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Biology

Chapter 14: Origin of Species

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Lecture

The Origin of Species

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Where do New Species come from?

Before we can talk about species in any detailed way, we need to make sure we have a good understanding of the term. Even among biologists, the term is used differently in different contexts, which can lead to confusion.

Species

The definition of species is a much-debated topic. The word species comes from the Latin for appearance and is commonly used to indicate a category of organisms with similar features. In the mid-1700s, Carolus Linnaeus gave the term a more particular meaning when he used it for part of his naming convention: a species is a subgroup of a genus, or kind, and together the genus and species names uniquely identify a given type of organism.

However, the advent of evolution theory and the mechanisms proposed for natural selection and mutations to new organisms have modified the definition to include considerations of inheritance and changes of allele frequencies in the population. Many biologists define a biological species as a population or group of populations whose members have the potential in nature to interbreed and produce fertile offspring (from the glossary of your text). Close inspection of the practical application of this definition raises certain problems.

These and similar problems should help us realize that the biological species concept definition is somewhat artificial, and depends to a great deal on what we are willing to consider natural mating patterns.

Some biologists use other concepts for "species":

As with any classification system, grouping organisms by different definitions of species is useful at different times, or for different purposes. However, the multiple conflicting definitions of "species" can muddy the waters in any debate about origins of species, whether among creationists and evolutionists, or even among scientists disposed to favor an evolutionary history of life on Earth.

Reproductive Barriers

Individuals in a population are considered members of a unique species if they cannot interbreed with individual organisms outside the population, or if the product of such a mating is infertile. This means there are two kinds of mechanisms possible: those that prevent mating and occur before the formation of a zygote from two gamete cells. These prezygotic barriers include:

Type of isolation which means Examples
temporal The individuals of the two species mate at different times of the day, season, or year. Two different species of fruit flies can inhabit the same geographic location, but drosophila pseudobscura mates in the afternoon, while drosophila persimilis mates in the morning, so the two never interbreed.
habitat The individuals of the two species don't live in the same area. Geographic isolation is discussed in more detail below.
behavioral The mating rituals of the two species are so different that the individuals of one species are never attracted by the mating antics of the other species. Many birds have very specific mating rituals, including dances and songs. A female of one species may not recognize the mating ritual of a male of a different species for what it is.
mechanical The structural differences in the organisms prevent a successful mating. The flowers of some closely related plants are of different sizes; for example, black sage must be pollinated by smaller bees than those which can enter the larger flowers of white sage.
gametic The gametes fail to unite because the sperm or ova can't survive long enough to form a zygote in the environment in which the mating takes place. Most cells have surface proteins (HLAs) which identify the cell to its own body and also to cells of organisms of the same species. These surface proteins on an ovum may reject sperm from a different species.

Suppose two individuals manage to mate. The second criteria for identification of a species, production of fertile offspring, now becomes the crucial point for determine whether we are talking about two individuals of the same species, or two individuals of different species. There are three types of such postzygotic barriers:

Type of isolation which means Examples
Hybrid inviability The hybrid offspring dies before birth or soon after. Crosses between bullfrogs and leopard frogs die as embryos, and crosses between different kinds of irises die before reaching reproductive maturity.
Hybrid sterility The hybrid can't produce offspring of its own because its chromosome number is abnormal and doesn't pair properly during meiosis. Mules (the children of a female horse and male donkey) have a chromosome number (63) which is different from both their mother (64) and their father (62).
Hybrid breakdown The first generation hybrid can reproduce, but the subsequent generations become sterile, usually be the fifth generation. This usually occurs in plants, such as hybrids between two sunflower species. The first generation of hybrids is healthy, but the second generation is defective.

If any one of the eight conditions listed above exists for two individual organisms, we define the organisms as members of different species, regardless of genetic similarities, general appearance, or the ability to breed fertile offspring in captivity.

Geographic barriers

It is within the context of geographic isolation that most speciation (creation of new species) is thought to occur. A change in environment which isolates individuals of a population into two or more separate areas without a chance to continue to intermingle gives rise to allopatric speciation. As time passes, the allele frequencies exhibited by separated groups diverge. If one population had a significant trait which was present trivially or not at all in the second population, then the allele frequencies may be radically different, even after a short time.

The standard example is the pupfish of the Death Valley region in California, which biologists and geologists think was once covered by a single glacial lake about 10000 years ago. As the lake dried up, it left behind individual small lakes, each with its own pupfish population. Over time, the allele frequencies of specific traits in the pupfish have changed, partly by natural selection, until there are marked differences between the pupfish of different lakes or pools. This process is called allopatric speciation, or speciation by removal from the "fatherland". The populations change because they are each subjected to a different set of environmental factors and adapt through natural selection differently.

