* For example, Haeckel proposed that the pharyngeal grooves between the pharyngeal arches in the neck of the human embryo not only roughly resembled gill slits of fish, but directly represented an adult "fishlike" developmental stage, signifying a fishlike ancestor. Embryonic pharyngeal slits, which form in many animals when the thin branchial plates separating pharyngeal pouches and pharyngeal grooves perforate, open the pharynx to the outside. Pharyngeal arches appear in all tetrapod embryos: in mammals, the first pharyngeal arch develops into the lower jaw (Meckel's cartilage), the malleus and the stapes. But these embryonic pharyngeal arches, grooves, pouches, and slits in human embryos can not at any stage carry out the same function as the gills of an adult fish, thus a homology. * Gene duplications are an essential source of genetic novelty that can lead to evolutionary innovation. Duplication creates genetic redundancy and if one copy of a gene experiences a mutation that affects its original function, the second copy can serve as a 'spare part' and continue to function correctly. Thus, duplicate genes accumulate mutations faster than a functional single-copy gene, over generations of organisms, and it is possible for one of the two copies to develop a new and different function. This is an examples of neofunctionalization.
* Gene duplication is believed to play a major role in evolution; this stance has been held by members of the scientific community for over 100 years. It has been argued that gene duplication is the most important evolutionary force since the emergence of the universal common ancestor.
* Another possible fate for duplicate genes is that both copies are equally free to accumulate degenerative mutations, so long as any defects are complemented by the other copy. This leads to a neutral "subfunctionalization" model, in which the functionality of the original gene is distributed among the two copies. Neither gene can be lost, as both now perform important non-redundant functions, but ultimately neither is able to achieve novel functionality. Subfunctionalization can occur through neutral processes in which mutations accumulate with no detrimental or beneficial effects. However, in some cases subfunctionalization can occur with clear adaptive benefits. If an ancestral gene is pleiotropic and performs two functions, often times neither one of these two functions can be changed without affecting the other function. In this way, partitioning the ancestral functions into two separate genes can allow for adaptive specialization of subfunctions, thereby providing an adaptive benefit
* Halobacteria sp. - Halobacteria are halophilic microorganisms, which means they grow in extremely high salinity environments. This archaeon can act as a good model for some aspects of eukaryotic biology, such as DNA replication, transcription, and translation.
* Methanopyrus sp. - a species of hyperthermophilic, halotolerant, methanogenic archaeon, originally isolated from sediments associated with undersea, hydrothermal, vent fields.rod-shaped cells, growing singly or in short chains of generally less than 10 individuals.
* Methanococcus jannaschii - n autotropic hyperthermophillic organism that belongs to the kingdom of Archaea. They were found to live in extreme environments such as hypothermal vents at the bottom of the oceans in which water reaches boiling temperature or pressure is extremely high (Bult, C.J. et. al., 1996). The evolutionary of these organisms and the biological mechanisms that they use to not only survive but also thrive in such extreme environments are of great interest to current researches.
* So if we accept the idea (at least tentatively) that animals had the genetic mechanisms to become large well before the Cambrian, why didn't they?
- Here we may need to look at extrinsic explanations of diversification
* Ancient atmosphere contained insufficient O2 to allow evolution of active life styles
- O2 didn't approach current levels until sometime in the Ediacaran
- Without sufficient O2, large animals are possible, but not large, active animals
- This may be the reason there are reasonably large animals in the Ediacaran, but not, apparently, very active ones
* Possibly a mass extinction of the former biota allowed new forms to radiate at the start of the Cambrian
- This has happened after previous mass extinctions
- We know the Ediacaran animals disappeared rapidly just before the Cambrian, but whether this was due to a mass extinction is not yet certain
* Another ecological explanation envisions an ecosystem that reached a tipping point in complexity resulting in widespread co-evolution
- For instance, there may have been an "arms race" between predator and prey
- Each advance in predatory ability requires a countermeasure by prey, and vice versa
- Greater incorporation of mineralized hard parts may have started such an arms race
- This would also explain the sudden appearance of fossils at this time
* The theory of the Cambrian Explosion holds that, beginning some 545 million years ago, an explosion of diversity led to the appearance over a relatively short period of 5 million to 10 million years of a huge number of complex, multi-celled organisms. Moreover, this burst of animal forms led to most of the major animal groups we know today, that is, every extant Phylum. It is also postulated that many forms that would rightfully deserve the rank of Phylum bothCambrian Fossils appeared in the Cambrian only to rapidly disappear. Natural selection is generally believed to have favored larger size, and consequently the need for hard skeletons to provide structural support - hence, the Cambrian gave rise to the first shelled animals and animals with exoskeletons (e.g., the trilobites). With the innovation of structural support, the early Cambrian period also saw the start of an explosion in the size of many animals.
