Gene FrequenciesGenes can be found in different versions and these are called alleles. Some genes have only a single allele,and these are said to be "conservative", but others have two, three, or, in some cases, many alleles, each of which encodes for a slightly different expression of the particular gene. Allele frequencies refer to the proportion of each allele within a population. Microevolutionary change is any change in allele frequency in a population through time. The "gene pool" is the sum of all the alleles in a given population. Darwin's theory was that small, microevolutionary changes (primarily via natural selection) in allele frequency can accumulate over time and lead to the formation of new species. The actual transformation of one species into another is referred to as macroevolution.
Factors that Alter Gene FrequenciesMUTATION- error in nucleotide sequence: may be detrimental, neutral, or beneficial. An example of a repeated mutation is achondroplastic dwarfism.
MIGRATION- individuals moving between populations
GENETIC DRIFT- gene pool changes due to chance or random events. This always involves small populations. The "Founder effect" occurs with a new, small, isolated population. An example is the high incidence of polydactlyly among Amish populations in the United States. The "bottleneck effect" is when only a few individuals survive for an entire species (genetic diversity decreases) following some sort of disturbance. Thoroughbred horses went through such a bottleneck in the late 16th century. Virtually all thoroughbreds are descended from 6 Arabian stallions in the late 1500's. This is why even the most distantly related thoroughbreds are closer genetically (share more alleles) than you (humans) are to your first cousin. Other examples of species that have experienced genetic bottlenecks include cheetahs and elephant seals.
NONRANDOM MATING- certain individuals mate more or less than others, changing the frequency of their alleles in subsequent generations
NATURAL SELECTION- This is the main focus of Darwin's theory. Environmental conditions result in differential survival and reproduction. Individuals are selected on the basis of how well their alleles function in their specific environment1. no selection -all individuals have equal chance of survival
2. directional selection- one extreme more likely to survive
3. stabilizing selection -extremes less likely to survive
4. disruptive selection- middle of the distribution less likely to survive
Patterns in Macroevolution
A species is defined as one or more populations that interbreed (potentially or actually) and produce viable offspring. The members are reproductively isolated from other organisms. When this occurs due to geographic separation (thought to be most common), this is called allopatric speciation.. When populations become isolated due to other factors (e.g. reproductive behavior or foraging activity), then it is called sympatric speciation. Genetic divergence, mutation and drift result in selection on a new genetic arrangement. When separated populations diverge enough that they can no longer breed-> speciation has occurred.
Reproductive isolating mechanism: 1) geographic barriers (allopatric), 2) mating rituals (sympatric), 3) seasonal/temporal changes in breeding (symptaric)
Pace of Evolution: Gradualism vs Punctuated equilibrium
Adaptive radiation: Development of many species from a common ancestor.
Extinction: On a small scale- species that fail to adapt to a changing environment. On a large scale- many groups of unrelated species due to a catastrophe (astroidal impact). At least five separte catastrophic incidents in the history of the earth eliminated as many as three-fourths of the organisms living at the time of the event.
Convergent Evolution: Evolution of similar adaptation in different groups of distantly related species. If convergence occurs in closely related species, this is called parallel evolution.
Continental Drift: Creates reproductive isolation and changing climates on land masses. Distribution of both fossils and living species on the continents can be understood and explained through evolutionary history and continental drift.
Evidence Supporting The Theory of Evolution
Fossil RecordHuge amount of direct evidence, especially when progressive change in a phylogeny is shown (horse ancestors with 4 toes -> horses with 2 toes). Many groups of organisms have a relatively complete fossil record linking ancestors and descendants (vertebrates). But many groups have incomplete fossil records. Fossilization occurs under specific conditions, so many group will be poorly represented.
Molecular RecordGenetic changes (nucleotide substitutions) should accumulate over time and reflect the changes seen in the fossil record. Recently diverged groups have fewer substitutions (horse/donkey) and more distantly related groups have more (horse/cow). The globin gene that encodes for hemoglobin is well understood and a molecular 'family tree' can be obtained.
Comparative EmbryologyThe development of embryos offers a window into the past. Early in development ancestral features are expressed. Then newer features develop. Fish/reptiles/mammals all similar. Human embryos- tail, gill slits and fur (5 mos.).
Comparative MorpholologyRelated groups often have the same inherited features (i.e. homologous stuctures) but they MAY be used for different purposes. Vertebrate limbs can be used as wings, legs, or flippers, but they are all constructed from the same basic body plan.
Vestigal OrgansInherited structures from the ancestral form but no longer serve a purpose: human appendix & goosebumps, pelvic bones of dolphins, "spurs" on some snakes
Observations of natural selectionArtificial selection in domestic plants and animals shows powerful evidence of how profound changes can occur in just a short period of time (e.g. breeds of domestic dogs in comparison with wolves). Many examples have been found for evidence of natural selection operating in the world today. Examples include Peppered moths in pre- and post industrial England. At one point, 98% of all moths were dark-colored, now light-colored moths are once again making a comeback. Antibiotic resistence in bacteria is yet another example of evolution at work.