Mechanisms of Evolution. How Does Evolution Work?. Individual organisms cannot evolve. Populations of a particular species evolve. Natural selection acts on the range of phenotypes in a population. Microevolution occurs as the frequency of alleles in a population changes.
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How Does Evolution Work? • Individual organisms cannot evolve. Populations of a particular species evolve. • Natural selection acts on the range of phenotypes in a population. • Microevolution occurs as the frequency of alleles in a population changes.
Evolution-What Happens? • Macroevolution or Evolution occurs when there is a change in allele frequency which produces a new species.
Definitions • Gene pool: All alleles of the population’s genes. • Allelic frequency: % of a specific allele in the gene pool. • Example: Approximately 75% have dominant allele for tongue rolling. 25% non-rolling • Genetic Equilibrium: This exists when the frequency of alleles remains the same over generations. The population is not evolving.
When Does Evolution Occurs? • Evolution results when there are Forces that change allelic frequencies. • Forces that cause Evolution: 1. Geneflow: Transport of genes by migrating individuals. 2. Nonrandom Mating: Mating based on preferences Example: a female may choose a mate based on male size, color, or ability to gather food.
Forces of Evolution Continued 3. Mutation: Change in DNA 4. Genetic Drift: chance event changes allelic frequencies – Greatly affect small populations such as the animals of the Galapagos Islands or Amish.
Causes of Genetic Drift • Mating over a long time period in a small population. • Little movement of males or females into or out of the population.
3 Types of Natural Selection • Stabilizing selection – favors average individuals • Directional selection – favors one of the extreme variations of a trait • Disruptive selection – favors individuals with both extremes of a trait (eliminates intermediate phenotypes)
What is a Species? • A population or group of populations whose members have the ability to breed with one another and produce fertile offspring
Evolution of Species (Speciation) • Significant changes in the gene pool can lead to evolution of a new species over time. • Speciation occurs when members of similar populations no longer interbreed to produce fertile offspring within their natural environment.
Artificial Speciation Diane Dodd’s fruit fly lab, 1989
Why Don’t the Populations Interbreed? • 1. Geographic isolation – physical barrier divides a population. • 2. Reproductive isolation – formerly interbreeding organisms can no longer mate to produce offspring.. • 3. Change in niche -- Change in food source. Example Finches
1. Geographic Isolation A physical barrier that separates a population into groups. Can be • Mountains or Rivers • Islands with water in between Darwin’s 13 finches on Galapagos • Valleys caused by lava flow • Roads/Highways
2. Reproductive Isolation • Prevents closely related species from interbreeding • Timing • Behavior • Habitat
Timing Similar species have different breeding seasons Eastern Spotted Western Spotted Skunk Skunk
Behavior Similar species may have different courtship or mating behaviors. Ex: Eastern & Western meadowlarks almost identical in color shape and habitat, but difference in courtship rituals differ different species
Habitat • Species remain reproductively isolated because they are adapted to different habitats. Ex: Stickleback fish one is a bottom feeder, one spends time in the top open layers of lakes in British Columbia, Canada
Patterns of Evolution • Divergent Evolution – evolutionwhere species diverge or become less and less alike as they adapt to different environments. • Adaptive Radiation-ancestral species evolves into an array of species to fit diverse habitats. This is a type of divergent evolution
Both the wooly mammoth, which occupied parts of North America, and the elephant, still found in Asia and Africa are presumed to have evolved from a common ancestor. • Their geographical isolation and environmental selection pressures caused further evolution of the species. • Each, in its own location, occupies(d) a similar niche.
Patterns of EvolutionContinued 2. Convergent Evolution– Unrelated species occupy similar environments in different parts of the world. Similar pressures of natural selection lead to similar adaptations.
Example of Convergent Evolution A Hummingbird Moth A Humming Bird
Rhea Emu Ostrich
Speciation can occur quickly or slowly • Gradualism – idea that species originate through a gradual accumulation of adaptations. • Punctuated equilibrium – hypothesis that speciation occurs relatively quickly, in rapid bursts, with long periods of genetic equilibrium in between.
Gradualism • Gradual changes in species over time • Evidence of many intermediate forms in fossil records
Punctuated Equilibrium • Scientists found remains of intermediate forms • Also saw that populations remained the same over large periods of time then suddenly changed
Phyletic Speciation Phyletic speciation is a process of gradual change in a single population. The modern form of the organism differs from the original form so much that the two can be considered separate species. Phyletic speciation could be drawn as a line. Species A becomes species B, which becomes species C, etc. In the past, phyletic speciation has been proposed for human evolution and the evolution of the horse. The problem with phyletic speciation is that it would only occur if there were a gradual change in the selective regime that progressively favored the modern form. This seems an unlikely occurrence in nature and the fossil record does not support phyletic speciation for either human or horse evolution. For these reasons, natural phyletic speciation is believed to be rare. Artificial selection in domestic animals and plants approximates phyletic speciation, however. The familiarity of this sort of evolution is probably the only reason that phyletic speciation was ever considered as a hypothesis of natural speciation.
Divergent Speciation If phyletic speciation is drawn as a line, divergent speciation has the form of a branching tree. Species A splits into species A and B. Species B may subsequently branch into species C, and so on. Species A, B and C may exist all at the same time and any of them may be ended by extinction at some point in the process. Divergent speciation is consistent with fossil evidence of biological evolution and with the known mechanisms of biological evolution Divergent speciation, the branch points in the tree described above, results from reproductive isolation of two parts of a population. Reproductive isolation means that interbreeding between the two groups is prevented by some barrier. Once interbreeding ends, two processes cause the isolated group to become different from the parent population: 1) Genetic variation occurs independently in the two groups. Lack of interbreeding prevents sharing of these independent genetic variations. Thus, the genetic variation on which natural selection acts is different in the two groups. 2) Selection may be different for the two groups, especially if they live in different places. If selection differs, different variants will be favored in the two groups. Over time, the two populations become sufficiently different that they can no longer interbreed even if barriers to interbreeding are removed. Speciation has occurred.