* The Cambrian Explosion is the outcome of changes in environmental factors leading to changes in selective pressures, in turn leading to adaptive diversification on a vast scale. By the start of the Cambrian, the large supercontinent Gondwana, comprising all land on Earth, was breaking up into smaller land masses. This increased the area of continental shelf, produced shallow seas, thereby also expanding the diversity of environmental niches in which animals could specialize and speciate.
* The Earth is 4.5 billion years old (give or take 200 million years!) formed from a rotating disk of gas and dust in the young Solar system. The earliest evidence we have for life on Earth comes from rocks 3.75 years old in Greenland. clock
* These sedimentary rocks record the cooling and stablisation of the Earth's crust, and contain a ratio of 12C/13C isotopes which indicate an organic origin for the rock carbon. In other words, life had originated nearly 3.8 billion years ago, about as early as was possible considering the time required for the young Earth to cool.
* The earliest morphological evidence for life is 3.5 billion years old, fossils of stromatolites (colonies of cyanobacteria) and single, undifferentiated cells, or Prokaryotes. For 1.6 billion years these simple cells were the only kind of living organism, until the arrival of Eukaryotes, or single cells with differentiated nuclei and cell organelles. Although representing a large leap in complexity, the Eukaryotes were still only single cells or cell aggregates. It was another 1.4 billion years before complex, multicellular life made an appearance in the form of the Ediacaran faunas (see fossils), followed by all the variety of the true Cambrian animals about 550 million years ago.
* Therefore 80% of the history of life on Earth is exclusively single or undifferentiated multi-cellular. 3 billion years went by before complex multicellular life appeared, but when it did it only took between 5 and 10 million years for all the basic body plans of the organisms we see around us today to be established. This is why the origin of multicellular life, in particular the metazoans or large animals with complex body plans, is termed the Cambrian explosion.
* Often brought up in the origins debate is how evolution does not explain the origin of life. Let's get something abundantly clear: abiogenesis and evolution are two completely different things. The theory of evolution says absolutely nothing about the origin of life. It merely describes the processes which take place once life has started up. There may also be multiple pathways to producing naturally occurring "life". Depending, of course, on the definition of life. This is something that Ben Stein is apparently willfully ignorant of.
* An objection to the distinction is that it is goalpost moving but this would only be true if evolution at some point did try to explain the origin of life and then people moved away from it. This is not the case at all. Evolutionary theory started with the observation of the mutability of species - a property that only exists once life has begun, indeed later definitions of "life" have often used the ability to evolve as a key component. This, of course, has been known for some time as animals and crops have been selectively bred for thousands of years. Later, the idea was refined by Charles Darwin in the form of natural selection, where nature provides the selection criteria to drive evolution. At no point was evolution, nor natural selection, about explaining the origin of life.
* One objection is that explaining the origin of life is a natural extension to what evolution has to explain. In fairness this is true, and theories surrounding abiogenesis often use natural selection as a jumping point for how organised molecules could themselves develop further (thus making such molecule groups "alive" by the definition discussed above). But whether evolution and natural selection can explain this stage in the development of life is absolutely irrelevant to its validity to living creatures post abiogenesis. A common analogy to the fallacy of rejecting evolution due to it not explaining the origin of life is that gravity doesn't explain the origin of life. Another might be that it is akin to confusing a university's admission process with grading, advancement, and graduation once students are admitted.
The discovery of ribozymes supported a hypothesis, known as the RNA World Hypothesis, that earlier forms of life may have relied solely on RNA to store genetic information and to catalyze chemical reactions. This hypothesis was proposed independently by Carl Woese, Francis Crick and Leslie Orgel in the 1960s -- decades before the discovery of ribozymes -- and soon after the double-helical structure of DNA was determined. According to the RNA World Hypothesis, life later evolved to use DNA and proteins due to RNA's relative instability and poorer catalytic properties, and gradually, ribozymes became increasingly phased out.
* The ribosome, a large molecular machine that drives protein synthesis, is a ribozyme. Roll over to compare the ribosome structure with and without proteins. Proteins are shown in green, and RNA is shown in blue and white.
