Macro, Micro, Neither?

Transcription

1 Workshop on Macroevolution by Julian Lee (revised by Dana Krempels) Evolutionary Processes In this workshop, we will contrast macroevolutionary patterns and processes with microevolutionary ones. This will help you (1) have a better understanding of the similarities and differences between these two categories of evolutionary mechanisms, (2) help enhance your vocabulary of evolutionary terms and concepts, and (3) help you understand some of the ways in which small genetic changes can translate into large phenotypic differences, thereby contributing to large-scale evolutionary change. Macroevolution encompasses evolutionary processes and patterns at or above the level of species. Examples include Reproductive isolation (speciation) Appearance of major evolutionary novelties (e.g., the evolution of vertebrate jaws from precursor characteristics in jawless ancestors) Patterns of evolutionary diversification (e.g., adaptive radiation) These are in contrast to processes of microevolution, that operate below the level of species. Specifically, microevolution is both 1. change the genetic constitution of populations, and 2. the processes that produce such changes (e.g., mutation, migration, genetic drift, nonrandom mating, natural selection). 3. Given these distinctions, consider each of the following scenarios and determine whether they are more appropriately associated with microevolution, macroevolution, or neither. Scenario 1. changes in allele frequencies within a population from one generation to the next 2. evolutionary diversification of flowering plants in the late Cretaceous 3. changes in genotypic frequencies in a population within a single generation 4. differential survivorship by varied phenotypes within a population 5. evolutionary transition from theropod dinosaurs to birds Macro, Micro, Neither? 6. adaptive radiation of marsupials in Australia 7. decrease in the allele that causes sickle cell anemia in parts of the world where malaria has been eradicated Ideas about the Nature of Evolutionary Change Although Darwin's theory of evolution by natural selection is by far the most powerful and well-supported idea so far, there have been many new proposals and modifications to his original ideas, some of which are more controversial than others.

2 Phyletic Gradualism Darwin proposed that evolutionary forces (especially natural selection, including sexual selection), operating generation after generation on individual variation within populations would, over the immensity of geological time, bring about ancestor-to-descendent evolutionary change. To use his expression, it would result in "descent with modification." Darwin was thus an advocate of phyletic gradualism, the idea that evolution is a consequence of the culmination over many generations of small differences in reproductive success among individuals within populations. Is phyletic gradualism sufficient to explain macroevolutionary events? Can the evolution of structures as complex as the photoreceptors of arthropods, cephalopod molluscs, and vertebrates, for example, be properly attributed to the cumulative effects of microevolutionary processes operating over the long term? Darwin thought so, but even his staunch supporter and advocate, Thomas Huxley ("Darwin's Bulldog"), chided Darwin for his unwillingness to embrace other evolutionary scenarios that involved more rapid and abrupt evolutionary changes. Can you think of some complex characteristics (in any living organisms) that seem unlikely to have resulted solely from very gradual genetic changes within populations? List them here. Punctuated Equilibrium Only a small portion of the world has been geologically explored. Only organic beings of certain classes can be preserved in a fossil condition, at least in any great number. Widely ranging species vary most, and varieties are often at first local--both causes rendering the discovery of intermediate links less likely. Local varieties will not spread into other and distant regions until they are considerably modified and improved; and when they do spread, if discovered in a geological formation, they will appear as if suddenly created there, and will be simply classed as new species. - Charles Darwin, 1859 in On the Origin of Species In 1972, Niles Eldredge and Stephen Jay Gould revisited this idea, noting that while the fossil record did contain records of what appeared to be "smooth transitions" from one ancestral form to new species, there were also--as Darwin had suggested--many examples of apparently rapid changes from one form to another, in which transitional fossil forms were lacking. In their paper, they suggested that long periods of evolutionary stasis--during which a population underwent little or no evolutionary change--were "punctuated" by relatively short spans during which a smaller subset of the population underwent relatively rapid evolutionary change. Such punctuated equilibrium would be more likely to occur in a small, genetically isolated deme of a parent population. What is meant by "rapid" change is relative. Eldredge and Gould were not proposing that species undergo a sudden, one-generation split from a parent species. Both authors have suggested that speciation via punctuated equilibrium could take place over periods of as little as

