Chapter 4 The Evolution and Classification of Species (Macroevolution)

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1 Chapter 4 The Evolution and Classification of Species (Macroevolution) Overview This chapter focuses on the long-term perspective of evolutionary theory (macroevolution) and the fossil record. o The first part of the chapter covers speciation and other macroevolutionary patterns. o The second part of the chapter deals with popular misconceptions about the evolutionary process. I find it useful to deal with these early in the course because they are part of the baggage that students bring into the class, and often still hold onto even at the end of the semester unless specifically addressed. o The third part of the chapter provides a brief review of methods for analyzing the fossil record, focusing on dating methods. o The chapter concludes with a very brief review of the major changes in vertebrate and mammalian evolution prior to the origin of primates. The debate o The debate over evolution is usually over macroevolution and not over microevolution. The processes of microevolution also apply to macroevolution and give us data to explain these long-term events When we talk about macroevolution we refer to the long-term pattern of evolution, including the origin and evolution of species The argument against macroevolution is that we can not observe over millions of years. The mistake is to ignore those sciences (such as astronomy and geology) that rely on historical data In macroevolution the fossil and geological records tell us about changes. o Macroevolution deals with the following: The nature of species, the process by which species change over time, and the relationship between macroevolutionary change and the classification of living organisms. The Birth and Death of Species 1 The origin of new species has been observed in historical times and in the present. For instance: New diseases such as Ebola, many of the tropical fish, and types of fruit flies When it comes to the birth and death of species, we need to look at a these questions: o How do species come into being? o Why do some species die out? What is a species? o The most common definition of a species is that described by the biological species concept (BSC) wherein species can be defined in terms of reproductive capability. o Organisms are classified in the same species if: 1) Individuals from two populations are capable of breeding naturally and 2) They produce fertile offspring. o The mule is the most often cited example The result of breeding a female donkey (62 chromosomes) with a male horse (64 chromosomes) is a mule (63 chromosomes) The opposite (male donkey and female horse) is called a hinney and is harder to produce When two mules or hinnies are breed there are no offspring All males are infertile and so are most females This means some female mules can be breed with donkeys or horses, but not male mules. The Birth and Death of Species 2 What is a species? (continued) o The phylogenetic tree is a diagram that shows the evolutionary relationship between species, includes the point at which two species separate. This is an idealized version. o The BSC assumes two organisms either belong to same species or not (mutually exclusive), but this may not be always clear: Example of gypsy moths shows how this is not an absolute Far away populations produce infertile offspring Closer together produce fertile offspring Another example is the dog-wolf-coyote-jackal issue We think of them as different species, but actually different subspecies

2 Another question, not yet settled, is if they can breed with the fox (very different number of chromosomes). Photo: Phylogenetic tree of the dog The Birth and Death of Species 3 Species change The BSC is useful when comparing two species living today, but how can we look at species over time? Across time study requires looking at two different modes of evolutionary change of species: Anagenesis is linear evolution. It assumes one species evolved directly into a new species over time. But at what point is it species A or species B? Researchers often modify the species concept to deal with complications in naming species. Chronospecies is a term used to label different stages of biological change over time, but there is only one species at any given point in time. Later in the book the term paleospecies is used to describe the species identified from the fossil record. Problems with this mode include that biological species concept does not apply. The time gap precludes the testing of breeding capacity. There needs to be a stand-in for mating and this is usually the analysis of different physical forms along a single lineage. Many contemporary evolutionary biologists tend not to use this method today Cladogenesis is the formation of one or more new species from another over time (branching evolution). In cladogenesis, both species A and species B can exist at the same time We will see that this is an issue as we explore paleoanthropology later. The Birth and Death of Species 4 Speciation o Speciation is the process wherein genetic differences between populations prevent successful interbreeding with the parent species. o The precondition for speciation is