Chapter 10 Notes, Part II The Theory of Natural Selection.

Chapter 10 Notes, Part IIThe Theory of Natural Selection

Publication of On the Origin of Species

• In 1858, Darwin received an essay from Alfred Russel Wallace, a naturalist working in Malaysia. Wallace’s thoughts on evolution were very similar to Darwin’s. This finally got Darwin to publish his theory, after almost 25 years.

Publication of On the Origin of Species

• In 1859, On the Origin of Species by Means of Natural Selection was published.

• In this book, his theory of evolution, called natural selection, was explained.

• It caused a sensation; some were appalled and some thought it to be brilliant.

Publication of On the Origin of Species

Inherited Variation

• One of Darwin’s most important insights is that members of any species have variations, or differences, among each other.

• He had no idea of how heredity, or genes, worked.

• He argued that variation was important. This was revolutionary, b/c in his day, variations were thought to be imperfections.

• He observed and reasoned that some variations were inherited.

Inherited Variation

• Darwin noted that farmers (both plant and animal) used this hereditable variation to improve their breeds.

• Ex: some hogs are “meatier” than others, some horses are faster than others, some cows give more milk, some sheep have better wool

Inherited Variation/ Artificial Selection• These farmers, or

breeders, only selected the most favorable traits to breed. Over many generations, the species would be different than the original population.

• Population is defined as all the individuals of a species living in a particular area.

• Nature provides the variation, but since it is humans SELECTING which traits are favorable/useful (not nature), this is called artificial selection.

Artificial Selection

Artificial Selection

Artificial Selection

Evolution by Natural Selection

• Natural selection is also called “survival of the fittest.”

• This theory is an explanation for how life changes over time, or evolves.

• 5 main parts of this theory.

Evolution by Natural Selection

I. In nature, organisms produce more offspring than can survive. This sets up a struggle for existence.

Ex. A female cod can produce 4-6 million eggs in one spawning; a female mouse can have 48 babies each year; a fly lays about 250 eggs at a time

Evolution by Natural Selection

II. In any population, variation exists among individuals.

Ex. Human skin color, speed of gazelle, size of a deer’s antlers

Evolution by Natural SelectionIII. Some variations give an

individual an advantage in surviving and reproducing in their environment

• These variations are called adaptations.

• Can be physical or behavioral

• Fitness is a measure of an organism’s ability to survive and reproduce in its specific environment.

Evolution by Natural Selection

IV. The offspring of these individuals, if the adaptation is hereditable, are likely to have the adaptation, too. They will likely live longer and produce more offspring b/c they also have the adaptation.

Evolution by Natural Selection

V. Over time, offspring with certain adaptations make up most of the population and may look entirely different from their ancestors.

• Darwin reasoned that each living species has descended, with changes, from other species over time. This is referred to as descent with modification.

• Implies all living things are related to one another through common descent.

Example of Natural Selection Today

Antibiotic-resistant bacteria• Natural populations of bacteria contain

considerable variation, primarily as the result of mutations. When exposed to antibiotics, most bacteria die quickly, but some may have mutations that make them slightly less susceptible.

• These surviving bacteria will then reproduce again, producing the next generation. This population contains more bacteria that have some resistance against the antibiotic.

Example of Natural• At the same time, new mutations occur,

contributing new genetic variation to the existing genetic variation. Spontaneous mutations are very rare, and advantageous mutations are even rarer. However, populations of bacteria are large enough that a few individuals will have beneficial mutations. If a new mutation reduces their susceptibility to an antibiotic, these individuals are more likely to survive when next confronted with that antibiotic. Given enough time, and repeated exposure to the antibiotic, a population of antibiotic-resistant bacteria will emerge.

Evidence of Evolution

The Fossil Record• Darwin saw

fossils as a detailed record of the history of life on Earth.

Evidence of EvolutionHomologous Body

Structures• There are striking

similarities in the body parts of different species.

• Ex: Consider the limbs of a human, cat, whale, and a bat . Though they look different and function differently, they all are formed by the same bones.

Evidence of Evolution• Each of these limbs has

adapted in ways that enable the animal to survive in different environments.

• A body part with the same basic structure and embryonic origin as that of another organism, though not necessarily sharing the same function, is a homologous structure.

Evidence of Evolution• Not all homologous

structures serve important functions (caution! Next slide… )

• Some organs that don’t function are so reduced in size that they are just traces of homologous organs in other species.

• Such organs are called vestigial organs.

Evidence of Evolution

Vestigial Structures Vestigial Structures• Go to

http://www.livescience.com/animals/top10_vestigial_organs.html

As far as science “knows,” they serve no important function. But, be careful…scientific knowledge grows everyday. An organ thought to be vestigial may be found to have function.

Example: human appendix

Evidence of Evolution

Similarities in Embryology• The embryos of many

vertebrates, for example, are very similar.

• The same groups of embryonic cells develop in the same order and in similar patterns to produce the tissues and organs in all vertebrates.

• What does this imply?

Evidence for Evolution

• These are just a few of the lines of evidence for evolution. Others include geographic distribution, distribution in time, evidence by example, comparative biochemistry, etc…