Natural Selection
and Speciation

Some Definitions

Evolution is the change in frequencies of the various alleles for some gene in a given population through time. These changes in allelic frequencies are brought about by the effects of natural selection and may lead, eventually, to speciation. Thus, we need to examine, “What is a species?” and “How does one species become two (or more) species?”

In 1942, Ernst Mayr defined a species as a group of actually or potentially interbreeding populations that are reproductively isolated from other such groups.

The term isolating mechanisms refers to any means by which the many, diverse species remain distinct. These may be morphological (the bodies don’t fit together), behavioral (day vs. night, spring vs. fall, courtship rituals), and/or geographical (separated by mountain, desert, lake, river, ocean, or other barrier) barriers. There are several different types of species, based on the mechanism(s) which keep those species isolated from other, similar species.

Types of Species

• Sympatric species are species which, because they occupy the same area at the same time, have the opportunity to interbreed if they could. However, due to other isolating mechanisms, they cannot and/or do not interbreed. Sympatric species are frequently subject to convergent evolution, which causes them to develop similar traits. For example, fish, whales (a mammal), penguins (a bird), sea turtles (a reptile), and diving beetles (an insect) all have similar, streamlined body shapes and oar-like appendages as adaptations for aquatic life. Convergent evolution also includes mimicry — it may be advantageous to look like someone else if that species is noxious and you aren’t.
  

  Koi	Penguin	Whirligig Beetle		Wasp Model	Beetle Mimic

• Allopatric species are species which, because they occupy areas separated by space (geographic isolation) or time, are reproductively isolated. It is often unknown whether, if brought together, they could actually interbreed. As an example, dogs and coyotes were originally allopatric species. However, when humans and their dogs moved into coyote-inhabited areas, it was discovered that dogs and coyotes are capable of interbreeding and producing viable offspring. By now, there are so many dog genes in the coyote population that there may no longer be any such thing as a 100% coyote.
• Parapatric species are species which occupy adjacent areas with a common border. Typically the species meet and come into contact with each other only along the border. Often, these may be closely-related species that must maintain reproductive isolation, and interestingly, members of the two species from the farthest portions of their respective ranges may look the most similar to each other, while along the border where the two species are more likely to come into contact, differences in appearance between the two species are more pronounced.

Variation within a Species

Within a species there is genetic variation due to mutations and crossing over in meiosis (as recognized by Darwin and Wallace). These variations may be grouped into several categories.

• A cline is a gradual geographical variation in some phenotypic character such as body size, color, etc. For example, in the common eastern North American Leopard Frog, individuals from Minnesota or Vermont probably would have a hard time coping with the heat on a typical Florida or Alabama summer day, while frogs of the same species from somewhere like Florida would probably not survive a Minnesota winter. Other organisms (for example, Luna Moths and American Robins) from the northern U. S. typically have one brood of young per year. By about here in Cincinnat or so, that may be up to about two broods (early spring and mid-summer), and down in Alabama, that may be up to three or even four broods per year.
• An ecotype is a specific genetic strain of a population that is specially adapted to some unique, local environmental factors.
• Polymorphism is the occurrence of several distinct forms or types of a species in the same habitat at the same time, and there are several, interesting sub-types that fall under polymorphism. In Central America, there are a number of closely-related species of butterflies in the genus Heliconius. Color patterns among these species are quite complicated: in any given area, there may be several species that are mimicking each other, and in a different part of their ranges, they’re still all mimicking each other, but based on a totally different color pattern. Many species of animals exhibit sexual dimorphism, in which the males and females may be told apart by various sexually-specific phenotypic characteristics. There are some species of butterfly in which the males and females look so different, that for years, entomologists did not know they were actually the same species. Industrial melanism in the English Peppered Moth (see below) is an example of balanced polymorphism, where an equilibrium between the normal, speckled and the melanistic (black) phenotypes has been reached, and these two forms occur together throughout the moth’s range.
  

