Lecture 13: Microevolution: Population genetics and speciation (Major Concepts)

Summary of Chapter 14

* From a genetic perspective, a population is identified by the particular collection of alleles in its gene pool. * Genetic diversity, as reflected by the number of different alleles in a population's gene pool, is important for the continued survival of populations, especially in the face of changing environments. * Evolution is a change in allele frequencies in a population over time. Evolution can be adaptive or nonadaptive. Natural selection is an adaptive form of evolution. Mutation, genetic drift, and gene flow are nonadaptive forms of evolution. * The founder effect is a type of genetic drift in which a small number of individuals establishes a new population in a new location, with reduced genetic diversity as a likely result. * The bottleneck effect is a type of genetic drift that occurs when the size of a population is reduced, often by a natural disaster, and the genetic diversity of the remaining population is reduced. * Inbreeding of closely related individuals may occur in small, isolated populations, posing a threat to the health of a species. * Gene flow is the movement of alleles between different populations of the same species, often resulting in increased genetic diversity of a population. * Genetic diversity can be assessed by using DNA sequences to determine allele frequency. * The Hardy-Weinberg principle describes the frequency of alleles and genotypes in a nonevolving population. It can be used to detect evolutionary change in a population, and to calculate allele frequencies when at least one genotype frequency is known. * According to the biological species concept, a species is a population of individuals that can interbreed to produce fertile offspring. * Speciation can occur when gene pools are separated, gene flow is restricted, and populations diverge genetically over time.

A population of aliens with similar genetics as humans is affected by a homozygous recessive condition known as "xbziq." This condition causes aliens to die at young ages because their digestive tracts are not able to effectively process food and obtain the necessary nutrients and energy from their diets. Approximately 2% of the newborn aliens have the xbziq condition. What is the carrier (heterozygote) frequency in the population?

24% * The frequency of homozygous recessives (q2) is 2% (0.02). Therefore, the frequency of q is the square root of 0.02 = .1414. P = 1 - q = 0.86. The carrier frequency is 2pq = 24.1%

Polyploidy

A chromosomal alteration in which the organism possesses more than two complete chromosome sets.

Nonadaptive evolution

Any change in allele frequency that does not by itself lead a population to become more adapted to its environment; the mechanisms of nonadaptive evolution are mutation, genetic drift, and gene flow. Nonadaptive evolution isn't necessarily "bad," or maladaptive. If mutations didn't introduce variation into a population, there would be no evolution at all. And many nonadaptive changes in allele frequency can be considered "neutral"—neither "good" nor "bad." But nonadaptive evolution can greatly influence the fate of a species, and so researchers are keen to study it.

Hybridization

Breeding technique that involves crossing dissimilar individuals to bring together the best traits of both organisms

Genetic drift (founder effect, bottleneck)

Genetic Drift: Random changes in the allele frequencies of a population between generations; genetic drift tends to have more dramatic effects in smaller populations than in larger ones. * "Genetic drift is a bit like rolling the evolutionary dice. By simple chance, some individuals survive and reproduce, and others do not." Those that pass on their genes aren't necessarily more fit or better adapted; they're just lucky—perhaps their nest or burrow wasn't swept away in a flash flood, for example. * Over time, genetic drift tends to decrease the genetic diversity of a population, as some alleles are lost completely and others sweep to 100% frequency. Genetic drift will have more dramatic effects in smaller populations than in larger ones. In a population with few individuals, any single individual that does not reproduce could spell the loss of alleles from the population. But all populations experience some measure of genetic drift, since chance is a fact of life. Founder Effect: A type of genetic drift in which a small number of individuals leaves one population and establishes a new population, resulting in lower genetic diversity than in the original population. * Biologists refer to two general types of genetic drift: founder effects and bottlenecks. A founder effect occurs when a small group of settlers ("founders") splits off from a main population and establishes a new one. Because a founding population is by definition small, there is a good chance that the particular alleles it carries will not be fully representative of the population it left. Thus, founder effects tend to reduce the genetic diversity of the new population. * If the founder population happens to contain a rare allele, then this allele may become much more common in the new population. Polydactyly (having extra fingers or toes) is an unusually common trait in the Amish population of eastern Pennsylvania, the result of Ellis-van Creveld syndrome, a recessive inherited condition. The Amish are a rural, isolated population stemming from a small number of German immigrants who moved to the area in the 18th century. The allele for Ellis-van Creveld syndrome arrived with a single couple who immigrated to the area in 1744 and has since spread throughout the population. Bottleneck: A type of genetic drift that occurs when a population is suddenly reduced to a small number of individuals, and as a result alleles are lost from the population. * When a population is cut down sharply—forced through a "bottleneck"—there's a good chance that the remaining population will possess a less-diverse gene pool. Bottlenecks can occur from natural causes—say, a flood that sweeps through the city, killing many individuals—or from human interference, such as the clearing of a forest. Either way, a population that is forced through a genetic bottleneck usually contains a fraction of the original diversity in the population (INFOGRAPHIC 14.3).

