Theory of EvolutionChapter 15

Charles Darwin•Who was he?

- English naturalist (studied and collected

biological specimens)- sailed around the world on ship HMS

Beagle- known for his studies of the

Galapagos Islands- came up with idea of how organisms

change over time (natural selection)

Natural Selection•A mechanism for change in populations.

•It happens when organisms with the best variations survive, reproduce, and pass their variations on to the next generation (population increases)

•Organisms that don’t have these variations are less likely to survive and reproduce (population decreases)

How does Natural Selection Work

1. Organisms produce more offspring than can survive

2. Individuals have variations3. Individuals with certain useful

variations, such as speed, survive and pass on those variations

4. Over time, those individuals will make up most of the population

“Survival of the Fittest”

Evidence for Evolution

Adaptations develop over many generations

1. Structural Adaptations:mimicry – when one species

resembles another species

camouflage – enables species to blend in

with their surroundings

Mimicry

Camouflage

Camouflage

Camouflage

Canouflage

Camouflage

Camouflage

Camouflage

Camouflage

Camouflage

Natural Selection Example

1. Variation in Traits – some beetles are green and some are brown.

2. Green beetles tend to get eaten by birds and survive to reproduce less often than brown beetles do.

3. The surviving brown beetles have brown baby beetles because this trait has a genetic basis.

4. The more advantageous trait, brown coloration, which allows the beetle to have more offspring, becomes more common in the population. If this process continues, eventually, all individuals in the population will be brown.

2. Fossils3. Anatomy:

homologous structures – structures with

common evolutionary origins; can be similar

in arrangement, function, or both(ex. Forelimbs of whales, alligators,

birds)

analogous structures – body parts of

organisms that are similar in function, but

have no common evolutionary origin.(ex. Wings of birds, butterflies, and

bats)

vestigial structures – structure that has

no function in a present-day organism, but

was probably useful to an ancestor

4. Embryology (embryos)- The embryos of a fish, reptile, bird,

and mammal are all similar in

appearance

5. Biochemistry- comparisons of DNA show

evolutionary relationships

Homologous Structures

Analogous Structures

Population Genetics

•Can individuals evolve?

•Natural selection acts on the range of phenotypes in a population. (Strong genes get passed on to the next generation)

•Evolution occurs as a population’s genes and their frequencies change over time

How Can A Populations Genes Change Over Time?

•All of the alleles of a population’s genes are known as the gene pool.

•The percentage of any specific allele in the gene pool is called the allelic frequency.

•A population is in genetic equilibrium if its alleles remain the same for generations.

•A population that is in genetic equilibrium is not evolving.

•Anything that affects the genes in the gene pool can change allelic frequencies, disrupting a population’s genetic equilibrium, which would lead to evolution.

Things that can affect genetic equilibrium:•Mutation•Genetic drift – the change of allelic

frequencies by chance events

(random)* greatly affects small populations

•Gene Flow – the movement of individuals in

and out of the population.•Natural Selection

Natural Selection Acts on Variation•All traits have variation (look at everyone

in the class)

•Some variations increase or decrease an organism’s chance for survival in an environment.

•There are three types of natural selection that act on variation:

1. Stabilizing Selection

•Favors average individuals

2. Directional Selection

•Favors one extreme variation of a trait

3. Disruptive Selection

•Favors both extreme variations

The Evolution of Species

•How do changes in the gene pool result in the evolution of a new species?

•Species – a group of organisms that look alike

and can interbreed to produce fertile

offspring.

•Speciation – evolution of a new species

Things That Cause Speciation:

•Geographic Isolation – happens when a physical barrier divides a population.

•Reproductive isolation – when formerly interbreeding organisms can no longer mate and produce fertile offspring.

Patterns of Evolution

•Divergent Evolution – where species that once were similar become increasingly different.

*Adaptive Radiation – when a species evolves into a number of different species to fit a lot of different habitats.

•Convergent Evolution – where distantly related organisms evolve similar traits.