Patterns of Evolution
Allopatric Speciation
It's a bit like a character in a movie going off and having adventures that change him so drastically that when he
returns, the folks in his hometown no longer recognize the way he looks and behaves. The biological equivalent is
"allopatric speciation," an evolutionary process in which one species divides into two because the original
homogenous population has become separated and both groups diverge from each other.
In their separate niches, the two groups go
their own evolutionary ways, accumulating
different gene mutations , being subjected to
different selective pressures, experiencing
different historical events, finally becoming
incapable of interbreeding should they ever
come together again. For many years this has
been regarded as the main process by which
new species arise.
Often this type of speciation occurs in three
steps. First, the populations become
physically separated, often by a long, slow
geological process like an uplift of land, the
movement of a glacier, or formation of a body of water. Next, the separated populations diverge, through changes in
mating tactics or use of their habitat. Third, they become reproductively separated such that they cannot interbreed
and exchange genes.
Under normal conditions, genes in a given population are exchanged through breeding, so that even if some
variation occurs, it is limited by this "gene flow." But gene flow is interrupted if the population becomes divided
into two groups. One way this happens is by "vicariance (change in form)," geographical change that can be slow
or rapid.
An example of vicariance is the separation of marine creatures on either side of Central America when the Isthmus
of Panama closed about 3 million years ago, creating a land bridge between North and South America. Nancy
Knowlton of the Smithsonian Tropical Research Institute in Panama has been studying this geological event and its
effects on populations of snapping shrimp. She and her colleagues found that shrimp on one side of the isthmus
appeared almost identical to those on the other side -- having once been members of the same population.
But when she put males and females from different sides of the isthmus together, they snapped aggressively instead
of courting. They had become separate species, just as the theory would predict.
Questions and Tasks
1.
Use your text book or the internet to write definitions for the words in bold type.
2.
Why does the geological history of the Isthmus of Panama make it a good place to look for evidence of
allopatric speciation in marine organisms?
3.
What kind of life history strategy (mating patterns, number of offspring, degree of mobility of larvae and adults,
etc) do you think might encourage allopatric speciation of populations along the isthmus?
Convergence
Marsupial and placental mammals diverged from a common ancestor more than 100 million years ago, and have
evolved independently ever since. The two groups have different modes of reproduction, yet within each group
individual species that occupy similar niches have evolved similarities in overall shape, locomotion, and feeding
habits. This widespread evolutionary phenomenon is known as convergence.
Questions and Tasks
1.
2.
Read and complete the worksheet Convergence: Marsupials and Placental Mammals
Discuss how the geological history of land masses in the northern and southern hemispheres, combined
with the timing of early events in mammalian evolution, led to the separation of ancient marsupial and
placental mammals.
3.
Discuss the concepts of adaptive radiation and convergent evolution, and how they relate to one another in
explaining the evolution of the separate, yet parallel, marsupial and placental faunas found today in
Australia and North America.
4.
South America was historically home to many more marsupial species (along with some other odd, extinct
mammals) than live there today. Discuss how continental drift and competition explain this current
situation.
Reproductive Barriers and Speciation
Have a look at this example of how reproductive barriers can lead to speciation.
Watch the video on Hummingbirds
The hummingbird study that Tom Smith and Chris Schneider are conducting in Ecuador is part of a much larger
research program spanning three continents. Evolutionary biologists are fanning out and tramping through varying
ecosystems in Africa, Australia, and South America, catching and meticulously describing the animals that live
there. It's a new venture aimed at answering an old question, one that underlies all of evolutionary science: What
drives the formation of new species?
The prevailing theory goes back almost 60 years, to when biologist Ernst Mayr of Harvard University proposed the
"reproductive isolation" theory. When a population of, say, lizards or birds becomes divided by geographical
barriers, small changes over time will alter the genetic makeup of the separated groups. Eventually, they differ
enough that, should they encounter each other again, they can no longer interbreed. The offshoot group has become
a new species.
But in recent years an even older, contrasting view dating back to Darwin has been gaining ground. The globetrotting biologists are discovering an intriguing pattern: In many places, species appear to have emerged at the
transition zones between different ecosystems, without ever being geographically cut off from the parent stock. They
are examples of how natural selection can act through ecological differences to spawn new species.
The Andean hummingbirds are not an isolated case. The force of ecology has also been studied in the leaf-litter
skink, a small lizard found in Australia. Two populations living close to each other but in different ecosystems show
in their DNA that they're genetically distinct. Chris Schneider figured out that the two populations had adapted to
different ecosystems -- one an open forest and the adjacent one a closed rainforest.
The open forest lizards are smaller, have shorter limbs and bigger heads, and become sexually mature earlier. The
reason: Predator birds more easily pick off lizards in the open forest, so the skinks there have evolved to reproduce
earlier, generating offspring before they become a bird's dinner. The genetic differences, shaped by selection, have
produced two distinct species living next to each other.
And the converse can be true: Populations separated by geography but living in similar environments may be almost
indistinguishable. Says Schneider: "Time and isolation alone don't necessarily result in new morphologies -whereas a new environment does."
Questions and Tasks
1.
Explain the hypothesis presented by the scientists profiled in this segment to explain the process of
speciation in hummingbirds and possibly other species.
2.
How does this hypothesis differ from the traditional view that speciation often requires geographic
separation of populations?
3.
Why were the researchers collecting blood from the populations they studied? Discuss at least two possible
analyses that could be performed on those samples and, identify at least two different questions that might
be answered with sufficient data.
Finally look at Darwin’s theory of evolution in this interactive activity.
1. Choose one of the areas that pollenpeepers ended up on. As the years progress, explain what
happens to the populations of pollenpeepers.
2. Have new species of pollenpeepers evolved? If so explain why and by what mechanisms this
may have occurred.
3. Complete chapter review question 10.