* Perhaps the strongest evidence for the RNA World Hypothesis is the fact that the ribosome, a large molecular complex that assembles proteins, is a ribozyme. Although the ribosome is made up of both RNA and protein components, structural and biochemical analyses revealed that the mechanisms central for translation (the process of assembling a peptide chain based on a RNA sequence) is catalyzed by RNA, not protein. This suggests that the use of RNA by early lifeforms to carry out chemical reactions preceded the use of proteins.
* Organisms in the domain Eukarya are eukaryotic cells, or consist of them, which have membranes that are similar to those of bacteria. Eukaryotes are further grouped into Kingdom Fungi (yeast, mold, etc.), Kingdom Plantae (flowering plants, ferns, etc.) and Kingdom Animalia (insects, vertebrates, etc.) and the now-defunct, paraphyletic Kingdom Protista (algae, protozoans, etc.).
* Not all Eukaryotes have a cell wall, and even in those which do, the walls do not contain peptidoglycan, which bacteria do have. While cells are organized into tissues in the kingdom Plantae as well as the kingdom Animalia, cell walls are never found in animal cells.
* Forty or fifty years ago, thanks to antibiotics, scientists thought medicine had all but eradicated infectious agents as a major health threat. Instead, the past two decades have seen an alarming resurgence of infectious diseases and the appearance of new ones.
* Today, the AIDS virus, tuberculosis, malaria, diarrheal diseases and other infectious agents pose far greater hazards to human existence than any other creatures.
* This upsurge of infectious disease is a problem we have unwittingly created for ourselves. The rise of rapid, frequent, and relatively cheap international travel allows diseases to leap from continent to continent. Inadequate sanitation and lack of clean drinking water are another factor. A third is the "antibiotic paradox" -- the overuse of the "miracle drugs" to the point that they lose their potency.
* Whenever antibiotics wage war on microorganisms, a few of the enemy are able to survive the drug. Because microbes are always mutating, some random mutation eventually will protect against the drug. Antibiotics used only when needed and as directed usually overwhelm the bugs. Too much antibiotic use selects for more resistant mutants. When patients cut short the full course of drugs, the resistant strains have a chance to multiply and spread.
* In some countries, such as the United States, patients expect and demand antibiotics from doctors, even in situations where they are inappropriate or ineffective. Our immune systems will cure many minor bacterial infections on their own, if given the chance, and antibiotics have no effect on viral infections at all. Every time antibiotics are used unnecessarily, they add to the selective pressure we are putting on microbes to evolve resistance. Then, when we really need antibiotics, they are less effective.
* While drug companies race to develop new antibiotics that kill resistant microbes, scientists are urging patients and doctors to limit antibiotic use.
* That means not asking for penicillin when all you have is a cold, since colds are caused by viruses that are not affected at all by antibiotics. It means taking all the pills that are prescribed, even if you're feeling better. Physicians have to resist prescribing the strongest and most broadly effective drugs unless the disease absolutely requires it. If society adopts these measures rigorously, the drugs may regain at least some of their lost "miracle" powers.
* Plant-like protists include different groups of organisms, such as different types of algae, diatoms, dinoflagellates and euglenoids. Protists can be either uni-cellular or multicellular.
* For example, in the algae group, red algae are multicellular, but certain types of green algae can be either uni-cellular or multicellular.
* Seaweed and kelp are types of multicellular algae, while phytoplankton are unicellular. Diatoms, dinoflagellates and euglenoids are typically uni-cellular organisms. Dinoflagellates have flagella for movement in water. If red dinoflagellates reproduce quickly, red tides, which can be toxic if eaten, may appear in bodies of water.
* Protists are eukaryotic organisms that live in water or moist regions. Some, such as green algae, can make their own food through photosynthesis, while others are heterotrophs that eat other protists. For example, some types of euglenoids do not contain chloroplast, so they eat other protists.
* Plant-like protists
* Used as a source of food for humans
- Sherbets, chocolate milk, cheeses, instant pudding, mayonnaise, ice cream,
* Used in many other things as well...
- Fertilizer, soil conditioner, animal feed, facial mask, massage gel, vitamins, seaweed baths, toothpaste, make-up, soaps, shampoo, shaving cream, shower gel
* Animal-like protists are called protozoa. Protozoa are single-celled eukaryotes that share some traits with animals. Like animals, they can move, and they are heterotrophs. That means they eat things outside of themselves instead of producing their own food.
* Animal-like protists are very small, measuring only about 0.01-0.5mm. Animal-like protists include the flagellates, ciliates, and the sporozoans.