isolation; as long as gene flow is possible then mutations tend to mix back into the population. Isolation can occur a number of ways: Geographic isolation, reproductive isolation (pre-mating or post-mating incapabilities), and behavioral isolation. o Speciation model: Step 1: A and B have not yet diverged Step 2: A and B are just beginning to diverge. Genetic differences accumulate and are acted on by the forces of evolution: Mutations increase variation within a population Genetic drift may contribute to differences in small populations Without gene flow these accumulated differences can not be shared between populations. As the populations are in different environments, natural selection also contributes to genetic divergence To accomplish this reproductive isolation must occur first; this is the genetic change that can lead to an inability to produce fertile offspring. Geographic isolation is the most common means of reducing gene flow between populations. This is the result of reduced or eliminated gene flow between populations. Behavioral isolation also occurs. o Step 3: A and B have diverged to a point where they re no longer able to reproduce; speciation so that genetic divergence is the result. Speciation is complete. The Birth and Death of Species 5 Speciation (continued) o There is a debate over the role of the various evolutionary forces in producing genetic divergences o At one time only natural selection was seen as the force behind speciation This suggests that natural selection operates on large populations gradually and is the consequence of differential adaptation. Today, the other forces are also seen as contributors. Relethford Chapter 4 Page 2

3 Many think that speciation occurs in small populations and are extensively by mutation and genetic drift. Genetic differences can come about as a result of all the evolutionary forces. Adaptive radiation o Adaptive radiation is the rapid formation of many new species following the availability of new environments or the development of a new adaptation. o A species, or group of species, will diverge into as many variations as two factors allow: Its own adaptive potential The adaptive opportunities of the available niches. o One example was the death of the dinosaurs, which allowed for the mammalian radiation o On a smaller scale, Darwin s finches are another example. Box 4.1: Rate of Speciation 1 The rate at which evolutionary change occurs came into a debate in the 1970s o Prior to that time, most evolutionists were of the same mind as Darwin, that change was gradual (called phyletic gradualism). o Gradualism is the view that macroevolution is a slow and gradual process. o Charles Darwin saw evolution as a process of millions of years. o He saw natural selection as the primary mechanism Mutation and drift have little effect on a specific generation. Natural selection acting on initial mutation results in speciation. o With gradualism, the fossil record will be a smooth, gradual transition (no gaps). Anagenesis is the term used to refer to this linear evolution It assumes one species evolved directly into a new species over time But at what point is it species A or species B? Box 4.1: Rate of Speciation 2 In some cases we lack transitional forms, o For some this is evidence of the fallacy of evolution o For researchers this became a challenge that was finally explained b y the concept of punctuated equilibrium After the 1970s, evolutionists came to realize that some cases of change went much faster and punctuated equilibrium (PE) as a mechanism was added. o Punctuated equilibrium is the view that the pattern of macroevolution consists of long periods of time when little change occurs (stasis) and short periods of time when rapid evolutionary change occurs. o This model infers that most genetic change occurs during speciation. Mutation occurs in a small, isolated population and then spreads rapidly due to inbreeding and genetic drift. o Stabilizing selection and other factors act to keep a species the same over time o Because some new species appear so rapidly we do not see transitional forms. The fossil record usually will not show the initial changes. We now know that within a species there can ALSO be punctuated gradualism (statis and then rapid changes in the species morphology. Gould acknowledged this alteration of PE You can watch a myriad of videos on evolution topics here History of Mass Extinctions The Birth and Death of Species 6 Relethford Chapter 4 Page 3

