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McGillivary Biology

Last Updated:

April 26, 2008

Welcome to the Evolutionary Biology page!

The sections of your text that apply to this section are: (p. 368 - 390).

•  There is no theory today as widely misunderstood as evolution. There is likely no scientist that has had as great an impact on the way we think as Darwin and yet Darwin himself was as much a product of the ideas of his time as we are now.
•  What we will do in this section is follow a series of logical steps that will allow us to carefully set the framework for Darwin's ideas, and look at their influence in Biological today.

Setting the groundwork:

1. Adaptation and change.

This image of an Bengal tiger skull is quite different from that of the not that long ago extinct Smiledon

(http://images.google.ca/imgres?imgurl=http://www.gutenberg.org/files/19550/19550-h/images/053skull.jpg&imgrefurl=http://www.gutenberg.org/files/19550/19550-h/19550-h.htm&h=489&w=600&sz=45&hl=en&start=8&tbnid=qRC5vUs9202B3M:&tbnh=110&tbnw=135&prev=/images%3Fq%3Dsmilodon%2Bskull%2Bsketch%26gbv%3D2%26hl%3Den)

Simple evidence of this would have inferred two ideas to a natural historian. The first is that organisms are adapted to their environment (aspects of their behaviour, structures, appearance and niche). The second idea is that there are organisms that exist today that did not exist long ago and, conversely, organisms that existed long ago that are not present today (although it would be cool to have saber-toothed cats!).

 

2. Pre-Darwin ideas.

1. Hutton (late 1700's): The Earth is very old. It is shaped by geological events that took place over millions of years (not thousands).

2. Geologic layering reinforces Hutton's ideas.

3. Malthus (~1800): His essay contains predictions that the human population growth rate will outpace its ability to feed everyone.

4. Cuvier (1800): The deeper the strata, the less similar fossils are to today's organisms.

5. Lamarck (1809): Evolution is based on the inheritance of acquired characteristics. Stretch your neck, it will grow. Your offspring will be born with longer necks.

6. Lyell (1833): In Principles of Geology, he explains that processes occurring now have shaped Earth's geological features over long periods of time and can be used to explain events in the past.

7. Fossils were being found in greater numbers, taking on something of a 'gold rush' mentality among collectors.

8. Similarities between organisms can be seen as evidence of relatedness.

3. Darwin's voyage on the Beagle (1831). This voyage provides Darwin with vast amounts of evidence leading to his theory of evolution by natural selection.

Darwin recorded evidence of all kinds. In particular, on the Galapagos Islands, Darwin noted 13 variety of finches, giant land tortoises, sea dwelling iguanas and many other varied organisms.

4. Darwin's logic and theory.

Darwin’s observations, and later reflection at home, lead him to develop a theory about the origins of life, the relatedness of living things and a mechanism by which changes in populations take place over great time periods.

In 1859, to great uproar, and ridicule, Darwin published his famous On the Origin of Species by Means of Natural Selection.

 

Darwin's Theory has four main parts:

1. Organisms have changed over time, and the ones living today are different from those that lived in the past. This Earth is not constant, it is changing.
2. All organisms are derived from common ancestors. Over time, populations evolve into different species, which are related because they are descended from a common ancestor.
3. Change is gradual and slow. The idea that all change is gradual and slow is not supported by all evidence in the fossil record. There are times when it appears that there are great and rapid increases in bio-diversity over a fairly short geologic time. In 1984, Stephen J. Gould and other evolutionary biologists suggested the theory of Punctuated Equilibrium to describe this episodic nature of bio-diversity.

4. The mechanism of evolutionary change is natural selection. This was the most important and revolutionary part of Darwin's theory.

5. A closer look at natural selection. First of all, we will define natural selection as the differential reproduction of genotypes. There are also four key components to natural selection.

1. The initial statements that Darwin makes is that organisms have the tendency to over reproduce.

2. Fortunately, there are environmental restrictions (such as limited resources, space, disease, etc.) that lead to ...

3. A [metaphorical] struggle for existence (competition).

4. Genetic (heritable) variation within populations means that some individuals have characteristics that make them more likely to have offspring or have more offspring that survive to reproduce.

As a result of these four conditions, adaptive traits will persist (this is Natural Selection)

 

Activity: Use the links on Natural Selection from SciLinks (look to the right) and then download this pdf file (you'll need adobe reader) to help you assess the value of two resources on this topic.

 

 

Interestingly, if you tie in the idea that the environment always changes (thus changing the environmental restrictions), the result is the selected trait changes and this is Evolution!

 

 

6. Modern areas of study have provided evidence of and applications for evolutionary biology.

1. Artificial selection. A new area of study looks at how human activity is influencing selective pressures on the Earth's organisms. For example, habitat loss, climate change, over harvesting, pollution, and genetic modification are human activities that have immense and profound consequences for the species on this planet. With extinction rates, currently rivaling that which took place 65 mya, societies are at a critical juncture in human history - asking the question "What next?".

2. Modern evidence:

   1. Geographic distribution of living species. Different and similar selective pressures mean that related species living in different geographic areas may have evolved very unique and dissimilar adaptations. At the same time, unrelated species may have evolved similar adaptations to address similar environmental conditions.

   2. Homologous body structures. Structures that appear different in adult forms (i.e. flippers and wings) derive from the same embryonic tissues and have similar bones.

   3. Embryology. Scientists once thought that 'phylogeny recapitulates phylogeny'. This means that similar embryonic developmental patterns were evidence of evolution and ancestry. This is not the case. Rather, embryonic development is simply evidence of common developmental patterns and the conservative nature of evolution.

   4. Molecular homology. Here we look at similar biological molecules (proteins) and the differences in amino acid sequence as evidence for relatedness. For example, if we look at the protein hemoglobin, humans and gorillas have only one amino acid that is different, while humans and dogs have 15 different amino acids, and humans and mice have 25.

   5. DNA-DNA hybridization. measures the degree of genetic similarity between pools of DNA sequences. It is usually used to determine the genetic distance between two species. When several species are compared that way, the similarity values allow the species to be arranged in a phylogenetic tree; it is therefore one possible approach to carrying out molecular measures for relatedness.

 

Chapter self-test:

 

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This site was last updated 04/26/08

by Lawrence McGillivary