Smokin Notes Macroevolution Part 2 Study Guide in word.docx

The Tempo of Speciation Recall that, so far, we have discussed the role of evolution (descent with modification) and the effects of natural selection (differential reproductive success) on the process of evolution. We have also discussed how microevolution (a change in the frequencies of genotypes i...

The Tempo of Speciation Recall that, so far, we have discussed the role of evolution (descent with modification) and the effects of natural selection (differential reproductive success) on the process of evolution. We have also discussed how microevolution (a change in the frequencies of genotypes in a population) leads to macroevolution (the formation of new species). In this section, we will be considering the rate of evolution and extinction. That is: What controls the speed with which evolution progresses and species are formed? Dr. Gillooly reminded students again that natural selection acts on individuals, and evolution acts on populations. Two Paradigms for the Tempo of Speciation There are two different ideas as to how the tempo of speciation is controlled: • Gradualism - DNA mutates slowly, but gradually over time. Therefore, we will see a slow divergence of isolated populations over a period of millions and millions of years. This was the primary model used by evolutionary biologists until about 50 years ago. • Punctuated equilibrium - Species maintain prolonged periods of stasis, during which no major speciation occurs. Then, suddenly, these periods are interrupted by sudden episodes of speciation. (Note that, although this speciation is relatively rapid, it still takes several thousands of years.) The work of Stephen J. Gould, an influential evolutionary biologist, describes punctuated equilibrium extensively. Gould was even more famous for his popular science literature, which explained science in a way that the common layperson could understand and be interested in. Which of these mechanisms of evolution do we see on Earth? The answer is that evolution on Earth exhibits both gradualism and punctuated equilibrium. Fossil records show periods of slow evolution (gradualism), punctuated by periods of rapid evolution (punctuated equilibrium). Consider the following examples: • Cichlids (punctuated equilibrium) - Fossil records show that cichlid fish in Lake Victoria in Africa underwent rapid evolution, as 400 species evolved in just 12,000 years. This is almost instantaneous in terms of evolutionary time. It is thought that differences in resource availability led these fish to occupy different parts of the lake and use different strategies. This is a good example of punctuated equilibrium. • Horseshoe crabs (gradualism) - Fossil records show that horseshoe crabs underwent a phenotypic stasis and, thus, only gradual evolution for 100 million years. This is a good example of gradualism. (Interestingly, horseshoe crabs are critical for sterilizing human drugs and vaccines, as their blood, which is blue and contains copper, is used to identity toxins. For this reason, horseshoe crab blood is very expensive- up to $80,000 per gallon!) Gradualism and punctuated equilibrium are closely related to evolutionary concepts called anagenesis and cladogenesis. Anagenesis occurs when gradual changes accumulate within a lineage, while cladogenesis occurs when a parent species splits into two or more distinct species as populations adapt to new environments, forming a clade. Question: Which model of evolution involves slow, progressive changes at a roughly constant rate? a. Punctuated equilibrium b. Divergent evolution c. Convergent evolution d. Gradualism e. Genetic equilibrium The answer is “e.” Gradualism is the model of evolution that involves slow, progressive change at a roughly constant rate. This model of evolution holds that DNA mutates slowly, but gradually over time, so we should see a slow, but constant, divergence of isolated populations over a long period of time. Question: Which of the following best describes cladogenesis? a. Evolution of a new species from an ancestral species b. Diversification Into two or more species as populations adapt to new environments c. Convergent evolution d. None of these is correct. e. All of these are correct. Cladogenesis occurs when a parent species splits into two or more distinct species as populations adapt to new environments. What Controls the Rate of Speciation? A combination of the following factors controls the rate of speciation: • Rates of mutation - The faster the mutation of an organism's genome, the faster the rate of speciation. Recall that mutation is the ultimate source of genetic diversity. • Rates of selection - Rates of selection can be controlled by competition among the species for food, mates, or space. They can also be impacted by generation time. For example, bacteria have shorter generation times than desert tortoises. All other things the same, you would expect bacteria to evolve more rapidly than desert tortoises. • Rates of environmental change - Periods of extreme environmental change can result in fast mutations. For example, as we will see later, mass extinctions are generally followed by rapid speciation. Or. Gillooly's research focuses on the impact of