DESCRIBE CELL THEORYAll organisms are made of cells, and all cells come from preexisting cells.Where do cells come from?1. Spontaneous generation hypothesis versus all-cells-from-cells hypothesis a.Pasteur’s experiment supported the all-cells-from-cells hypothesis. (next slide)2. All individuals in a population of single-celled organisms are related by ancestry.3. All cells in a multicellular organism are descended from the same ancestral cell.
DESCRIBE NATURAL SELECTION1. Natural selection occurs whenever two conditions are met: a. Individuals within a population vary in characteristics that are heritable. b. Certain heritable traits help individuals survive and produce offspring.2. How do these two conditions lead to evolution? a. If certain heritable traits help individuals produce more offspring, then those traits become more common in the population over time. 3. Natural selection acts on individuals, but evolutionary change affects only populations.
NATURAL SELECTION EXAMPLEGalapagos FinchesThere are 13 types of Galapagos finches, also known as Darwins Finches, and they share the same habits and characteristics except for one: All 13 have different beaks. The differences in their beaks might be the most important aspect of their survival. Two documented examples of that survival occurred in the late 1970s and mid 1980s. First, when a large drought affected the island in 1977, seeds became scarce. Finches with the largest, toughest beaks were able to eat larger seeds that werent typically part of their diet. As a result, they survived. Finches with smaller beaks were unable to crack the tough seeds and many died. Through the process of natural selection, the birds that were able to adapt reproduced and thrived, while the others dwindled in number. However, adaptation can go both ways, as was seen during rainy weather in 1984 and 1985. The heavy rains created more of the small, soft seeds and few of the large ones. This time, more birds with the smaller beaks survived and produced more offspring.-Source
PROPERLY FORMATTED SCIENTIFIC NAME Genus (capitalized) followed by species (lower case). Underlined if written or italicized if typed. Geospiza fortisGenus: a taxonomic category of closely related species.Species: a distinct, identifiable group of populations that is thought to be evolutionarliy independent of other populations and whose members can interbreed.
EVOLUTION & EVOLUTIONARY THOUGHTPlato and typological thinking1. Plato saw species as unchanging, perfect "types" created by God.2. Plato thought individual variation was an unimportant deviation from the true "type." B. Aristotle and the great chain of being (scale of nature) 1. Aristotle, like Plato, thought species were unchanging types. 2. Aristotle thought species could be organized into a sequence or ladder of increasing complexity, with humans at the top.
EVOLUTION & EVOLUTIONARY THOUGHT CONT…C. Lamarck and the idea of evolution as change through time1. Lamarck noticed that organisms changed over time.2. Lamarck thought animals progressed over time from "lower" to "higher" forms (like Aristotles ladder) via inheritance of acquired characteristics. Darwin and Wallace and evolution by natural selection 1. Species change over time, but they do not "progress." 2. A species does not have a single true "type." 3. Individual variation is important; variation is what drives evolution. 4. This theory made predictions and was testable; that is, it was scientific.
EVIDENCE OF CHANGE THROUGH TIME1. The fossil record and geologic time a. A fossil is any trace of an organism that lived in the past. b. The fossil record was initially organized based on the relative age of the fossils. c. The geologic time scale (1) Sedimentary rocks form layers over long times. These layers form in a chronological sequence (the geologic time scale). (2) From the number of layers and the time it takes to deposit each one, geologists realized that the Earth must be very old. d. Radiometric dating enables us to date rocks directly. (1) The Earth is 4.6 billion years old. (2) The earliest signs of life are in rocks that are 3.4−3.8 billion years old.2. Extinction changes the species present over time. a. The fossil record shows that more than 99% of all the species that have ever lived are now extinct b. This is evidence that the species composition on Earth has changed over time.
EVIDENCE OF CHANGE THROUGH TIME CONT…3. Transitional features link older and younger species. a. Law of succession: Fossils found in a certain geographic region frequently resemble the species currently living in that region. (1) This is evidence that the extinct species are related to existing species. b. Fossils with transitional features (traits intermediate between those of older and younger species) are compelling evidence that species change over time. Example: the fins-to-feet transition. 4. Vestigial traits are evidence of change through time. a. Vestigial traits are traits that have reduced or no function but are clearly related to functioning organs in related species. (Fig. 24.5) b. The existence of these traits challenges the theory of special creation that organisms were designed by a perfect creator and are static. c. Biologists interpret the existence of these traits as evidence that organisms change over time.
