Descent with Modification: A Darwinian View of life - Charles Darwin published On the origin of Species by means of Natural Selection on November 24, 1859. The Origin of Species focused biologists attention on the great diversity of organisms their origins and relationships, their similarities and differences, their geographic distribution, and their adaptations to surrounding environments. - Darwin made two major points in The Origin of Species. -First, he presented evidence that the many species of organisms presently inhabiting Earth are descendants of ancestral species that were different from the modern species. - Second, he proposed a mechanism for this evolutionary process, which he termed natural selection. - Natural selection: The basic idea of natural selection is that a population can change over generations if individuals that possess certain heritable traits leave more offspring than other individuals. - Evolutionary adaptation: - The result of natural selection is evolutionary adaptation - Evolutionary adaptation is an accumulation of inherited characteristics that enhance organisms ability to survive and reproduce in specific environments. Evolution: - In modern terms, we can define evolution as a change over time in the genetic composition of a population. - Eventually, a population may accumulate enough change that it constitutes a new species a new life form. 1
- We can also use the term evolution on a grand scale to mean the gradual appearance of all of biological diversity, from the earliest microbes to the enormous variety of organisms alive today. 1- The Darwinian revolution challenged traditional views of a young Earth inhabited by unchanging species Resistance to the Idea of Evolution - Darwin s view of life contrasted sharply with traditional beliefs of an Earth only a few thousand years old, populated by forms of life that had been created at the beginning and remained unchanged ever since. The Scale of Nature and Classification of Species - Aristotle (384 322 b.c.), viewed species as fixed (unchanging). - Aristotle recognized certain affinities among living things, leading him to conclude that life forms could be arranged on a ladder, or scale, of increasing complexity, later called the scala naturae ( scale of nature ). - Each form of life, perfect and permanent, had its location on this ladder. - The Old Testament holds that species were individually designed by God and therefore perfect. - In the 1700s, many scientists interpreted the superb adaptations of organisms to their environments as evidence that the Creator had designed each species for a particular purpose. - One such scientist was Carolus Linnaeus (1707 1778), a Swedish physician and botanist who wanted to classify life s diversity for the greater glory of God. - Linnaeus adopted a nested classification system, grouping similar species into increasingly general categories. For example, similar species are grouped in the same genus, similar genera (plural of genus) are grouped in the same family, and so on. - To Linnaeus, the observation that some species resemble each other did not imply evolutionary kinship, but rather the pattern of their creation. 2
Fossils, Cuvier, and Catastrophism - Fossils in each layer (stratum) provide a glimpse of some of the organisms that populated Earth at the time that layer formed. - Georges Cuvier (1769-1832), a French scientist (paleontologist) noted that the deeper (older) the strata, the more dissimilar the fossils are from current life. - He also observed that from one stratum to the next, some new species appear while others disappear; extinctions must have been a common occurrence in the history of life. - Yet Cuvier opposed the idea of gradual evolutionary change. Instead, he advocated catastrophism, speculating that each boundary between strata represents a catastrophe, such as a flood or drought that destroyed many of the species living at that time. Theories of Gradualism - Other scientists promoted the concept of gradualism---the idea that profound change can take place through the cumulative effect of slow but continuous processes. - In 1795, Scottish geologist James Hutton (1726 1797) proposed that Earth s geologic features could be explained by gradual mechanisms currently operating in the world. - Charles Lyell (1797 1875), incorporated Hutton s thinking into a more comprehensive theory known as uniformitarianism---the idea that the same geologic processes are operating today as in the past, and at the same rate. - Hutton and Lyell s ideas exerted a strong influence on Darwin s thinking. - Darwin agreed that if geologic change results from slow, continuous actions rather than sudden events, then Earth must be much older than the 6,000 years that theologians estimated. - He later reasoned that perhaps similarly slow and subtle processes could act on living organisms over a long period of time, producing substantial change. - However, Darwin was not the first to apply the principle of gradualism to biological evolution. 3
Lamarck s Theory of Evolution - By comparing current species with fossil forms, Lamarck (1744-1829) had found what appeared to be several lines of descent, each a chronological series of older to younger fossils leading to a living species. - He explained this with two principles: - The first was use and disuse, the idea that parts of the body that are used extensively become larger and stronger, while those that are not used deteriorate. As an example, he cited a giraffe stretching its neck to reach leaves on high branches. - The second principle, inheritance of acquired characteristics, stated that an organism could pass these modifications to its offspring. - Lamarck reasoned that the long, muscular neck of the living giraffe had evolved over many generations as giraffes stretched their necks ever higher. - However, Lamarck deserves credit for his insightful observations of nature and recognition of gradual evolutionary change as the best explanation for his observations. 2- In The Origin of Species, Darwin proposed that species change through natural selection - As the 19th century dawned, it was generally believed that species had remained unchanged since their creation. - A few clouds of doubt about the permanence of species were beginning to gather. 2.1- Darwin s Research 2.1.1- The Voyage of the Beagle - In 1831, the 22 year old Darwin left England aboard the Beagle. Biological observations: - Darwin noted that the plants and animals in temperate regions of South America more closely resembled species living in the South American tropics than species in temperate regions of Europe. 4