3 five- to fifty-thousand years, or as long as hundreds of thousands to millions of years. Although this may sound like a long time, such periods are negligible in terms of the geologic time scale. The idea of punctuated equilibrium has generated a great deal of controversy, partly because many people have confused it with the (largely discounted) idea of saltational evolution, whose proponents suggest that a new species may arise in single generation through a macromutation. While there are documented cases of reproductive isolation taking place in one generation in flowering plants via polyploidy, such events are rare in other types of organisms. Unlike saltation, punctuated equilibrium is more an expansion of Darwin's original gradualist model, adding the dimension of differential rates of change to the evolutionary equation. Can you think of some complex characteristics (in any living organisms) that might be good "candidates" for evolution via punctuated equilibrium? List them below. Forces of Evolutionary Change: Heterochrony Humans (Homo sapiens), chimpanzees (Pan troglodytes) and Bonobos (Pan paniscus) are each other's closest living relatives, sharing about 98% similarity of their genomes. Notwithstanding the fact that chimps and humans share many phenotypic traits, they differ in many obvious and important respects. For example, chimps are not fully bipedal, have a smaller cranial capacity than humans, have forward-projecting jaws, heavy brow ridges, and a sagittal crest on the skull that serves as a surface for the attachment of powerful jaw muscles. Chimps and humans are placed in different genera, and--until recently--were considered members of different families (Pongidae, and Hominidae, respectively; currently accepted classification places all Great Apes, including humans, in Homindae). Despite this close genetic relationship, chimp-human hybrids are unknown, and members of the two species do not normally attempt to breed with one another. Clearly, in the evolutionary divergence between the lineage that led to chimps and the lineage that led to humans, the accumulation of small genetic differences has had profound phenotypic consequences. Heterochrony--an evolutionary change in the timing of developmental events--provides a plausible explanation for how this might have happened. Even small genetic changes in the developmental program, when expressed early in development, can have profound and farreaching effects on the resultant adult phenotype.

4 Imagine an evolutionary change in the timing of developmental events such that, relative to the development of an ancestral species, 1. development of the reproductive system is accelerated with respect to the same process in an ancestral lineage (PAEDOGENESIS) 2. development of non-reproductive (somatic) tissues is retarded with respect to the same process in an ancestral lineage (NEOTENY) The result could be an organism that is a "mosaic" of adult characteristics (e.g., reproductive competence) and juvenile characteristics (body form). An reproductively competent species that retains the juvenile form of an ancestral species is said to be paedomorphic (from the Greek paed meaning "child") and morph meaning "form"). Its relatively juvenile form can be generated via one of the two types of heterochrony described above, or possibly a combination of both. Figure 1. Comparison of developmental change in human and chimpanzee skulls. More specifically, paedomorphosis describes the appearance of juvenile (or larval) features of an ancestor in the adult of a descendant. The phenomenon of paedomorphosis appears to have been important in the evolution of our own species from the ape-like ancestor we shared with chimpanzees. As an example, depicted are fetal, juvenile and adult skulls of human and chimp (Figure 1). Superimposed on the skulls are transformation grids, which allow visualization of the changes in shape that occur during development. Notice that the fetal skulls of the chimp and human are quite similar in shape, much more so than in the adult. The juvenile skull of the chimp, though already showing more of a protruding jaw than the human skull, is almost more similar to that of the human than it is to that of the adult ape. Questions 1. In what ways do the fetal skulls of the two species differ? 2. In what ways do the adult skulls differ? 3. Which developmental sequence (chimp or human) involves the greatest amount of change in shape (i.e., which shows the greatest deformation of the transformation grid?)? 4. Which species, then, is more "child shaped" as an adult? Apparently, in the evolution of our species certain terminal developmental stages--those that would further transform our skulls into those resembling a chimp's--have been suppressed, producing a decidedly juvenile-shaped skull in adult humans. 5. How do we know that human evolution has involved suppression of terminal developmental stages rather than the addition of developmental stages in the chimp lineage?

5 Allometric and Isometric Growth Allometry is the study of the relationship between the size of an organism and the size of any of its parts. One can consider allometry at different levels, including during the growth and development of a single organism between different members of the same species (ontogenetic allometry) between different species (evolutionary allometry) In graphic an allometric relationship at any of these levels, one would typically use body size as the independent variable (x axis), and some body part s measure as the dependent variable (y axis). The points on the graph could reflect measurements from a single individual measured at different ages different individuals of a single species (scatter may be due to age differences) (ontogenetic allometry) different species within a taxon of higher rank (genus; family, etc.) (evolutionary allometry) Note: phylogenetic allometry is a type of evolutionary allometry in which analysis are constrained to taxa within a monophyletic (i.e., having a single common ancestor) lineage Differential timing of developmental events may occur within a species, as well as across species over evolutionary time. Figure 2. salamander Isometric growth in a Figure 3. Allometric growth in a human. Ontogenetic Allometry As an organism grows larger during ontogeny, or as an evolving lineage of organisms increases in size over evolutionary time, the various parts of the organisms also increase in size. Bigger organisms have bigger parts. But not all of the parts necessarily grow at the same rate. Such differential growth of different parts is termed allometry, and it results in shape changes as the animal grows. When all parts of an organism grow at the same rate, the organism is said to exhibit isometric growth, and this results in a change in size without a change in shape. Although most organisms exhibit allometric growth, some, such as certain salamanders (Figure 2) are essentially isometric in their growth. Note that the various body proportions remain more or less constant as the animal increases in size. Growth in our own species (Figure 3) is decidedly allometric, most obviously with respect to