EVIDENCE THAT SPECIES ARE RELATED BYCOMMON ANCESTRY1. Similar species are found in the same geographic area. a. Similar, but distinct, species are often found living close together in the same geographic area, implying that they are linked by a common ancestor. Example: Galápagos mockingbirds. (Fig. 24.6a) b. These similar species are part of a phylogeny (a family tree), and their relationships can be diagrammed on a phylogenetic tree (a branching diagram that indicates genealogy). (Fig. 24.6b)
EVIDENCE THAT SPECIES ARE RELATED BYCOMMON ANCESTOR CONT…2. Homology: the occurrence of similar features in different species because they both inherited the trait from a common ancestor. a. Different kinds of homology: (1) Genetic homology—similarities in DNA sequences (Fig. 24.7) (2) Developmental homology—similarities in the morphology of embryos and the fate of embryonic tissues (Fig. 24.8) (3) Structural homology—similarities in the structure of body parts (Fig. 24.9) b. The three levels of homology interact: Genetic homologies cause developmental homologies, which cause structural homologies. c. Hypotheses about homology can be tested experimentally. (Fig. 24.10) d. Homology is used extensively in contemporary biology. Examples: use of model organisms, comparative genomics.
THE PROCESS OF EVOLUTION: HOW DOESNATURAL SELECTION WORK?A. Darwins four postulates, the outcome of which is evolution: 1. Individuals vary. 2. Some variations are heritable. 3. More offspring are produced than can survive. 4. Individuals with traits that confer an advantage are more likely to survive and reproduce.Summary: Evolution by natural selection occurs whenever heritable variation leads to differential success in survival and reproduction.
MECHANISMS OF EVOLUTIONARY CHANGE1. Natural selection increases the frequency of alleles that contribute to improved reproductive success.2. Genetic drift changes allele frequencies randomly.3. Gene flow changes allele frequencies when individuals join or leave a population.4. Mutation continually introduces new alleles.
GENETIC DRIFTA. is any change in the allele frequencies in a population that is due to chance (luck, sampling error). 1. causes allele frequencies to drift up and down randomly over time. 2. Computer simulations can model genetic drift over many generations. (Fig. 25.6)3. Key points about genetic drift: a. Genetic drift is random with respect to fitness. b. Genetic drift is most pronounced in small populations. (Fig. 25.6) c. Over time, genetic drift can lead to the random loss or fixation of alleles.
GENETIC DRIFT CONT…What causes genetic drift in natural populations? 1. Founder effects (Fig. 25.8a) a. Drift occurs when a group of individuals immigrate to a new geographic area and establish a new population (founder event). b. If the new population is small, the allele frequencies will likely be different from those in the original population (founder effect). 2. Population bottlenecks (Fig. 25.8b) a. A bottleneck occurs when a disease outbreak, a natural catastrophe, or another event causes a sudden reduction in population size. b. The remaining individuals likely have different allelic frequencies than the original population has.
GENE FLOWA. is the movement of alleles from one population to another. (Fig. 25.9)C. How does gene flow affect fitness? 1. Gene flow is random with respect to fitness. 2. Gene flow tends to equalize allele frequencies among populations. a. For example, human migration across continents is homogenizing allelic frequencies in human populations.
GENE FLOW CONT…B. Gene flow in natural populations 1. Example: great tits on an island in the Netherlands. (Fig. 25.10a) a. Birds on the side of the island nearest the mainland, where gene flow is greatest, are less adapted to the island environment. (Fig. 25.10b)
MUTATIONA. How does mutation affect genetic variation in a population? 1. Mutation constantly introduces new alleles at all loci. 2. Mutation increases genetic diversity in populations. 3. Mutation introduces new alleles into every individual, every generation. a. For example, each human contains an average of 1.1 new alleles created by mutation (new mutations unique to that individual). 4. Mutation is the ultimate source of all genetic variability. 5. Without mutation, evolution would stop. 6. Mutation is random with respect to fitness; most mutations are deleterious, but a few are advantageous.
MUTATION CONT….B. Mutation as an evolutionary mechanism 1. Mutation happens too infrequently to cause dramatic changes in allele frequencies on its own. a. Example: human mutation rates. It would take 4000 years for a mutation to produce a change in allele frequency of 1%. 2. However, when considered across entire genomes and combined with natural selection, mutation becomes an evolutionary force.
