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Tutorial 1 - What is Life? Transcript


  • Slide 1:  As you have just read, Biology is the study of life.  But what is life?  Defining life can be challenging - we often depict life using an image like you see here: a tree of life.  In this image, the base of the tree (which I have just circled) is an ancestor to all of the organisms found at the tips of the branches.  Trees like this can be relatively simple like the tree here - a simple tree of eukaryotic life.  


  • Slide 2:  Or these trees can be rather complex - like the tree you see here.  This is a tree that was generated by Biologist David Hillis at the University of Texas.  This image shows the relationships of about 3000 different species and these organisms have been classified based on genetic similarity.   This classification system results in all of the animals being put in one group, plants in another group, fungi in a group, and all of the protists in a group - those are the eukaryotic organisms.  And this diagram also shows all of the bacteria and archaebacteris which are prokaryotic organisms.


  • Slide 3:  This image also includes humans in the red circle which says "You are Here".  Because all living things share a common ancestor, this image shows them in the middle.


  • Slide 4:   Because all organisms share a common ancestor they also share some common features.  For example, all organisms have the ability to reproduce and grow, have metabolism (which is the collection of chemical reactions that occurs inside an organisms cells to maintain life), all organisms maintain homeostasis (a relatively stable internal environment), and all organisms have the ability to respond to their external environment (stimuli).  For example, to turn toward or away from sunlight. Another commonality of life is that all living things are acted upon by natural selection.  And through natural selection organisms are able to adapt to their environment in successive generations.


  • Slide 5:You have most likely heard the terms "evolution" and "natural selection" by perhaps you have not stopped to think about the real definitions of these terms.  Do these terms mean the same thing?  Or do they have distinct meanings?


  • Slide 6:  The answer to this question is "Yes".  Natural selection and evolution are distinct concepts - they do not have the same meaning.  Evolution is a term that refers to the change in a population's genetic structure over time.  So if we see a population of organisms getting larger over time - that is an example of evolution because there are genes that contribute to body size and as the genes for body size increase in frequency in the population that is a change in the genetic structure of that population.  This definition of evolution will become more clear when we discuss genetics later in the course.  Natural selection on the other hand, is one means by which evolution can occur.  We will look at other mechanisms of evolution later on in the course but for now we will focus on natural selection.


  • Slide 7: Natural selection is a process by which a population becomes adapted to an environment over time.  And we can even be more succinct in our description of natural selection  and define it as differential reproduction. What this means is that individuals that are better adapted to their environment are better able to survive, reproduce, and pass on their traits (their genes) to the next generation.


  • Slide 8:  We can see an example of evolution via natural selection by looking at a population of finches on the Galapagos islands.  These are the finches made famous by Darwin when he found them on the Galapagos islands and wrote about them in the "Origin of Species".  The particular species we will look at is known as Geospiza fortus.  This species lives on Daphne Major and during the years 1976-1978 this island experienced a severe drought.  The image is not very clear, and you are not responsible for the details of the image, but appreciate the general story.  What we see is that the population of finches declined dramatically and the reason is because there was also a decline in food supply (seeds).  The population of seeds declined as a result of the drought.  The drought meant that the plants were not able to produce as many seeds and the seeds that they did produce became harder and thicker.  So the overall size and thickness of the seed coat increased during the drought.  In response to that increase in seed coat thickness, the birds that did survive the drought became larger because larger birds were able to crack the seed coats of the thicker seeds.    Individuals that were likely to survive during the drought were the larger individuals and the overall average size of the birds increased during the drought. And this is an example of evolution because body size is controlled by genes but natural selection (or differential reproduction) is causing the evolution.   Larger finches are better able to access larger harder seeds so they are better able to get food and energy for reproduction and they then pass those genes on to their offspring.  The population overall becomes larger and this is a nice example of evolution that is often used in textbooks because it is such a nice example.  


  • Slide 9: Natural selection and evolution will be recurring themes in this course.