Here, you can explore different ways to define a species and learn about the various processes through which speciation can occur. This section also addresses the topics of cospeciation — when two lineages split in concert with one another — and modes of speciation that are specific to plants. Show Let’s start by defining a species. A species is often defined as a group of individuals that actually or potentially interbreed in nature. In this sense, a species is the biggest gene pool possible under natural conditions. For example, these happy face spiders look different, but since they can interbreed, they are considered the same species: Theridion grallator. That definition of a species might seem cut and dried, but it is not — in nature, there are lots of places where it is difficult to apply this definition. For example, many bacteria reproduce mainly asexually. The bacterium shown at right is reproducing asexually, by binary fission. The definition of a species as a group of interbreeding individuals cannot be easily applied to organisms that reproduce only or mainly asexually.Also, many plants, and some animals, form hybrids in nature. Hooded crows and carrion crows look different, and largely mate within their own groups — but in some areas, they hybridize. Should they be considered the same species or separate species? If two lineages of oak look quite different, but occasionally form hybrids with each other, should we count them as different species? There are lots of other places where the boundary of a species is blurred. It’s not so surprising that these blurry places exist — after all, the idea of a species is something that we humans invented for our own convenience! Speciation is a lineage-splitting event that produces two or more separate species. Imagine that you are looking at a tip of the tree of life that constitutes a species of fruit fly. Move down the phylogeny to where your fruit fly twig is connected to the rest of the tree. That branching point, and every other branching point on the tree, is a speciation event. At that point genetic changes resulted in two separate fruit fly lineages, where previously there had just been one lineage. But why and how did it happen? The branching points on this partial Drosophila phylogeny represent long past speciation events. Here is one scenario that exemplifies how speciation can happen:
This is a simplified model of speciation by geographic isolation, but it gives an idea of some of the processes that might be at work in speciation. In most real-life cases, we can only put together part of the story from the available evidence. However, the evidence that this sort of process does happen is strong. Geographic isolation It doesn’t even need to be a physical barrier like a river that separates two or more groups of organisms — it might just be unfavorable habitat between the two populations that keeps them from mating with one another. Reduction of gene flow However, speciation might also happen in a population with no specific extrinsic barrier to gene flow. Imagine a situation in which a population extends over a broad geographic range, and mating throughout the population is not random. Individuals in the far west would have zero chance of mating with individuals in the far eastern end of the range. So we have reduced gene flow, but not total isolation. This may or may not be sufficient to cause speciation. Speciation would probably also require different selective pressures at opposite ends of the range, which would alter gene frequencies in groups at different ends of the range so much that they would not be able to mate if they were reunited. Even in the absence of a geographic barrier, reduced gene flow across a species’ range can encourage speciation. The environment may impose an external barrier to reproduction, such as a river or mountain range, between two incipient species but that external barrier alone will not make them separate, full-fledged species. Allopatry may start the process off, but the evolution of internal (i.e., genetically-based) barriers to gene flow is necessary for speciation to be complete. If internal barriers to gene flow do not evolve, individuals from the two parts of the population will freely interbreed if they come back into contact. Whatever genetic differences may have evolved will disappear as their genes mix back together. Speciation requires that the two incipient species be unable to produce viable offspring together or that they avoid mating with members of the other group. Here are some of the barriers to gene flow that may contribute to speciation. They result from natural selection, sexual selection, or even genetic drift:
In our fruit-flies-in-rotten-bananas-in-a-hurricane example, allopatry kicked off the speciation process, but different selection pressures on the island caused the island population to diverge genetically from the mainland population. Geographic isolation can instigate a speciation event — but genetic changes are necessary to complete the process.What might have caused that to happen? Perhaps, different fruits were abundant on the island. The island population was selected to specialize on a particular type of fruit and evolved a different food preference from the mainland flies. Differing selection pressures on the two islands can complete the differentiation of the new species.Could this small difference be a barrier to gene flow with the mainland flies? Yes, if the flies find mates by hanging out on preferred foods, then if they return to the mainland, they will not end up mating with mainland flies because of this different food preference. Gene flow would be greatly reduced; and once gene flow between the two species is stopped or reduced, larger genetic differences between the species can accumulate. Speciation in action? A plausible model Scientists have found a lot of evidence that is consistent with allopatric speciation being a common way that new species form:
If the association between two species is very close, they may speciate in parallel. This is called cospeciation. It is especially likely to happen between parasites and their hosts. To see how it works, imagine a species of louse living on a species of gopher. When the gophers get together to mate, the lice get an opportunity to switch gophers and perhaps mate with lice on another gopher. Gopher-switching allows genes to flow through the louse species. Consider what happens to the lice if the gopher lineage splits into lineages A and B:
Evolutionary biologists can often tell when lineages have cospeciated because the parasite phylogeny will “mirror” the host phylogeny. Observing parallel host and parasite phylogenies is evidence of cospeciation.This example is somewhat idealized — rarely do scientists find hosts and parasites with exactly matching phylogenies. However, sometimes the phylogenies indicate that cospeciation did happen along with some host-switching. Attribution“Speciation.” Understanding Evolution. University of California Museum of Paleontology. 8 June 2020 <https://evolution.berkeley.edu/evolibrary/article/evo_40>. “Defining a species.” Understanding Evolution. University of California Museum of Paleontology. 8 June 2020 <https://evolution.berkeley.edu/evolibrary/article/evo_41>. “Defining speciation.” Understanding Evolution. University of California Museum of Paleontology. 8 June 2020 <https://evolution.berkeley.edu/evolibrary/article/evo_42>. “Causes of speciation.” Understanding Evolution. University of California Museum of Paleontology. 8 June 2020 <https://evolution.berkeley.edu/evolibrary/article/evo_43>. “Reproductive isolation.” Understanding Evolution. University of California Museum of Paleontology. 8 June 2020 <https://evolution.berkeley.edu/evolibrary/article/evo_44>. “Evidence for speciation.” Understanding Evolution. University of California Museum of Paleontology. 8 June 2020 <https://evolution.berkeley.edu/evolibrary/article/evo_45>. “Cospeciation.” Understanding Evolution. University of California Museum of Paleontology. 8 June 2020 <https://evolution.berkeley.edu/evolibrary/article/evo_46>. All of the genes in a population. Any genes that could wind up in the same individual through sexual reproduction are in the same gene pool. The movement of genes between populations. This may happen through the migration of organisms or the movement of gametes (such as pollen blown to a new location). A group of organisms that is about to become a separate species from other, related individuals. Speciation that depends on an external barrier to gene flow (such as geographic isolation) to begin or complete the process of speciation. What evidence exists for speciation?Scientists have found a lot of evidence that is consistent with allopatric speciation being a common way that new species form: Geographic patterns: If allopatric speciation happens, we'd predict that populations of the same species in different geographic locations would be genetically different.
What are 3 ways that speciation can happen?There are four major variants of speciation: allopatric, peripatric, parapatric, and sympatric.
In what 2 ways can speciation occur?Speciation can take place in two general ways. A single species may change over time into a new form that is different enough to be considered a new species. This process is known as anagenesis. More commonly, a species may become split into two groups that no longer share the same gene pool.
What is the main reason for speciation?The factors responsible for speciation are: Geographical barrier. Natural selection. Genetic drift.
|