Inbreeding and Inbreeding depression

Mating between closely related individuals. Inbreeding does not change the allele frequency within a population, but it does increase the proportion of homozygous individuals to heterozygotes. * One reason gene flow is important is that small, isolated populations can be damaged by lack of genetic diversity. Take the Florida panther (Puma concolor), for example. In the past, Florida panthers mated with puma populations from neighboring states where their ranges overlapped. This interbreeding—breeding among different populations of the same species—fostered an exchange of alleles that continually enriched the local populations' genetic diversity. By the mid-20th century, however, hunting and development had squeezed the Florida panther population into an isolated region at the state's southernmost tip. By 1967, only 30 panthers remained, and the U.S. Fish and Wildlife Service listed them as endangered. By 1980, the panthers showed unmistakable signs of ill health—birth defects, low sperm count, missing testes, and bent tails—that resulted from inbreeding, mating between closely related members of a population. The negative reproductive consequences for a population associated with having a high frequency of homozygous individuals possessing harmful recessive alleles. * Inbreeding can have dangerous consequences for a population. Because closely related individuals are more likely to share the same alleles, the chance of two recessive harmful alleles coming together during mating is high. When that happens, homozygous recessive genotypes are created, and previously hidden recessive alleles start to affect phenotypes in negative ways. This effect is called inbreeding depression.

Types of Prezygotic Isolation Mechanisms:

Mechanical Isolation Mechanical isolation is probably the simplest concept that keeps individuals from being able to reproduce offspring with each other. Simply put, mechanical isolation is the incompatibility of sexual organs. They just do not fit together. It may be the shape of the reproductive organs not being compatible, or size differences that prohibit the individuals from coming together. In plants, mechanical isolation is a bit different. Since size and shape are irrelevant to reproduction in plants, mechanical isolation is usually due to the use of a different pollinator for the plants. For instance, a plant that is structured so a bee can pollinate it will not be compatible with a flower that relies on hummingbirds to spread its pollen. This is still a result of differing shapes, but not the shape of the actual gametes. Instead, it's the incompatibility of the shape of the flower and the pollinator. Temporal Isolation Different species tend to have different breeding seasons. The timing of when females are fertile leads to temporal isolation. Similar species may be physically compatible, but may still not reproduce due to mating seasons being different times of the year. If the females of one species are fertile during a given month, but the males are not able to reproduce at that time of the year, then there will be reproductive isolation between the two species. Sometimes, mating seasons of very similar species will overlap somewhat. This is especially true if the species live in different areas where there is no chance for hybridization. However, it has been shown that similar species that live in the same area will not have an overlapping mating time even if they do when they are in different environments. Most likely, this is an adaptation caused by reducing competition for resources and mates. Behavioral Isolation Another type of prezygotic isolation between species has to do with the behaviors of the individuals, and, in particular, the behaviors around mating time. Even if two populations of different species are both mechanically compatible and temporally compatible, their actual mating ritual behavior could be enough to keep the species in reproductive isolation from each other. Mating rituals, along with other necessary mating behaviors like mating calls, are very necessary for males and females of the same species to indicate it is time to sexually reproduce. If the mating ritual is rejected or not recognized, then not mating will occur and the species are reproductively isolated from each other. For instance, the blue-footed booby bird has a very elaborate mating "dance" the males must do to woo the female. The female can either then accept or reject the advances of the male. Other species of birds do not have the same mating dance and will be fully ignored by the female, meaning they have no chance at reproducing with a female blue-footed booby. Habitat Isolation Even very closely related species have a preference of where they live and where they reproduce. Sometimes, the preferred locations of the reproductive events are not compatible and this leads to what is known as habitat isolation. Obviously, if individuals of two different species live nowhere near each other, there will be no opportunity to reproduce and reproductive isolation will lead to even more speciation. However, even different species that live in the same area may not be compatible due to their preferred place of reproduction. There are some types of birds that prefer different types of trees, or even different parts of the same tree, to lay their eggs and make their nests. If similar species of birds are in the area, they will choose a different location and they will not interbreed. This keeps the species separate and unable to reproduce with each other.​ Gametic Isolation During sexual reproduction, the female egg is fused with the male sperm and, together, they create a zygote. If the sperm and egg are not compatible, this fertilization cannot occur and the zygote will not form. The sperm may not even be attracted to the egg due to the chemical signals released by the egg. Other times, the sperm just cannot penetrate the egg because of its own chemical make-up. Either one of these reasons is sufficient enough to keep fusion from happening and the zygote will not form. This type of reproductive isolation is especially important for species that reproduce externally in the water. For instance, most species of fish have females that will just release her eggs into the water. Male fish of that species will come along and release their sperm all over the eggs. However, since this happens in the water, some of the sperm will get carried away by the water molecules and moved around the area. If there were not gametic isolation mechanisms in place, any sperm would be able to fuse with any egg and there would be hybrids of just about everything floating around. Gametic isolation ensures that only sperm of the same species can penetrate the egg of that species and no others.

Reproductive isolation (Prezygotic vs. postzygotic)

Mechanisms that prevent mating (and therefore gene flow) between members of different species. * Members of different species cannot mate and produce fertile offspring with each other because their populations are reproductively isolated. Such reproductive isolation can be caused by a number of factors. For example, the two species may have different mating times, locations, or mating rituals—so, like ships passing in the night, they may never meet. This is true of many ant species, for example, which breed at different times of year. Or, two species may be able to mate—as zebras and horses can—but the hybrid offspring they produce is infertile (INFOGRAPHIC 14.6).

