EVOLUTION Objective Notes

Evolution: The change in the kind of organism over time. The process by which modern organisms have descended from ancient organisms. From:

New combinations of genetic material (biodiversity in the gene pool) occur through mutations and through sexual reproduction. Some combinations enable individuals to outcompete others, and thus be better able to pass those combos to their offspring.

A great diversity of species (species biodiversity) also increases the chance that at least some living organisms will survive in the face of cataclysmic changes in the environment. From:

Living things can be classified based on: -similar structures -embryological similarities (how much their embryos look alike) -molecular (DNA) evidence: the degree of kinship between organisms or species can be estimated from similarity of their DNA and protein sequences

Before the advent of modern, genetically based evolutionary studies, European and American biology consisted primarily of taxonomy, or classification of organisms into different categories based on their physical characteristics. The leading naturalists of the 18th and 19th centuries spent their lives identifying and naming newly discovered plants and animals. However, few of them asked what accounted for the patterns of similarities and differences between the organisms. This basically non-speculative approach is not surprising since most naturalists two centuries ago held the view that plants and animals (including humans) had been created in their present form and that they have remained unchanged. As a result, it made no sense to ask how organisms have evolved through time. Similarly, it was inconceivable that two animals or plants may have had a common ancestor or that extinct species may have been ancestors of modern ones. Carolus Linnaeus One of the most important 18th century naturalists was a Swedish botanist and medical doctor named Karl von Linné. He wrote 180 books mainly describing plant species in extreme detail. Since his published writings were mostly in Latin, he is known to the scientific world today as Carolus Linnaeus, which is the Latinized form he chose for his name. In 1735, Linnaeus published an influential book entitled Systema Naturae in which he outlined his scheme for classifying all known and yet to be discovered organisms according to the greater or lesser extent of their similarities. This Linnaean system of classification was widely accepted by the early 19th century and is still the basic framework for all taxonomy in the biological sciences today. (from:

A comparison of vertebrate embryos. Notice that all the embryos shown here begin with the same number of gill arches. From:

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With the advent of molecular techniques to compare similarities in our DNA starting in the 1960s, most experts have come to accept the fact that humans and chimps are most closely related. Studies indicate that humans and chimps are between 95 and 98.5 percent genetically identical. -From: impanzees.html From: From:

NATURAL SELECTION Populations can grow exponentially Populations can grow exponentially Genetic variation in offspring exists due to mutations and genetic recombination Genetic variation in offspring exists due to mutations and genetic recombination Limited resources necessary for life Limited resources necessary for life Selection from environmental pressure leaves some of those organisms better able to survive and leave offspring Selection from environmental pressure leaves some of those organisms better able to survive and leave offspring

Let’s try a demo:

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Natural selection acts on individuals, but it is populations that evolve. You had a good example of one way evolution works with the cricket article. The following cartoon shall illustrate:

Living things have been evolving on Earth for millions of years. Evidence is found in: -the fossil record -the geographic distribution of living species -homologous structures of living organisms -similarities in embryonic development -genetic kinship

Evolutionary theory explains that biological diversity results from the descendants of local or migrant predecessors becoming adapted to their diverse environments. This explanation can be tested by examining present species and local fossils to see whether they have similar structures, which would indicate how one is derived from the other. Also, there should be evidence that species without an established local ancestry had migrated into the locality. Wherever such tests have been carried out, these conditions have been confirmed. A good example is provided by the mammalian populations of North and South America, where strikingly different native organisms evolved in isolation until the emergence of the isthmus of Panama approximately 3 million years ago. Thereafter, the armadillo, porcupine, and opossum— mammals of South American origin—migrated north, along with many other species of plants and animals, while the mountain lion and other North American species made their way across the isthmus to the south. From:

HOMOLOGOUS STRUCTURES Structures that have different mature forms in different organisms but develop from the same embryonic tissues

SPECIATION When a population becomes isolated, by changes in geography or by other methods, it may evolve down separate paths, and even become different species, as each population evolves to become well suited to their different environments. Different selection pressures affect the two populations and support different genes. Genetic drift now occurs differently too. If the two populations don’t tend to mate or cannot mate (physically or genetically) even if they by chance encounter each other, then they are considered to be new species.

GENETIC DRIFT An allele may increase — or decrease — in frequency simply through chance. Not every member of the population will become a parent and not every set of parents will produce the same number of offspring. From: From:

Two examples of reduced polymorphism because of genetic drift: By 1900 hunting of the northern elephant seal off the Pacific coast had reduced its population to only 20 survivors. Since hunting ended, the population has rebounded from this population bottleneck to some 100,000 animals today. However, these animals are homozygous at every one of the gene loci that have been examined. Cheetahs, the fastest of the land animals, seem to have passed through a similar period of small population size with its accompanying genetic drift. Examination of 52 different loci has failed to reveal any polymorphisms; that is, these animals are homozygous at all 52 loci. The lack of genetic variability is so profound that cheetahs will accept skin grafts from each other just as identical twins (and inbred mouse strains) do. Whether a population with such little genetic diversity can continue to adapt to a changing environment remains to be seen. From: From: From: