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9/29/2013 Definitions The Importance of Theory in Biology An Introduction to Some Big Ideas Refer to text pgs. 169-173; 368-386 The Cell Theory There are three parts to the Cell Theory: • A law is a description of natural phenomena. • A hypothesis is a testable suggested explanation for a set of observations. • A scientific theory is a hypothesis that is so well supported by evidence that it is generally considered true. It explains the natural phenomena. More on Cells… • Organisms may be: – unicellular (made up of one cell) OR 1) Cells are the basic units of structure and function of all organisms. 2) All cells come from pre-existing cells (one cell divides into two cells during mitosis). 3) All organisms are made up of one or more cells. ANOTHER BIG IDEA: NATURAL SELECTION • For a long time people didn’t know how organisms changed over time (evolved). • There was plenty of evidence of evolution, but no one could suggest a mechanism for how it worked. • Many scientists had some ideas, but we will just look at two of the most popular ideas at the time. – multicellular (consisting of many specialized cells) • Cells may be many different shapes and sizes. • Cells contain organelles: structures that perform various functions within the cell. Lamarck’s Hypothesis • A French scientist named Lamarck had an idea that organisms gained or lost certain organs or traits with use or disuse and those traits (or lack thereof) were passed onto the next generation. • So if an organism used a certain body part a lot, it would develop a lot and the organism’s offspring would show similar development. A BIG IDEA: THE CELL THEORY There were several important events that helped develop the Cell Theory. Below is an example of such an event: • Two scientists (Hooke and Leeuwenhoek) in the mid-1600s independently used microscopes to make discoveries about the microscopic world. • Without these discoveries and those like them cells might not have been discovered for a long time. More on Cells… Cells are either prokaryotic or eukaryotic. •Prokaryotes have no nucleus and few organelles; eukaryotes have a nucleus and many different organelles. •Prokaryotes are small; eukaryotes are much bigger. •Prokaryotes are mostly unicellular; eukaryotes can be unicellular but many are multicellular. •Prokaryotes were the first type of life on Earth; eukaryotes came later and probably developed from merging prokaryotes. Lamarck’s Hypothesis • What do you think of this idea? It was popular in Lamarck’s time as it seemed to explain some characteristics of living things. • There were, however, some problems with the hypothesis. Can you think of any? 1 9/29/2013 Voyage of the Beagle • In the 1831 a ship called the Beagle needed a naturalist (as many ships of the day did). • The position was taken by a young man named Charles Darwin. • Among the many places that the Beagle visited over the next few years Darwin was able to see patterns in the diversity of living things. Voyage of the Beagle • During the expedition the ship went to the Galapagos Islands. It was there that Darwin noticed that some very similar finches on different islands had slight differences. • The birds’ bills were perfectly adapted to the type of food the finch ate – seeds, insects, plants, etc. • He got the idea that the finches started out with a common ancestor, but changed over time as they adapted to their surroundings on new islands. Ideas That Influenced Darwin • Geological events could shape the Earth over millions of years. • His findings in the Galapagos and other areas, especially with finches and tortoises. • Books on economics and human population growth. One book suggested that if the population growth stayed the same there would not be enough resources for everyone. The Theory of Natural Selection Darwin published a book called On The Origin of Species. This landmark work outlined his theory: 1) Individual organisms differ, and some of this variation is inherited. 2) Organisms produce more offspring that can survive. 3) Individual organisms have different advantages/disadvantages. 4) Competition for limited resources leads to “survival of the fittest.” 5) Species today are descended with modification from successful ancestors. 2