  Male Downy Woodpecker	Female Downy Woodpecker

Because of this variation, some individuals area better suited to cope with their environments than others, or in other words, the environment provides selective pressure on the species/population. Two false assumptions students typically make are worth noting:

• Contrary to what some students think, organisms and/or populations or species neither cannot nor do not willingly decide to change to better match with environmental conditions. Lamarckian evolution was disproved long ago.
• Contrary to what some students think, other than a handful of chemicals known to be mutagenic agents, in general the environment does not cause changes in the individual organisms to make them better suited to itself.

Rather, as stated by Darwin and Wallace, random mutations just happen, and some make the organism better or worse able to get along, given existing environmental conditions.

Natural Selection

Natural selection is the differential reproductive success of individual organisms (given existing environmental conditions), or in other words, the ability of individuals to

1. survive, and
2. leave the most reproducing offspring.

The relationship between sickle-cell anemia and malaria is an interesting example of natural selection. Sickle-cell is caused by a mutation in one nucleotide in the sequence which codes for hemoglobin, resulting in one different amino acid in the sequence, and thereby causing the person’s RBCs to “sickle” under stress (which leads to other problems). Sickle-cell is a “recessive” allele, but unlike other recessive alleles, it does not code for “I don’t know how to make this,” but rather “I’m going to make this, but make it wrong.” Thus, an individual who is SS will make all normal hemoglobin, a person who is Ss has two sets of instructions and will make both normal and sickle-cell hemoglobin, and a person who is ss will make only abnormal, sickle-cell hemoglobin. The rather pointed, jagged, sickled RBCs of a person with sickle-cell tend to get caught in capillaries and trigger blood clots. Thus, many people who are ss (sickle-cell disease) die from complications of the sickle-cell (natural selection against homozygous recessive individuals).

However, malaria is also present in the part of the world where the sickle-cell allele arose, and as it turns out, sickle-cell confers resistance against the malaria parasite. When a malaria parasite invades a normal RBC, it then goes on to reproduce and invade other RBCs, but when a malarial parasite invades a RBC containing abnormal, sickle-cell hemoglobin, the cell is put under so much stress by the presence of the parasite, that the cell sickles (crinkles up), killing the parasite in the process. Thus, normal SS people often die due to causes associated with the malaria (natural selection against the homozygous dominant). Because Ss (sickle-cell trait) individuals have some normal hemoglobin, it takes a higher level of stress on their systems to cause their RBCs to sickle. Also, they are somewhat resistant to malaria (RBCs containing abnormal hemoglobin will sickle if attacked, but RBCs with regular hemoglobin won’t). Thus, most of the people in those populations are heterozygotes, and in that environment, end up being more fit than either of the homozygotes.

According to various sources (including the Merck Manual), in some areas of Africa 64% of the people are SS, 32% are Ss, and 4% are ss. From this, it is possible to calculate that the selective pressure against ss is 4 × the selective pressure against SS (4 × more people die from sickle-cell than from malaria). However, among African Americans, 90.25% of the population is SS, 9.50% is Ss, and 0.25% is ss. From this, it is possible to calculate that the selective pressure against ss is 19 × the selective pressure against SS (19 × more people die from sickle-cell than from malaria). Comparatively more people here die from sickle-cell vs. from malaria than in Africa (fewer people here die from malaria).

In discussing natural selection, a textbook we were using several years ago says that, “Those individuals that contribute the most to the gene pool are said to be the most fit, and those that contribute little or nothing to the gene pool are said to be the least fit. The fitness of an individual is measured by its reproducing offspring. . .” In general, while dealing with “wild” organisms this seems to make sense, but an interesting rhetorical question or paradox arises if this is applied to humans. Ironically, we criticize rapidly-growing “poor” or “Third-World” populations, virtually telling them that if they were as smart, as well off, or whatever, as us (read: as fit as) that they should have less offspring like we do. Here in “Western Civilization,” despite lower rates of diseases, more food, better medical care, etc., the reproduction rate in some countries is not even high enough to maintain current population levels, and population sizes are actually declining. Here, then, is something to consider: just who is the most fit?

Reference: Smith, Robert Leo  1992.  Elements of Ecology, 3rd Ed.  HarperCollins Publ., NY.  p. 18.

Peppered Moths

One of the best documented examples of natural selection in modern times is the English Peppered Moth, Biston betularia. Typically, this moth is whitish with black speckles and spots all over its wings. During the daytime, Peppered Moths are well-camouflaged as they rest on the speckled lichens on tree trunks. Occasionally a very few moths have a genetic mutation which causes them to be all black, so they are said to be melanistic. Black moths resting on light-colored, speckled lichens are not very well camouflaged, and so are easy prey for any moth-eating birds that happen by. Thus, these melanistic moths never get to reproduce and pass on their genes for black color. However, an interesting thing happened to these moths in the 1800s. With the Industrial Revolution, many factories and homes in British cities started burning coal, both for heat and to power all those newly-invented machines. Coal does not burn cleanly, and creates a lot of black soot and pollution. Since lichens are extremely sensitive to air pollution, this caused all the lichens on city trees to die. Also, as the soot settled out everywhere, this turned the tree trunks (and everything else) black. This enabled the occasional black moths to be well-camouflaged so they could live long enough to reproduce, while the “normal” speckled moths were gobbled up. Studies done in the earlier 1900s showed that while in the country, the speckled moths were still the predominant form, in the cities, they were almost non-existant. Nearly all the moths in the cities were the black form. It was evident to the researchers studying these moths that the black city moths were breeding primarily with other black city moths while speckled country moths were breeding primarily with other speckled country moths. Because of this, any new genetic mutations in one or the other of those populations would only be passed on within that population and not throughout the whole moth population. Additionally, because the city and country environments were different, there were different selective pressures on city vs. country moths that could potentially drive the evolution of these two populations of moths in different directions. The researchers pointed out that if this were to continue for a long enough time, the city and country moths could become so genetically different that they could no longer interbreed with each other, and thus would be considered distinct species. In this case, what actually happened is that the people of England decided they didn’t like breathing and living in all that coal pollution, thus found ways to clean things up. As the air became cleaner, lichens started growing on city trees again, thus the direction of the selective pressure (birds) was once again in favor of the speckled moths. By now, English cities, as well as countrysides, all have speckled moths, and all those moths are interbreeding at random, thus they were not separated for long enough to develop into separate species.

Dr. H. B. D. Kettlewell, the main researcher who worked on Peppered Moths discovered the following:

• Industrial melanism is characterized by rapid spread, increased genetic fitness, and occurrence among moths that depend on camouflage against a background of lichens, etc. for survival.
• In most cases, melanism is inherited as a simple Mendelian dominant allele.
• Work by earlier researchers indicated that melanistic forms of some species of moths went from less than 1% to around 95% of some urban populations within a period of 50 years, indicating around a 30% selective advantage over the light forms of those species.
• This 30% selective advantage was a very unusual record high for wild populations of organisms.
• By the time Kettlewell was doing his research, unpolluted areas of Scotland and England had almost 100% of the light, speckled form of the Peppered Moths, while urban areas and areas subject to drifting air pollution had 80% or more of the melanistic form.
• Typically, under polluted conditions, the period from the initial black mutation(s) until 1% of the population is melanistic is relatively long. Once the 1% level is reached, the percentage of the melanistic allele in the population increases rapidly, until it reaches about 80 to 95%, at which point the increase in frequency slows.
• Thus, with an assumed mutation rate of 1 in 1,000,000, a graph of numbers of melanistic moths in a population vs. time will typically yield an “s”-shaped (sigmoid) curve.
• Even in areas where the melanistic form of the Peppered Moth has made up 95 to 99% of the population for over 60 years, the speckled form has never totally disappeared, resulting in a balanced polymorphism.
• Industrial melanism is an indirect result of air pollution because the air pollution kills the lichens on the tree trunks and turns the trunks black. The soil around these trees is also affected (acid rain), and even the caterpillars of these moths must cope with eating leaves that are covered with “fallout” and/or which contain chemicals absorbed into the plant.
• Bird predators on Peppered Moths include an European relative of chickadees and titmice called the Great Tit, a species of flycatcher, a nuthatch, the European Robin, and several other species of birds.
• Kettlewell and N. Tinbergen working together showed that, in one non-polluted woodland, 164 speckled to 26 melanistic Peppered Moths were eaten by birds, while in a polluted woodland, 43 speckled and only 15 melanistic moths were eaten.
• Kettlewell also conducted mark-and-release studies. In a polluted woodland, out of 137 speckled and 447 black Peppered Moths released, 27.5% of the black ones and only 13% of the speckled ones were recaptured. Keep in mind that if there is no selective pressure against either form, the null hypothesis would predict capture of equal percentages of the two forms released.
• In an unpolluted woodland, out of 473 black and 496 speckled Peppered Moths released, 12.5% of the speckled were recaptured, as compared to only 6.3% of the black.
• From these results, Kettlewell concluded that there was a definite “cryptic advantage” of the speckled moths against lichens and of the black moths against blackened tree trunks.
• Kettlewell also conducted experiments which showed that the moths themselves can and do distinguish between a white and a black background and in most cases correctly choose a perching site that matches their coloration.

