Jean Baptiste Lamarck (1744-1829). Published his theory of evolution in 1809. Mechanism -- Inheritance of acquiredcharacteristics.
Darwin not first to suggest evolution occurs.
Inheritance of acquired characteristics:
Characteristics an organism acquires during its lifetime are passed on to its offspring.
Example: Giraffes stretch their necks toreach leaves. As a result their young born with longer necks.
Lamarck’s mechanism does not workbecause DNA is not changed by anorganism’s behavior or experiences.
One-way flow of information fromDNA to body.
Charles Darwin (1809-1882).
Son of a wealthy doctor.Studied medicine. Dropped out.Then studied theology.Keen naturalist and hunter.
Voyage of The Beagle (1831-1836)
Darwin companion for Captain Fitzroy. (Darwin not the official naturalist.)
Beagle sailed around the World. Mission to map coast of South America.
Sites visited included Galapagos Islands.
Volcanic islands (hence of recent origin) off coast of Ecuador.
Unique animals on Galapagos includegiant tortoises, marine iguanas, and Darwin’s finches.
On voyage Darwin read Lyell’sPrinciples of Geology.Book emphasized:
1. Gradualism: Geological features can be explained by gradual action of processes we see around us. E.g. wind, waves, rivers.
2. Great age of the earth
What Darwin observed
1. South American fossils resembledliving animals.
e.g. Extinct glyptodont (2,000 kg) resembles modern-day armadillo (2 kg).
What Darwin observed:
2. Parts of the world with similar climates (e.g. Australia, South America)populated by very different organisms.
What Darwin observed:
3. Plants and animals on each continent are distinctive.
E.g. kangaroos in Australia, guinea pigs,armadillos in South America.
What Darwin observed:
4. Many species on oceanic islands are found only there (endemic).
Often groups of very similar species found on these islands.
E.g. Galapagos finches and tortoises. These are examples of adaptive radiation.
Galapagos Giant Tortoise
Large CactusGround Finch
Sharp-beakedGround Finch
Warbler Finch
Galapagos Woodpecker Finch
What Darwin observed:
5. Endemic species on islands closelyresemble species on adjacent mainland.
These observations suggested to Darwinthat species are not unchanging.
Instead species change over time i.e., Evolution occurs.But how does evolution take place?
Key breakthrough when Darwin read:Malthus’s Essay on the Principle of population.
Malthus: Populations increase geometrically (multiplicatively), but resources do not.
Populations will outgrow their foodsupply.
Darwin concluded that among organisms there is a struggle for existence. Only the best adapted organisms survive to breed.
Darwin called the mechanism that causes
evolution to occur: natural selection.
Darwin’s logic
5 observations and 3 inferences (or conclusions).
Observation 1. If all organisms bred successfully populations would increase exponentially.
Observation 2. However, populations generally stay stable over time.
Observation 3. Resources are limited.
Inference 1.
Because resources are limited, more individuals are born than the environment can support.
There is a struggle for existence (and ultimately for reproduction.)Only some individuals survive to breed.
Observation 4. Individuals differ in their characteristics. There is variation.
Observation 5. Much of that variation is heritable.
Inference 2. Survival and reproduction are not random. The characteristics of an organism, which it inherits, affect its chances of survival and reproduction.
Organisms best adapted to the environment on average leave the most offspring.
Inference 3. Because variation is heritable, differences between individuals in their reproductive success lead to changes in the characteristics ofthe next generation. Evolution occurs.
Darwin developed his ideas in 1842, butdid not publish them immediately.
In 1858 Alfred Russel Wallace wrote to Darwin. He had independently developed the theory of evolution by natural selection.
Joint presentation of their ideas in 1858.
Darwin published On the Origin of Species in 1859.
Explained in detail his ideas aboutevolution and natural selection.
Major difficulty for Darwin was thathe did not understand how inheritanceoccurs.
Darwin did not express his ideas in termsof genes. He had no idea what genes were or what DNA was.
DNA is the genetic material. Theinstructions for making and “operating”an organism are written in DNA.
DNA is divided into sections called genes.
Basic genetics terminology
Each gene codes for a protein.
Together the genes determine the characteristics of an organism.
Alleles are different versions of agene.
If a single gene codes for flowercolor, white and blue flowers would be coded for by 2 different alleles.
You possess two copies of each gene in your body*.
One copy is inherited from each parent.
For a given gene you may have two different alleles or two copies of the same allele. (* excluding genes on sex chromosomes in males).
A homozygous individual has two copies of a particular allele. (AA)
A heterozygous individual has twodifferent alleles. (Aa)
Genotype and phenotype
An organism’s genes (its genotype) play a large role in determining its physical appearance (its phenotype).
