jimtrue.com : school : BSC2011 : CH 22-24: Evolution

Posted by Jim True on December 9, 2004 2:45 PM. Last Updated October 22, 2006 9:23 PM

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CH 22-24: Evolution

Evolutionary "Players"

• Aristotle (4th Century BC) - Proposed what came to be called the scala naturae ("natural scale"), in which all living things were organized into a system of increasing complexity.
• This was the prevailing concept until about the 1700's, along with the concept of fixity (immutability) of species - All species on Earth date to eone point in time and are unchanging.
• Fossils and extinct organisms killed off by catastrophic events in time.
• Important individuals and their roles in the 1700's and later included:
• Linnaeus: Devised teh system of taxonomy and binomial nomenclature, which is universally used to provide unique names to individual species.
• Initially, the taxonomic schemes did not include phylogeny, but this would change over time so that modern schemes are constructed around evolutionary relationships.
• Cuvier (1769-1832) - Pioneered the science of paleontology and was a reknowned scientist of his time.
• However, he was a firm believer in the fixity of species and proposed catastrophism - fossils and extinct organisms killed off by catastrophic events in time.
• Hutton (1726-1797) - Geologist who proposed the concept of gradualism, that dramatic changes we observe in the Earth occurred gradually over time.
• Lyell (1797-1875) - A contemporary of Darwin's, in 1830 he authored the concept of Uniformitarianism - Laws of physics and chemistry remain the same through Earth's history.
• Lamarck (1744-1829) - Another famous and well respected scientist, produced a transformational hypothesis of evolution.
• Lamarck believed that there is an innate tendency towards increasing complexity which all organisms strive towards. (Believed all organisms strive to become better organisms, with obviously humans being at the pinnacle of perfection).
• Organisms pass changes to their offspring directly, thus changing them over time.
• He has been a source of ridicule for this conclusion (Lamark's giraffes 'stretched their necks' over time to become more able to eat from the taller trees; implied CONSCIOUS evolutionary change). In fact, Lamarck was the first to correctly propose that organisms can and do change in response to environmental change, ie, they exhibit adaptation.
• Malthus (1765-1835) - In 1799, proposed that organisms produce more offspring than can survive. If all offspring survive, the environment would be overwhelmed. Some must die. (Malthus was a mathematical biologist who presented his findings as justification for population control). Population growth is geometric (2 x 2 = 4 x 2 = 8 x 2 = 16); Resource growth is arithmetic (2 + 2 = 4 + 2 = 6 + 2 = 8)

Darwin's Theory

• Darwin (1809-1882) - In 1858, published and presented a treatise entitled "On the Origin of Species By Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Survival".
• We refer to this today as the Theory of Evolution.
• Darwin's concept differed in ONE major respect from Lamarck's.
• He presented a variational hypothesis.
• Darwin believed in descent with modification, that all organisms were ultimately related forms that had just changed over time by gradually acquiring new and different characteristics in response to the environment.
• Darwin's hypothesis was as follows [Basic Darwinian Concept]:
1. All populations exhibit variation. Variations are usually caused by response to the environment and can be passed on.
2. From Malthus' concept - More offspring produce than survive - some must die.
3. Struggle for survival results - living space, food, ates.
4. Some individuals, because of variations (adaptations are better suited for the particular conditions - they exhibit fitness. (More capable of dealing with environmental condition of a particular time.)
5. This results in Natural Selection - Adaptive traits allowing an advantage are passed on from parent in reproduction. At least some offspring will be successful. Because environment (Nature) dictates fitness, Nature "selects" the survivors (fittest)
6. Natural Selection over Time results in new species from the accumulation of adaptive changes (Environment induces fitness; environment is nature; thus nature selects.)
• Lyell's projection of a very old Earth (millions of years old) suggested to Darwin that evolution was possible because there was enough time for it take place.
• Wallace (1823-1913) - British naturalist who independently arrived at the same hypothesis as Darwin.
• Darwin had more evidence to back his hypothesis, but suggested that they co-present the ideas to the Royal Academy in 1858. (The book 'On the Descent of Man' came out the following year).
• In the ensuing clamor over the ideas presented, Wallace recanted his ideas, but later continued work on evolution.