- Furthermore, the fossils he found, though clearly different from living species, were distinctly South American in their resemblance to the living organisms of that continent. Geologic observations: - Darwin experienced geologic change when a violent earthquake rocked the coast of Chile, and he observed afterward that the coastline had risen by several feet. - Finding fossils of ocean organisms high in the Andes Mountains, he inferred that the rocks containing the fossils must have been raised there by a long series of similar earthquakes. - i.e. the physical evidence did not support the traditional view of a static Earth only a few thousand years old. Observations on Galápagos: - The Galápagos, a group of geologically young volcanic islands located near the equator about 900 km (540 miles) west of South America. - Among the birds he collected on the Galápagos were several kinds of finches that, although quite similar, seemed to be different species. - Some were unique to individual islands, while others were distributed on two or more adjacent islands. -The animals on the Galápagos resemble species living on the South American mainland, but most of the Galápagos animals live nowhere else in the world. - He hypothesized that the Galápagos had been colonized by organisms that had strayed from South America and had then diversified on the various islands. 2.1.2- Darwin s Focus on Adaptation - Darwin began to perceive adaptation to the environment and the origin of new species as closely related processes. - Could a new species arise from an ancestral form by the gradual accumulation of adaptations to a different environment? 5
-From studies made years after Darwin s voyage, biologists have concluded that this is indeed what happened to the Galápagos finches. - Galápagos finches beaks and behaviors are adapted to the specific foods available on their home islands. - Darwin realized that an explanation for such adaptations was essential to understanding evolution. - By the early 1840s, Darwin had worked out the major features of his theory of natural selection as the mechanism of evolution. - In 1844, Darwin wrote a long essay on the origin of species and natural selection. However, he was reluctant to introduce his theory publicly. - In June 1858, Lyell s prediction came true. Darwin received a manuscript from Alfred Russel Wallace (1823 1913), a young British naturalist working in the East Indies who had developed a theory of natural selection similar to Darwin s. - Darwin quickly finished The Origin of Species and published it the next year. - Within a decade, Darwin s book and its proponents had convinced most biologists that biological diversity was the product of evolution. 2.2- The Origin of Species In publishing his theory, Darwin developed two main ideas: - First, evolution explains life s unity and diversity. - Second, natural selection is a cause of adaptive evolution. 2.2.1- Descent with Modification - The phrase descent with modification summarized Darwin s view of life. - Darwin perceived unity in life, with all organisms related through descent from an ancestor that lived in the remote past. 6
- As the descendants of that ancestral organism spilled into various habitats over millions of years, they accumulated diverse modifications, or adaptations, that fit them to specific ways of life. - In the Darwinian view, the history of life is like a tree. - Closely related species, such as the Asian elephant and African elephants, are very similar because they shared the same line of descent until a relatively recent divergence from their common ancestor (Figure 22.7) - To Darwin, the Linnaean hierarchy reflected the branching history of the tree of life, with organisms at the various taxonomic levels related through descent from common ancestors. 2.2.2- Natural Selection and Adaptation - How does natural selection work, and how does it explain adaptation? - Evolutionary biologist Ernst Mayr has dissected the logic of Darwin s theory of natural selection into three inferences based on five observations: - OBSERVATION #1: For any species, population sizes would increase exponentially if all individuals that are born reproduced successfully (Figure 22.8). -OBSERVATION #2: Nonetheless, populations tend to remain stable in size, except for seasonal fluctuations. -OBSERVATION #3: Resources are limited. - INFERENCE #1 : Production of more individuals than the environment can support leads to a struggle for existence among individuals of a population, with only a fraction of their offspring surviving each generation. - OBSERVATION #4: Members of a population vary extensively in their characteristics; no two individuals are exactly alike (Figure 22.9). 7
- OBSERVATION #5: Much of this variation is heritable. - INFERENCE #2: Survival depends in part on inherited traits. Individuals whose inherited traits give them a high probability of surviving and reproducing in a given environment have higher fitness and are likely to leave more offspring than less fit individuals. - INFERENCE #3 : This unequal ability of individuals to survive and reproduce will lead to a gradual change in a population, with favorable characteristics accumulating over generations. - Darwin perceived an important connection between natural selection and the capacity of organisms to over-reproduce. Differential reproductive success: - In each generation, environmental factors filter heritable variations, favoring some over others. - Organisms with traits favored by the environment tend to produce more offspring than do organisms without those traits. - This differential reproductive success results in favored traits being excessively represented in the next generation. - Increases in the frequencies of favored traits in a population are an important source of evolutionary modification. Artificial Selection - The selective breeding of domesticated plants and animals to encourage the occurrence of desirable traits. Summary of Natural Selection. Let s once more state the main ideas of natural selection: 8
