THE THEORY OF EVOLUTION
Chapter 13 Notes
Charles Darwin
He sailed around the world, collecting specimens of plants, animals and fossils everywhere he went The scientific thought at the time was that species did not change
Darwin’s Finches
On the Galapagos Islands, Darwin found species of finches that each had specialized beaks to catch food in different ways
Evolution by Natural Selection
Evolution – change in a species over time Darwin’s book, On the Origin of Species by Means of Natural Selection more commonly known as The Origin of the Species), presented his evidence that evolution occurred by means of natural selection – the process by which organisms with traits well suited to an environment are more likely to survive and produce more offspring than organisms without these favorable traits
Natural Selection
Adaptation – a feature that has become common in a population because the feature provides a selective advantage OR the process by which an organism becomes better suited to its environment
Four major points of Darwin’s theory:
a. There is variation in the genes of every population or species (because of random mutations and translation errors) b. In a particular environment, some individuals of a population or species are better suited to survive (because of genetic variation) and have more offspring (natural selection)
More major points of Darwin’s theory:
c. Over time, the traits that make certain individuals able to survive and reproduce tend to spread in that population d. Evidence from fossils and other sources suggests that species living today evolved from species that are extinct
Darwin’s ideas updated
Knowledge of genetics now helps scientists understand the process of natural selection better; natural selection causes the frequency of certain alleles in a population to increase or decrease over time Reproductive isolation – when two populations of the same species do not breed with each other because they live in different places; the two species will usually become increasingly different
Evidence of evolution
Fossils – include any traces of dead organisms (footprints, animal tracks, insects, impressions of leaves or skin, bones, etc.) Because new species form from existing species, Darwin predicted that transitional forms – intermediate stages between older and newer species would be found in the fossil record
More evolution evidence
Comparing the anatomy of different organisms
Comparing the way organisms are put together provides evidence for evolution
Vestigial structures – structures with no function, they are considered to be evidence of an organism’s evolutionary past
Example: the hind limbs bones of a whale; human tailbones
More comparing anatomy of different organisms
Homologous structures – structures that share a common ancestry; they are similar because they are modified versions of structures that occurred in a common ancestor As a human embryo develops, it shows similarities to all other vertebrate embryos, which has also been used as evidence to support the theory of evolution
DNA evidence
DNA and proteins are being analyzed to see how closely related species are to one another
If two species are very closely related, they will share more of the same nucleotide sequences (remember the A, T, C and G?) in their DNA and more of the same resulting amino acid sequences in their proteins (because the DNA sequence determines the order amino acids are put together, which determines the protein)
Four factors in natural selection:
1. 2.
3.
4.
All populations have genetic variation The environment presents challenges that organisms must overcome in order to successfully reproduce and pass on their genes Individuals tend to produce more offspring than the environment can support, which creates competition for survival (for food, territory, etc.) Individuals that are better able to cope with the challenges presented by their environment tend to leave more offspring than those individuals less suited to the environment
Examples of natural selection
1. Antibiotic resistant bacteria (tuberculosis) 2. The peppered moth and Kettlewell’s experiment:
1.
Until the 1850’s (Industrial Revolution) dark gray peppered moths were rare because the trees where they lived had light colored bark, so the dark moths were more likely to be seen and eaten by birds. After the pollution turned the bark black, the dark moths became more common. In the 1950’s Kettlewell tested this by releasing equal #s of light and dark moths in a polluted forest (dark colored bark on the trees). He recaptured 2/3 more dark moths than light moths. Then he released equal #s in a non-polluted forest (light bark on the trees) and recaptured 2/3 more light moths than dark moths.
The puzzle of sickle cell anemia
People in Africa who are heterozygous for the sickle cell allele are much less susceptible to malaria Deaths in Africa from sickle cell anemia are about 1 in 100 (1%), while 1 in 5 (20%) individuals are heterozygous for the allele and survive malaria
This means natural selection has favored the sickle cell allele in central Africa because the payoff in survival from malaria (for heterozygotes) is higher than the rate of death from sickle cell anemia (for homozygotes)
More sickle cell
This is an example of balancing selection – a situation in which an allele remains at the same frequency because the natural selection for an allele is balanced by the rate of selection against it
This means that the number of people dying from sickle cell anemia (which removes some of the homozygous alleles from the population) is balanced by the number of people who survive a malaria infection because they are heterozygotes for the sickle cell allele (and live to pass on the sickle cell allele to their offspring)
What about in the U.S.?
In the U.S. the frequency of sickle cell anemia is 1 in 500 (0.2%) for African Americans because malaria is so rare here, which means there is no selective advantage to being a heterozygous individual. There is only the selective DISadvantage of being homozygous. This type of unopposed (or one-sided) selection is called directional selection, which moves the frequency of an allele (and its trait) in one direction.