environmental change- particularly changes in temperatures rates of evolution. Exaptation Speciation does not occur from out of nowhere. That is, natural selection must start with structures that are already in place. An exaptation occurs when a structure evolves for one purpose and comes to be used for another. Once an evolutionary change occurs, it may serve a different function when introduced to a new context. Moreover, it can be further modified to serve a different function. This can lead to an evolutionary novelty. For example, Archaeopteryx (a bird that was alive during the Jurassic Period), had some of the first wings that ever evolved. How did dinosaurs develop wings? They started with hollow bones, which existed before birds could fly but are integral parts of wings that allow for flight. Dinosaurs probably originally had hollow bones so they could move quickly to catch food. Moreover, recent research indicates that flight evolved in birds that escaped from predators by climbing up trees. Eventually, their appendages became modified enough that they could use the tree as a launching platform for flight. The point is that the wing did not just appear; it developed through a series of morphological changes to an existing structure. Causes of Rapid Speciation The following factors can cause rapid speciation: • Key genetic processes that change the organism's phenotype but maintain its function. For example, polyploidy occurs when gamete fusion accidentally results in a doubling or tripling of the genome. Polyploidy prevents the new organism (with multiple copies of the genome) from mating with individuals with only one copy of the genome, thus leading to the rapid propagation of a new species. This phenomenon is especially prevalent in plants. • Key developmental processes that change the organism's phenotype but maintain function. Alteration of the genes that affect the structure of the organism can lead to rapid speciation. For example, homeotic (Hox) genes are relatively short (around 200 base pairs), but they are important for early organization and development of the body. A small mutation in a homeotic gene can lead to dramatic changes. For example, a mutation in a Hox gene may cause a fly's legs to grow where its antennae should be. A different mutation might cause the development of a second set of wings. Also, changes in developmental timing or the sequence of events (referred to as heterochrony) can lead to significant phenotypic changes. Two kinds of heterochrony are as follows: o Allometric heterochrony is a change in the rate of growth or development. For example, most breeds of domesticated dogs reach their maximum size at an earlier point in time than wolves, which is why adult dogs are smaller than adult wolves. This change in the rate of growth or development has led to a significant phenotypic change that allowed domesticated dogs to diverge from wolves. o Sequence heterochrony is a change in the time at which a structure appears. Consider, for example, the Axolotl-a large Mexican salamander that, unlike other salamanders, does not internalize its gills as it goes from the juvenile stage to the adult stage. (This is an example of neoteny, which occurs when adults retain juvenile traits.) This kind of delayed development has allowed the Axolotl to live in aquatic environments, leading to vast differences in behavior and anatomy. Some scientists believe that there is evidence of sequence heterochrony and neoteny in humans, as well. The theory is that humans have retained the "baby faces of other primates, as human faces are flatter than the adult faces of most primates. This may have conveyed some sort of evolutionary advantage that allowed humans to thrive. Professor Gillooly hinted that “neoteny” would definitely be tested on the exam! The Tempo of Extinction As speciation adds new species to the Earth's collection over time, extinction removes species from the Earth. Just like speciation, extinction is a continuous process that constantly occurs at some background rate due to chance and natural selection, is accelerated by environmental change, and can occur in rapid bursts. To understand the dynamics that establish biodiversity on earth, you must understand the dynamic interplay between speciation and extinction. Scientists have estimated that the current extinction rate is higher than ever in history, in part because humans are modifying the environment over large portions of the Earth. The current rate is about 100 times higher than extinction rates that we are able to discern from the available fossil record. Scientists project that the extinction rate in the future will be much higher. The exponential increase in the number of extinction events in the last 250 years has corresponded with an exponential increase in the number of humans on the planet. There is little doubt that human activity is driving a large share of the extinctions that are occurring today. As a result, scientists call the current geological age the anthropocene – the period during which human activity is the dominant influence on climate and the environment. Extinction threatens some organisms more than others. For example, almost half of all amphibians are "threatened" with extinction. In