02/13/02

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In this lesson, we wish to ask:

• What is biological evolution?
• How are theories of microevolution and macroevolution related?
• What is a species, and what are the different ways it can be defined?
• What are the limitations of each definition?
• How is reproductive isolation important to speciation, and what forms can it take?
• Why should natural selection reinforce reproductive isolation?
• Can species be formed in ways other than geographic isolation?
   

Evolution and Its Many Forms

The word "evolution" does not apply exclusively to biological evolution. The universe and our solar system have developed out of the explosion of matter that began our known universe. Chemical elements have evolved from simpler matter. Life has evolved from non-life, and complex organisms from simpler forms. Languages, religions, and political systems all evolve. Hence, evolution is an appropriate theme for a course on global change.

The core aspects of evolution are "change" and the role of history, in that past events have an influence over what changes occur subsequently. In biological evolution this might mean that complex organisms arise out of simpler ancestors - though be aware that this is an over-simplification not acceptable to a more advanced discussion of evolution.

A full discussion of evolution requires a detailed explanation of genetics, because science has given us a good understanding of the genetic basis of evolution. It also requires an investigation of the differences that characterize species, genera, indeed the entire tree of life, because these are the phenomena that the theory of evolution seeks to explain.

We will begin with observed patterns of similarities and differences among species, because this is what Darwin knew about. The genetic basis for evolution only began to be integrated into evolutionary theory in the 1930's and 1940's. We will add genetics into our understanding of evolution through a discussion activity.
 

 Definitions of Biological Evolution

• Definition 1:

Changes in the genetic composition of a population with the passage of each generation
 
• Definition 2:

The gradual change of living things from one form into another over the course of time, the origin of species and lineages by descent of living forms from ancestral forms, and the generation of diversity

A full explanation of evolution requires that we link these two levels. Can small, gradual change produce distinct species? How does it occur, and how do we decide when species are species? Hopefully you will see the connections by the end of these three lectures.

Today we will discuss how species are formed. But to do this, we need to define what we are talking about.
 

What is a Species?

• Morphological species concept: Oak trees look like oak trees, tigers look like tigers. Morphology refers to the form and structure of an organism or any of its parts. The morphological species concept supports the widely held view that "members of a species are individuals that look similar to one another." This school of thought was the basis for Linneaus' original classification, which is still broadly accepted and applicable today.

Mimicry complexes supplied further evidence against the concept, as organisms of the same species can look very different, depending upon where they are reared or their life cycle stage (some insects produce a spring brood that looks like one host plant and a summer brood that looks like another).

The morphological species concept was replaced by another viewpoint that puts more emphasis on the biological differences between species.

• Biological species concept: This concept states that "a species is a group of actually or potentially interbreeding individuals who are reproductively isolated from other such groups."

This concept also emphasized that a species is an evolutionary unit. Members share genes with other members of their species, and not with members of other species.

Although this definition clearly is attractive, it has problems. Can you test it on museum specimens or fossil data? Can it explain the existence of species in a line of descent, such as the well-known lineage of fossil horses? Obviously not.

In fact, one cannot apply this definition easily, or at all, with many living organisms. What if species do not live in the same place? What about the hybrids that we know occur in zoos? These problems are serious enough that some biologists recently argued for a return to the morphological species concept.

So what is the best way to define a species?

Most scientists feel that the biological species concept should be kept, but with some qualifications. It can only be used with living species, and cannot always be applied to species that do not live in the same place. The real test applies to species that have the potential to interbreed.

Most importantly, the biological species concept helps us ask how species are formed, because it focuses our attention on the question of how reproductive isolation comes about. Let us first examine types of reproductive isolation, because there are quite a few.  
 

Types of Reproductive Isolation

This suggests a simple and useful dichotomy, between pre-mating or prezygotic (i.e., pre-zygote formation) reproductive isolating mechanisms, and post-mating or postzygotic isolating mechanisms. Remember that a zygote is the cell formed by the union of two gametes and is the basis of a developing individual.

Prezygotic isolating mechanisms

1. Ecological isolation: Species occupy different habitats. The lion and tiger overlapped in India until 150 years ago, but the lion lived in open grassland and the tiger in forest. Consequently, the two species did not hybridize in nature (although they sometimes do in zoos).
2. Temporal isolation: Species breed at different times. In North America, five frog species of the genus Rana differ in the time of their peak breeding activity.
3. Behavioral isolation: Species engage in distinct courtship and mating rituals (see Figure 1).
4. Mechanical isolation: Interbreeding is prevented by structural or molecular blockage of the formation of the zygote. Mechanisms include the inability of the sperm to bind to the egg in animals, or the female reproductive organ of a plant preventing the wrong pollinator from landing.

Postzygotic isolating mechanisms Hybrid inviability. Development of the zygote proceeds abnormally and the hybrid is aborted. (For instance, the hybrid egg formed from the mating of a sheep and a goat will die early in development.)  Hybrid sterility. The hybrid is healthy but sterile. (The mule, the hybrid offspring of a donkey and a mare, is sterile; it is unable to produce viable gametes because the chromosomes inherited from its parents do not pair and cross over correctly during meiosis (cell division in which two sets of chromosomes of the parent cell are reduced to a single set in the products, termed gametes - see Figure).  Hybrid is healthy and fertile, but less fit, or infertility appears in later generations (as witnessed in laboratory crosses of fruit flies, where the offspring of second-generation hybrids are weak and usually cannot produce viable offspring).  ** Post-zygotic mechanisms are those in which hybrid zygotes fail, develop abnormally, or cannot self-reproduce and establish viable populations in nature. ** So species remain distinct due to reproductive isolation. But how do species form in the first place? An abbreviated illustration of meiosis, by which reproductive cells duplicate to form gametes.