~ Flagellates have long flagella, or tails. Flagella rotate in a propeller-like fashion, pushing the protist through its environment (Figure below). An example of a flagellate is Trypanosoma, which causes African sleeping sickness.
~ Other protists have what are called transient pseudopodia, which are like temporary feet. The cell surface extends out to form feet-like structures that propel the cell forward. An example of a protist with pseudopodia is the amoeba.
~ The ciliates are protists that move by using cilia. Cilia are thin, very small tail-like projections that extend outward from the cell body. Cilia beat back and forth, moving the protist along. Paramecium has cilia that propel it.
~ The sporozoans are protists that produce spores, such as the toxoplasma. These protists do not move at all. The spores develop into new protists.
* The Kingdom Fungi includes some of the most important organisms, both in terms of their ecological and economic roles. By breaking down dead organic material, they continue the cycle of nutrients through ecosystems. In addition, most vascular plants could not grow without the symbiotic fungi, or mycorrhizae, that inhabit their roots and supply essential nutrients. Other fungi provide numerous drugs (such as penicillin and other antibiotics), foods like mushrooms, truffles and morels, and the bubbles in bread, champagne, and beer.
* Fungi also cause a number of plant and animal diseases: in humans, ringworm, athlete's foot, and several more serious diseases are caused by fungi. Because fungi are more chemically and genetically similar to animals than other organisms, this makes fungal diseases very difficult to treat. Plant diseases caused by fungi include rusts, smuts, and leaf, root, and stem rots, and may cause severe damage to crops. However, a number of fungi, in particular the yeasts, are important "model organisms" for studying problems in genetics and molecular biology.
* Fungi are Heterotrophic. Fungi are not able to ingest their food like animals do, nor can they manufacture their own food the way plants do. Instead, fungi feed by absorption of nutrients from the environment around them. They accomplish this by growing through and within the substrate on which they are feeding. Numerous hyphae network through the wood, cheese, soil, or flesh from which they are growing. The hyphae secrete digestive enzymes which break down the substrate, making it easier for the fungus to absorb the nutrients which the substrate contains.
* This filamentous growth means that the fungus is in intimate contact with its surroundings; it has a very large surface area compared to its volume. While this makes diffusion of nutrients into the hyphae easier, it also makes the fungus susceptible to dessication and ion imbalance. But usually this is not a problem, since the fungus is growing within a moist substrate.
* Most fungi are saprophytes, feeding on dead or decaying material. This helps to remove leaf litter and other debris that would otherwise accumulate on the ground. Nutrients absorbed by the fungus then become available for other organisms which may eat fungi. A very few fungi actively capture prey, such as Arthrobotrys which snares nematodes on which it feeds. Many fungi are parastitic, feeding on living organisms without killing them. Ergot, corn smut, Dutch elm disease, and ringworm are all diseases caused by parasitic fungi.
* Mycorrhizae are a symbiotic relationship between fungi and plants.
* Most plants rely on a symbiotic fungus to aid them in acquiring water and nutrients from the soil. The specialized roots which the plants grow and the fungus which inhabits them are together known as mycorrhizae, or "fungal roots". The fungus, with its large surface area, is able to soak up water and nutrients over a large area and provide them to the plant. In return, the plant provides energy-rich sugars manufactured through photosynthesis. Examples of mycorrhizal fungi include truffles and Auricularia, the mushroom which flavors sweet-and-sour soup.
* In some cases, such as the vanilla orchid and many other orchids, the young plant cannot establish itself at all without the aid of its fungal partner. In liverworts, mosses, lycophytes, ferns, conifers, and flowering plants, fungi form a symbiotic relationship with the plant. Because mycorrhizal associations are found in so many plants, it is thought that they may have been an essential element in the transition of plants onto the land.
* Lichens display a range of colors and textures. They can survive in the most unusual and hostile habitats . They cover rocks, gravestones, tree bark, and the ground in the tundra where plant roots cannot penetrate. Lichens can survive extended periods of drought: they become completely desiccated and then rapidly become active once water is available again. Lichens fulfill many ecological roles, including acting as indicator species, which allow scientists to track the health of a habitat because of their sensitivity to air pollution.
