Lecture 3.docx

Lecture 3: Getting our Bearings – Evolution of Life Why do organisms behave? All organisms need to eat, avoid being eaten, and reproduce Natural Selection By identifying why some individuals in a population survive and reproduce better than others, Darwin recognized that evolutionary changes can be...

Lecture 3: Getting our Bearings – Evolution of Life Why do organisms behave? All organisms need to eat, avoid being eaten, and reproduce Natural Selection By identifying why some individuals in a population survive and reproduce better than others, Darwin recognized that evolutionary changes can be explained. Organisms with the best-suited characteristics for their environment are more likely to survive and reproduce. Pangenesis: Each body cell produces hereditary particles that circulate in the blood before collecting in the reproductive cells. Through reproduction, adaptations become diluted, not strengthened Problem: Pangenesis works against natural selection Mendel’s Solution Discovered some traits are dominant and some recessive Heritable Particles Heritable particles do not blend and so solves Darwin’s problem Heritable particles known to be located in chromosomes and called ‘genes’ Watson and Crick confirmed that genes are made of DNA and proposed the famous double helix model. Modern Synthesis Combination of Natural Selection and ‘Genes’ The main source of variation in traits is mutation Natural selection acts on genetic variants that produce ‘fitter’ phenotypes (i.e., ones that pass on more copies of those genes) Another Implication of Evolution Life evolves by diversification from a common ancestor Descent with Modification How to Classify Living Things Linnaeus clustered plants based on similarity, with differing degrees of similarity leading to a hierarchy. Problem of Homoplasy The development of organs or other bodily structures within different species, which resemble each other and have the same functions, but did not have a common ancestral origin. E.g., both birds and mammals are warm-blooded, does that similarity make them related? Hennig – Cladistics Shared derived characters are the most informative for deciding relatedness (synapomorphies) Convergence The process whereby distantly related organisms independently evolve similar traits to adapt to similar necessities. Hennig Shared primitive characters (plesiomorphies) and unique characters (apomorphies) are not useful for identifying which species are most closely related. Clades vs Grades A grade represents a grouping with certain features in common Grades do not inform us about genealogy A clade represents a grouping based on relatedness, and hence shows the pattern of evolutionary diversification (shown in cladograms)