I. Evolution Evolution:

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1 I. Evolution Evolution: a) Over time, species gradually acquire structural, biochemical, & behavioral changes (selected for by the environment). These Adaptations facilitate survival & reproductive fitness in a particular environment. II. Early Views on Diversity of Life Aristotle ( BC): visualized physical entities & biological organisms as occupying a chain in the order of their perceived complexity. According to this Great Chain of Being, all entities occupied fixed positions above which they never progressed (evolved). This idea persisted in one form or another until the Age of Enlightenment. Figure 1: Aristotle s Great Chain of Being Georges Cuvier ( ): believed each species was a divine, permanent creation. While documenting fossil succession in the outcrops around Paris, Cuvier noted that each stratum (layer) was characterized by a unique collection of species. The more deeply buried the stratum, the more dissimilar the species from modern life. Figure 2: Cuvier s Interpretation of Paris Outcrops Unique fossil species limited to a SINGLE stratum (layer). To reconcile his observations with the accounts of Genesis, which suggested a young (6,000 year old) Earth with all species present in their modern forms, Cuvier proposed the idea of Catastrophism. This idea contended that each boundary between strata represented a RAPID, catastrophic event that eliminated species from the local region. To explain the appearance of new species in the strata immediately above, Cuvier suggested that they migrated into the area after the region had stabilized. 1

2 Contributions of catastrophism to a young Earth worldview: a) Geologic changes due to rapid, catastrophic events allows for a young Earth. b) Significant biological evolution is not likely assuming a young Earth (not enough time). c) Appearance of new species following each catastrophe is accounted for by migration, not species evolution. Charles Lyell ( ): in contrast to Cuvier, Lyell contended that the geologic features of the landscape (mountains, valleys, etc) occurred gradually over long periods of time. This idea of Gradualism was in direct opposition to catastrophism & depended on the assumption that geologic forces are constant over time, a concept known as Uniformitariansim. Contributions of gradualism to an old Earth worldview: d) Most geologic changes occur in small increments whose rate remains relatively constant over time. e) The long expanse of time needs for significant geologic change to form landscape features provides a timeframe suitable for the possible evolution of species. Erasmus Darwin ( ): Grandfather of Charles Darwin & early proponent of species evolution. Was one of the first to suggest that all life descended via evolutionary change from a single common ancestor. Jean Basptiste de Lamarck ( ): expands on ideas first proposed by Erasmus Darwin to present a possible mechanism for evolution that did not rely on divine guidance: a) Use & Disuse: according to Lamarck, a changing environment causes an organism to alter its behavior, thereby using some body parts more & others less. Over its lifetime, these traits will become increasingly well-developed or degenerate. Examples of use & disuse include: Increased skin pigmentation due to sun exposure. Thickening of the soles of the feet due to walking barefoot. b) Transmission of Acquired Traits: it was thought that physical characteristics acquired throughout the life of the individual are passed to offspring & further improved upon in future generations (i.e. elongated giraffe necks). Figure 3: Lamarckian Evolution via Transmission of Acquired Traits Deficiencies of Lamarckism Physical changes, like the lengthening of the giraffe s neck, do NOT become inscribed onto DNA. Therefore, it is impossible for such somatic changes to be passed onto subsequent generations. Most physical changes acquired over an organism s lifetime are the result of injury & disease. Reproduction of these individuals may result in future generations become progressively more decrepit. Lamarck s mechanism can only account for structures becoming more or less pronounced, but does NOT sufficiently explain the origin of complexity. III. Charles Darwin The Voyage of the HMS Beagle In 1831, Charles Darwin set sail on the survey ship HMS Beagle, whose primary mission was to circumnavigate the globe & verify longitude measurements. Upon reaching the east coast of South America, Darwin spent many weeks ashore studying geological formations & collecting animal, plant, & fossil specimens. 2

3 Figure 4: Voyage of the Beagle ( ) In 1835, The HMS Beagle spent 5 weeks at the Galapagos Islands, a geologically young volcanic island archipelago 600mi west of Ecuador. During his excursions to the islands, Darwin collected numerous plant & animal specimens (many of these specimens were sent back to England to be examined by Darwin s colleagues in the scientific community). Figure 4.1: Galapagos Specimens Collected by Darwin **Initially, Darwin assumed (wrongly so) that each plant & animal specimen collected throughout the islands (as well as those collected along the S. American coast) belonged to the SAME species. Shaping the Theory of Natural Selection Upon returning to England in 1836, Darwin is made aware that many of the specimens he collected throughout S. America & the Galapagos did not belong to the same species but were actually DISTINCT SPECIES! 3