* Lichens are not a single organism, but, rather, an example of a mutualism in which a fungus (usually a member of the Ascomycota or Basidiomycota phyla) lives in close contact with a photosynthetic organism (a eukaryotic alga or a prokaryotic cyanobacterium). Generally, neither the fungus nor the photosynthetic organism can survive alone outside of the symbiotic relationship. The body of a lichen, referred to as a thallus, is formed of hyphae wrapped around the photosynthetic partner . The photosynthetic organism provides carbon and energy in the form of carbohydrates. Some cyanobacteria fix nitrogen from the atmosphere, contributing nitrogenous compounds to the association. In return, the fungus supplies minerals and protection from dryness and excessive light by encasing the algae in its mycelium.
* The Plantae includes all land plants: mosses, ferns, conifers, flowering plants, and so on—an amazing range of diverse forms. With more than 250,000 species, they are second in size only to the arthropoda.
* The most striking, and important, feature of plants is their green color, the result of a pigment called chlorophyll. Plants use chlorophyll to capture light energy, which fuels the manufacture of food—sugar, starch, and other carbohydrates. Without these food sources, most life on earth would be impossible. There would still be mushrooms and algae, but there would be no fruits, vegetables, grains, or any animals (which ultimately rely on plants for their food too!)
* Another important contribution of plants is their shaping of the environment. Think of a place without plants. The only such places on earth are the arctic wastelands, really arid deserts, and the deep ocean. Everywhere else, from the tundra to the rainforest to the desert, is populated by plants. In fact, when we think of a particular landscape, it is the plants which first come to mind. Try to picture a forest without trees, or a prairie without grasses. It is the plants which produce and maintain the terrestrial environment as we know it.
* There are four main ways that plants adapted to life on land and, as a result, became different from algae:
1. In plants, the embryo develops inside of the female plant after fertilization. Algae do not keep the embryo inside of themselves but release it into water. This was the first feature to evolve that separated plants from green algae. This is also the only adaptation shared by all plants.
2. Over time, plants had to evolve from living in water to living on land. In early plants, a waxy layer called a cuticle evolved to help seal water in the plant and prevent water loss. However, the cuticle also prevents gases from entering and leaving the plant easily. Recall that the exchange of gasses—taking in carbon dioxide and releasing oxygen—occurs during photosynthesis.
3. To allow the plant to retain water and exchange gases, small pores (holes) in the leaves called stomata also evolved (Figure below). The stomata can open and close depending on weather conditions. When it's hot and dry, the stomata close to keep water inside of the plant. When the weather cools down, the stomata can open again to let carbon dioxide in and oxygen out.
4. A later adaption for life on land was the evolution of vascular tissue. Vascular tissue is specialized tissue that transports water, nutrients, and food in plants. In algae, vascular tissue is not necessary since the entire body is in contact with the water, and the water simply enters the algae. But on land, water may only be found deep in the ground. Vascular tissues take water and nutrients from the ground up into the plant, while also taking food down from the leaves into the rest of the plant. The two vascular tissues are xylem and phloem. Xylem is responsible for the transport of water and nutrients from the roots to the rest of the plant. Phloem carries the sugars made in the leaves to the parts of the plant where they are needed.
Modern classification systems, based largely on molecular evidence, divide living organisms into three domains: Bacteria (also called Eubacteria), Archaea, and Eukarya.
* Plants are classified as a kingdom (Plantae) within the Eukarya; organisms that possess a nucleus , mitochondria , an internal cytoskeleton , and, in photosynthetic species, chloroplasts.
* Most scientists recognize three other eukaryotic kingdoms: Protista (most of which are single-celled organisms), Fungi, and Animalia (animals). The fungi, plants, and animals are thought to have evolved from different groups of protists.
* Plants are multicellular organisms that have evolved the ability to live on land. The vast majority can carry out photosynthesis, but they are not the only organisms with this ability: many protists can photosynthesize too, as can several important groups of bacteria.
* Photosynthetic protists (commonly called algae) are a diverse group of organisms and are divided into several phyla. Many are unicellular, including most euglenoids (phylum Euglenophyta) and dinoflagellates (Dinophyta), and some diatoms (Bacillariophyta) and green algae (Chlorophyta).
* These, along with the cyanobacteria (often misleadingly called blue-green algae), form the phytoplankton of aquatic ecosystems . Others, including all brown algae (Phaeophyta), most red algae (Rhodophyta), and many green algae are multicellular. The large marine forms of these phyla are usually called seaweeds.
* Plants are thought to have evolved from a class of freshwater green algae called the charophytes. Two particular groups of charophyte, the Coleochaetales and the Charales, resemble the earliest land plants (bryophytes) in a variety of ways, including the structure of their chloroplasts and sperm cells, and the way their cells divide during mitosis .
