Evolution Review

Evolution Review

DNA Pioneers Hershey-Chase Two American scientists: Alfred Hershey and Martha Chase Collaborated on studying viruses that infect living organisms Bacteriophage: type of virus that infects bacteria

Hershey-Chase Experiments

DNA Pioneers Erwin Chargaff, American biochemist Puzzled by relationship between DNA’s nucleotides Discovered that guanine and cytosine are nearly equal in any DNA sample; same went for adenine and thymine

Chargaff’s Rules

Nucleotides Are made of – A simple sugar (Deoxyribose) – A phosphate group – A nitrogen base

DNA Structure Each side of the double helix is made of alternating sugar (deoxyribose) and phosphates Each strand is linked to the other by nitrogen bases

DNA Nitrogen Bases Four nitrogen bases make up the “stairs” of the DNA double helix Adenine Guanine Cytosine Thymine

DNA Nitrogen Bases Adenine and Guanine are purines – Made of two rings of carbon and nitrogen atoms Cytosine and Thymine are pyrimidines – Made of one carbon and nitrogen ring

DNA to RNA to Proteins RNA takes the genetic code from the nucleus to the ribosomes in a process called transcription. RNA differs from DNA in 3 ways: – It is a single strand (alpha helix) – It contains the sugar ribose – It has a different nitrogen base: uracil Uracil replaces thymine

A sea voyage helped Darwin frame his theory of evolution Darwin was particularly intrigued by the geographic distribution of organisms on the Galápagos Islands, including – marine iguanas – giant tortoises. – finches – Their features were very like the animals on the mainland

A sea voyage helped Darwin frame his theory of evolution By the early 1840s, Darwin had composed a long essay describing the major features of his theory of evolution by natural selection. But he delayed publishing his essay, continued to compile evidence in support of his hypothesis, and finally released his essay to the scientific community when learning of the work of another British naturalist, Alfred Wallace, who had a nearly identical hypothesis.

The study of fossils provides strong evidence for evolution Some fossils are not the actual remnants of organisms. The 375-million-year-old fossils in Figure 13.2B are casts of ammonites, shelled marine animals related to the present-day nautilus.

Figure 13.2b

The study of fossils provides strong evidence for evolution The fossil record is the chronicle of evolution over millions of years of geologic time engraved in the order in which fossils appear in rock strata.

SCIENTIFIC THINKING: Fossils of transitional forms support Darwin’s theory of evolution Thousands of fossil discoveries have since shed light on the evolutionary origins of many groups of plants and animals, including – the transition of fish to amphibian, – the origin of birds from a lineage of dinosaurs, and – the evolution of mammals from a reptilian ancestor.

SCIENTIFIC THINKING: Fossils of transitional forms support Darwin’s theory of evolution Whales are cetaceans, a group that also includes dolphins and porpoises. – They have forelimbs in the form of flippers but lack hind limbs. – If cetaceans evolved from four-legged land animals, then transitional forms should have reduced hind limb and pelvic bones.

SCIENTIFIC THINKING: Fossils of transitional forms support Darwin’s theory of evolution Beginning in the late 1970s, paleontologists unearthed an extraordinary series of transitional fossils in Pakistan and Egypt. The new fossil discoveries were consistent with the earlier hypothesis, and paleontologists became more firmly convinced that whales did indeed arise from a wolf-like carnivore.

Figure 13.3b Pakicetus Rodhocetus Dorudon Living cetaceans Key Pelvis Femur Tibia Foot

Homologies provide strong evidence for evolution Evolution is a process of descent with modification. – Characteristics present in an ancestral organism are altered over time by natural selection as its descendants face different environmental conditions. – Evolution is a remodeling process. – Related species can have characteristics that have an underlying similarity yet function differently. – Similarity resulting from common ancestry is known as homology.

13.4 Homologies provide strong evidence for evolution Darwin cited the anatomical similarities among vertebrate forelimbs as evidence of common ancestry. As Figure 13.4A shows, the same skeletal elements make up the forelimbs of humans, cats, whales, and bats, but the functions of these forelimbs differ. Biologists call such anatomical similarities in different organisms homologous structures.

Figure 13.4a Humerus Radius Ulna Carpals Metacarpals Phalanges Human Cat Whale Bat

13.5 Homologies indicate patterns of descent that can be shown on an evolutionary tree Homologous structures can be used to determine the branching sequence of an evolutionary tree. These homologies can include – anatomical structure and/or – molecular structure.

Figure 13.5 Each branch point represents the common ancestor of the lineages beginning there and to the right of it Lungfishes Amniotes Mammals 2 Tetrapod limbs Amnion Lizards and snakes 3 4 5 Ostriches 6 Feathers Hawks and other birds Birds A hatch mark represents a homologous character shared by all the groups to the right of the mark Crocodiles Tetrapods Amphibians 1

Figure 13.14 Frequency of individuals Original population Original population Evolved population Stabilizing selection Phenotypes (fur color) Directional selection Disruptive selection

The origin of species is the source of biological diversity Microevolution is the change in the gene pool of a population from one generation to the next. Speciation is the process by which one species splits into two or more species. Each time speciation occurs, the diversity of life increases.

There are several ways to define a species The word species is from the Latin for “kind” or “appearance.” Although the basic idea of species as distinct lifeforms seems intuitive, devising a more formal definition is not easy and raises questions. In many cases, the differences between two species are obvious. In other cases, the differences between two species are not so obvious.

