Charles Darwin

Darwinian Natural Selection

1. Individuals within populations are variable.

2. The variations among individuals are, at least in part, passed from parents to offspring.

3. In every generation, some individuals are more successful at surviving and reproducing than others.

4. The survival and reproduction of individuals are not random. The individuals with the most favorable variations, those who are better at surviving and reproducing, are naturally selected.

Darwinian Fitness

• Darwinian Fitness is the ability of an individual to survive and reproduce in its environment.

• Fitness of an organism is measured by how many offspring it produces compared to other individuals of its species.

• Adaptation refers to a trait that increases an organisms fitness relative to individuals without the trait (ie..Reverse Transcriptase).

Is Darwinian Natural Selection Testable?

• Yes, each of the 4 postulates that make up the theory are testable.– Section 3.3 The evolution of flower color in an

experimental Snapdragon population.

– Section 3.4: The evolution of Beak Shape in Galapagos Finches

The Evolution of Flower Color in Snapdragons

• Question: Can Darwinian natural selection by bumble bees influence the evolution of a floral trait?

• Hypothesis: Yes.– There is variation among individuals– The variation is heritable– Individuals vary in their reproductive success– Reproduction is nonrandom– The above will lead to evolution of the species.

Prediction

• Based upon the theory of Natural Selection, We predict that If you allow wild bumblebees to pollinate a population of Snapdragon in a meadow, there will be a difference in reproductive success among the plants based on the different colors of the flowers. The bees will preferably pollinate one type of flower over the other and the population will change over time.

Experiment and Results

Postulate 1: There is variation among individuals

• ¾ of the plants had flowers that were almost pure white, with just two spots of yellow on the lower lip.

• ¼ of the population of plants had flowers that were solid yellow.

• Therefore Postulate 1 is True; there is variation within the population.

Postulate 2: Some of the Variation is Heritable

• The difference in flower color for the population of snapdragons was shown to be held at a single locus.

• The dominant allele S expressed white flowers with two yellow spots, while the recessive trait ss is expressed as the yellow flower.

• Genotypes of the 48 plants were as follows– SS = 12– Ss = 24– ss = 12

Postulate 3: Do individuals vary in Their Success at Reproducing?

• Researchers measured reproductive success by measuring the plants ability to export pollen and making seeds.– researchers tracked the number of times each flower

was visited by a bee.– Researchers counted seeds produced from each fruit.

• Consistent with Darwin’s 3rd postulate, the plants showed considerable variation in reproductive success, both as pollen donors and as seed producers.

Postulate 4:Is Reproduction nonrandom?• White flowers attracted twice as many bees as yellow

flowers

• White flowers also produced slightly more seeds than the yellow flowering plant.

• Therefore the data shows that reproductive success is not random. The white flowers attract more bees.– Could create a new hypothesis from the analysis. The yellow dots on

the white plants serve as a guide to pollinating bees.

Did the Population Evolve?

• Since bees selected white flowers more often than yellow and color is carried on a single gene the next generation should have more white flower than the last.

• Results– White flowers went from 75% to 77%

The Evolution of Beak Shape in Galapagos Finches.

• Use the Galapagos Finch population to demonstrate that Charles Darwin’s theory of natural selection can be verified by the scientific method.

3.5 The Nature of Natural Selection

How selection does and does not operate

Natural Selection Acts on Individuals, but its Consequences Occur in Populations

• How did the finches on Daphne Major react when a drought caused seeds to change from small soft seeds to larger harder seeds.– Did birds with short shallow beaks start to

grow longer and deeper beaks in order to survive?

– Why did beak size change over time?

Natural Selection Acts on Phenotypes, but Evolution consists of changes in Allele Frequencies

• If beak size was not genetically controlled but rather a product of the environment, would deep beaks still have been favored during the drought?– Would the phenotypic

frequencies in beak size change after the drought?

• Evolution can only occur when the traits are genetically based.

• Evolution is measured by changes in Allele frequency within a Population

Natural selection is Not Forward Looking

• Natural selection adapts populations to conditions that prevailed in the past, not conditions that might occur in the future.

– The finch population became better adapted to the drought only after the drought occurred.

– Following the drought in 1977, the finch population was not better adapted to survive the flooding that took place in 1983.

Can Natural Selection Create New traits?

• Natural selection can select only from the variations that already exist in a population.– Example: Natural

selection cannot instantly create a new and optimal beak

• How then do new unique traits come into existence?

Evolution of New traits

1. Mutations produce new alleles in all species

2. Meiosis and fertiliation reomine existing alleses to create new genotypes

• Natural selection acts upon these new traits.

Corn oil

The Panda’s Thumb

• Natural selection is able to “repurpose” existing behaviors, structures or genes.

• A trait that is used in a new way and is elaborated on by selection into a completely new structure is called a preadaptation.

Is Natural Selection random?Is Natural Selection progressive?• Natural Selection is nonrandom. It is predictable

in that it increases a populations adaptation to the environment.

• Natural Selection is not progressive. It does not necessarily push organism toward more complex forms of life.– Examples of organisms becoming less complex

overtime?– Are some animals considered higher than other

animals?

In nature, are there true altruistic behaviors

• Altruistic behaviors reduce the bearer’s fitness and increases the fitness of others.

• If such behavior is inherit or genetic, it will be strongly selected against.

• Individuals do not do things for the good of the species. They behave in a way that maximizes their individual fitness.

Altruistic behavior reduces the bearer’s fitness and increases the fitness of others

Evolution of Darwinism

• Is the amount of variability in populations strictly limited?

• If a new trait emerges in a large population, won’t it be eventually be swallowed up in the population no matter how advantageous it is?

• Was Lord Kelvin right about the age of the earth?