4 o Extinctions and mass extinctions o We need to remember that 99% of all species that have ever existed have become extinct. In modern times we see evidence of species extinction The dodo bird, the passenger pigeon, and many others. o What causes extinction? One source is where a species is no longer adapted to an environment, it may die. Or another species becomes better adapted to the same environment and out-competes (competitive exclusion principle) In fact, extinction seems to be the fate of all species o Occasionally, large numbers of species become extinct at the same time and this is called mass extinction. o In fact, many of the geological terms for eras, epochs, and so forth are named for mass extinction events/adaptive radiations o For instance: First mass extinction: At the end of the Ordovician period and the start of the Silurian period (444 mya) the first mass extinction event documented. Second mass extinction: At the end of the Devonian period and the start of the Carboniferous period (364 mya), 30% of animal species extinct, while advent of first reptiles. Third mass extinction: At the end of the Permian period the start of the Triassic period (251 mya) the biggest extinction of 75-95% of all species, 60% of all families went extinct but the first dinosaurs and mammals appeared. The Birth and Death of Species 7 Extinctions and mass extinctions (continued) o Fourth mass extinction: At the end of the Triassic period (200 mya) and start of the Jurassic is the period of the first birds, but also extinction of 35% of animal families o Fifth mass extinction: During the Cretaceous period is the time where the flowering plants were introduced, but at the end of the Cretaceous period (65 mya) dinosaurs are out and mammals dominate. o Sixth mass extinction: Now Mammalian evolution o The Cenozoic era, the Age of Mammals. Mammals exhibit a number of shared traits: The enlargement of the cerebrum, especially the neocortex, which controls higher brain functions, resulting in more nerve cells A longer, more intense period of growth in utero Distinctive dentition, termed a heterodont (different kinds of teeth), with 3 incisors, 1 canine, 4 premolars, and 3 molars in each quarter of mouth Maintenance of constant internal body temperature, warm-bloodedness, and endothermic (Able to maintain internal body temperature by producing energy through metabolic processes within cells; characteristic of mammals, birds, and perhaps some dinosaurs) o Emergence of the major mammalian groups Egg-laying mammals, or monotremes - extremely primitive Pouched mammals, or marsupials - young are born extremely immature and must complete development in an external pouch. Placental mammals develop over a longer period of time inside the mother, made possible by development of the placenta which provides for fetal nourishment. Misconceptions about Evolution The nature of selection o Bigger is better implies that natural selection will always lead to larger structures is not accurate. o Newer is better implies that traits more recent in origin are superior because they are newer is not accurate. o Natural selection always works implies that natural selection always provides opportunity for some members of a species to survive is not accurate. o There is an inevitable direction in evolution implies the notion of orthogenesis, the notion that evolution would continue in a given direction because of a vaguely defined non-physical force. Structure, function and evolution o Natural selection always produces perfect structures reflects the tendency to view nature as the product of perfect natural engineering. Biological structures are far from perfect. Relethford Chapter 4 Page 4

5 o o All structures are adaptive overlooks the fact that many structures are simply a by-product of other biological changes and have no adaptive value of their own. Current structures always reflect initial adaptations which is an assumption that any given structure with an associated function originally evolved specifically for that function. Classification of Species 1 Classification systems in daily use are used in a general way. o Biology uses systems of classification to show relationships between different groups of organisms, a rather difficult task. o Biological classification reflect certain common characteristics. Classification is required and useful in reflecting evolutionary relationships. Biological classification should reflect evolutionary processes, but it requires careful analysis of both living and extinct forms to determine evolutionary relationships. Taxonomic categories o The Linnaean system is a hierarchical classification with each category containing a number of subcategories containing other subcategories. Biological classification uses a number of categories. Commonly used categories in anthropology include: kingdom, phylum (plural phyla), class, order, family, genus (plural genera), and species. Further breakdowns are distinguished by adding prefixes (i.e., subphylum and infraorder). o The scientific name given to an organism consists of the genus and species names in Latin or Latinized form (Homo sapiens). o A given genus may contain a number of different species. They are placed in the same genus because of certain common characteristics, such as a large brain size. The Human Place in the Organic World To deal scientifically with the tremendous diversity of life on the planet, biologists develop a system of classification. o Classification: Organizes diversity into categories Indicates evolutionary and genetic relationships Well after I got my biology degree, a new level of classification was added: domain. o It groups the 5 (some say 6) kingdoms into three larger groups. o These groups consist of Archaea (single-celled organisms), Bacteria, and Eukaryota (having a nucleus and are multi-cellular) Organisms that move about and ingest food (but don t photosynthesize,) are animals. o The kingdom Animalia includes 20 major phyla (singular, phylum) Chordata is one phyla and