Why do organisms ‘match’ their environment?

• The ‘match’ is genetic and induced by environment.

Natural selection: differential survival and reproduction of individuals based on genetic differences in some trait(s)

fitness

adaptation

• offspring resemble parents

• variation in traits can be inherited

• life is full of challenges!

• variation in traits variation in fitness

Fitness:

- ability to survive and reproduce

- depends on the environment

Charles Darwin figured out how natural selection could lead to adaptive evolution.

Resistance to antibacterial soapGeneration 1: 1.00 not resistant

0.00 resistant

How natural selection works

Generation 1: 1.00 not resistant 0.00 resistant

Resistance to antibacterial soap

How natural selection works

Resistance to antibacterial soap

mutation!

Generation 1: 1.00 not resistant 0.00 resistant

Generation 2: 0.96 not resistant 0.04 resistant

How natural selection works

Resistance to antibacterial soapGeneration 1: 1.00 not resistant

0.00 resistant

Generation 2: 0.96 not resistant 0.04 resistant

Generation 3: 0.76 not resistant 0.24 resistant

How natural selection works

Resistance to antibacterial soapGeneration 1: 1.00 not resistant

0.00 resistant

Generation 2: 0.96 not resistant 0.04 resistant

Generation 3: 0.76 not resistant 0.24 resistant

Generation 4: 0.12 not resistant 0.88 resistant

How natural selection works

Fitness depends on the environment.

local adaptation

For adaptations to evolve:

- differences in the trait must cause differences in fitness

- differences in the trait must be heritable

Natural selection – key points

differential survival and reproduction of individuals based on genetic differences in some trait(s)

Natural Selection• Directional Selection: natural selection

removes organisms at one extreme and in the middle, favoring one extreme:– The peppered moth– Bacterial resistance to antibiotics– Insect resistance to pesticides

Natural Selection

• Stabilizing Selection: natural selection removes organisms at the extremes of a population:– Birth weight in humans (heaviest and

lightest babies have the highest mortality)

Natural Selection

• Disruptive selection: natural selection that favors two extremes of a characteristic– Grass plants in Welsh copper mines– Sickle cell heterozygotes in Africa

Observing Variation

• (1) Variation exists in living populations.

• (2) Some variations are helpful and increase life span.

• (3) Some variations are harmful and decrease life span.

Observing Variation

• (4) A population may become physically separated, so two groups form.

• (5) As mutations and meiosis occur, new variations will appear.

• (6) Some offspring will survive better than others.

Observing Variation

• (7) Offspring that survive in one area may not survive in another area.

• (8) Over time, more variations will accumulate in the two populations.

• (9) The two populations will become different because different variations occur in the two groups.

Observing Variation

• (10) Individuals that die out do not get to reproduce.

• (12) Sometimes, a new species has formed.

• (11) In some cases, the two groups become so different from each other that they can no longer interbreed.

What is this Process?

• Evolution of species

• A set of natural processes that causes change in a population of living things over time.

• Biological evolution, simply put, is descent with modification.

Why was this little sticker so controversial?

Source: http://www.swarthmore.edu/NatSci/cpurrin1/textbookdisclaimers/CobbDisclaimer.jpg

Understanding Evolution

• Scientists study natural processes and look for natural explanations.

• I will introduce you to some of the evidence that leads biologists to view evolution as the great unifying theory of the field.

• Science cannot answer every question.• What if you disagree with photosynthesis?

Evolution by Natural Selection

• Variation exists in every population.

• Natural selection (predators, disease, drought, cold, famine, flood, heat…..) causes some individuals to survive while others do not.

• Sources of inherited variation: meiosis, crossing over, mutation.

• Allele frequency change is called evolution. Sometimes it causes speciation.

Charles Darwin

• Age 25: set sail on 5 year voyage on HMS Beagle as “naturalist”.

Darwin’s Ideas Did Not Develop in a Vacuum

Contributor’s to Darwin’s thinking included:

Charles Lyell – uniformitarianism.

1797-1875

Georges Cuvier – species extinction.

1769-1832

Darwin’s Ideas Did Not Develop in a Vacuum

Contributor’s to Darwin’s thinking included:

Thomas Malthus – struggle for existence.