4 This observation, coupled with that fact that Galapagos Islands are young relative to the South American landmass, led Darwin to suspect that the Galapagos species initially migrated from South America & subsequently evolved into unique species. This would explain why the species observed in both regions were so similar to one another. In considering a possible mechanism for how species evolved over time, Darwin drew upon several sources & experiences: a) Geological Observations: reads Charles Lyell s Principles of Geology, which contends that given the slow pace of many geological forces, Earth must be far older than the 6,000 years purported by theologians. This revelation that provided Darwin with a suitable time scale over which substantial biological evolution could occur. b) Artificial Selection / Domestication: practice by which breeders & farmers develop many varieties of domesticated plants & animals in just a few generations by choosing certain desirable traits & breeding only those individuals that exhibited the desired traits. Darwin hypothesized that a similar selective process occurred in nature to favor individuals having traits better adapted for survival. Figure 5: Artificial Selection / Domestication of Wild Mustard c) Thomas Malthus: noted that populations have the capacity to increase faster than the food supply. In the case of humans, this may lead to famine, disease, & war, which serve as checks on population growth. Darwin observed a parallel struggle for existence in the animal & plant kingdoms in which only the best adapted survived. *It was Malthus essay that provided Darwin with the driving force behind his burgeoning evolutionary mechanism. Without a struggle for existence, differential reproductive rates based on favorable adaptations would not be observed. 4

5 Figure 5.1: Thomas Malthus -Population Growth & Food Supply The aforementioned sources helped Darwin construct an evolutionary mechanism, which he called Natural Selection. Figure 6: Natural Selection Galapagos Tortoises a) Fragmentation & Isolation: populations may break into subgroups as a result of migration or by adopting different behaviors. The geographic features or behavioral differences between the groups act as Reproductive Barriers, preventing interbreeding (gene flow) that would otherwise maintain similarities between them. b) Overproduction & Variation: overproduction within each population produces a wide range of variants (speed, strength, camouflage, etc). The source of variation, unknown to Darwin, arises via mutation & genetic recombination. c) Struggle for Existence: overproduction results in intra & interspecific competition among individuals. Factors of the environment affecting the survival & reproductive success of individuals w/in populations (living space, predation, disease, food, mates) are called Selecting Agents. d) Survival of Fittest: in each subgroup, only those variants best equipped for survival (best able to cope with existing selecting agents) will reach reproductive age, passing these variations to their offspring. 5

6 e) Divergence: the adaptive traits will increase in frequency within each population, causing them to Diverge (become more dissimilar) from both each other & the ancestral population. After many generations, as more & more adaptive traits accumulate via natural selection, the populations will continue to diverge. f) Speciation: ultimately, each population will acquire so many new traits that they will no longer resemble each other or their ancestral form. Even if reunited, they will no longer be able to successfully interbreed. At such a point, Speciation is said to have occurred. On the Origin of Species In 1859, Darwin published On the Origin of Species by Means of Natural Selection. In which he presented overwhelming evidence for the evolution of species over time in addition to his controversial mechanism of natural selection. This book provided the foundation for understanding the relationships between all life & the origin of biological diversity. Key concepts to keep in mind when considering natural selection include: IV. Speciation via Natural Selection Figure 7: Allopatry The ancestors of the Galapagos species initially arrived via migration. In their new environments, they began to diverge from their S. American ancestors Allopatrically, whereby geographic barriers (i.e. oceans) prevented interbreeding & instead promoted speciation. As a consequence of being isolated on islands that typically exhibit a low amount of biodiversity & thus many unoccupied niches, many members of the migrant population began to assume different ecological roles (i.e. eating different foods, living in different habitats, etc). This resulted in further reproductive isolation between the resulting subgroups, leading to a rapid diversification or Adaptive Radiation into many species. 6

7 Figure 8: Allopatry & Adaptive Radiation: Galapagos Finches Upon reaching the Galapagos, finches began to occupy niches inaccessible to them on the S. American mainland. Individuals begin to assume different roles in their new environment (i.e. diets, living spaces, etc) due to genetic variations between them. This may result in the establishment of isolated groups Figure 8.1: Allopatry & Adaptive Radiation: Galapagos Finches Ultimately, the isolated groups w/in each niche acquired so many genetic differences as to establish distinct species via Adaptive Radiation. This method of speciation via natural selection occurs mostly on islands or after extinction events whereupon a species can rapidly diversify into many species by fragmenting & assuming previously unoccupied niches. 7