Postzygotic Isolation

Speciation is the divergence of two or more lineages from a common ancestor. In order for speciation to occur, there must be some sort of reproductive isolation that occurs between formerly reproducing members of the original ancestor species. While most of these reproductive isolations are prezygotic isolations, there are still some types of postzygotic isolation that leads to making sure the newly made species stay separate and do not converge back together. Before the postzygotic isolation can happen, there must be an offspring born from a male and female of two different species. This means there was no prezygotic isolations, like the fitting together of the sex organs or incompatibility of the gametes or differences in mating rituals or locations, that kept the species in reproductive isolation. Once the sperm and the egg fuse during fertilization in ​sexual reproduction, a diploid zygote is produced. The zygote then goes on to develop into the offspring that is born and hopefully will then become a viable adult. However, offspring of two different species (known as a "hybrid") are not always viable. Sometimes they will self-abort before being born. Other times, they will be sickly or weak as they develop. Even if they make it to adulthood, a hybrid will most likely be unable to produce its own offspring and therefore reinforce the concept that the two species are more suited to their environments as separate species as natural selection works on the hybrids. Below are the different types of postzygotic isolation mechanisms that reinforce the idea that the two species that created the hybrid are better off as separate species and should continue with evolution on their own paths. The Zygote is not Viable Even if the sperm and the egg from the two separate species are able to fuse during fertilization, that does not mean the zygote will survive. The incompatibilities of the gametes may be a product of the number of chromosomes each species has or how those gametes are formed during meiosis. A hybrid of two species that do not have compatible chromosomes in either shape, size, or number will often self-abort or not make it to full term. If the hybrid does manage to make it to birth, it often has at least one, and more likely multiple defects that keep it from becoming a healthy, functioning adult that is able to reproduce and pass down its genes to the next generation. Natural selection ensures that only the individuals with the favorable adaptations survive long enough to reproduce. Therefore, if the hybrid form is not strong enough to survive long enough to reproduce, it reinforces the idea that the two species should stay separate. Adults of the Hybrid Species are not Viable If the hybrid is able to survive through the zygote and early life stages, it will become an adult. However, it does not mean that it will thrive once it reaches adulthood. Hybrids are often not suited for their environment the way a pure species would be. They may have trouble competing for resources such as food and shelter. Without the basic necessities of sustaining life, the adult would not be viable in its environment. Once again, this puts the hybrid at a distinct disadvantage evolution wise and natural selection steps in to correct the situation. Individuals that are not viable and not desirable will most likely not reproduce and pass down its genes to its offspring. This, again, reinforces the idea of speciation and keeping the lineages on the tree of life going in different directions. Adults of the Hybrid Species are not Fertile Even though hybrids are not prevalent for all species in nature, there are many hybrids out there that were viable zygotes and even viable adults. However, most animal hybrids are sterile at adulthood. Many of these hybrids have chromosome incompatibilities that make them sterile. So even though they survived development and are strong enough to make it to adulthood, they are not able to reproduce and pass down their genes to the next generation. Since, in nature, "fitness" is determined by the number of offspring an individual leaves behind and the genes are passed on, hybrids are usually considered "unfit" since they cannot pass down their genes. Most types of hybrids can only be made by the mating of two different species instead of two hybrids producing their own offspring of their species. For instance, a mule is a hybrid of a donkey and a horse. However, mules are sterile and cannot produce offspring so the only way to make more mules is to mate more donkeys and horses. * Hybrid inviability. Hybrid embryo forms, but of reduced viability. * Hybrid sterility. Hybrid is viable, but resulting adult is sterile. * Hybrid breakdown. First generation (F1) hybrids are viable and fertile, but further hybrid generations (F2 and backcrosses) may be inviable or sterile.

Observed vs. expected allele frequency

The Hardy-Weinberg principle can help researchers figure out, say, if genetic drift or natural selection is operating in a given population. Let's say biologists obtain samples of DNA from a random sampling of mice in a population and they look at the frequencies of genotypes at 10 different regions of DNA. Nine of those regions have genotype frequencies predicted by the Hardy-Weinberg principle, but one does not—it is far from Hardy-Weinberg equilibrium. Researchers then know that something interesting is happening at that one DNA location—some force of evolution is acting. In fact, this is how Munshi-South and his colleagues identified the candidate genes to compare between city and country mice. "You can use certain deviations from Hardy-Weinberg equilibrium to find parts of the genome that are under selection," he says. "So, if they strongly deviate from Hardy-Weinberg, whereas the rest of the genome roughly fits it, those outliers are likely to have something interesting going on, like natural selection." By understanding how city life has changed mice genetically, researchers will have a better understanding of how human activity is influencing mice evolution. That might not sound like a hugely important goal, especially if you're not a fan of mice. But there are larger lessons to take away. According to Munshi-South, "Manhattan offers a preview of what human activity will do to many other species in the coming years."

Biological Species Concept

The definition of a species as a population whose members can interbreed to produce fertile offspring.