1766-1834

Jean Baptiste de Lamarck – evolution by acquired characteristics.

1744-1829

Jean Baptiste de Lamarck“Inheritance of Acquired

Characteristics”• Animals face a need to change.• Animals make themselves change• Animals pass on the changes to

offspring.

Darwin observed animals and plants in the Caribbean, South America, Galapagos, Hawaii, Indonesia, Africa & Europe.

• He wrote three books about his experiences.

• He took massive notes in several notebooks, working on Origin of Species 20 years!

Common AncestorThe central idea of biological evolution is that all life on Earth shares a common ancestor, just as you and your cousins share a common grandmother.

A reconstruction of the HMS Beagle sailing off Patagonia.

Darwin’s Voyage of Discovery

The Voyage of the Beagle

Galapagos Islands and South American Mainland

Intra-Galapagos Variability:Darwin’s Finches

Evolution by Natural Selection

• Variation exists in a population. (Darwin did not know how the variation was passed…genetics was not yet understood)

• Some organisms survive and pass on genes…some do not.

• Causes of this evolution: migration, “heredity”, mate selection, predators, disease, famine, drought,

Lamarck and Darwin: How would they explain these observations?

• In 1960, a pesticide was sprayed at Tybee Island, GA, killing 97% of all mosquitoes.

• In 1979, the same pesticide killed less than 30% of all the mosquitoes.

• Lamarck: Darwin:

Darwin’s Observations and Inferences

Observation 1: Left unchecked, the number of organisms of each species will increase exponentially, generation to generation.

Observation 2: In nature, populations tend to remain stable in size.

Inference 1: Production of more individuals than can be supported by the environment leads to a struggle for existence among individuals, with only a fraction of offspring surviving in each generation.

Observation 3: Environmental resources are limited.

Darwin’s Observations and Inferences

Observation 4: Individuals of a population vary extensively in their characteristics with no two individuals being exactly alike.

Observation 5: Much of this variation between individuals is heritable.

Inference 2: Survival in the struggle for existence is not random, but depends in part on the heritable characteristics of individuals. Individuals who inherit characteristics most fit for their environment are likely to leave more offspring than less fit individuals.

Darwin’s Observations and Inferences

Inference 3: The unequal ability of individuals to survive and reproduce leads to a gradual change in a population, with favorable characteristics accumulating over generations (natural selection).

Taken together, these three inferences are a statement of Darwin’s Theory of Evolution.

Darwin’s Observations and Inferences

The Modern Synthesis1932-1953

1) Mutation and segregation result in large variability within populations

2) Individuals pass alleles to offspring

3) Differential reproductive success

4) Adaptation increases allelic fitness

How can allele frequency change?

2. Meiosis and crossing over

1. Migration

3. Mutation

5. Genetic drift (chance changes to small populations)

4. Natural Selection (predators, disease, famine, drought)

How can evolution occur?

2. Meiosis and crossing over

1. Migration

3. Mutation

5. Genetic drift (chance changes to small populations)

4. Natural Selection (predators, disease, famine, drought)

Evidence evolution

• Homologous or vestigial structures

• Biogeography

• Homologous DNA and proteins

• Fossil evidence

• Evolution observed right now

Why use the same skeletal plan for these very different appendages?

Homologous skeletal structure

Vestigial whale pelvis bones

Vestigial anthers and pollen: Dandelions are asexual

Why does organism X have a feature which looks exactly like a weakened, not fully effective, overly-complex version of an organ found in its evolutionary ancestors, if not for common descent via natural selection and mutation?

Why does the degree of relationship of genes match their degree of relationship established by other methods?

Why should different organism possess related genes?

Evidence for Evolution – Evolution Observed

Evolution of pesticide resistance in response to selection.

Explain how this is perceived as an example of evolution.

Biogeography

Evolution of new species• A population becomes divided by a physical

barrier (water, mountains, desert, or just a large distance.

• The two populations experience different selection pressures and will evolve separately.

• Even if the two populations meet again, they are now so different that they can no longer interbreed. They are reproductively isolated and are two distinct species.

Convergent Evolution – process by which unrelated organisms independently evolve similarities when adapting to similar environments.

Nothing in biology makes sense except in the light of evolution. Theodosius Dobzhansky