Charles Darwin Born the same day as Abraham Lincoln (2/12/1809), he has contributed more to our...
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Transcript of Charles Darwin Born the same day as Abraham Lincoln (2/12/1809), he has contributed more to our...
Charles Darwin
Born the same day as Abraham Lincoln (2/12/1809), he has contributed more to our knowledge and understanding of evolution than any one person
Traveled the world (on the HMS Beagle)
Made numerous observations and collected evidence that led him to propose a revolutionary hypothesis about the way life changes over time (remember, evolution is the process by which modern organism have descended (or do we mean ascended??) from ancient organism – it is the process of change over time)
• 1835 – The beagle arrives in the Galapagos Islands.
• Collects several types of small brown birds – not particularly unusual or important
• Examined birds, noticed they had differently shaped beaks
• Thought some were wrens, some warblers and some blackbirds.
• Came to no other conclusions – at first.
Journey Home
• Spends great deal of time thinking about findings, examining different birds from Galapagos
Notices individual birds from one island looked different from those of a different island
• Did not immediately understand reason for diversity
At home, Darwin wondered if animals living on different islands had once been members of the same species
Works of other scientists of Darwin’s time
• Charles Lyell – geologist – explained how geological features could be built up or torn down over long periods of time
• This lead Darwin to thinking that if the Earth could change over time, might life change as well?
Darwin realized that it would have taken many, many years for life to change in this way and would be possible only if the Earth were extremely old
(The common thinking of that day was that the Earth was only a few thousand years old and since the original creation, neither the Earth nor the living species had changed.)
Spontaneous Generation• Early attempts to explain origin of life
• Concept that living things come from nonliving things.– Ex.: toads come from mud– Flies from rotting bodies of animals– Mice from cheese
• Made sense to people because they could not see tiny eggs laid by organisms, therefore, they didn’t know that the organisms came from eggs.
• Theory widely accepted until late 1800’s when Louis Pasteur disproved it.
Pasteur’s Experiment that finally disproves spontaneous generation
Jean Lamarck
• Theory of use and disuse
Thought organisms were able to developed new structures because they needed the structures
Believed that the size of an organ is determined by how much the organ is used
OR structures disappeared from species as a function of the degree to which these adaptations were needed by members of the species
TRANSMISSION OF ACQUIRED TRAITS
• Traits developed as needed within the lifetime of an organism (acquired traits), according to Lamarck & could be passed on to future generations thru the process of reproduction
Lamarck’s theories were an important 1st step in the development of a scientific view of the evolutionary process
August Weismann
• Measured and surgically removed the tails of lab mice over several generations
Hypothesized that, if Lamarck’s theory of transmission of acquired traits were correct, the offspring of each successive generation should show diminished tail length
Actual experimental data, however, showed no reduction of tail length in these mice
These experiments had the effect of DISPROVING Lamarck’s theory
Back to Darwin
• 1836 – returns to EnglandLearns that the brown birds were actually
finches found nowhere else in the worldLooked a great deal like a similar species on
the South American mainland yet island species were clearly different
Began filling notebooks with his ideas about species diversity and t he process that would later be called evolution.
• Publishes his work in 1859 (with the incentive from fellow naturalist Alfred Russel Wallace) in his book, On the Origin of Species
A very important observation of Darwin’s was the variation he saw among the different species.
And…he noted that this variation existed both in nature and on farms
Ex.: Some cows give more milk, some trees larger
fruits• Darwin learned that some of these differences were passed
from parent to offspring
(but he had not knowledge of genetics, therefore, he
couldn’t explain this variation – the big weakness in
his theory)
Darwin knew this variation was important though (in his day, variation were thought to be unimportant)
• Darwin noticed that breeders used inheritable variations to improve their herds and their crops
• He called this process artificial selection
• Artificial selection –
Nature provides the variation and humans selected those variations that they find useful (selective breeding)
• Darwin's greatest contribution and break from the past…
He compared processes in nature to artificial selection
And developed a scientific hypothesis to explain how evolution occurs
Darwin’s theory of evolution was based on VARIATION and NATURAL SELECTION (he compares processes in nature to artificial selection).
• This theory forms the basis of the modern theory of evolution.
• Included in this theory were 5 main ideas:1. Overproduction2. Competition3. Variations4. Natural Selection5. Survival of the Fittest PlusInheritance of VariationsEvolution of New Species
Overproduction
A population generally produces more offspring than can survive in the environment.
Ex.: a fish must lay millions of eggs to reproduce a small number of new fish
Competition
Because of overproduction, there is competition, or a struggle for survival, between organisms for space, food, water, light, minerals or other limited resources
• The struggle for existence (or survival) means that members of each species compete regularly to obtain food, living space and other necessities of life.
Variations
Members of a population show variations (differences in traits) that make certain individuals better adapted to survive.
Differences in structure, size, and color are examples of variations
Note: Darwin could not explain the genetic variations – remember, his work is published in the same time period as Mendel.
Natural Selection
Since some variations are more helpful than others, there is a natural selection against organisms that cannot adapt.
Organisms that cannot adapt – die
Loser
Survival of the Fittest• Survival of the fittest applies to those individuals
that have variations that enable them to live and reproduce
In a woodland environment,brown fur color would be a helpful variation and white fur color would not be helpful.
Fitness
• Darwin called the ability of an individual to survive and reproduce in its specific environment fitness
He proposed that fitness is the result of adaptations
An adaptation is any inherited characteristic that increases an organism’s chance of survival.
