THE THEORY OF EVOLUTION Natural Selection & Speciation.
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Transcript of THE THEORY OF EVOLUTION Natural Selection & Speciation.
THE THEORY OF EVOLUTION
Natural Selection & Speciation
Fossils shape ideas about evolution
Scientists used fossils as a basis for evolutionary concepts
When geologists provided fossil evidence of the age of Earth, biologists began to suspect that life changes slowly over time
Evolution = the change in populations over time
Charles Darwin
English Scientist Lived from 1809 – 1882
Took a job as a naturalist on the HMS Beagle (1831) Sailed to South America & the
South Pacific on a 5 yr journey Darwin studied & collected
biological specimens every time the ship stopped He became curious about the
possible relationships between species.
The Galapagos Islands
A group of small islands ~ 1000 miles off the coast of S. America Near the equator
Darwin studied species unique to the islands Similar to species
elsewhere Could not explain how
such changes would occur
Figuring out why
Thomas Malthus – English economist Ideas about human population growth Said that the human population grows
faster than Earth’s food supply Darwin related this to his studies:
Many species produce more offspring than can survive
They compete for resources like food, space, predation, mating, and shelter
Only some individuals survive
Two Kinds of Selection
Natural Selection a mechanism for change in populations –
“survival of the fittest” Organisms with certain variations survive &
reproduce to pass on their variations to offspring Organisms without beneficial variations are less
likely to survive and reproduce Result = each generation inherits beneficial
variations and others die out Artificial Selection
Organisms are purposely bred for desirable traits Examples: Horses, dog breeds
Types of Natural Selection
Stabilizing Selection = favors average individuals Spiders: Large individuals may be easier prey, small
individuals may not be able to get enough food Directional Selection = one extreme is favored
Woodpeckers: short or average beak cannot reach deep enough to get food, long beaks can feed on insects deeper within the tree trunk
Disruptive Selection = either extreme is favored Dark brown & white marine limpets: white limpets
have an advantage on light rocks, brown limpets have an advantage on darker/wet rocks. Tan limpets are easily seen and preyed upon by birds - tan limpets have the least favored adaptation.
Alfred Russell Wallace
Reached a similar conclusion Darwin & Wallace presented their ideas to
the scientific community Darwin published On the Origin of Species in
1859
There have been many advances since Darwin: Any change in the gene pool of a population
is now considered part of evolution
Adaptations
Structural Adaptations: Adaptations = any structure, behavior, or
internal process that makes an organism more likely to survive
develop over generations Examples: Giraffe height, mole-rats
Physiological Adaptations Develop more quickly than structural
adaptations Examples: antibiotics (penicillin) & pesticides
Other Adaptations
Mimicry = one species resembles another “warning” coloration =
red, orange, and yellow Camouflage =
organisms are able to blend into surroundings Insects
Other Evidence of Evolution
1. Fossils – provide a record of early life and evolutionary history
2. Embryology Embryos at early stages of development
are very similar in appearance
3. Biochemistry Comparisons of DNA or RNA of different
species
Other Evidence of Evolution4. Anatomy
Homologous structures – similar structures with different functions Example: animal forelimbs have the same bone
structure even though they have different functions Analogous structures – similar functions with
different structure Example: wings of a butterfly & wings of a bird have the
same function but different structure
15.2
Population Genetics
Darwin developed his theory without an understanding of genes & inheritance
Population genetics = studies of the behavior of genes in populations of plants or animals
Natural selection acts on a range of phenotypes in a population
If a variation of a phenotype is beneficial, it will be more common
Evolution of Populations
All of the alleles of a population’s genes = its’ gene pool
Genetic Equilibrium
Genetic equilibrium = the frequency of alleles remains the same over generations Populations in genetic equilibrium are not
“evolving” Disruptions to genetic equilibrium:
Mutations Genetic drift (chance alterations to allele
frequency) Movement of individuals in or out of the population Size of population – can cause recessive alleles to
become more common
Evolution of a species Speciation = evolution of a new species
Occurs when members of similar populations no longer interbreed to produce fertile offspring
Causes of speciation: Geographic isolation – physical barriers
separate populations & each develops its own gene pool
Reproductive isolation – formerly interbreeding organisms can no longer mate (may be caused by behavior such as breeding seasons)
Chromosomal change – mistakes during cell division may result in polyploidy
Speciation can occur at different rates Gradualism = species originate through
gradual changes and adaptations Punctuated equilibrium = change
occurs in rapid bursts with period of genetic equilibrium in between May be caused by environmental changes or
competitive species Scientists agree that either method can
occur, depending on the circumstances
Patterns of Evolution
Adaptive radiation = when a species evolves into an array of species Example: Hawaiian Honeycreepers Common on islands
Divergent evolution – a diverse group of species share a common ancestor A type of adaptive radiation
Convergent evolution – distantly related species develop similar traits Similar environmental