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Whose theory is this? What is this theory called? . Lamarckian Theory. Acquired characteristics Use and disuse . Evolution. Notice the similar bone structure on the head of both animals. Scientists believe that the Giraffe evolved from an animal very similar to the Okapi .

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lamarckian theory
Lamarckian Theory
  • Acquired characteristics
  • Use and disuse


Notice the similar bone structure on the head of both animals.

Scientists believe that the Giraffe evolved from an animal very similar to the Okapi.

How might this have happened?



i contrasting early theories

Debate in the 1800s

I. Contrasting Early Theories

Lamarck: thought that if an organism used a part, it would get bigger or better. Likewise, if it was not used, that part would shrink or get weaker. This is called his theory of use and disuse. He thought that this “acquired characteristic” would be inherited by an organism’s descendents. This is called his theory of inheritance of acquired characteristics.

B. Darwin: thought that parents had a varietyof children, and those best suited to their environment survived to reproduce in greater numbers.

ii causes of evolution

Our modern understanding

II. Causes of Evolution

A. Nonrandom mating -

the probability of two organisms mating depends on their phenotypes (therefore their genotypes).

Inbreeding - mating between relatives (usually in isolated populations).

*Usually disadvantageous - harmful traits may show up because of increased pairing of recessive alleles (homozygous recessive genotype).

ex. - first cousin marriage - Hopi Indians - high rates of albinism;

Royal European families had hemophilia due to intermarriages with descendents of Queen Victoria of England.

ii causes of evolution1

II. Causes of Evolution


Bright male plumage is preferred by females in many bird species.

In the 20th century, tall women more often married tall men than short men.

Sexual selection - mating between unrelated individuals of the same species, to a greater or lesser frequency than is expected by chance.

(i.e. mates are being chosen)

Female and Male Peacock

ii causes of evolution2

II. Causes of Evolution

B. Genetic Drift –

when gene frequencies in a small population change due to random events or chance.

Bottleneck effect – when only a small portion of the original population serves as the sole source of a new population.

Ex: Today’s world-wide cheetah population

Founder effect – a few individuals leave a large population and start a new population.

Ex: Various species on Galapagos Islands

ii causes of evolution3

II. Causes of Evolution

1. Alleles are transferred from one population to another, changing the allele frequencies in both populations.

C. Migration(called “gene flow”)*

The movement of individual organisms into or out of a population.

ii causes of evolution4

II. Causes of Evolution

D. Mutation – a physical change in a gene or a chromosome (DNA).

Mutations (and recombinations) provide the variations on which natural selection acts upon.



ii causes of evolution5

II. Causes of Evolution

E. Natural Selection – (Darwin’s theory)

1. Variations - characterize a population and may be beneficial, detrimental, or make no difference.

2. Nature selects those individuals with the beneficial variation that permits it to survive/reproduce.

3. Adaptations passed down from parents to offspring.

Over many generations, the populationchanges– not the individual.

This is evolution.