How can a river influence evolution?
Imagine a group of small organisms, such as mice, that become separated by a mighty river. This group has now become isolated, and formed two separate groups. The groups are obviously no longer able to breed together. Over many generations, each group will evolve separately, eventually forming two completely new species of mice.
Origin of Species
Macroevolution is evolution over geologic time above the level of the species. One of the main topics in macroevolution is how new species arise. The process by which a new species evolves is called speciation. How does speciation occur? How does one species evolve into two or more new species?
To understand how a new species forms, it’s important to review what a species is. A species is a group of organisms that can breed and produce fertile offspring together in nature. For a new species to arise, some members of a species must become reproductively isolated from the rest of the species. This means they can no longer interbreed with other members of the species. How does this happen? Usually they become geographically isolated first.
Assume that some members of a species become geographically separated from the rest of the species. If they remain separated long enough, they may evolve genetic differences. If the differences prevent them from interbreeding with members of the original species, they have evolved into a new species. Speciation that occurs in this way is called allopatric speciation. An example is described in the Figure below.
Allopatric Speciation in the Kaibab Squirrel. The Kaibab squirrel is in the process of becoming a new species.
Less often, a new species arises without geographic separation. This is called sympatric speciation. The following example shows one way this can occur.
- Hawthorn flies lay eggs in hawthorn trees (see Figure below). The eggs hatch into larvae that feed on hawthorn fruits. Both the flies and trees are native to the U.S.
- Apple trees were introduced to the U.S. and often grow near hawthorn trees. Some hawthorn flies started to lay eggs in nearby apple trees. When the eggs hatched, the larvae fed on apples.
- Over time, the two fly populations—those that fed on hawthorn trees and those that preferred apple trees—evolved reproductive isolation. Now they are reproductively isolated because they breed at different times. Their breeding season matches the season when the apple or hawthorn fruits mature.
- Because they rarely interbreed, the two populations of flies are evolving other genetic differences. They appear to be in the process of becoming separate species.
Sympatric Speciation in Hawthorn Flies. Hawthorn flies are diverging from one species into two. As this example shows, behaviors as well as physical traits may evolve and lead to speciation.
A Great Leap To Speciation?
Cane toads (Bufo marinus) were introduced to Australia in 1935 to aid sugar cane farmers in NE Australia. Their hope was that they would control a beetle that was damaging their crops. The cane toads had already been used successfully for this purpose in the Hawaiian Islands. At the time, cane toads were considered less destructive than pesticides and were seen as a "natural" form of pest control. But Australia isn’t Hawaii. Hawaii is a much smaller contained environment compared to Australia, and Australia didn't have any native toad species at all. When the cane toads were introduced well…things didn’t go that well.
Since their introduction and spread in Australia, cane toads have become a classic example of a runaway invasive species and caused great damage in Australia. But along with all the carnage, they have also provided valuable information. At first, the cane toads spread out from their initial location at a rate of 10 km per year, but scientists noted that this rate was consistently increasing until today it stands around 50 km per year. This unexpected occurrence raised the question of, "How was this happening?" Scientists looked at toads from both extremes of their range and found something interesting. The toads at the advancing edge of the population looked and behaved differently than the toads from Northern Queensland, where they were initially introduced. The advance toads have longer legs, jump farther, jump in a straight line more frequently, and have greater endurance than toads from Northern Queensland. Scientists realized that as the population spread, toads that had characteristics (like jumping farther) that allowed them to travel farther began to pull away from the rest of the population. When it came time to breed, the leading edge toads bred with each other, increasing the frequency of the characteristics that had placed them at the leading edge (it is important to note here that the characteristics for leg size, hopping characteristics, and endurance are from different genes, so there is more than one way for a toad to be at the advancing edge). They realized toads in the population were starting to differentiate from each other and that gene frequencies were changing within the population. They realized they were witnessing what might be an unappreciated mechanism of evolution.
Natural selection says gene frequency changes within a population based on breeding success. Those individuals who are more successful breeders leave more offspring and more copies of their genes in the next generation. In this way, gene frequencies in populations change through time and evolution can occur. What scientists were seeing with the cane toad was different. The gene frequency was changing in the population, but it did not change through reproductive success through time; it was changing because the population was spatially spreading out and certain traits determined who were the best dispersers. They were seeing an evolutionary mechanism acting through space instead of through time. They call this phenomena "spatial sorting."
So are there now two species of cane toads in Australia? Well the short answer is no. The populations are still connected, and they can still interbreed. But members of the population are becoming increasingly different and will probably continue to do so as long as cane toads can continue to advance into unoccupied territory. The question of species will have to wait until the toads run out of new territory. At that point, if something has happened to prevent gene flow throughout the population, then we will be looking at a new species. But if there is still gene flow throughout the population at the time their advance stops, then the slow jumpers will eventually catch up with the fast jumpers, and the gene frequency in the population will start to move back to its original point. This is because if they are in the same location, the slow dispersers have higher reproductive success than the fast dispersers, so the slow disperser genes will increase in frequency in the population while the fast disperser genes decline in frequency. If that does happen, we will wind up with only one species of cane toad, which will probably be very similar to the one that originally arrived in Australia.
- What other characteristics can you think of that make an organism a good disperser?
- How many of the characteristics you came up with in question #1 are genetically based?
- Why is it important that more than one gene is involved in determining how far a cane toad disperses in its lifetime?
- Do you think the definition of natural selection should be changed to include spatial sorting?
- http://animals.nationalgeographic.com/wild/shows Youtube by NatGeoWild
- New species arise in the process of speciation.
- Allopatric speciation occurs when some members of a species become geographically separated. They then evolve genetic differences. If the differences prevent them from interbreeding with the original species, a new species has evolved.
- Sympatric speciation occurs without geographic separation.
Use these resources to answer the questions that follow.
- http://www.hippocampus.org/Biology Biology for AP* Search: Speciation: Overview
- Why are animals on opposite sides of the Grand Canyon different?
- Define speciation.
- http://www.hippocampus.org/Biology Biology for AP* Search: Geographic and Reproductive Barriers
- Define allopatric speciation. How is allopatric speciation related to gene flow?
- Give three examples of geographic barriers.
- Describe sympatric speciation.
- Give an example of a prezygotic barrier.
1. Define speciation.
2. Describe how allopatric speciation occurs.
3. Why is sympatric speciation less likely to occur than allopatric speciation?