6 the growth of the head relative to the body. At birth, the head is relatively enormous, and comprises nearly a third of the length of the infant. As we grow larger, our body grows more rapidly than the head such that the body "catches up" eventually to produce the proportions that we recognize as normal in the adult. Maturation in humans thus involves both increase in size and a change in shape. Questions 1. In humans (and in vertebrates in general), the head is large at birth relative to the rest of the body. Why is this so? 2. An increase in cranial capacity to accommodate the expansion of the cerebral hemispheres in our species is considered one of the evolutionary innovations that has contributed to our success as a species. (But note that the average cranial capacity of Neanderthal humans (Homo neanderthalensis) slightly exceeded that of modern humans (Homo sapiens). What do you suppose sets the upper limit on cranial capacity (and thus brain size) in our species? Evolutionary Allometry A lineage of organisms can also exhibit allometric growth over evolutionary time, as exemplified by the brontotheres (pictured at the right), an extinct group of large mammals. The brontothere lineage shows a clear trend of increasing size, as seen in the comparison of these four species. From a small, hornless ancestral species, the trend has been for an increase in overall body size, but an even more rapid increase in horn size. Over evolutionary time, the horns have increased in size more rapidly than the head or body, producing a change in shape as well as a change in size. Questions 1. What might be the adaptive significance of horns in brontotheres? 2. If horns in brontotheres are restricted to males only, what would this suggest concerning the selective agents involved in their evolution? Figure 4. Evolutionary allometry in brontothere species. The Results of Organic Evolution Every person alive today is the descendant of ancestors who successfully mated and left offspring. Those ancestors of ours faced various pressures of natural selection, and we are left with much of that "evolutionary baggage" in our own morphology and even in our behaviors, though we may not recognize them as such. With a little bit of training, however, you can teach yourself to recognize traits in yourself and other humans that have been handed down from a time (and environment) both different and the same as the one in which we now live.

7 Cashing in on Ancestral Hardwiring: The Evolution of Mickey Mouse Stephen Jay Gould noted that during the fifty years since his inception in the animated cartoon Steamboat Willie in 1928 (Figure 5, Stage 1), Mickey Mouse has been gradually transformed by Disney artists, from a small-headed, long-snouted, beady-eyed rodent (Stage 1) to the large-eyed, more loveable version (Stage 3). Figure 5. The evolution of Mickey Mouse from 1926 to the present. For each of the evolutionary stages in Figure 5, calculate the following ratios: 1. EYE SIZE/HEAD HEIGHT X 100 stage 1: stage 2: stage3: 2. HEAD HEIGHT/BODY LENGTH X 100 stage 1: stage 2: stage3: Figure 6. Mouse-o-gram indicating proper measurement points. 3. HEAD HEIGHT/HEAD LENGTH X 100 stage 1: stage 2: stage3:

8 Now quantify this "evolutionary" transition: record the following measurements on the mouse at each stage. characteristic stage 1 stage 2 stage 3 EYE SIZE (maximum height) HEAD HEIGHT (base of snout to top of rear ear) (1 in the diagram) HEAD LENGTH (base of snout to posterior margin of the anterior ear) (2 in the diagram) BODY LENGTH (bottom of foot to margin of top ear) (3 in the diagram) Questions 1. Over the fifty years of mouse "evolution" represented here, what has happened to the relative size of the head? 2. What has happened to the size of the head relative to the overall body size? 3. What has happened with respect to the extent of the cranial bulging, as reflected by the rearward displacement of the rear ear? 4. Assuming that Stage 1 is the ancestral configuration and that stage 2 is transitional to the modern mouse of Stage 3, is the modern adult mouse more similar or less similar to a juvenile mouse? 5. What motive(s) might Disney Studios have had in transforming Mickey this way?