But remember an organism’s phenotype is also affected by the environment.
Today we express evolutionary ideas interms of genes because genes are the only things passed from one generationto the next.
In the process of natural selection, genes that help organisms to surviveand reproduce become more common.
Genes that help less or are harmfulgradually are eliminated from thepopulation.
Only those individuals that are themost successful, e.g. the fastest, the best camouflaged, the best foragers, etc. succeed in breeding.
The genes of other individuals are not passed on to the next generation.
Natural selection is the term used to describe the process by which thebest genes are chosen in each generation.
Evolution occurs when gene frequencieschange from one generation to the next.
As a result of natural selection organismsbecome increasingly well adapted totheir environments.
Adaptations are characteristics of organisms that enable them to survive in their environments.
Evolution is a population process. Anindividual does not evolve.
Natural selection in action.
Peppered Moth and industrial melanism.
Peppered moth occurs in two forms: light, speckled form and dark (melanic) form.
In early 1800’s dark form very rare.
Dark form caused by dominant mutationthat occurs spontaneously.
A mutation is a change in gene’s DNA .
Dominant allele is expressed even if onlyone copy is present. (i.e., is expressed even in heterozygotes).
Peppered moths rest on trees and depend on camouflage for protection.
In unpolluted areas trees coveredin lichens. Light form of moth hard forbirds to see.
In mid 1800’s air pollution in Britishcities covered trees in soot.
In cities dark form of moth became common and light form rare.
In mid 1950’s pollution controls reduced air pollution in Britain.
Frequency of dark form has declined steadily since then.
Darwin’s Finches on Galapagos
Peter and Rosemary Grant of Princetonhave studied Medium Ground Fincheson Daphne Major for almost 30 years.
Banded and measured the island’s entire population and followed it over many years.
Rainfall on Galapagos Islands is unpredictable.
In wet years lots of seeds produced.
In dry years few or no seeds produced.
Finches with larger bills can eat largeseeds and small seeds.
Finches with smaller bills cannot eat large seeds, but are more efficient at eating small seeds.
In droughts large-billed finches (and genesfor large bills) survive better than small-billed finches (and genes for small bills).
Mean beak depth of the finch populationfluctuates in synchrony with climate.
Sickle Cell Anemia
Sickle cell anemia is a condition commonin West Africans (and African Americans of
West African ancestry).
In sickle cell anemia red blood cells aresickle shaped.
Usually fatal by about age 10.
About 1% of West Africans have sickle cell anemia.
A single mutation causes a valineamino acid to replace a glutaminein an alpha chain of the hemoglobin molecule.
Mutation causes molecules to stick together.
Why isn’t mutant sickle cell geneeliminated by natural selection?
Only individuals homozygous forsickle cell gene get sickle cell anemia.
Individuals with one copy of sickle cell gene(heterozygotes) get sickle cell trait (mild form of disease).
Individuals with sickle cell trait don’tget malaria.
MalariaMalaria is one of the most important diseases in the world.
About 500 million cases and an estimated 700,000 to 2.7 million deaths occur worldwide each year (CDC).
Malaria was well known to the Ancient Greeks and Romans. The Romans thought the disease was caused by bad air (in Latin mal-aria) from swamps, which they drained to prevent the disease.
Malaria is caused by infection with a single–celled sporozoan parasite called Plasmodium.
It is transmitted by the bite of an Anopholes mosquito
Plasmodium has two hosts: mosquitoes and humans and a complex life cycle.
Sexual reproduction takes place in the mosquito and the parasite is transmitted to humans when the mosquito takes a blood meal.
In a human Plasmodium goes reproduces in liver cells and then red blood cells before being picked up by a mosquito to continue the cycle again.
The severity of a malaria infection may range from asymptomatic (no apparent sign of illness) to the classic symptoms of malaria (fever, chills, sweating, headaches, muscle pains), to severe complications (cerebral malaria, anemia, kidney failure) that can result in death.
Sickle cell allele protects against malaria
People with the sickle cell allele are protected against Plasmodium because their hemoglobin under low oxygen conditions contracts into needle-shaped clumps.
This contraction not only causes the sickling of the cell, but harms the parasite. Parasites are impaled on the clumps and the cell loses its ability to pump potassium, which the parasite needs.
Heterozygotes (those with one copy of the sickle cell Allele have higher survival thaneither homozygote. Heterozygote advantage.
Sickle cell homozygotes die of sickle cell anemia
“Normal” homozygotes more likelyto die of malaria.
Stabilizing selection for sickle cell allele.