8 Figure 8.2: Allopatry & Adaptive Radiation: Galapagos Tortoises Upon arriving to the Galapagos, the tortoises began to adapt to the available food sources on each island. The geographic isolation between groups on different islands lead to adaptive radiations resulting in distinct changes in the shell morphology on each island. Tortoises from Isabela Island exhibited dome-shaped shells, adaptive for eating low-growing vegetation. Those from Hood Island exhibited saddle-backed shells, allowing the tortoises to raise their heads high in order to consume the higher growing vegetation on the island. V. Evolutionary Arms Races Figure 9: Arms Race 8

9 An Evolutionary Arms Race is a struggle between populations that develop adaptations & counter-adaptations against each other. At any point in an arms race, one side is better equipped. This forces the opposition to acquire better equipment to neutralize the threat. In response, the other side acquires even better equipment. Figure 9.1: Asymmetric Arms Race During an Asymmetric Evolutionary Arms Race, the parties involved are trying to accomplish DIFFERENT goals. For example, in the arms race between predator & prey species (i.e. cheetah & gazelle), the predator endeavors to capture & kill the prey, whereas the prey evolves adaptations to avoid detection & capture. Figure 9.2: Symmetric Arms Race During a Symmetric Evolutionary Arms Race, the parties involved are trying to accomplish the SAME goals. For example, in the arms race between trees in a forest, each individual grows upward to maximize its exposure to light. 9

10 Improved equipment without benefit, typical of arms races, is akin to running in place (energy expenditure w/o covering ground). This phenomenon is known as the Red Queen Effect. Examples include: a) As cheetahs & gazelles have driven adaptations in each other over time, each group has become WORSE off than before. This is because each group had to invest greater amounts energy & resources to maintain their respective equipment. Despite this, cheetahs are no more efficient at killing nor gazelles at evading attacks. b) As trees grow tall to outcompete their rivals for sunlight, the differential between the energy created via photosynthesis & the energy invested in growth progressively increases. All trees would be much better off if they remained shorter (greatest differential between photosynthetic output & energy invested in growth). Arms races end when further investments in developing survival equipment become too costly. For the cheetah, growing bigger leg muscles may result in more kills. To do this, an even greater energy investment is required. This energy would have to be donated from another vital life function (i.e. reproduction). VI. Evidence for Evolution Geographical Evidence: Species Distribution Figure 10: Biogeography: Wallace s Line Both Darwin & Wallace noted similarities among species living in the same geographic region. In general, species that live in the same geographic range are more likely to share a common ancestor. During his time in Indonesia, Wallace observed that mammal species on the islands to the west were placental, whereas those to the east were marsupial. Wallace concluded that the mammals inhabiting the western islands were descended from placental ancestors in Asia, while those inhabiting the eastern islands descended from marsupial ancestors from Australia. Since species are a reflection of the environment in which their ancestors evolved, they are usually found within a relatively narrow geographic range. Thus, introducing a species to an environment radically different from its own may impair its ability to survive (i.e. displacing lions from Africa to Antarctica). Biogeography: 10

11 Geologic Evidence: Radiometric Dating The geologic record provides a means by which the age of rock & the fossils within them can be accurately dated. Radioactive isotopes found in minerals decay at constant rates or Half Lives & act as clocks to date rocks. If the half-life of a radioisotope is known, the time since radioactive decay began (age of rock) can be determined: Figure 11: Absolute Age of Rock via Radiometric Dating It is found that ALL the radioactive clocks in the oldest rocks converge on an age of the Earth that is approximately 4.6 billion years, far older than the 6,000 years claimed by young earth creationists. This age provides a suitable amount of time for significant biological evolution. Geologic Evidence: Fossil Sequences The geologic record, largely incomplete in Darwin s day, now illustrates a general progression from simple to more complex forms (each form modified toward improvement). At NO place do we observe the remains of an organism before it could have evolved (i.e. fossil rabbit remains in the Devonian period, prior to the known appearance of mammals). If such remains were ever uncovered, it would imply that ALL life was created at the SAME time in accordance with creationist dogma. 11

12 Figure 11.1: Logical Fossil Sequences Geologic Evidence: Transition Fossils Natural selection predicts that, within any group, intermediate forms between primitive & more modern forms should exist within the fossil record. Such intermediates are called Transitional Fossils, rare in Darwin s day, but now so abundant that the evolutionary history of most species can be accurately reconstructed. Figure 11.2: Transitional Fossils (Fish to Tetrapods -4 legged land animals) Geologic Record: 12

13 Anatomical Evidence: Homologous Structures Figure 12: Homologous Structures (Mammalian Forelimbs) Figure 12.1: Homologous Structures via Common Ancestry Homologous Structures: 13