* Because of the need for water, bryophytes are especially common in wet habitats such as bogs, streambanks, and in moist forests. However, they are not restricted to these habitats, and some mosses thrive in deserts, above the treeline, and in the Arctic tundra.
* Among the living bryophytes, liverworts are probably most closely related to the earliest land plants, since unlike hornworts, mosses, and all vascular plants they do not possess stomata . Indeed, the fact that stomata first appeared in hornworts and mosses is evidence that vascular plants evolved from one of these two groups.
* Vascular plants appear to be more closely related to mosses than to hornworts, because some mosses possess food-conducting cells (leptoids) and water-conducting cells (hydroids) that resemble the phloem and xylem of vascular plants
* The next step in the evolutionary story is the development of seed plants. The first to evolve were the gymnosperms, cone-bearing plants like conifers, ginkgo, and cycads. These plants had an adaptive advantage over the pteridophytes and dominated the forests of the late Paleozoic era.
* The evolution of flowers and seed protected in a fruit led to the next branch, the angiosperms. The production of flowers and fruits has a cost, but this cost is outweighed by the benefits. And many adaptations have led to diverse types of flowering plants. Along with these new information-gaining changes, many intricate symbiotic relationships supposedly developed. Many exclusive symbiotic relationships exist between fungi, bacteria, and insects. If these relationships are disturbed, the plants either do not survive or are less able to compete.
* The scenario that explains how a complex symbiotic relationship evolves seems to be a Catch-22 in evolutionary scenarios. If the bee doesn't have a certain shape, it can't get to the nectar, but it is the only bee with the right body shape to distribute the pollen for the flower. If either is not in place, the other does not survive, or it is not as fit as its competitors.
* Plants are classified into two main groups: the bryophytes (nonvascular plants) and the tracheophytes (vascular plants). Both groups have multicellular embryos, which indicates that they are closely related to each another and distinguishes them from the green algae.
Indeed, true plants are often referred to as embryophytes because of this feature.
* The bryophytes consist of the liverworts, hornworts, and mosses, and as their name implies none of these plants possess vascular tissues.
* All other plants, including the ferns, gymnosperms, and angiosperms, are classified as tracheophytes. These possess specialized vascular tissues— phloem and xylem —to transport sugars, water, and minerals throughout their bodies. The oldest known vascular plants appeared in the middle Silurian period (439-409 million years ago); the oldest known bryophytes appeared later, in the Devonian (409-354 million years ago).
* Despite this, most scientists believe that bryophytes evolved before vascular plants, and that the earliest bryophytes have not been found because they fossilize poorly. This belief is supported by a variety of evidence, including morphological traits, ultrastructural features visible under the electron microscope, and molecular information obtained from gene sequencing
* The first detailed vascular plant fossils appear in rocks from middle Silurian, about 425 million years ago. The oldest of these, including a plant called Aglaophyton, appear to have possessed conducting cells similar to the hydroids of mosses. These ancient plants, which are sometimes called prototracheophytes, may have been an evolutionary link between the bryophytes and the true tracheophytes.
* Early vascular plants possessed two features that made them especially well adapted to life on land. First, their vascular tissues transported sugars, nutrients, and water far more efficiently than the conducting cells of mosses. Second, they evolved the ability to synthesize lignin , which made the cell walls of their vascular tissues rigid and supportive. Taken together, these features allowed them to grow much larger than their bryophyte ancestors and considerably reduced their dependence on moist habitats.
* There are three major groups of tracheophytes: seedless vascular plants, gymnosperms, and angiosperms. Since the first appearance of tracheophytes in the Silurian, the fossil record shows three major evolutionary transitions, in each of which a group of plants that were predominant before the transition is largely replaced by a different group that becomes predominant afterward. The first such transition occurred in the late Devonian, approximately 375 million years ago.
* Prior to this time the most common plants were simple, seedless vascular plants in various phyla, several of which are now extinct. However, one phylum from this time, the Psilophyta, still has two living genera, including a greenhouse weed called Psilotum.
From the late Devonian until the end of the Carboniferous period (290 million years ago) larger, more complex seedless plants were predominant. The main phyla were the Lycophyta, the Sphenophyta, and the Pterophyta.
* All three groups contain living relatives, including club mosses (Lycopodiaceae) in the Lycophyta, Equisetum which are the horsetails (the only living genus of sphenophytes), and ferns, which are pterophytes. Only the ferns, which have about 11,000 living species, are common today, but in the Carboniferous these three phyla comprised a large fraction of the vegetation on the planet.