Speciation

The genetic divergence of populations, leading over time to reproductive isolation and the formation of new species. * Reproductive isolation explains why species remain separate—as do the variety of ant species that share a median strip—but how did the species form in the first place? New species form when a strong barrier to gene flow occurs between populations. That barrier could be physical—like a road or river that divides a forest in two—or climatic, like the different temperatures that occur at different elevations on a mountainside. Once this barrier forms, the separated gene pools will evolve independently by the mechanisms we have already encountered: mutation, genetic drift, and natural selection. Eventually, if enough genetic changes accumulate between populations of the same species to make them reproductively isolated, the two populations may diverge into separate species, a process called speciation. * Speciation is happening all the time in nature, but it can be hard to see because it occurs so slowly. It generally takes many thousands of years for species to diverge. We see the results of speciation whenever we look at the diversity of nature—there are more than 12,000 known ant species, for example—but observing speciation as it happens is much harder.

Gene flow

The movement of alleles from one population to another, which may increase the genetic diversity of a population. * Once a population has lost genetic diversity because of genetic drift, there are only two ways that genetic diversity can be reintroduced: (1) by mutation, which as we saw in Chapter 10 continually introduces new alleles into the population, and (2) by gene flow, in which alleles move between populations as individuals leave and enter populations and breed with members of other populations. Like genetic drift, gene flow is a type of nonadaptive evolution that does not lead to a population becoming more adapted to its environment. Unlike genetic drift, gene flow tends to increase the genetic diversity of a population, not decrease it (INFOGRAPHIC 14.4).

Gene pool / allele frequency

The total collection of alleles in a population; the relative proportion of an allele in a population. * From a population genetics perspective, each distinct population of organisms—whether mice in Manhattan or elephants in Africa—has its own particular collection of alleles, which together constitute its gene pool. Within the gene pool, each allele is present in a certain proportion, or allele frequency, relative to the total number of alleles for that gene in the population. For example, if a particular allele for a gene is present 50 times out of a total of 1,000 alleles, its allele frequency is 0.05. Over time, several forces can change the frequency of alleles—that is, how common they are in the population. When the frequency of alleles changes over time, a population evolves. Recall from Chapter 13 that this is the definition of evolution (INFOGRAPHIC 14.1). * Evolutionary changes in a gene pool can have lasting consequences for a population. They can, for example, result in the population becoming more adapted to its environment—think of the antibiotic-resistant bacteria we met in Chapter 13. The evolutionary mechanism that results in adaptation is natural selection.

Assortative mating

The type of mating that occurs when an organism selects a mating partner that resembles itself.

Reproductive isolating mechanisms:

• Prezygotic barriers: Anything that prevents mating and fertilization is a prezygotic mechanism. Habitat isolation, behavioral isolation, temporal isolation, mechanical isolation and gametic isolation are all examples of prezygotic isolating mechanisms. Some species of fruit flies in the genus Rhagoletis provide an example of habitat and behavioral isolating mechanisms. Different species are reproductively isolated because each species lays its eggs on different host species. Adults return to lay eggs on the hosts from which they emerged. Some species of fruit flies in the genus Drosophila are reproductively isolated because of mechanical incompatibility of their genitalia. • Postzygotic barriers: Postzygotic barriers prevent a hybrid zygote from developing into a viable, fertile adult. The mule is a typical example. Reduced viability or fertility of hybrid individuals or reduced viability or fertility of the offspring of hybrid individuals are evidence of postzygotic reproductive isolation. Differences in chromosome number or arrangement of genes on chromosomes usually result in postzygotic isolation because chromosomes may not pair normally during mitosis or meiosis. The formation of new species requires some initial reduction of gene flow Two processes of speciation are thought to be important: allopatric speciation and sympatric speciation. • Allopatric speciation: The first step in allopatric speciation is the restriction of dispersal between two or more populations that would otherwise freely interbreed. One possibility is that a geographic barrier such as a river or desert forms. The separation of two populations by such a barrier is called a vicariance event. A second possibility is that individuals from a source population colonize a new geographic area that is separated from the source population by a nearly complete barrier to gene flow. The establishment of a new population is a founder event. Both vicariant and founder events may reduce gene flow sufficiently that reproductive isolating mechanisms can evolve afterwards. Whether a geographic barrier leads to allopatric speciation or not depends on dispersal ability. A barrier may lead to speciation in some groups but not in others. For example, a river may be a barrier for a snake but not a bird. In the Origin, Darwin emphasized that isolation led to the evolution of separate species. The neo-Darwinian theory is essentially the same. If there is no gene flow between two populations, they will evolve independently under the combined effects of mutation, natural selection, and genetic drift and will eventually be recognized as different species. Reproductive isolating mechanisms will eventually evolve in the absence of gene flow. Female choice can play an important role in species formation, as illustrated by the cichlid fishes and birds of paradise. Accidental differences in female preference can lead to rapid evolution of reproductive isolation. Cichlid fishes have evolved into hundreds of species in a relatively short time. • Founder events: Founder events are thought to be important for species formation because they create opportunities for rapid changes in allele frequencies both because of natural selection and genetic drift. If the founder group is small, genetic drift alone can cause substantial changes in allele frequencies in the new population, and some of those changes can result in morphological change and reproductive isolation. In the extreme case of a population founded by a single individual capable of self-fertilization, a gene can become homozygous for an allele that is in low frequency in the source population. A population arriving in a new area may experience new environmental conditions that lead to rapid evolution as the population adapts to the new conditions. • Adaptive radiation: An adaptive radiation is the evolution of numerous species from a common ancestor in an environment that presents new opportunities. Cactuses are the result of an adaptive radiation in the deserts of North and South America. Many adaptive radiations occur on remote islands or in island-like habitats such as lakes because there are many opportunities for founder events. Both natural selection and genetic drift may lead to rapid evolution. Hawaiian honeycreepers and cichlid fishes found in African lakes are good examples of adaptive radiations in restricted areas. • Laboratory experiments: Reproductive isolation between two groups can evolve in a laboratory experiment. In two groups of fruit flies, a prezygotic barrier evolved in a laboratory experiment when flies were raised on starch or maltose. Sympatric speciation can occur. Other processes can reduce gene flow and lead to the evolution reproductive isolation of sympatric populations without an initial restriction in dispersal. • In animals, sympatric speciation can occur because of differences in habitat or food preference: Host shifts combined with extreme habitat fidelity can lead to rapid speciation, as has occurred in fig wasps. Each species of fig has its own wasp species. New species in the genus Rhagoletis have evolved when new host species were colonized. Example test questions Q1. Mules are sterile because they cannot produce functional gametes. This is evidence that A. horses and donkeys are members of the same species. B. horses and donkeys are separated by genetic drift. C. horses and donkeys are separated by a founder event. D. horses and donkeys are separated by a prezygotic barrier. E. horses and donkeys are separated by a postzygotic barrier. Q2. Which one of the following is a necessary step in the process of species formation? A. The appearance of a new morphological structure. B. A change in the number of chromosomes C. A change in geographic location. D. A reduction in gene flow. E. The formation of a new genus. Q3. Which pair of words best completes the following sentence: In the __________ theory of speciation, a barrier to ______ is the initial step. A. allopolypoloid, autopolyploid. B. sympatric, dispersal. C. allopatric, dispersal. D. allopatric, selection. E. sympatric, selection. Q4. What has favored the evolution of many species of fig wasps? A. Fig wasps are very small. B. Fig wasps can fly long distances. C. Fig wasp species are very different in size. D. Each fig wasp species lays eggs on only one species of fig. E. Fig wasp species are hermaphroditic. Q5. Snapdragons are plants capable of self-fertilization. Flower color is controlled by a single gene with two alleles: RR individuals have red flowers; RW individuals have pink flowers; WW individuals have white flowers. A new population of snapdragons is established by a seed from a source population in which the frequency of R is 0.2. If the source population has genotypes in the Hardy-Weinberg frequencies, what is the chance that the first plant in the newly founded population will have red flowers? A. 0.01 B. 0.04 C. 0.2 D. 0.36 E. 0.64 Correct answers: E, D, C, D, B,