addition, insect populations are on the decline. A recent study showed a stunning 76% decline in the biomass of insects in Germany between 1990 and 2015. Rapid bursts of extinction (where more than 50% of species are lost) are known as mass extinction events. There have been five mass extinction events in the Earth's history. Two of the most severe mass extinction events in the Earth's history were the following: • The Cretaceous mass extinction, which wiped out about 71% of all terrestrial species 65 million years ago. This is the mass extinction that wiped out the dinosaurs and most marine life. • The Permian mass extinction wiped out about 83% of all genera and 97% of all aquatic genera 250 million years ago. Only a handful of animals remained, such as Lystrosaurus. a weird herbivore the size of a pig, which was the most common terrestrial vertebrate in its day. Lystrosaurus is thought to be the common ancestor of all modern mammals. What caused these mass extinctions? Geological evidence has led scientists to believe that the Permian extinction was due to continental drift, volcanoes, or asteroids, though scientists are still not exactly sure what the cause was. Scientists are pretty sure, however, that the Cretaceous mass extinction was caused by either volcanoes or a meteor. Satellite imaging has allowed scientists to identify a 112- mile-wide crater at the bottom of the ocean near the Yucatan Peninsula, where scientists believe the meteor that caused the Cretaceous mass extinction hit the Earth. The impact of this meteor immediately killed everything in North America and threw so much dust into the air that it inhibited photosynthesis and dramatically changed the Earth's climate. This climate change caused the extinction of many other species worldwide-even species that were not located near the meteor's impact. Note that periods of mass extinction are almost always followed by periods of punctuated speciation because the post-mass-extinction environment favors the propagation of new speciation. For example, the age of the flowering plants (angiosperms) followed the Permian mass extinction, and the age of the mammals followed the Cretaceous mass extinction. You can think of a "tree of life" as a snapshot of the interaction between speciation and extinction, which has led to the divergence of different groups of organisms over time. Most phylogenetic trees, including the ones that we will discuss later in this course, show organisms in a hierarchical relationship. This leads many people to incorrectly presume that certain species are "more evolved" than others. trees, including the ones that we will discuss later in this course, show organisms in a hierarchical relationship. This leads many people to incorrectly presume that certain species are "more evolved" than others. Dr. Gillooly showed a different version of the "tree of life," which is shown to the right. This version plots 3,000 species based on ribosomal RNA sequences, and it shows that all living things are related to one another in a non-hierarchical fashion. Practice Question: Identify the TRUE statement about speciation below. a. Allopatric speciation is less common than sympatric speciation in animals. b. Speciation always requires a geographic barrier that separates populations. c. Mistakes in cell replication leading to polyploidy can cause sympatric speciation. d. None of these is correct. e. All of these are correct. The correct answer is “b” Sympatric speciation occurs among individuals living in the same area, while allopatric speciation occurs among individuals that are geographically separated. While geographic separation can lead to allopatric speciation, it is not necessary for sympatric speciation. One way that sympatric speciation can occur is via mistakes in cell replication that lead to polyploidy individuals who become reproductively isolated from individuals who are not polyploidy. Practice Question: Male damselflies of one species are unable to mate with females of another species because of anatomical differences in the male copulatory organ. This is an example of ______ isolation, which is a ______ reproductive barrier. a. temporal; prezygotic b. mechanical; postzygotic c. behavioral; postzygotic d. mechanical ; prezygotic e. gametic; postzygotic The correct answer is “d” This is an example of mechanical reproductive isolation, which occurs when two species physically cannot undergo fertilization. Mechanical reproductive isolation is a prezygotic reproductive barrier because it prevents fertilization; that is, it prevents a zygote from forming in the first place. Practice Question: When reconstructing evolutionary history using the parsimony principle, you would ... a. select a tree with the fewest shared, derived characters. b. choose a tree that groups species that look alike together. c. choose a tree that groups species that have the fewest genetic differences. d. not consider homologous traits using sequence data. e. None of these is correct. The correct answer is “a”- When applying the parsimony principle to reconstruct evolutionary history, scientists select trees that have the fewest shared, derived characters because this is the simplest explanation for the evolution of traits among taxa.