1- Natural selection is the differential success in reproduction (the unequal ability of individuals to survive and reproduce) that results from the interaction between individuals that vary in heritable traits and their environment. 2- Over time, natural selection can increase the adaptation of organisms to their environment 3- If an environment changes over time, or if individuals of a particular species move to a new environment, natural selection may result in adaptation to these new conditions, sometimes giving rise to new species in the process. - Three important points about evolution by natural selection: - First, although natural selection occurs through interactions between individual organisms and their environment, individuals do not evolve. A population is the smallest unit that can evolve. (Evolution can be measured only as changes in relative proportions of heritable variations in a population over a succession of generations.) - Second, natural selection can amplify or diminish only heritable traits that is, traits that are passed from organisms to their offspring. (An organism may acquire modified characteristics through its own interactions with the environment during its lifetime. There is no evidence that such acquired characteristics can be inherited by offspring. We must distinguish between adaptations that an organism acquires during its lifetime and inherited adaptations that accumulate in a population over many generations as a result of natural selection.) - Third, environmental factors vary from place to place and from time to time. A trait that is favorable in one situation may be useless or even detrimental in different circumstances. (Natural selection is always operating, but which traits are favored depends on the environment.) 9
- Thus, life is evolving through gradual accumulation of small changes. - Natural selection, operating in varying contexts over vast spans of time, could account for the entire diversity of life. 3- Darwin s theory explains a wide range of observations 3.1- Natural Selection in Action 3.1.1- Differential Predation and Guppy Populations 3.1.2- The Evolution of Drug Resistant HIV - The evolution of drug resistant pathogens directly affects our own lives. - In bacteria and viruses (because of rapid rates of reproduction) a variation that makes individuals resistant to a particular drug can increase in frequency very quickly in the population. - The example of HIV (human immunodeficiency virus), which causes AIDS: (the evolution of HIV resistance to the drug 3TC) - Researchers have developed numerous drugs to combat this pathogen, but using these medications selects for viruses resistant to the drugs. - A few drug resistant viruses may be present by chance at the beginning of treatment. - Those that survive the early doses pass on the genes that enable them to resist the drug to their progeny, rapidly increasing the frequency of resistant viruses. - Case study: Evolution of drug resistance in HIV: - Rare resistant viruses multiply quickly when each patient is treated with the anti HIV drug 3TC. - Within just a few weeks, 3TC resistant organisms comprise 100% of the virus population in each case. - The 3TC resistant varieties of HIV carry slightly different versions of reverse transcriptase that are able to discriminate between the drug and the normal C nucleotide 10
-. The viruses that carry these genes have no advantage in the absence of 3TC; in fact, they replicate more slowly than those that carry the more common form. - But once 3TC is added to their environment it becomes a powerful selecting force, favoring reproduction of resistant individuals. - Two key points about natural selection: - First, natural selection is more a process of editing than a creative mechanism. - A drug does not create resistant pathogens; it selects for resistant individuals that were already present in the population. - Second, natural selection depends on time and place: - It favors those characteristics in a genetically variable population that increase fitness in the current, local environment. - What is adaptive in one situation may be useless or even harmful in another. 3.2- Homology, Biogeography, and the Fossil Record - Darwin s theory provides a cohesive explanation for observations made by researchers in the fields of anatomy, embryology, molecular biology, biogeography, and paleontology. 3.2.1- Homology - Certain characteristics in related species have an underlying similarity even though they may have very different functions. - Such similarity resulting from common ancestry is known as homology. Darwin s concept of descent with modification can explain why certain characteristics in related species have an underlying similarity even though they may have very different functions. Such similarity resulting from common ancestry is known as homology. 11
Homology Similarity in characteristics resulting from a shared ancestry. 3.2.1.1- Anatomical Homologies Homologous structures Structures in different species that are similar because of common ancestry. Vestigial organ A structure of marginal, if any, importance to an organism. Vestigial organs are historical remnants of structures that had important functions in ancestors. 3.2.1.2- Molecular Homologies 3.2.1.3- Homologies and the Tree of Life 3.2.2- Biogeography Biogeography The study of the past and present distribution of species. Endemic species Species that are confined to a specific, relatively small geographic area. 3.2.3- The Fossil Record What Is Theoretical about the Darwinian View of Life? 12