Note that this example does not depend on mutations, nor do biologists claim that the current pupfish populations have any features that were not evident in the original population of pupfish. It may be possible to get pupfish from different pools to successfully mate if humans breed them in captivity.

Biologists extrapolate from such examples, however, to speculate that over a long enough period of time, genetic drift and mutations will change each individual population to the point where one of the other prezygotic or postzygotic barriers prevents interbreeding. In the 15th century, some European rabbits were introduced to the island of Porto Santo, off Portugal. Over the next 400 years, the Porto Santo rabbits developed different traits from the original European stock: they are only about half as large, they tend to be more active at night, and they have different color patterns. All of this could be attributed to genetic drift, where alleles are lost but no new genetic material is introduced. However, when they were mated (in captivity) with the current European rabbits, they were unable to produce any offspring. For some reason, the Porto Santo rabbits can no longer interbreed with the European rabbits, and so according to the definition of biological species, they are a new species.

Polyploidy

Sometimes species -- especially plant species -- can diverge while sharing the same geographic region. Normally, when gametes form the chromosome number is halved as part of meiosis. However, sometimes the chromosome number doubles before meiosis, and the resulting gametes contain two pair of a particular chromosome. The possession of two sets of a given chromosome is polyploidy--note that this is a different phenomenon from trisomy, which results three chromosomes, but not four.

As mentioned in a previous lecture, polyploidy is usually fatal in animals, but in plants is occasionally beneficial. A successful polyploid offspring cannot be crossed with plants of the parent generation, since it has a different number of chromosomes than either parent type. However, the polyploid may be able to self-pollinate and thus perpetuate itself with subsequent generations of offspring whose individuals are fertile and can be mated to each other--in which case it meets the criteria of isolation and fertility for a new species.

If the new species has traits which fit it for a different niche in the same habitat as the parent, then both can coexist. This is sympatric speciation (in the same "fatherland"). However, if it isn't different enough from the parent, it will compete, and one may eventually be wiped out by the better adaptation of the other to the current environment.

Sympatric speciation has been observed in a number of plant species, both in nature (hemp nettle and bread wheat) and by experimental reproduction of the process in the lab (with hemp nettle). Most of our current food sources are thought to be the result of polyploidy genetic changes which resulted in species that were large, better adapted to their environments, and more resistant to disease than the original native plants.

Punctuated vs. Gradual Evolution

One of the major goals of evolution theory is to account for the observations in the fossil record, where there are often large gaps between the appearance of an organism with one set of traits and the appearance of a similar organism with enough differences to be considered a new species. Two mechanisms have been proposed to account for the record; for awhile, the theories were in competition, but now most biologists seem comfortable with a kind of coexistence, since neither mechanism actually excludes the possible of the other operating at least some of the time. Neither theory accounts for all the observations, however.

Gradualism is the older theory associated with Darwin. According to this theory, evolutionary changes occur at a statistically constant rate over long periods of time. Since most organisms decompose as they die (fossil formation is the exception, rather than the rule), they leave behind no record of their existence. Long periods of no change in the fossil record are interpreted as the result of stabilizing selection, where natural selection tends to weed out the extremes.

However, some parts of the fossil record seem to indicate that there were periods of rapid evolution (where rapid is the emergence of new major traits within 10000 to 50000 years). There are few transitional fossils, because there are few transition generations to produce them. The overall pattern of this model, punctuated equilibrium, was originally proposed as periods of no change (stasis) interrupted by these periods of rapid change. Most biologists now combine the two and assume that the no-change periods of punctuated equilibrium are really slow-change periods of gradualism, and that occasional extreme conditions (such as cataclysmic earthquakes, collisions with comets, or other catastrophes) give rise to periods of rapid change.

We have covered most of the mechanisms which make up the model of evolution. However, we do need to recognize the role of extinction of a species as part of evolution. Since we have defined evolution to be any change to the gene pool (that is, a change in the frequency of alleles for one or more genes), the disappearance of an entire species with all the alleles and genes unique to that species is definitely an evolutionary event. In the last century, we have recorded the disappearance of several species of birds; there are many environmentalists anxiously watching the decreasing populations of frogs and toads in many regions of the world.

Since the mutations thought to create new genetic sequences are also assumed to be rare events, the disappearance of a single genetic sequence is probably permanent. Assuming that the original mutation was beneficial and preserved, the probability that the same mutation could occur again to an organism in the same genetic state as the original mutation is very small. This has serious ramifications where the genes involved provide disease resistance or a trait on which many different organisms depend. The same considerations which make us leary of using genetic engineering indiscriminately to change the gene pool also figure in our decisions to change the environment in ways which may wipe out even "minor insect" species.