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Theory of Evolution Chapter 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
Organisms produce more offspring than can survive Individuals have variations Individuals with certain useful variations, such as speed, survive and pass on those variations 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
Natural Selection Example
Variation in Traits – some beetles are green and some are brown. 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. Fossils 3. 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
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.
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