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1. The Big Picture on Evolution.docx

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University of Toronto Scarborough
Larry Sawchuk

ANTB15 – The Big Picture on Evolution - “Nothing in biology makes sense except in the light of evolution” - By biologist Theodosius Dobzhansky reflecting on the coming together of two strands of thinking: evolutionary change, kick started by Darwin in the mid-19 century, and genetics. - Genetics provided the mechanism by which natural selection could occur. This was famed the ‘modern synthesis’. - Darwinian evolution has been widely accepted, but some in the wider society (outside the scientific world) remain skeptical about it. Darwin’s Big Idea (2) - Alfred Russel Wallace and Charles Darwin shared the inside that living species were not fixed, but were the product of a gradual process of change driven by natural selection: the survival and reproduction of organisms suited to their environment, at the expense of those less successful. What is Natural Selection? - Organisms produce more offspring than survive to reproduce. - Their offspring vary slightly. - Characteristics can be passed on from generation to generation. - Those most suited to their environment survive at the expense of those less ‘fit’. Sexual Selection - According to natural selection, heritable features should have survival advantage. - Sexual selection is a variation of natural selection also developed by Charles Darwin. In the Beginning… (2) - Mainstream scientific theories hold that life came into existence and developed through natural causes. But many cultures have developed alternative explanations for our existence. Need to Know (2) - Genotype: Genetic characteristic of an individual (e.g. having gene or genes for red hair). - Phenotype: The physical traits of an individual (e.g. having red hair). Phenotypes are dependent on genotype and interaction with the environment. - Allele: A version of a gene. Many genes have a number of different alleles, owing to small variations in gene sequence. - Genome: The complete set of genetic instruction of an individual or species. - Sexual reproduction: Where an offspring’s genes come from two parents. - Asexual reproduction: Where an offspring is genetically identical to its (single) parent. Cultural Evolution (3) - Biological evolution is central to our existence and acts through our genes. Genes are not the only way information can be passed on. - Culture, our system of beliefs, values and knowledge, is passed on through interaction between people. - One of the strongest cultural systems is language, which shows clear signs of evolutionary change. - Many social customs are deeply ingrained. - In some cases, it can be difficult to disentangle biological and cultural explanations. Caught in a Drift (3) - A random process of drift can also change the genetic make-up of a species. - A neutral change is a change in DNA that won’t make a blind bit of difference. - Neutral changes are important thanks to a random process known as genetic drift. - Because of chance events in breeding, the frequency of alleles may fluctuate or ‘drift’ over time. - Changes in allele frequency due to chance have probably had a big impact on people because we are derived from a very small founder population. One Big Family (4) Tree of Life (4) - Living things fall into three major divisions: Bacteria, Archaea, and Eukarya. - Tree of Life: Trunk represents the common ancestor, major brances are the high-level taxonomic groups, and the twigs are the individual species themselves. - It used to be only prokaryotes and eukaryotes, now a new group has emerged: Archaea (single-celled and lacking a nucleus, but are more closely related to eukaryotes than bacteria). Convergent Evolution (4) - Evolution usually works by diversification, but sometimes a similar solution is arrived at by different routes – convergent evolution. - Convergent evolution – an environmental challenge has been overcome in a similar way but independently in different species. Whose Gene is it Anyway? (5) - Genes are widely shared but become adapted to take on new roles in different organisms. - Hox genes are master control genes that coordinate the activity of other genes and they are found throughout the animal world. - There are many ways in which genes can take on new functions. The classical way is by mutation, a change in DNA sequence altering the properties of the protein (or RNA) that it codes for. If the mutation affects control regions, the protein may be made in a different place in the body or at a different time during an organism’s life. - Sometimes genes or collections of genes are duplicated. Following duplication, genes can be free to take on new functions, as one gene continues to perform its original role. - Genes may become inactivated but remain in the genome as pseudogenes. Building Family Trees (5) - DNA sequence analysis is now used to build family trees. - Many genes or even entire genomes can be compared. - From a collection of sequences, family relationships can be deduced. Evolution in Action (6) Speciation (6) - A species is a group of animals or plants that are able to interbreed. - A new species is formed when two population of animals or plants accumulate so many different genetic changes that they cannot interbreed and produce fertile offspring. - Typically this happens when two populations become geographically separated. - Inter-species crosses mostly generate no viable offspring because of chromosomal or genetic incompatibilities. - Even simple genetic changes can create reproductive barriers, and typically, such genes affect reproductive biology (usually in males) and are under selective pressures. Pathogen Evolution (7) - Bacteria, a single-cell pathogen, have one huge advantage in the evolutionary game; they can reproduce in minutes relative to humans (who take approx.. 25 years). - Viruses, tiny particles that infect and take over a cell, also have numbers on their side. A single infected cell can spew out millions of new
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