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Chapter Three: BIOLOGY AND EVOLUTION Link to the Canadian Association for Physical Anthropology What Forces Are Responsible For The Diversity Of Primates In The World Today? What Is Evolution? How Does Evolution Produce New Forms of Organisms? What Are The Forces Responsible For Evolution? Our group of animals is so diverse because it is a product of evolution “Descent with modification” (Charles Darwin) Linnaeaus classified living things: as a way of creating order and naming the plants and animals to the glory of God’s creation on the basis of overall similarities in small groups, or species Modern classification distinguishes superficial similarities (analogies) from basic ones (homologies) homologous structures are possessed by organisms that share a common ancestry analogous structures may look similar and may serve the same purpose, but do not arise in similar fashion from a common ancestor Bird and bat wings are analogous — that is, they have separate evolutionary origins, but are superficially similar because they evolved to serve the same function. Analogies are the result of convergent evolution. Interestingly, though bird and bat wings are analogous as wings, as forelimbs they are homologous. Birds and bats did not inherit wings from a common ancestor with wings, but they did inherit forelimbs from a common ancestor with forelimbs. On the basis of homologies, as in Linnaeus’ system groups of like species are organized into larger groups, genera (singular, genus) Species Population or group of populations capable of interbreeding but that is reproductively isolated from other such populations Characteristics used by Linnaeus to classify: 1. 2. 3. Body structure Body function Sequence of bodily growth Characteristics used by Modern Taxonomy: 1. 2. Body structure, function and growth Chemical reactions of blood, protein structure, genetic material, parasite comparison In Linnaeus’ time species were thought to be unchangeable since the time of creation Today creationism is supported by fundamentalist Christian groups like the Creation Research Society (CRS) Creationism is based on a belief system rather than on scientific evidence Lamarck – species can change Cuvier – different sedimentary layers held different types of fossils, consistent with existing 18th century view called catastrophism (evidence of new acts of divine creation) Lyell – gradual process shaped the earth over a long period of time and are no different today, uniformitarianism By the 19th century many naturalists had come to accept the idea that life had evolved Charles Darwin and Alfred Russell Wallace independently discovered how evolution works Their idea was natural selection Darwin and Wallace based their idea on two observations: 1. All organisms display a range of variation 2. All organisms have the ability to expand beyond their means of subsistence The evolutionary process through which factors in the environment exert pressure that favours some individuals over others to produce the next generation “I am, and shall ever remain in a hopeless muddle,” Darwin wrote to Asa Gray (a friend) in November of 1860. “I cannot think that the world, as we see it, is the result of chance, and yet I cannot look at each separate thing as the result of design.” …Chance… …Design… First…genetic diversity. Thanks to the random sorting out of traits inherited by offspring from their parents, no two individuals (except identical twins) are alike. Second…the ultimate source of variation is genetic mutation. The genetic material, the DNA molecule, is intrinsically vulnerable, due to its structure. Here, spontaneous changes occur frequently. They are random. Most are harmful. Due to the fact that there is variation to select from, an individual with a particular trait may be better able to survive and pass on its genetic heritage to its offspring. The trait allows them to cope with change in their environment better than others of their species. So… The environment has the power to design its eventual occupants. The size, shape, behavioral features, anatomical devices, and even intelligence, of all animals, are dictated by the interaction of the species with the environment. V= Variation. All life forms vary genetically within a population. It is the genetic variation upon which selection works. I = Inheritance. Genetic traits are inherited from parents and are passed on to offspring. S = Selection. Organisms with traits that are favorable to their survival have a chance to live and pass on their genes to the next generation. T = Time. Evolution takes time. Evolution can happen in a few generations, but major change, such as speciation, often takes long periods of time. Gene Portion of DNA molecule containing several base pairs that directs the synthesis of a protein, e.g. gene for ABO blood type Mendel Law of Segregation Genes, or the units controlling the expression of visible traits, retain their separate identities over generations Mendel Law of Independent Assortment Genes controlling different traits are inherited independently of one another DNA -- A complex molecule with information to direct the synthesis of proteins and the ability to produce an exact copy of itself RNA – carries instructions from DNA to make amino acids Genetic Code Three-base sequence (codon) of a gene that specifies production of an amino acid Amino acids strung together make a protein Genome The complete sequence of DNA for a species Human Genome 3 billion chemical bases, with 30,000 functioning genes 30,000 genes account for 1-1.5% of the human genome The rest is non-coding “junk DNA” Two types of genes are responsible for what organisms actually become: Structural genes – contribute directly to actual formation of the structure, e.g. eye colour Regulatory genes – control expression or activity of other genes, e.g. homeobox genes control whether an organism has fins or legs Chromosomes Long strands of DNA in a protein matrix 23 pairs in humans Each chromosome in the pair contains genes for the same traits, e.g. gene for A-B-O blood group Variant forms of these genes are called alleles, e.g. A, B and O Mitosis Cell division that produces new cells having exactly the same number of chromosome pairs, and hence, genes as the parent cells Meiosis Cell division that produces the sex cells, each of which has half the number of chromosomes, and hence genes, as the parent cell Homozygous Refers to a chromosome pair that bears identical alleles for a single gene, e.g. each of the pair has an A allele Heterozygous Refers to a chromosome pair that bears different alleles for a single gene, e.g. one chromosome has A allele, one has O allele Genotype actual genetic makeup of an organism, e.g. AA or AO Law of dominance and recessiveness Certain alleles are able to mask the presence of others; one allele is dominant, the other is recessive Phenotype The physical appearance of an organism; may or may not include recessive alleles Polygenetic inheritance Two or more genes (as opposed to just two or more alleles) work together to produce one particular phenotype, e.g. skin colour, stature Concept of the population A group of individuals within which breeding takes place It is within the population that natural selection takes place, and at this level that evolutionary change occurs The stability of the population In theory, the gene pool of a population should remain the same generation after generation, i.e. the alleles should occur in the same frequency Gene pool The genetic variants available to a population The percentage of individuals that are homozygous dominant, homozygous recessive, and heterozygous will remain the same from one generation to the next provided certain conditions are met: Random mating Large population No new variants Equal survival and reproductive success Sources of change: 1. Mutation – chance alteration that produces a new gene 2. Genetic drift – chance fluctuations of allele frequencies in the gene pool 3. Gene flow – introduction of new alleles from nearby populations 4. Interspecies gene transfer – transfer of DNA between species 5. Natural selection – adaptation Evolutionary process through which the environment exerts pressure that selects some individuals over others to reproduce 1. 2. 3. Directional selection – a particular allele may be favoured Disruptive selection – individuals at both extremes of the distribution are favoured Stabilizing selection – populations are already well adapted Gradualism if isolated over a long period of time, species evolve from subspecies (or races) through accumulation of genetic differences races – in biology, populations within a species that are capable of interbreeding but may not regularly do so **this concept has no biological validity in humans; races are merely social categories Punctuated Evolution new species appear quickly, in geological terms, and this dramatic change lasts for a long time with little significant change rapid appearance of novelty likely due to involvement of homeobox genes Factors that separate breeding populations, leading to the appearance first of divergent races and then divergent species Isolating factors: Geographical Anatomical structure Early miscarriage of offspring Early death of offspring due to weakness/maladaptation Sterility of hybrid offspring Genetic Social Isolation may cause a single ancestral species to give rise to two or more descendant species This divergent evolution is probably responsible for much of the diversity of life today Two distant forms develop greater similarities because their structures serve similar functions, e.g. birds and bats Monkeys, Apes, and Humans: The Modern Primates