Reference: Kettlewell, H. B. D.  1961.  The phenomenon of industrial melanism in Lepidoptera.  Ann. Rev. of Entomol.  6: 245 - 262.

However, an article in the 24 May 1999 issue of The Scientist 13(11) presents and discusses data which may refute this long-held idea.

Try It Out:

An organism’s alleles are subjected to natural selection. Some of the alleles code for certain body features or behaviors which are better suited to the environment than others. Organisms with those alleles are more likely to reproduce and pass on those alleles. Other alleles are not as well suited, and an organism with that genotype dies or is not able to reproduce, so the alleles never get passed on. Over time, this leads to speciation, the division of one species into two or more species.

Natural selection may, then, lead to cladogenesis, which is the splitting of one species into two or more species, in other words, speciation. There are several theories on exactly how this happens. Some biologists feel that the most common means is gradual speciation, in which allopatric or geographically isolated populations of an organism, each subject to unique selective pressures (climate, etc.), gradually become so genetically different that they can no longer interbreed, even if the geographical barrier is subsequently removed. This long-held belief is based on the works of Darwin, Wallace, and others who said that species are always slowly evolving. In contrast, abrupt speciation as part of punctated equilibrium is an “instantaneous” speciation (taking thousands of years rather than tens of thousands or millions), followed by long periods of species stability (with relatively little change to the gene pool). This fairly-recent and somewhat controversial theory, proposed by paleontologists to explain holes in the fossil record, postulates that new species may arise “spontaneously” (for example, via polyploidy), and then are fairly stable.

One important thing to note here is that populations, not individuals, evolve: individuals can have genetic mutations, but unless those mutations are passed along to offspring and thus into the gene pool of the population, no real change has occurred. An individual who has a mutation but does not pass it on (for whatever reason) has no long term effect on the genes present in the population as a whole. Another important point is that natural selection operates on the genetics of a species, including whatever genes affect instinctive behavior patterns as well as those affecting the looks of the individuals. Thus, genetics, looks, behavior, and external environmental factors are all interwoven. Sometimes, there are tradeoffs between various genes and selective pressures. For example, the bright red color of a male Cardinal might be very attractive to a female, such that the brighter a male is, the greater his chances of mating, but that same bright red color, especially against a snowy background, makes him much more visible to predators.

Species of organisms may spread out into nearby, available habitats (and if the selective pressure is different there, may eventually give rise to another species). Adaptive radiation is the process whereby a species spreads into and makes use of a new environment. In order for this to occur, several conditions must be met:

1. The organisms must be able to get there — there must be physical access to the new habitat.
2. There must be empty/open niche(s) to be filled.
3. The new arrivals must be able to fill/exploit those niches.

Probably one of the most famous and best studied examples of adaptive radiation is Darwin’s finches. It is thought that these 14 species of birds on the Galapagos Islands arose from a few birds of one species which originally came from the Ecuadorian mainland and eventually adapted to a variety of diets and habitats. Another similar example involves a number of species of fruit flies on the Hawaiian Islands.