* Many grew to the size of trees and dominated the tropical and subtropical swamps that covered much of the globe at this time.
* The gymnosperms probably evolved from an extinct phylum of seedless vascular plants, the progymnosperms, that appeared about 380 million years ago. The fossils of these plants, some of which were large trees, appear to form a link between the trimerophytes (another extinct phylum of seedless vascular plants) and true gymnosperms.
* Progymnosperms reproduced by means of spores like the former, but their vascular tissues were very similar to those of living conifers. The oldest true gymnosperms, which produce seeds rather than spores, first appeared about 365 million years ago.
* The evolution of seeds, with their hard, resilient coats, was almost certainly a key factor in the success of the group. A second factor was the evolution of pollen grains to protect and transport the male gametes. As a consequence of this, gymnosperms, unlike seedless vascular plants, were no longer dependent on water for successful fertilization and could broadcast their male gametes on the wind.
* Several early gymnosperm groups are now extinct, but there are four phyla with living representatives: the cycads, the gnetophytes, the conifers, and one phylum (Ginkgophyta) that has only a single living species, the ginkgo tree ( Ginkgo biloba ).
* Of these, the conifers are by far the most abundant and diverse, and many species are of considerable ecological and economic importance.
* Most conifers are well adapted to dry environments, particularly in their leaf morphology , and some can withstand severe cold. These features may have enabled them to thrive in the Permian, when Earth became much drier and colder than it had been in the Carboniferous.
* The angiosperms, or flowering plants, are all members of the phylum Anthophyta. There are at least 250,000 species, making the group easily the most diverse of all plant phyla. They share a number of features that distinguish them from other plant groups. The most obvious of these is the possession of flowers, highly modified shoots that carry the male and female reproductive structures.
* They also carry out a process called double fertilization, in which two male gametes (sperm nuclei) are released from the pollen tube into the ovule . One of these sperm nuclei fuses with an egg cell in a similar way to gymnosperms. The second nucleus (which degenerates in most gymnosperms) fertilizes other cells in the ovule called polar nuclei. Most commonly, two polar nuclei fuse with the sperm nucleus to form a triploid endosperm nucleus. The tissue that forms from this fusion is called endosperm, which in most angiosperms provides nutrients for the developing embryo.
* A third feature that separates angiosperms from gymnosperms is that angiosperm embryos are protected by an ovary wall, which develops into a fruit after fertilization has taken place. In contrast, gymnosperm embryos are held relatively unprotected on the surfaces of ovule-bearing scales in the female cones.
* Angiosperms first appear in the fossil record about 130 million years ago, and by 90 million years ago they had become the predominant group of plants on the planet. English naturalist Charles Darwin considered the sudden appearance of angiosperms to be an "abominable mystery," and scientists have debated about the origin of the group for many years.
* Comparative studies of living species suggest that angiosperms evolved from the gnetophytes, a group of gymnosperms with three living genera of rather strange plants: Ephedra, Gnetum, and Welwitschia. Double fertilization has been shown to occur in both Ephedra and Gnetum, and the reproductive structures (strobili) of all three genera are similar to the flowering stalks of some angiosperms.
*Some gene sequencing studies also indicate that gnetophytes and angiosperms are closely related to each other and to an extinct group of gymnosperms called the Bennettitales. However, more recent molecular studies suggest that gnetophytes are more closely related to conifers than they are to angiosperms.
* In 1998, the discovery of an angiosperm-like fossil called Archaefructus, which apparently existed 145 million years ago, also cast some doubt on the idea that angiosperms descended from gnetophytes or Bennettitales. Although a great deal of information has been obtained since the time of Darwin, the origin of angiosperms is still something of a mystery.
* Following fertilization of the egg, the ovule grows into a seed. The surrounding tissues of the ovary thicken, developing into a fruit that will protect the seed and often ensure its dispersal over a wide geographic range. Not all fruits develop from an ovary; such structures are "false fruits."
* Like flowers, fruit can vary tremendously in appearance, size, smell, and taste. Tomatoes, walnut shells and avocados are all examples of fruit.
* As with pollen and seeds, fruits also act as agents of dispersal. Some may be carried away by the wind. Many attract animals that will eat the fruit and pass the seeds through their digestive systems, then deposit the seeds in another location.
* Cockleburs are covered with stiff, hooked spines that can hook into fur (or clothing) and hitch a ride on an animal for long distances. The cockleburs that clung to the velvet trousers of an enterprising Swiss hiker, George de Mestral, inspired his invention of the loop and hook fastener he named Velcro.
