On the Origin of Species chapter IV outlines Darwin’s proposal on how a new species is generated via natural selection over a long period of time. In Darwin’s theory of speciation, he proposes that all organisms within a species must express some degree of variation, which allow for organisms with the most advantageous variations to produce the most offspring (Darwin 10) This also means that the variations must be heritable from one generation to the next.
In chapter IV Darwin uses the examples variants causing a wolf to be slimmer so it can run faster to escape predators and hunt down prey, and a bear may have a variant that allows it to prey on more abundant sources of food (Quote: “Let us take the case of a wolf, which preys on various animals, securing some by craft, some by strength, and some by fleetness; and let us suppose that the fleetest prey, a deer for instance, had from any change in the country increased in numbers, or that other prey had decreased in numbers, during that season of the year when the wolf was hardest pressed for food.
Under such circumstances the swiftest and slimmest wolves would have the best chance of surviving and so be preserved or selected,- provided always that they retained strength to master their prey at this or some other period of the year, when they were compelled to prey on other animals. ” Darwin page 6). In addition to natural selection, sexual selection plays a role as well.
Males that attract females and successfully mate with them will produce offspring, and thus, males that have a favorable variant that attracts the most females will have more offspring than other males that lack the variant. These sexually advantageous variants will also be hereditable. Such examples of this used by darwin is the ability of some male birds to have more brightly colored feathers, different songs, and different mechanisms to attract attention.
As mentioned by Darwin, these would all greatly increase the ability of the male to mate with more females, or to secure a female for each season (Quote: “The rockthrush of Guiana, birds of paradise, and some others, congregate; and successive males display with the most elaborate care, and show off in the best manner, their gorgeous plumage; they likewise perform strange antics before the females, which, standing by as spectators, at last choose the most attractive partner” Darwin 5). Darwin notes that isolation is essential for the generation of a new species via natural selection.
This isolation must be in a small geographic area (Quote: “Lastly, isolation will give time for a new variety to be improved at a slow rate; and this may sometimes be of much importance. If, however, an isolated area be very small, either from being surrounded by barriers, or from having very peculiar physical conditions, the total number of the inhabitants will be small; and this will retard the production of new species through natural selection, by decreasing the chances of favourable variations arising” Darwin 11).
If new organisms were introduced it could, and often does, interfere with the interaction between a species variant and the environment. The isolation will ensure that an advantageous variant will win out over a disadvantageous one. As time goes on, the advantageous variant will become established in the population, making it appear as if the species is perfectly adapted for their environmental niche.
At the end of chapter, Darwin notes that as advantageous variants are selected over disadvantageous one, it will lead to divergence from the parent species. When these diverged species start to diverge themselves, it creates a cascade and ultimately limits the ability for the population to grow further. Thus, those that diverged but lack a favorable variant will die. As more and more divergences come about the subspecies may develop into an entirely different species over tens of thousands of years.
To sum it up, in Darwin’s theory of speciation involves: variation amongst individuals in a species that must be inheritable, accumulation of these inheritable variants, competition between those with different variants, isolation in order for natural selection to occur, geographical differences so sub species niches can develop, sexual selection, natural selection, long periods of time, divergence from parent species. The modern theory of speciation adds more nuance to Darwin’s theory of speciation.
Today, we have numerous speciation concepts: ~25 that are all equally efficient in their own right. This lack of set definition, also adds further contention to the lumpers vs. splitters debate which additionally fuels misconceptions in society and can have major impacts on conservation efforts. One idea idea that Darwin had that compared completely to the modern speciation idea is the theory of speciation by sexual selection, in that the males that are able to mate more frequently by being more attractive to females via some sort of ornament, song, etc. nd can pass that variant on to their offspring are more fit and can cause divergence to start to take place.
For the most part, scientists would agree that like Darwin’s theory there must be a barrier to gene flow. In Darwins time, the term gene flow didn’t exist, but he described the definition in On the Origin of Species. What is different about this and the modern view is that instead of a geographic barrier situation (allopatry), there’s several other different types like reproductive barriers: both pre- and post-zygote, parapatric speciation, peripatric, and sympatric.
The last three are reproductive barriers in their own right after the diverged species and the parent species re-meet. So, while Darwin got the geographic barrier part right, he missed another part: barriers can occur by different mechanism. Another major idea by Darwin is that speciation takes thousands and thousands of years to occur, but that isn’t always true. In our evolution textbook, speciation can take place in a few generations via hybridization of plant species or alloploidy in plants.
Comparatively, it can take millions of years for a new mammalian, reptilian, or avian species to develop. Additionally, the knowledge of microbes has been around for several centuries, yet Darwin failed to address how speciation occurs in them, which could’ve added problems to his theory then but now we know that microbes have flexible genomes and a single individual can evolve in it’s lifetime via induction, conjugation, and transformation.
One thing I noticed that both Darwin and the evolutionary textbook failed to talk about in detail on the creation of a species is the ideas behind mutation and genetic drift. Mutations are typically overlooked as it’s most often than not going to result in a disadvantageous allele, but it can pose as a mode of speciation that Darwin overlooks, but really, that’s not too surprising since the knowledge of genetics didn’t exist at this time. Genetic drift can significantly impact the fitness of a species, and can lead to their extinction.
In this case allelic frequencies can alter the variation of individuals in a population and ultimately fix an allele, get rid of it quickly, or stabilize it. Genetic drift is another mechanism of speciation that isn’t looked at by Darwin, but, again, considering the lack of genetic knowledge, it’s not surprising. Artificial selection, which Darwin mentioned but not by term, is another factor that can create a new species. Artificial selection today is used on a wide scale to develop domesticated animals that are of some use to humans, for example, cattle, dog/cat breeds, lab rats, etc.
As I hinted above, it appears that Darwin’s model of speciation would most closely align with the books allopatric isolation speciation model, in the a physical geographical boarder must take form in order for two separate species populations to diverge enough so that one, or both, becomes a different species all together due to new environmental conditions and a new niche to fill. When these species re-meet there will be reproductive barriers that prevent hybridization from occurring through reinforcement. Now we know this isn’t the only way speciation can occur because there are three other modes: peripatric, parapatric, and sympatric.