2.6 Speciation and Extinction (macroevolution)

Microevolution, which we have already covered, deals with changes within populations or species. Macroevolution, on the other hand, deals with the gain and loss of species through time.

I. species concepts

We can't talk about how new species arise until we have some idea of what a species is. Unfortunately, the concept of species is a human invention and the natural world has no reason to try to fit itself into our definitions. So the question of what a species is has many answers. We will look at three of them: morphological, biological, and evolutionary/phylogenetic.

A. morphological species concept

This is the oldest concept and is just based on what the organism looks like. Specimens are compared for several physical features like size, feather color, bone structure, number and type of teeth, arrangement of organs, etc. And more recently, microscopic and chemical traits were also used to classify organisms.

B. biological species concept

The biological species concept is the one most people use when they refer to a species. It is based on whether or not organisms can reproduce with each other.

In the biological species concept, organisms are in the same species if:

All of the above conditions must be met for organisms to meet the requirements for a biological species. There are several problems in applying this concept: it won't work with organisms that reproduce asexually, it can't be applied to extinct organisms, and we can't tell with populations that live miles apart whether they don't reproduce with each other because of physical differences, or just because they can't get together.

C. phylogenetic (evolutionary) species concept

II. the process of speciation

Speciation occurs when two groups within the same species become unable to reproduce to form viable fertile offspring. This may involve some kind of physical separation accompanied by genetic divergence. It may also occur without physical separation. When speciation occurs in groups that are physically separated from each other it is called allopatric. When speciation occurs within a group of organisms that are not physically separated from each other it is called sympatric. The result of genetic divergence is one or more reproductive isolating mechanisms that prevent interbreeding between the two groups.

A. allopatric speciation

In allopatric speciation, geographical barriers separate two groups of individuals, and they subsequently diverge genetically. After some period of time, even if the two groups are rejoined, they are no longer able to reproduce to form viable, fertile offspring (reproductive barriers exist). Examples discussed in the textbook are the antelope squirrels of the Grand Canyon and the pupfishes of Death Valley. Adaptive radiation in island chains is a specific type of allopatric speciation.

1. Physical barriers can form through geological or climatic processes and separate populations into smaller groups that can no longer interbreed with each other. They include things like rivers, mountain ranges, and glaciers. Land areas that were once joined together can be separated if the water level rises; this occurs in lakes and in oceans. Conversely, parts of the ocean that were once connected to each other can be separated by the formation of land masses.

Obviously, what constitutes a physical barrier depends on the species we are considering. Whereas a river might be a barrier for snails and worms, it is hardly a barrier for birds, or even large mammals.

2. Genetic divergence can occur in several ways once the two groups are physically separated. We will cover two of them: adaptive divergence and genetic drift.

a. In adaptive divergence, geographically isolated populations adapt to different environmental conditions. The physical barrier blocks gene flow between the two groups (gene flow would decreased genetic differences between the groups). Mutations that arise in one group may not occur in the other. Because they are in different areas, environmental conditions are bound to be somewhat different. Their gene pools will be acted on by natural selection to favor the alleles that are best for the particular environment they are in Over time, the two groups accumulate enough genetic differences that they can no longer interbreed--now a reproductive barrier exists between them.

b. Genetic drift can lead to differences between the gene pool of an isolated group and the gene pool of its parent population. This could occur by the founder effect or a bottleneck event. Over time, the two groups accumulate enough genetic differences that they can no longer interbreed--now a reproductive barrier exists between them.

B. sympatric speciation

For speciation to occur when two groups are not physically separated, some genetic changes must arise in some individuals that are not shared with all other individuals. This can occur through non-random mating, a sudden change in chromosome number (polyploidy), or other genetic changes.

1. Polyploidy involves a sudden increase (2X, 3X, 4X) in the number of chromosomes
of one or more individuals in a population. The two ways this can happen (autopolyploidy, allopolyploidy) are described in the textbook. In most cases, organisms can only reproduce with other organisms if the chromosome numbers are the same (there are exceptions). So, once there is an individual with a different number of chromosomes, it is automatically isolated reproductively from the rest of the population. Polyploidy can cause speciation if:

a. it happens in more than one individual at about the same time and they can interbreed
b. it happens in one individual that can self-fertilize

2. Assortative mating can eventually lead to speciation. If, for example, the black-bellied seedcrackers shown on page 459 were to mate only with birds that have the same beak size, they might eventually fail to recognize birds with different beak sizes than theirs as potential mates.

3. If a mutation causes one or more individuals in a population to start using a different resource (food, shelter) than the parent population, enough genetic change could accumulate between them to cause speciation (examples in textbook include fig wasp and Lake Victoria cichlids).

C. reproductive isolating mechanisms

Once speciation occurs, the new species is prevented from rejoining its parent species by reproductive isolating mechanisms. These include anything that prevents two groups of organisms from producing viable, fertile offspring. They could be considered biological rather than physical barriers. They are generally separated into two groups: prezygotic and postzygotic. Prezygotic barriers are those that in some way prevent fertilization. Postzygotic barriers do not prevent fertilization, but they do either prevent the development of the offspring or make the offspring infertile. For both groups of barriers, you should know the examples used in the textbook:

a. prezygotic

b. postzygotic

III. extinction

Extinction is a normal and natural process on the earth. It is estimated that over 99% of all species that ever lived on the earth are now extinct. Because we are currently worried about the high rate of extinction, we sometimes forget that some extinction would occur even if humans did not have a significant impact on the environment. But in fact, humans seem to have speeded up the rate of extinction far beyond anything that has happened on this planet in the past.

There is an old saying that when the environment changes a species can either move (migrate), adapt, or die. Extinction occurs when species cannot migrate fast enough to escape adverse environmental changes, or when they do not have enough genetic variation to allow adaptation to the new conditions.

Before we look at the details we have to define two terms: local extinction and species extinction. When most of us use the word extinction we mean that all members of a species are dead. That is species extinction. But in population biology there is a concept called metapopulations in which only one population of a species may die out. That is a local extinction and will not be covered in this unit. So when we use the word extinction, we mean species extinction. All dead. Very dead.

A. temporal patterns in extinction rate

The rate at which new species arise and other species die out varies tremendously over time. On page 491 is a graph showing extinction rate (the blue line represents the percent of taxonomic families that died out over time. Notice that sometimes the blue line rises sharply. These are mass extinction events, when many more organisms died in a specific time period than usual. Note the Permian and the Cretaceous extinctions. You will need to know the dates of these extinctions and the factors that are believed to have caused them.

B. causes of extinction

Each species extinction may be caused by several factors acting together.

1. Extrinsic factors are those that come from outside and include:

Humans have affected extinction rate through habitat destruction, climate change, and the introduction of foreign species that compete with, prey on, or infect other species.

2. Intrinsic factors are biological characteristics of the species themselves:

Humans have indirectly affected intrinsic extinction factors. By building roads, railroads, pipelines, canals, and cities we have blocked the ability of some plant and animal species to migrate (disperse) in response to environmental change.

C. correlation between extinction rate and human population growth

read: "What is a Species" on pages 465-468
"Modes of Speciation" on pages 468-475 (stop at "The punctuated equilibrium…")
"The history of life is punctuated…" on pages 490-492

2. Which concept of species is more useful in conservation? Explain.

3. What are the requirements for speciation to occur?

4. Describe in detail the processes involved in
a. allopatric speciation
b. sympatric speciation

5. Describe the function of reproductive barriers in speciation. Describe the different kinds of reproductive barriers and give examples.

6. Explain the process by which adaptive radiation occurs on island chains.

7. Explain the difference between species extinction and local extinction.

8. Explain why the rate of extinction is not always the same.

9. Discuss the events that probably led to the
a. Permian extinction
b. Cretaceous extinction

10. Explain the difference between extrinsic and intrinsic reasons for extinction. Give examples.