Evolution Notes A. Theory of Evolution: Evolution is change in the traits of a population over time. All organisms developed from previously existing organisms. B. Mechanisms of Evolution: Organisms are very stable systems. There are many processes that keep the organism maintained and properly functioning. Likewise, populations tend to have a stable proportion of traits unless one or more of the following processes acts on the variations contained in the population: 1. Genetic Drift – changes in the traits in a population due to chance, for example unique traits of a colonizing population (founder effect), or loss of many individuals with different traits due to natural disasters (bottleneck effect). 2. Gene Flow – migration of individuals from one population to another, introducing new traits. 3. Non-random Mating – the choosing of mates because of certain traits that results in amplification of those traits. 4. Natural Selection – populations of organisms that become adapted to a particular environment due to the differential survival and reproduction of “fit” organisms. C. The Process of Natural Selection 1. Organisms in a population vary. Each organism has unique DNA. This variation comes from mutations, and crossing over and recombination during meiosis (sexual reproduction). These variations can be passed down to offspring. 2. Organisms have the potential to reproduce quickly. Populations tend to grow exponentially. More offspring are produced than can survive.
3. The ecosystem can only hold a limited number of organisms. Resources are limited; there is a carrying capacity for each species.
4. The organisms with the most favorable characteristics survive to reproduce. “Nature” selects the “fit” organisms in a population for the particular environment. Favorable characteristics become more widespread in the population.
D. Speciation: (formation of new species) A species is a group of organisms in a population that have the potential to interbreed. New species evolve due to a sub-population becoming geographically isolated from the rest of the population, then reproductively isolated through one of the barriers listed below.
Figure 1: A Model for Speciation Through Geographic Isolation
1. A population, A in Figure 1, becomes dispersed to an island from the mainland. The population adapts to the conditions on the island and becomes population B. 2. Population B then is dispersed to the second island. The population adapts to the conditions on the second island and becomes population C. 3. Population C is then dispersed to the first island. Population C and population B cannot interbreed: they are different species.
Figure 2: Reproductive Isolation
Habitat isolation – species use different habitats within the same location. Temporal isolation – species reproduces at a different season or time of day. Behavioral isolation – courtship behavior differs or they respond to different calls, songs, pheromones, or other signals. Mechanical and gamete isolation – sexual organs or gametes are unsuitable for one another. Zygote mortality – fertilization occurs but zygote does not survive. Hybrid sterility – hybrid survives but is sterile and cannot reproduce. Hybrid breakdown –offspring of hybrid have reduced viability or fertility. E. The Pace of Evolution: Although Darwin and Wallace, the originators of the theory of evolution through natural selection, thought the pace of evolution was gradual (occurring over millions of years), many scientists think that the pace varies. Punctuated equilibrium interprets the fossil record as showing rapid speciation followed by long periods of equilibrium with little evolution.