of their differences of living places, close species can be prevented from mating.

Behavorial isolation

Species can differ in their mating rituals : birds and many other species (e.g. frogs) use sound to attract mates, meaning that for one species there is one particular sound which is recognized by the other member of the same species. The other species' courtship rituals' sounds don't have any influence on individual which doesn't belong to the same group.
Another kind of mating ritual mechanism relies on the detection of chemical signals called pheromones. Some species emit pheromones when they are ready to breed in order to attract the partner, the point is that 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 necessarly the living place 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 trees closely related and which can produce viable hybrids rarely interbreed because some are blooming in spring and other ones in summer. Some frogs can also be subject of this particular kind of mating prevention since the peak breeding time doesn't occur at the same time.

Mechanical isolation

The shape of organism can also be a barrier to reproduction, since for obvious reason a member of a species which is big cannot breed with a little one, but the copulatory organs may also have an impact on the reproducing process. For example in many insects and other arthropod groups there exist such a big variety in sexual organs, particularly the male ones, such that it can a way to distinguish the species.

Prevention of gamete fusion

Gametes of one species can work extremly poorly depending on another species which prevents gamete fusion. The reason of why it may doesn't work is not only because of the gametes which could incompatible, but also because of the reproductive tract that can be just not have a perfect match.

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 split in two because of erosion, or if a mountain appeared because of an earthquake. Geographic barriers affect speciation because they separate a population in two. Then, because the populations live in different environments, they will develop unique characteristics. These characteristics won't be shared between the two 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. Their genes would not mix, because their pollen would be transported by different species : birds for one, and bees for the other[, better yet, their genes themselves would not be able to mix, because their genetic code would be too different!] They are 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 come from a common ancestor, who then went to form separate species of men and monkeys.

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, 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. This speciation occurs most of the time with plants, about half of the species of plants are estimated to have had a polyploid episod in their life. This event could also occurs with some animals, such as insects, fishes, salamanders, however it's much more rare than with plants.

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 #4 for an illustration of this example.

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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