Evidence for Evolution

1. Artificial Selection (Selective Breeding) - Enhancement or elimination of gene frequencies ALREADY PRESENT in population by human interference. e.g. Modern day crops, domestic animals (dogs) (animal and crop husbandry).
• Darwin himself used domestic breeding programs for cattle and sheep and crops as evidence for the power of selection. (Environment [US] can change genetic characteristics
2. Microevolution - Short term (generations to 100's of years) changes in gene frequencies ALREADY PRESENT in a population in response to environmental change. e.g. Peppered Moth in post-Industrial Revolution England; pesticide resistance in insects; drug resistance in HIV.
• Pre-Industrial, light peppered moths dominated; post industrial soot covers birch bark, dark peppered moths dominate. Demonstrated a shift in dominance of alleles already present in a population.
• It is evidence FOR evolution, but does NOT produce new species! Allelic forms are present; not forming a new species.
3. Homologous Structures:
• Homology - Structures or organs have common origin among different organisms, although the function differs. (Same origin; different function).
• Evolutionary relationship evident, e.g. Mammalian forelimb.
• Analogy - Common function from structures/organs that developed independent of one another. No evolutionary relationship, e.g. insect and bird wings.
• Hopping legs of a frog and cricket are an example of an analogous structure; bat and bird wings are actually both analogous and homologous.
4. Vestigial Structures - another type of anatomical homology.
• Nonfunctional remnants of once functional organs.
• Examples include hind limb bones in marine mammals and some snakes, human appendix.
5. Embryological Development - "Ontogeny recapitulates phylogeny" (Considered the 'biogenetic law', that the earliest development of an organism recapitulates evolutionary history of that organism over time).
• ("Biogenetic Law" of Haeckel). Can, in some but not all cases, show clear interrelationships between/among taxa., eg. four chordate characteristics, Protostomes & Deuterostomes.
• Another form of homology, but in this case, it is embryological only.
6. Molecular Biology - The DNA "genetic code" is found in ALL life on Earth. Four nitrogen bases link to 20 amino acids, strong evidence for common origin of life.
• Also, the more similar the genetic makeup of organisms the more closely related.
7. Biogeography - Species that most closely resemble each other usually associated with nearby geographic regions. e.g. Darwin's Finches
• However, it is possible to observe unrelated forms acquire similar characteristics based on the similarities in their environments.
• This is termed parallel or convergent evolution, where unrelated forms respond in a similar fashion to environmental selection.

Population Genetics

• Modern Synthesis of Evolution - Developed between 1920's and 1940's with advances in studies on genes - The adaptations that are produced and passed on are generated by MUTATION.
• Thus, modern idea incorporates Darwinian and Mendelian concepts. Puts evolution into a genetic perspective.
• Population genetics, which examines the extent of genetic variability within populations allows the genetic basis for natural selection to be examined. (Use populations to determine allelic frequencies)
• Population - A group of individuals of the same species co-occurring in a specific location at a specific time.
• To define a population, you must have 1) organisms, 2) a specific time frame, and 3) a specific geographical location.
• Species - Individuals that share common characteristics and that which, under natural conditions, can interbreed and produce fertile offspring. (This is known as the biological species concept).
• Gene Pool - the sum of genetic variability for a species; all alleles at all loci for all individuals in a population.
• SKIP all the way to Mechanisms of Speciation

Mechanisms of Speciation

• All mechanisms are based on reproductive isolation of at least some members of a population.
• Isolation can shift gene frequencies and allow for adaptive changes to affect the isolated groups differently.
• Two categories for producing isolation among unrelated species:
1. Prezygotic barriers, preventing different species from interbreeding (doesn't allow organisms to mate in the first place):
• Habitat Isolation - Species that live in different habitats within the same area.
• Behavioral Isolation - Courtship rituals, color patterns or other signals are not recognized by members of another species, even closely related species. iei Peacock's showy display has no effect whatsoever on a chicken.
• Temporal Isolation - Mating seasons occur at different times. ie, some go into estris in Summer, others in Spring.
• Mechanical Isolation - Anatomical incompatibility, ie different species of shrimp have different sperm carrying swimmerettes that will NOT fit with other species of female shrimp.
• Gametic Isolation - Chemical incompatibility between sperm and egg of different species, ie Chemical receptors, sperms and eggs have recognition proteins that won't allow the sperm to burrow into the egg and initiate fertilization.
2. Postzygotic Barriers - Mating and even fertilization occurs but do not produce either viable or fertile offspring. Includes:
• Reduced hybrid viability - either offspring do not fully develop or die quickly once born. Offspring are both sickly and infertile.
• Reduced hybrid fertility - Offspring may be healthy but infertile.
• Hybrid breakdown - First generation offspring may be healthy and fertile, but succeeding generations will be neither.

Modes of Speciation

• Two main modes on how gene flow becomes interrupted:
1. Allopatric speciation - Isolation accomplished by physical separation of parts of population.
• Vicariance - "physical barrier" separates once continuous population. e.g. mountain ranges, land submergence.
• Adaptive Radiation - Small portionof population is removed from main body into a new environment. If it becomes established, can begin to diverge rapidly into a variety of different species. Adaptive radiation can occur for two reasons:
• Gene frequency shift due to small population size ("founder effect").
• Adaptation can lead to invasion of previously unexploited habitats, e.g. "Darwin's finches"
• Original finch species on mainland (Ecuador) evolved into many new types when isolated on Galapagos Islands.
2. Sympatric Speciation - Isolation occurs WITHIN the population as adaptations allow individuals to exploit new niches (physical and biological role of an organism) and habitats (living space) within the population range. e.g Rift Lake cichlid fishes in Africa (all Cichlid fishes are in the same lake but have diverged into 100 different species from a common ancestor based on the variety of habitats within the lake and available niches).

Rates of Speciation

• There are two current models proposed that relate to the rate of changes resulting in speciation:
1. Gradualism - Proposed by Darwin. Adaptive changes are acquired over time gradually transforming the organism from one form (species) to another.
• Along the way, there are numerous intermediate forms produced.
2. Punctuated Equilibrium - Proposed by Eldredge (UF) and Gould (greatest modern day Evolutionist) (1972). The overall time frame for the production of new species is the same as Darwin suggested, but the rate at which accumulated changes transform species differs.
• Species remain static for long periods of time (equilibrium) and then suddenly (10,000's years) transform into new forms (punctuation). (the genome (or the record of all possible variations) is transformed gradually but the species remains at equilibrium, and then environmental factors suddenly force the unactivated changes and they are presented in the next generation of the species.)

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