includes all animals with a nerve cord, gill slits and supporting cord along the back. Most chordates are called vertebrates, animals with segmented, bony spinal columns. In addition to a vertebral column, vertebrates have a brain and sensory structures for sight, smell, and balance Includes 5 classes: bony fishes, cartilaginous fishes,, amphibians, reptiles/birds, and mammals. o For a very simply organized list of human classification from kindgom to species click here Classification of Species 2 Methods of classification o Classification involves making statements regarding similarities of traits between species. o The process can be confusing due to biological similarities which can arise for different reasons. o Classification systems will reflect evolutionary relationships if we focus on traits that exhibit homology, but not homoplasy Homology refers to similarity due to descent from a common ancestor. Homology is often apparent by comparing the actual structure of different species. Animals may use their anatomical features for different purposes, but the basic shape is the same. (i.e., whale, bird, and humans have similar limb bones). Humans and apes share certain features of their shoulder anatomy enabling them to hang by their arms. Remember, the shape is due to a shared descent Sometimes it can be confusing when one compares shape as there are independent evolution of a trait in different species Relethford Chapter 4 Page 5

6 o o Homoplasy refers to similarity due to the independent evolution of the same trait(s) in both species. Birds and flies are both capable of flight but not because of descent from a common ancestor but independent evolution. There are two different types of homoplasy, Parallel evolution and Convergent evolution. Parallel evolution is the independent evolution of similar traits in closely related species (i.e., increase in dental size among early human ancestors). Convergent evolution is the independent evolution of similar traits in more distantly related species (i.e., evolution of flight in birds and flies). Methods of Classification 3 Methods of classification (continued) o Some terms we need before we discuss the approaches to the classification and interpretation of evolutionary relationships Primates have remained quite generalized (non-specific) traits as compared to being specialized traits (evolved for a specific function) Primates have also retained many primitive traits (meaning ancestral mammalian traits) mammalian traits, rather than derived traits (meaning recent) that many mammals have acquired. The concept of primitive and derived traits is relative. What might be considered primitive at one level of comparison might be considered a derived trait at another level. Information on modern and fossil forms is necessary in order to determine whether a trait is primitive or derived. o Shared traits are exactly what they sound like, as is an unique trait. An example of a shared, derived trait is the absence of a tail in humans and apes An example of a unique, derived trait is habitual (obligate) bipedalism in humans. o Which traits are used as part of interpretations? Ancestral (primitive) traits are not diagnostic of groups that diverged after the character appeared Derived traits are diagnostic of particular evolutionary lineages. Shared traits are considered the most useful for making evolutionary interpretations Methods of Classification 4 Approaches to classification o Two approaches are used to interpret relationships: Evolutionary systematics and cladistics. o They have some similarities: Both trace evolutionary relationships and construct classifications that reflect these relationships. Both recognize that organisms must be compared for specific features. Both approaches focus exclusively on homologies o Evolutionary systematics (ES) is the traditional method of classification ES considers all homologous traits (primitive or derived) when classifying organisms into taxonomic groups. Species sharing the largest number of homologous traits are placed in the same group even if all these traits do not reflect an ancestor-descendent relationship (i.e., crocodiles, lizards, and birds are all grouped together). o Cladistics is a more recent approach but is that more often used in research today. Cladistics looks at only shared derived traits and classifies organisms based solely on their evolutionary relationship For instance, having 5 digits is a primitive, shared trait and so is of no use to separate primates For instance, the large brain of humans is a unique, derived trait and no use either. Determining if a trait is primitive or derived requires the comparison of groups of interest with an outgroup. Outgroups are those that are more distantly related to the species being classified. Uses the cladogram to illustrate relationships. A cladogram is a chart showing evolutionary relationships as determined by cladistic analysis. It s based solely on interpretation of shared derived characters. It contains no time component and does not imply ancestor-descendant relationships. Example, in cladistics, birds and crocodiles go into one taxonomic group, lizards in another. Read Box 4.2 and review Figure 4.15 for more details. Relethford Chapter 4 Page 6