Biology 1B Evolution Lecture 2 February 26 2010 Natural Selection Phylogenies 1 Natural Selection Darwin Wallace There are three conditions for natural selection 1 Variation Individuals within a population have different characteristics traits or phenotypes 2 Inheritance Offspring inherit traits from their parents An offspring does not receive the same spectra of traits as either parent but rather a mixture of both parents traits 3 Competition More offspring are produced than can survive so offspring with traits better matched to the environment will survive and reproduce more effectively than others 4 Natural selection states that given these three conditions a population will accumulate the traits that enable more successful competition Origin of the species by means of natural selection Darwin 1859 Darwin used several lines of evidence 1 Artificial Selection was used as an analogy for natural selection If humans can breed animals dogs horses crops for certain traits tiny size speed high yield then it makes sense that in a natural environment nature would select for the traits that allow organisms to survive and reproduce better 2 Biogeography Nested geographic distributions Species that occur on islands are often closely related to species each other species on the nearby mainland Wallace also contributed a lot in terms of examples of adaptive radiation for island evolution Repeated observations like this cemented the idea of descent with modification 3 Homology Figure 1 Homologies in mammal forelimbs Campbell 8th edition Copyright 2005 Pearson Education Inc Publishing as Pearson Benjamin Cummings pg 463 a Darwin used the example of homologous structures or variations on a structure present in a common ancestor For example a human arm a cat s leg a whale s flipper and a bat s wing all are adapted to different purposes but share the same bone structure This suggests one common ancestor with that common structure i Don t confuse a homology with the term homoplasy from lab which means the opposite similar structures functions that originated from different ancestors and converged A homoplasy can also be called an analogous structure b Embryological homologies developing embryos are identical at certain stages the longer two creatures developing embryos remain similar the more closely related they are c Creatures have vestigial structures or remnants of structures that were used for one function in an ancestor but are no longer used for that function Moritz Lecture 2 Notes page 1 Biology 1B Evolution Lecture 2 February 26 2010 Natural Selection Phylogenies 2 Figure 2 Hox genes controlling body segment development Campbell 8th edition Copyright 2005 Pearson Education Inc Publishing as Pearson Benjamin Cummings pg 463 d Molecular homologies while not available to Darwin provide further evidence for natural selection The genes that govern development of body sections are known and are expressed in the same anterior to posterior sequence in embryos of mice and fruit flies However in each animal the genes eventually specific different structures they form portions of the spinal cord in the mouse while forming the segments of the fly s body This suggests the two animals diverged from a common ancestor with the common genes 4 Population pressures Malthus population growth requires resources 5 Fossil Record At the time there was a fossil record and it was known that many of the fossilized species did not exist any longer and were organized into time series strata Descent with Modification Putting it Together Natural selection is different from descent with modification The first is the process the second is the result For example the famous diagram included in the Origin of the Species shows descent with modification From a single ancestor you have multiple branches lineages with diverging phenotypes Natural selection is the reason that these branches exist rather than a solid cone of phenotypes Not all phenotypes survive as intermediate phenotypes less able to survive die out creating the branching structure of the tree of life Extinction events also lead to phenotypic gaps in the tree Moritz Lecture 2 Notes page 2 Biology 1B Evolution Lecture 2 February 26 2010 Natural Selection Phylogenies 3 Figure 3 Darwin s Tree of Life diagram from the Origin of the Species The Tree of Life In the past species were viewed as fixed discrete categories of beings and were permanent and nonalterable e g Aristotle Under that system classifying all the creatures that exist is simple you simply need to record them If you accept that there is all this variation however then classifying the existing species becomes more difficult See Lecture 8 for more information about defining species Currently we use a phylogenetic hierarchy revealed by comparative DNA sequenceto define the domains of life There are three domains Archaea Bacteria and Eukarya Each of these branches contain related organisms that share a common ancestor Those domains split into further branches repeatedly until eventually the tips of those branches are the currently existing species or species that have gone extinct Estimation Interpretation of Phylogeny How do we know that species are descended from a common ancestor We use systematics or the science of estimating phylogenies We have a few sources of information the dashed line indicates some DNA acquired from recent fossils Key Concepts Moritz Lecture 2 Notes page 3 Biology 1B Evolution Lecture 2 February 26 2010 Natural Selection Phylogenies 4 A cladograph shows how different groups of organisms are related A phylogeny shows how different groups of organisms are related and provides times that each of these organisms existed A character is a characteristic or a trait A character state is what an organism has for that trait A taxon plural is taxa is a group species genera family order etc grouped by shared derived traits Taxa should be monophyletic that is all entities descended from the same common ancestor A clade is a monophyletic group where all descended extant taxa are from a common ancestor A paraphyletic group is where either some but not all taxa are from a common ancestor DEF below A polyphyletic group is when the taxa independently evolved analogous traits CDEFG below Figure 4 Different types of phyletic groups Campbell 8th edition Copyright 2005 Pearson Education Inc Publishing as Pearson Benjamin Cummings pg 542 For groups A and B and C trait a is a shared derived trait
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