 Species Formation

This question is critical, because it is what produces many species from few, and results in evolutionary trees of relatedness. The most common way for species to split, especially in animal species (we will talk more about the origin of new plant species later), is when the population becomes geographically isolated into two populations. This is referred to as allopatric (geographic) speciation (see Figure).

One model of allopatric speciation. A single population (a) is fragmented by a barrier (b); geographical isolation leads to genetic divergence (c); when the barrier is removed, the two populations come back into contact with each other, and there is selection for increased reproductive isolation (d); if reproductive isolation is effective, speciation is complete (e).

Geographic isolation leads to reproductive isolation. Once two populations are reproductively isolated, they are free to follow different evolutionary paths. They are likely to differentiate for two reasons:

1. Different geographic regions are likely to have different selective pressures. Temperature, rainfall, predators and competitors are likely to differ between two areas 100's or 1,000's of kilometers apart. Thus, over time, the two populations will differentiate.
2. Even if the environments are not very different, the populations may differentiate because different mutations and genetic combinations occur by chance in each. Thus, selection will have different raw material to act upon in each population.

Differentiation also depends upon the strength of selective pressures. Strong selection can cause rapid change.

Given time and selection, the two populations become two species. They may, at some later time, spread back into contact. Then we can ask, are these two "good biological species"?

The real test of the biological species concept is when two populations, on the threshold of becoming two species, come back into contact. They may simply merge. They may be so different that they do not even recognize one another as species.

Often, though, species may come into contact when not yet fully reproductively isolated. In that event, natural selection should reinforce the reproductive barriers. Why? Because individuals that waste their reproductive effort -- their gametes -- on individuals with whom they will produce inferior offspring are less likely to pass on their genes to the next generation.

Natural selection should reinforce reproductive isolation. Probably, species that are isolated only by post-zygotic barriers will subsequently evolve pre-zygotic barriers. Why should that occur?

To review: allopatric (geographic) speciation is the differentiation of physically isolated populations to the point that reunion of the two populations does not occur if contact is re- established.
 

Speciation as a Gradual Process

If speciation is a gradual process, species may not yet be fully separated. A continuum must exist from species that are in the process of splitting into two, to species that are fully formed. Surely we only expect the latter to behave as "good species."

We still haven't fully explained the speciation process. In our next lesson, we will examine the theory of natural selection, which helps to explain how localized populations become adapted to local conditions. By adapting to local conditions and accumulating genetic differences, isolated geographic races start down the path to becoming separate species and creating another pair of branches on the tree of life.

But now I want to point out that there are alternative models of species formation, and finally I want to conclude by linking the concept of species formation to the hierarchical structure of life.

Alternative Models of Species Formation -- Hybridization and Polyploidy

Even if hybrids are unable to undergo sexual reproduction because their chromosomes do not sort out properly in meiosis, they may reproduce vegetatively. The total chromosome number also may double by combining the chromosome sets of a single species.

Of the 260,000 known species of plants, as many as half may have originated in this way. Many commercially important plants are examples of polyploidy (e.g. bread wheat, cotton, tobacco, sugar cane, bananas, potatoes). Polyploidy is an example of sympatric speciation defined as species arising within the same, overlapping geographic range.
 

Conclusion: Species Formation and the Hierarchy of Life

There are two ways to construct a phylogenetic tree (see Figure). We can use a "perfect" fossil record to trace the sequence from beginning to end, or we can use similarities and differences among living things to reconstruct history, working from the endpoint toward the beginning.

In this course, we will not consider these two methods in detail. I introduce them to make the point that, ultimately, we want to understand how evolution produces not just two species from one but the entire tree of life. This requires that we make the transition from microevolution to macroevolution. To Darwin, and to modern evolutionary biologists as well, the answer simply is time. Given enough time and successive splittings, the processes that produce two species from one will result in the entire diversity of life.

In reality, deducing the historic record of branching is very difficult. Data are incomplete, scientists debate the pace of change, and sometimes species separated by many branching steps look more similar to one another than those separated by one or a few branches. Molecular biology offers exciting new opportunities to address these issues, by looking at similarities and differences in DNA sequences.

From here we will turn away from the macroevolutionary view and look more closely at how small changes occur and accumulate, by the processes of natural selection and genetic change.

Summary

The definition of a species is debatable. Most scientists adhere either to the morphological species concept (members of a species look alike and can be distinguished from other species by their appearance), or to the biological species concept (a species is a group of actually or potentially interbreeding individuals who are reproductively isolated from other such groups). Both definitions have their weaknesses.

Reproductive isolating mechanisms are either prezygotic or postzygotic. These mechanisms ensure that species remain distinct in nature.

Species formation can occur either through allopatric (geographic) speciation or through sympatric speciation.

We can construct phylogenetic trees that show the evolutionary relatedness among living things, though the building of such trees is as yet an imperfect science.

Suggested Readings:

• Futuyma, D.J. 1986. Evolutionary Biology. Sunderland, Mass: Sinauer Associates, Inc.
• Wessells, N.K. and J.L. Hopson. 1988. Biology. New York: Random House. Chapter 43.
• Rosenzweig, M.L. 1995. Species Diversity in Space and Time. Cambridge: Cambridge University Press.

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