* Unlike bryophyte and fern spores (which are haploid cells dependent on moisture for rapid development of gametophytes), seeds contain a diploid embryo that will germinate into a sporophyte. Storage tissue to sustain growth and a protective coat give seeds their superior evolutionary advantage.
Several layers of hardened tissue prevent desiccation, freeing reproduction from the need for a constant supply of water. Furthermore, seeds remain in a state of dormancy induced by desiccation and the hormone abscisic acid until conditions for growth become favorable. Whether blown by the wind, floating on water, or carried away by animals, seeds are scattered in an expanding geographic range, thus avoiding competition with the parent plant.
* Pollen grains are male gametophytes carried by wind, water, or a pollinator . The whole structure is protected from desiccation and can reach the female organs without dependence on water. Male gametes reach female gametophyte and the egg cell gamete though a pollen tube: an extension of a cell within the pollen grain.
* The sperm of modern gymnosperms lack flagella, but in cycads and the Gingko, the sperm still possess flagella that allow them to swim down the pollen tube to the female gamete; however, they are enclosed in a pollen grain.
* Vertebrates have a backbone or spine (vertebral column), and amount to less than five percent of all described animal species.
- They include fish, amphibians, reptiles, birds and mammals.
* The remaining animals are the invertebrates, which lack a backbone.
* These include molluscs (clams, oysters, octopuses, squid, snails); arthropods (millipedes, centipedes, insects, scorpions, crabs, lobsters, shrimp); annelids (earthworms, leeches), nematodes (filarial worms, hookworms), flatworms (tapeworms, liver flukes), cnidarians (jellyfish, sea anemones, corals), and sponges.
* The third phylum, vertebrata, is the most important, and is distinguished by a backbone (made either of bone or cartilage) containing interlocking vertebrae and a skull enclosing a brain. These two features serve to protect the entire central nervous system, and in addition give support and structure to the body; these bones also form part of a larger system of bones, the endoskeletal system.
* Unlike the exoskeleton of other phylums such as the arthropods, which must be shed periodically, this endoskeleton is permanent and can grow with the organism. This endoskeleton gives vertebrates a competitive edge over all other animals, as it can easily be scaled for use in large organisms, and it allows these organisms to be relatively light and fast-moving.
* In comparison, most organisms with an exoskeleton are small and slow-moving, due to the limitations of their large and bulky skeletal system.
* A community is made up of populations of different species, or animals, plants, fungi, and bacteria, living in the same area.
* In Nanortalik, the Inuit people share their land with polar bears, whales, seals, marine birds such as puffins, fish, crabs, willow trees, and lichens, among others. Together, these populations form a biological community, interacting and sharing natural resources.
* Abiotic pressure is created by non-living factors within the organism's environment, such as light, wind, and soil.
- Examples of abiotic selective pressures:
-- Temperature, sunlight, humidity, rainfall, snow, wind, soil minerals, salinity, fire...
* Mechanical defenses are an effective deterrent to both, predation and herbivory. Mechanical defenses are used by a multitude of different animals and also by plants.
* Mechanical defenses are physical additions that keep the organism from being eaten. These defenses include but are not limited to, horns, sticky skin, spikes, hard shells, or slippery skin.
- For example, the stems of roses are covered in short, prickly thorns in order to deter herbivores form eating the fragrant colorful flowers. When these thorns successfully keep predators away, the rose is able to reach its biological imperative which is to be pollinated and ultimately to create more roses.
- Another ex. is turtles also have a form of defense that is strictly mechanical. These creatures are notorious for their slow movement and they also have incredibly soft bodies. In order to keep these animals from becoming dinner, they have evoled in such a way that they have a very hard shell. Without a hard shell to cover all of their important organs, turtles would have gone extinct ages ago.
* The scarlet kingsnake, Lampropeltis elapsoides, copies the stripe patterns of deadly coral snakes, Micrurus fulvius, so well that people use mnemonic rhymes to tell them apart, such as: "If red touches yellow, you're a dead fellow; if red touches black, you're all right, Jack."
* The species live side by side across much of southeastern North America. The scarlet kingsnake uses mimicry to dupe predators, such as red-tailed hawks, keen to avoid attacking the venomous reptile.
* The Red Queen (from Lewis Carroll's Through the Looking-Glass)
* "Well, in our country," said Alice, still panting a little, "you'd generally get to somewhere else — if you run very fast for a long time, as we've been doing."
* "A slow sort of country!" said the Queen. "Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that!"