SUMMARY OF THE FOUR EVOLUTIONARYFORCES: SELECTION, GENETIC DRIFT, GENEFLOW, AND MUTATION (TABLE 25.3)
MODES OF NATURAL SELECTIONA. Directional selection1. One extreme is favored, and the average phenotype (and the other extreme) is selected against. (Fig. 25.3a) Examples: body size and cold tolerance in cliff swallows. (Fig. 25.3b)2. Directional selection changes the average value of a trait.3. It reduces genetic variation.a. If continued for a long time, the favored allele becomes "fixed" at 1.0, and the other allele(s) dies out.4. It is often counterbalanced by selection on a different trait (countervailing selection and fitness trade-offs).
MODES OF NATURAL SELECTION CONT… B. Stabilizing selection 1. The average phenotype is favored, and both extremes are selected against. (Fig. 25.4a) Example: human birth weight. (Fig. 25.4b) 2. The average value of the trait does not change over time. 3. Stabilizing selection reduces genetic variation.
MODES OF NATURAL SELECTION CONT… Disruptive selection 1. Both extremes are favored, and the average phenotype is selected against. (Fig. 25.5a) Example: beak length in black-bellied seedcrackers. (Fig. 25.5b) 2. Disruptive selection is relatively rare. 3. It maintains genetic variation. 4. It plays a role in speciation—the generation of new species.
HOW ARE SPECIES IDENTIFIED? 1. Biologists use different sets of criteria to identify species.The biological species concept 1. Species are defined by reproductive isolation. 2. Populations are reproductively isolated if they fail to interbreed or to produce viable, fertile offspring. a. Prezygotic isolation mechanisms prevent fertilization. b. Postzygotic isolation mechanisms prevent survival or reproduction of the hybrid offspring3. Reproductive isolation cannot be evaluated in fossils, asexual species, or species whose ranges do not overlap.
HOW ARE SPECIES IDENTIFIED?The phylogenetic species concept 1. A species is defined as the smallest monophyletic group on the tree of life. a. A monophyletic group consists of an ancestral population, all of its descendants, and only its descendants. b. A monophyletic group is identified by synapomorphies⎯homologous traits unique to that lineage. c. Example: elephants. (Fig. 26.2)
HOW ARE SPECIES IDENTIFIED?C. The morphospecies concept 1. Species are defined by differences in size, shape, or other morphological features.
STRENGTH & WEAKNESSES OF CONCEPTSBiological species concept Strength: Either individuals can or cannot produce viable offspring. Weakness: Many living species have never been observed mating and then those offspring mating and raising viable. Fossils certainly can’t mate any longer, although we have to classify them. Some living organisms, e.g. bacteria don’t have sex but can swap genetic material among themselves.Morphospecies concept Strength: widely applicable and can be used for fossil species and asexual species. Weakness: cannot identify cryptic species, and it is subjective.Phylogenetic species concept Strength: It can be applied to any populations. It is logical because populations are monophyletic only if they are independent of one another and isolated from gene flow. Weakness: It is time consuming and there isn’t a phylogenetic tree readily available for every species.
SPECIATION MECHANISMAllopatric: The divergence of populations into different species by physical isolation of populations in different geographic areas.Speciation that begins with physical isolation is called allopatric speciation.1. Physical isolation occurs in two ways. a. Dispersal—a population can colonize a new habitat. (Fig. 26.4a) b. Vicariance—a new physical barrier can split a widespread population into two or more isolated groups. (Fig. 26.4b)
SPECIATION MECHANISMS CONT…Dispersal and colonization isolate populations. 1. Example: Galápagos finches a. A population of ground finches colonized a new island. Later, they were found to have larger beaks that the original population. 2. Colonization, followed by drift and selection, is thought to be responsible for speciation in many groups.C. Vicariance isolates populations.1. Example: Snapping shrimp a. Many species of snapping shrimp in the Pacific have a sisterspecies in the Atlantic that is their closest relative. b. This pattern indicates that the formation of the land bridge of Panama was a vicariance event that separated many marine species into Pacific and Atlantic populations, which then speciated.
SPECIATION MECHANISMS CONT…Sympatric: The divergence of populations living within the same geographic area into different species as the result of their genetic (not physical) isolation.A. Sympatric populations live in the same area. Traditionally, biologists thought that speciation could not occur among sympatric populations because gene flow is possible. (Fig. 26.6)B. Can natural selection cause speciation even when gene flow is possible?1. Yes, particularly if populations prefer different habitats (so that their preference for specific habitats restricts gene flow).2. Example: apple maggot flies. a. Apple maggot flies occur only in North America and feed only on apple trees, but apple trees have been in North America for only 300 years. b. Apple maggot flies closest relatives are hawthorn flies. c. The two types of flies each prefer the scent of their own host tree, rarely land on other trees, and thus rarely meet each other. (Fig. 26.7) (1) There is very little gene flow, despite sharing the same range. d. Disruptive selection appears to be driving the two populations toward sympatric speciation.
