Speciation and variation – Evidence of common descent

As predicted by evolutionary theory, populations evolve in response to their surroundings. In any ecosystem there are finite opportunities to make a living. Organisms either have the genetic tools to take advantage of those opportunities or they do not.

House sparrows arrived in North America from Europe in 1800s. Since then, genetic variation within the population, and selection in different habitats, have spread them throughout most of the continent. Sparrows in the north tend to be larger and darker coloured than the ones in the south. Darker colours absorb sunlight better and a larger size allows less surface area to volume, therefore reducing heat loss, things which both lend to survival in a colder climate. This is a clear example of natural selection in a population, producing so-called “micro-evolution”.

Experiments conducted within the past few decades also show that populations evolve. John Endler from the University of California conducted experiments with the guppies on the Caribbean island of Trinidad that show the principles of selection at work. For mating purposes, female guppies prefer colourful males. Predatory fish also tend to go for colourful males as they are a more easily spotted source of food. Some portions of the streams where guppies live have fewer predators than others and in these locations the males tended to be more colourful.

guppy_1_2311When Dr. Endler moved predators to the regions with brightly colored male guppies, selection acted to produce a population of duller males. This demonstrates that when there are changes in environmental conditions persistent variation within a population leads to rapid evolution.

This raises the question, what is a species?

A species is often defined as a group of individuals that do or can interbreed in nature. 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.

National Geographic

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, 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, some species form rings of overlapping ability to interbreed, with adjacent populations being able to interbreed, but populations at either ends being unable.
Finally, many plants, and some animals, form hybrids. 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?

evolution.berkeley.edu

If two lineages of oak tree look quite different, but occasionally form fertile hybrids with each other, should we class them as different species? There are lots of other places where the boundary of a species is blurred. We shouldn’t be surprised though that these uncertain boundaries exist as the idea of a species is merely a human convention.


Defining speciation
I couldn’t do it much better than Berkeley:

“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:


“The scene: a population of wild fruit flies minding its own business on several bunches of rotting bananas, cheerfully laying their eggs in the mushy fruit…


“Disaster strikes: A hurricane washes the bananas and the immature fruit flies they contain out to sea. The banana bunch eventually washes up on an island off the coast of the mainland. The fruit flies mature and emerge from their slimy nursery onto the lonely island. The two portions of the population, mainland and island, are now too far apart for gene flow to unite them. At this point, speciation has not occurred — any fruit flies that got back to the mainland could mate and produce healthy offspring with the mainland flies.

 

“The populations diverge: Ecological conditions are slightly different on the island, and the island population evolves under different selective pressures and experiences different random events than the mainland population does. Morphology, food preferences, and courtship displays change over the course of many generations of natural selection.

drosophila_scene3

“So we meet again: When another storm reintroduces the island flies to the mainland, they will not readily mate with the mainland flies since they’ve evolved different courtship behaviors. The few that do mate with the mainland flies, produce inviable eggs because of other genetic differences between the two populations. The lineage has split now that genes cannot flow between the populations.


drosophila_scene4

“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”.

Causes of speciation
61283_evo_resources_resource_image_332_original
There are many causes of speciation including: geographic isolation, reduction of gene flow and reproductive isolation, the evolution of different mating location, mating time, or mating rituals, lack of “fit” between sexual organs (illustrated below).
damselflypenes

These damselfly penises illustrate the complexity of insect genitalia.


Observing speciation in action?
Around 15 iguanas survived the summer 1995 Hurricane Marilyn. For a month, these iguanas rode on a small raft of uprooted trees then settled on the small Caribbean island of Anguilla. What is going to happen to these iguanas is being closely followed by biologists. Will the iguanas die out, will they survive and change only slightly, or will they become reproductively isolated from other iguana and become a new species? We could be watching the first steps of a speciation event, but we can’t be sure what will occur.
It’s all well and good talking about how speciation could occur in theory, but what about the pratice? Have speciation events happened in ways that we can observe and measure?
If allopatric speciation happens, we’d predict that populations of the same species in different geographic locations would be genetically different. There are abundant observations suggesting that this is often true. For example, many species exhibit regional “varieties” that are slightly different genetically and in appearance, as in the case of the Northern Spotted Owl and the Mexican Spotted Owl. Also, ring species are convincing examples of how genetic differences may arise through reduced gene flow and geographic distance.
Spotted owl subspecies living in different geographic locations show some genetic and morphological differences. This observation is consistent with the idea that new species form through geographic isolation.
Experimental results
The first steps of speciation have been produced in several laboratory experiments involving “geographic” isolation. For example, Diane Dodd examined the effects of geographic isolation and selection on fruit flies. She took fruit flies from a single population and divided them into separate populations living in different cages to simulate geographic isolation. Half of the populations lived on maltose-based food, and the other populations lived on starch-based foods. After many generations, the flies were tested to see which flies they preferred to mate with. Dodd found that some reproductive isolation had occurred as a result of the geographic isolation and selection for different food sources in the two environments: “maltose flies” preferred other “maltose flies,” and “starch flies” preferred other “starch flies.” Although, we can’t be sure, these preference differences probably existed because selection for using different food sources also affected certain genes involved in reproductive behavior. This is the sort of result we’d expect, if allopatric speciation were a typical mode of speciation.
Diane Dodd’s fruit fly experiment suggests that isolating populations in different environments (e.g., with different food sources) can lead to the beginning of reproductive isolation. These results are consistent with the idea that geographic isolation is an important step of some speciation events.

REFERENCES
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