14 Anatomical Evidence: Vestigial & Analogous Structures Figure 12.2: Vestigial Structures (Whale Pelvis) Vestigial Structures: a) Vestigial structures provide evidence of evolution simply because we can observe their functional homologues in other related groups. These groups must share a common ancestor in which the structure was fully functional. Figure 12.3: Analogous Structures (Forelimb Structure Across Groups) *Structures 1-4 are HOMOLOGOUS. **Structures 4 & 5 are ANALOGOUS Analogous Structures: a) The presence of analogous structures across groups does NOT imply a common ancestry. They do however suggest that each species evolved in a convergent manner the non-related species adapted in similar ways to similar environmental demands. 14

15 Molecular Evidence: Immunological Studies Figure 13: Magnitude of Immune Response vs Species Relatedness Figure 13.1: Human & Chimpanzee Chromosome 2 Homology It is documented that humans possess 46chromosomes as opposed to 48 for all other primate species. This discrepancy in chromosome number is explained by the observation that human chromosome 2 was the product of the fusion of 2 chromosomes (corresponding to primate chromosomes 2A & 2B) that must have occurred at some point after our ancestors split from other primates. 15

16 Figure 13.2: DNA Hybridization Figure 13.3: Molecular Clocks The observed mutation rates w/in genes can be used as molecular clocks to estimate the time since two groups diverged from a common ancestor. In the above example, the time since the 2 modern species diverged can be calculated in the following manner: a) Count the number of base differences (or amino acid differences if comparing common proteins). b) Multiply this number by the estimated mutation rate. c) Divide this value by two. Thus the time since divergence is: 4 differences x 25 million years / 2 = 50 million years 16

17 Comparative Biochemistry: Embryological Evidence: Common Germ Layers Figure 14: Embryonic Germ Layers Early Embryonic Stages (Vertebrates) Comparative Embryology: a) Early embryologists discovered that all animal groups exhibited 3 distinct layers during development (ectoderm, mesoderm, & endoderm) that gave rise to the SAME adult tissues in all groups. They also noticed that features common to all animals (primitive features) appear early on in development while features unique to a group appear later on (derived characters). VII. Documented Examples of Natural Selection at Work The English Peppered Moth The English peppered moth occurs in two varieties that differ in coloration. The form for which the moth is named is light with splotches of pigment (recessive). The other variety is uniformly dark (dominant). Peppered moths rest on trees & rocks encrusted with light-colored lichens. Against this background, light moths are camouflaged, but the more conspicuous dark moths are susceptible to predation. Thus light colored individuals were more common prior to the industrial revolution. In the mid-1800s, entomologists recognized that dark-colored moths were becoming more common in industrialized areas. Some suggested pollution might be causing the moths to turn gray or black, a phenomenon called Industrial Melanism. Henry Kettlewell believed the shift toward the dark phenotype in industrialized regions was due to natural selection & the color of individual moths must have some effect on their survival rate. To test his hypothesis, he conducted a mark-release-recapture experiment in forest near polluted & unpolluted regions: 17

18 The next year, he conducted similar experiments in an unpolluted area. When he recaptured the moths, he found that marked light-colored moths were twice as likely to avoid predation by birds in the unpolluted forests. Conclusion: the presence or absence of pollution severed to shift selective pressure in the form of predation to one variety or the other. In unpolluted regions, there was more selective pressure against black morphs due to their more conspicuous coloration. Likewise, there was greater selective pressure against light morphs in unpolluted regions. Antibiotic Resistance In a bacterial colony, a small percentage of cells may be naturally resistant to antibiotics. This ability may also be acquired through chromosomal mutations or acquisition of plasmids that confers resistance. Antibiotics act as a selecting agent by eliminating nonresistant cells which normally keep the number of resistant varieties in check. Consequently, within hours a colony of 1 billion resistant bacteria (comprised of both pathogenic & nonpathogenic strains) may be produced. Via genetic recombination (transformation, conjugation, transduction), the resistant cells may acquire genes conferring resistance to additional antibiotics. Such multi-resistant bacterial strains are more difficult to kill. Pesticide Resistance In a population of insect crop-pests, a small percentage of individuals may possess a mutant allele that results in a low growth rate. Consequently, these individuals take longer to reach reproductive age & produce less offspring. In the presence of a pesticide, such a gene also may allow the individual to tolerate the harmful chemical. Thus, in the presence of such a selecting agent, these individuals will have a survival advantage & are more likely to reach reproductive age. After a several generations, the entire population may become resistant to the pesticide. 18