SPECIATION MECHANISMS CONT…Autopoly ploidy: The state of having more than two full sets of chromosomes (polyploidy) due to a mutation that doubled the chromosome number.Autopoly ploidy is a doubling of the chromosome number in the offspring of one species.b. This can result in a new tetraploid species. Example: maidenhair ferns became tetraploid due to an error in meiosis.
WHAT HAPPENS WHEN ISOLATED POPULATIONS COME INTO CONTACT?A. Possible outcomes of secondary contact (Table 26.4) 1. If prezygotic isolation mechanisms do not exist, the populations may start breeding again, leading to any of the following outcomes: a. Fusion⎯if gene flow erases differences between populations b. Reinforcement⎯selection against interbreeding, leading to further divergence and evolution of prezygotic isolation mechanisms c. Founding of a stable hybrid zone d. Extinction of one population e. Creation of a new species via hybridization
WHAT HAPPENS WHEN ISOLATEDPOPULATIONS COME INTO CONTACT?B. Reinforcement 1. If hybrid offspring have lower fitness than their parents, there will be strong selection against interbreeding (reinforcement). 2. Example: Closely related species of Drosophila fruit flies: Sympatric Drosophila species are seldom willing to mate with each other, but allopatric species are.D. New species through hybridization 1. If hybrids have higher fitness than either parent, a new species can arise. a. This can occur if hybrids have a unique combination of alleles that give it different characteristics than either parent, 2. Example: Helianthus sunflowers of the American West. (Fig. 26.11) a. H. anomalus appears to have arisen as a hybrid. b. It inherited a unique combination of alleles that allowed it to survive in drier habitats than either parent species. c. Researchers succeeded in recreating the speciation event in the lab.
WHAT HAPPENS WHEN ISOLATED POPULATIONS COME INTO CONTACT?C. Hybrid zones1. Hybrid zones are geographic areas where interbreeding occurs and hybrid offspring are common.a. This sometimes occurs when hybrids have intermediate traits that are well adapted to the environment where the two species overlap.b. Hybrid zones can be narrow or wide, and long or short lived.2. Example: Townsends and hermit warblers. Mitochondrial DNA (mtDNA) analysis shows that the hybrid zone has been expanding to the south, apparently driving out the hermit warblers. (Fig. 26.10)3. Hybrid zones can lead to the extinction of one of the parent species or to the formation of a new species.
ADAPTIVE RADIATIONA. Adaptive radiation is rapid diversification of a single lineage into a wide variety of adaptive forms. Example: Hawaiian silverswords. (Fig. 27.11)
ADAPTIVE RADIATION CONT…B. Why do adaptive radiations occur? 1. Adaptive radiations frequently occur when habitats are unoccupied by competitors. 2. Example: Anolis lizards in the Caribbean. (Fig. 27.12) a. From different starting points, an adaptive radiation occurred on each island, eventually producing the same four ecological types of lizards. 3. Morphological innovation as a trigger a. Morphological innovations (new traits) are a driving force behind many adaptive radiations. b. Examples: insect body plan, flowers, cichlid jaws, and bird flight. (Fig. 27.13)Figures on next slide.
PATTERNS/PROCESSES OF THE DESERT1. Arid environment -when input (precipitation) < output (evaporation) -receives less than 250 mm/yr of precipitation2. Found 30’ north of Tropic of Cancer & 30’ south of Tropic of Capricorn3. Four types - Trade Wind desert - Continental Interior desert - Costal Desert - Rain Shadow desert4. Environmental Challenges - finding food - finding mates - aridity - extreme temperatures - salinity
KEY TERMS OF THE DESERT1. Alluvial Fan - a fan-shaped deposit formed where a fast flowing stream flattens, slows, and spreads, typically at the exit of a canyon onto a flatterplain.2. Bajada - formed by two alluvial fans3. Playa -flat area, sandyHOW ORGANISMS COPE WITH THE DESERT
HOW ORGANISMS COPE WITH THE DESERTAvoiding Heat Many desert animals are nocturnal and come out to forage only at night, or are crepuscular and feed at dawn and dusk. During the heat of the day, they stay out of the heat in burrows or rock crevices.