The Theory of Evolution

 Darwin developed the first theory on evolution, which is the basis for modern evolutionary theory ◦ Darwin spent 5 years sailing around the world collecting organisms and fossils ◦ His idea that species can change over time is based on his observations on the Galapagos Islands where many species were similar to species in other parts of the world, yet still unique

 Darwin bred pigeons and studied how traits were inherited from one generation to the next ◦ He was able to breed for specific characteristics using artificial selection ◦ He then hypothesized that desirable traits were bred into the population through natural selection  The organisms without the desirable traits were less likely to survive and pass on their traits  As a result, only the desirable traits got passed on

 An adaptation is any variation that aid’s an organisms chances of survival in its environment ◦ Structural adaptations include any change to the physical appearance or body parts of an organism  These develop very slowly (over millions of years)  Examples include beak shape, neck length (giraffes), camouflage and mimicry  Mimicry is when one organism looks similar to another organism ◦ Physiological adaptations are changes in the metabolic processes of an organism  They develop over fairly short time periods (a few generations)  Examples include antibiotic and pesticide resistance

 Fossils are used to examine physical similarities in organisms ◦ Homologous structures are similar in function, arrangement or both between organisms with a common evolutionary origin  An example would be forelimbs of whales, crocodiles and bird wings ◦ Analogous structures are similar in function between organisms that do not have a common evolutionary origin  An example would be bird and bat wings ◦ Vestigial structures are those which probably used to serve a purpose but are no longer needed, yet still inherited  In humans, your appendix is a vestigial structure

 An embryo is the earliest stage of growth and development for both plants and animals ◦ Even though they develop into very different organisms, the embryos of fish, reptiles, birds and mammals are very similar, indicating that they may have had a common ancestor

 Scientists often compare amino acid or DNA sequences of organisms ◦ Organisms that have more in DNA or amino acid sequences in common are said to be more closely related in terms of biochemistry

 Genetic equilibrium is a state where the genes in a population are the same over several generations ◦ The combination of all the alleles of a population’s genes is called the “gene pool”  The percentage of any specific allele in the gene pool is called the allelic frequency  Any factor that changes the allelic frequency results in the process of evolution

 There are several factors that can cause changes within a population, some of which impact small, isolated populations such as islands, and others than impact larger populations ◦ Mutations, genetic drift and gene flow can have a big impact on small populations  Genetic drift is the alteration of allelic frequencies by chance events  Gene flow is the movement of genes in and out of a population through migration

 There are 3 types of natural selection: ◦ Stabilizing selection is natural selection that favors average individuals in a population ◦ Directional selection is natural selection that favors one extreme of a trait ◦ Disruptive selection is natural selection that favors both extremes of a trait

 Speciation occurs when new species are produced because members of similar populations can no longer breed to produce fertile offspring ◦ Geographic isolation occurs when a physical barrier divides a population and can lead to a new species ◦ Reproductive isolation occurs when populations who used to interbreed no longer mate and produce fertile offspring  This can be because of genetic changes, such as polyploidy, or behavioral changes

 Depending on the mechanism involved, speciation can occur quickly or gradually ◦ Gradualism is the idea that new species originate through a gradual change of adaptations ◦ Punctuated Equilibrium is the idea that speciation occurs relatively quickly, in short rapid bursts with long periods of genetic equilibrium in between  Both mechanisms are supported by the fossil record

 There are 2 main patterns of evolution: ◦ Convergent evolution occurs when unrelated species occupy similar environments in different parts of the world and develop similar traits due to natural selection ◦ Divergent evolution occurs when populations that were once similar become increasingly distinct as they adapt to different environments  Adaptive radiation is an example of divergent evolution  In adaptive radiation, one ancestral species evolves into many separate species to occupy different habitats