Figure 14.2a-0

There are several ways to define a species How similar are members of the same species? – Whereas the individuals of many species exhibit fairly limited variation in physical appearance, certain other species—our own, for example—seem extremely varied.

Figure 14.2b

There are several ways to define a species Reproductive isolation – prevents genetic exchange (gene flow) and – maintains a boundary between species. But there are some pairs of clearly distinct species that do occasionally interbreed. – The resulting offspring are called hybrids. – An example is the grizzly bear (Ursus arctos) and the polar bear (Ursus maritimus), whose hybrid offspring have been called “grolar bears or pizzly bears.”

Figure 14.2c-0 Grizzly bear Polar bear Hybrid “grolar” or “pizzly” bear

VISUALIZING THE CONCEPT: Reproductive barriers keep species separate Reproductive barriers – serve to isolate the gene pools of species and – prevent interbreeding. Depending on whether they function before or after zygotes form, reproductive barriers are categorized as – prezygotic or – postzygotic.

Reproductive barriers keep species separate Five types of prezygotic barriers prevent mating or fertilization between species. 1. In habitat isolation, there is a lack of opportunity for mates to encounter each other. 2. In temporal isolation, there is breeding at different times or seasons.

Figure 14.3-0 PREZYGOTIC BARRIERS Habitat isolation (different habitats) Temporal isolation (breeding at different times) Mechanical isolation (incompatible reproductive parts) Behavioral isolation (different courtship rituals) Gametic isolation (incompatible gametes) POSTZYGOTIC BARRIERS Reduced hybrid vitality (short-lived hybrids) Reduced hybrid fertility (sterile hybrids) Hybrid breakdown (fertile hybrids with sterile offspring)

Reproductive barriers keep species separate 3. In behavioral isolation, there is failure to send or receive appropriate signals. 4. In mechanical isolation, there is physical incompatibility of reproductive parts. 5. In gametic isolation, there is molecular incompatibility of eggs and sperm or pollen and stigma.

Figure 14.3-3 Behavioral isolation (different courtship rituals) The blue-footed booby (Sula nebouxii) performs an elaborate courtship dance. The masked booby (Sula dactylatra) performs a different courtship ritual.

Figure 14.3-4 Mechanical isolation (physical incompatibility of reproductive parts) Heliconia latispatha is pollinated by hummingbirds with short, straight bills. Heliconia pogonantha is pollinated by hummingbirds with long, curved bills.

Figure 14.3-5 Gametic isolation (molecular incompatibility of eggs and sperm or pollen and stigma) Purple sea urchin (Strongylocentrotus purpuratus) Red sea urchin (Strongylocentrotus franciscanus)

Reproductive barriers keep species separate Three types of postzygotic barriers operate after hybrid zygotes have formed. 1. In reduced hybrid viability, interaction of parental genes impairs the hybrid’s development or survival. 2. In reduced hybrid fertility, hybrids are vigorous but cannot produce viable offspring. 3. In hybrid breakdown, hybrids are viable and fertile, but their offspring are feeble or sterile.

Reproductive barriers keep species separate Three types of postzygotic barriers operate after hybrid zygotes have formed. 1. In reduced hybrid viability, interaction of parental genes impairs the hybrid’s development or survival. 2. In reduced hybrid fertility, hybrids are vigorous but cannot produce viable offspring. 3. In hybrid breakdown, hybrids are viable and fertile, but their offspring are feeble or sterile.

Figure 14.3-7 Reduced hybrid fertility (vigorous hybrids that cannot produce viable offspring) A mule is the sterile hybrid offspring of a horse and a donkey.

In allopatric speciation, geographic isolation leads to speciation A key event in the origin of a new species is the separation of a population from other populations of the same species. – With its gene pool isolated, the splinter population can follow its own evolutionary course. – Changes in allele frequencies caused by natural selection, genetic drift, and mutation will not be diluted by alleles entering from other populations (gene flow).

In allopatric speciation, geographic isolation leads to speciation In allopatric speciation, the initial block to gene flow may come from a geographic barrier that isolates a population.

In allopatric speciation, geographic isolation leads to speciation Several geologic processes can isolate populations. – A mountain range may emerge and gradually split a population of organisms that can inhabit only lowlands. – A large lake may subside until there are several smaller lakes, isolating certain fish populations. – Continents themselves can split and move apart. – Allopatric speciation can also occur when individuals colonize a remote area and become geographically isolated from the parent population.

In allopatric speciation, geographic isolation leads to speciation How large must a geographic barrier be to keep allopatric populations apart? – The answer depends on the ability of the organisms to move. – Birds, mountain lions, and coyotes can easily cross mountain ranges. – In contrast, small rodents may find a canyon or a wide river a formidable barrier. The Grand Canyon and Colorado River separate two species of antelope squirrels.

Sympatric speciation takes place without geographic isolation Sympatric speciation occurs when a new species arises within the same geographic area as its parent species. How can reproductive isolation develop when members of sympatric populations remain in contact with each other? Gene flow between populations may be reduced by – polyploidy, – habitat differentiation, or – sexual selection.

Sympatric speciation takes place without geographic isolation Many plant species have originated from sympatric speciation that occurs when accidents during cell division result in extra sets of chromosomes. New species formed in this way are polyploid, in that their cells have more than two complete sets of chromosomes.

Sympatric speciation takes place without geographic isolation Sympatric speciation can result from polyploidy – within a species (by self-fertilization) or – between two species (by hybridization).