**Note: Fitness involves passing inherited traits to the next generation
• Concept of fitness was central to the process of evolution by natural selection.
• Individuals with characteristics that are not well suited to their environment –low levels of fitness-either die or leave few offspring.
• Individuals that are better suited to their environment-with adaptations that enable fitness-survive and reproduce successfully.
THIS WAS (and IS)…SURVIVAL OF THE FITTEST !
Because of the similarities to artificial selection, Darwin referred to the survival of the fittest as natural selection.
• Darwin proposed that over long periods, natural selection produces organisms that have different structures, establish different niches, or occupy different habitats.
• As a result,
Species today look different from their ancestors.
Caveman of 1 million years ago
Modern man
Inheritance of Variations(Reproduction)
• Organisms with helpful variations are more likely to survive and to reproduce, passing these variations to their offspring.
Evolution of New Species(Speciation)
• Over long periods of time, variations accumulate in a population
• Eventually, there are so many variations that the population becomes a new species.
Weakness of Darwin’s Theory
The main weakness of Darwin’s theory was its inability to explain the sources of the variations that he observed. It should be remembered that Darwin’s work was performed before there was accurate scientific understanding of the mechanisms of genetic inheritance.
Genes and Variation
• Darwin had no idea how inheritable traits pass from one generation to the next.
• Although variation in heritable traits was central to his theory, he had no idea how that variation appeared.
• 1930’s – evolutionary biologist connect Mendel’s work to Darwin’s
Variation and Gene Pools
• Genetic variation is studied in populations
Because members of a population interbreed, they share a common group of genes called a gene pool
A gene pool consists of all genes, including all the different alleles, that are present in a population
The relative frequency of an allele is the number of times that the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur.
Example
• In the mouse population the relative frequency of the dominant B allele (black fur) is 40% and the relative frequency of the recessive b allele (brown fur) is 60%
• Natural selection on single-gene traits can lead to changes in allele frequencies and ----to evolution.
Ex: population normally brown; mutations cause red (seen by predators easily and eliminated from gene pool); black becomes more common over time.
Speciation• As natural selection and chance events can change
the relative frequencies of alleles in a population• How do these changes form new species????• Redefine species………..
Species – a group of organisms that breed with one another and produce fertile offspring.
• If a genetic change increases fitness, that allele will eventually be found in many individuals of that population
What are some possible ways a new species might arise?
Reproductive isolation
As new species evolve, populations become reproductively isolated from each other.
When the members of two populations cannot interbreed and produce fertile offspring,
reproductive isolation has occurred.
Behavioral IsolationWhen two populations are capable of successfully
interbreeding but have different courtship rituals.
Ex.: eastern and western meadowlarks
Similar birds
Habitats overlap
Do no mate because they have different songs to attract mats
Do not responds to each others songs.
Geographic Isolation
• When two populations are separated by geographic barriers
Rivers
Mountains
Bodies water
Even a road (for small insects or other inverts)
Kaibab
Abert
Ex: two populations of squirrels were separated by the Colorado River.
The two species have similar characteristics but don’t interbreed.
Temporal Isolation
When two or more species reproduce at different time.
Ex: 3 species of orchid in the same rain forest
Each releases pollen only on a singe day
Cannot pollinate one another
Let’s examine how a new species of our friend, the fruit fly, might happen.
The branching points on this partial Drosophila phylogeny represent long past speciation events. Here is one scenario that exemplifies how speciation can happen:
•The scene: a population of wild fruit flies minding its own business on several bunches of rotting bananas, cheerfully laying their eggs in the mushy fruit...
Disaster strikes: A hurricane washes the bananas and the immature fruit flies they contain out to sea. The banana bunch eventually washes up on an island off the coast of the mainland. The fruit flies mature and emerge from their slimy nursery onto the lonely island. The two portions of the population, mainland and island, are now too far apart for gene flow to unite them. At this point, speciation has not occurred—any fruit flies that got back to the mainland could mate and produce healthy offspring with the mainland flies.
The populations diverge: Ecological conditions are slightly different on the island, and the island population evolves under different selective pressures and experiences different random events than the mainland population does. Morphology, food preferences, and courtship displays change over the course of many generations of natural selection.
So we meet again: When another storm reintroduces the island flies to the mainland, they will not readily mate with the mainland flies since they’ve evolved different courtship behaviors. The few that do mate with the mainland flies, produce inviable eggs because of other genetic differences between the two populations. The lineage has split now that genes cannot flow between the populations.
Often, studies of fossils or of living organisms
show that a single species or a small group of
species has evolved into several different
forms that live in different ways.
This process is known as adaptive radiation.
How quickly does evolution operate?
Evidence that horseshoe crabs have changed little from the time they first appeared in the fossil record
Much of the time these species are in a state of equilibrium, which means they do not change very much
At several points in the fossil record, changes in animals and plants occurred over relatively short periods of time.
Some biologists suggest that most new species are produced by periods of rapid change
(maybe when a small population becomes isolated fromThe main part of the population-That’s what happened withthe Galápagos finches) Scientists use the term punctuated equilibrium to describe
this pattern of long, stable periods interrupted by brief periods of more rapid change.
According to Darwin, evolution occurs slowly an continually over millions of years.
This time frame for evolution is known as gradualism
It is supported by fossil records of species that display slight changes in each sedimentary layer.
