Evolution and heritable characteristics

Created April 2020, Offline version here
Videos by Paul Andersen. Visit his website: Bozeman Science.

    Evolution can be described as the change in heritable characteristics over time. There are multiple mechanisms that give rise to evolution and one such mechanism is speciation. Speciation increases biodiversity with one species of organism branching off into separate species,whereas the opposite is where that one specie ceases to exist, more commonly known as . Mass extinctions can occur if multiple extinctions happen at once. There have only been mass extinctions, on record, in the history of life on Earth. Within speciation, there are different mechanisms that give rise to different species. One of them is natural selection where the environment favors certain traits amongst an organism’s population so that the ones that survive are able to reproduce and continue their gene line. In the example of the stickleback fish, the dragonfly larva is the environmental pressure that increased the number of low-armored stickleback as the dragonfly larva prey on more of the fully armored stickleback and the low-armored stickleback takes less time to develop. The environment favored the , therefore its population increased. If a population of organisms were to inhabit a new area with unfilled niches, the population will undergo in that the organisms with varying traits will “specialize” in their own traits. This gives rise to pronounced variations of the traits. An example of this are the honeycreeper birds. When the birds first arrived at the island, there were no predators and an abundance of various resources. Some birds had shorter beaks while others had slightly longer, thinner beaks. Those with the slightly longer, skinnier beaks were able to survive and as they reproduced over time, the offspring resulted in long skinny beaks. As the island had an abundance of different species of flowers, all with different shapes, the birds with their varied beaks, began to adapt to different species of flowers. Another mechanism that leads to speciation is reproductive isolation. Reproductive isolation acts on the reproductive process of organisms whether is the before or after fertilization or a physical isolation on the organisms. A species can be defined as a population of organisms that can interbreed and produce offspring. The fertility of the offspring is important as that is the only way for the gene line to continue. Geographic isolation refers to geographic events that happen to the environment the organisms live in. Two main types are allopatric and sympatric speciation. refers to the environment being physically split into different areas and then overtime the environment remerges into one. However, during the time of separation, the organisms have already diverged so that when the land remerged, they no longer can interbreed. Sympatric refers to the same environment that a population shares but through genetic mutation, the resulting organisms can no longer breed with the original population. This gives rise to two new species. Peripatric and parapatrice are similar in that both refer to a closely distanced niche. In peripatric, the niches are and in parapatric, the niches are . Prezygotic and postzygotic barriers also play a role in speciation in that they directly act on the reproductive process. barriers refer to the barriers before fertilization of the egg and barriers refer to after the fertilization of the egg. Prezygotic barriers include behavioral, temporal, and mechanical isolation. isolation is when certain mating traits, calls or dances displayed by males are species specific and do not affect other females of other species.
    isolation refers to the season or period of time the organism is actively reproducing. In other words, different organisms have different mating seasons and if the times do not coincide, then mating would not occur.
    isolation refers to the physical compatibility or incompatibility of the organisms. If the physical anatomy does not complement each other, then fertilization cannot occur. Postzygotic behaviors include zygote mortality and hybrid sterility. Zygote mortality refers to the death of the zygote. An example would be that fertilized coral zygotes die in the ocean currents as they try to anchor themselves on a surface to take root. Hybrid sterility refers to the infertile offspring produced, if two organisms that do not naturally reproduce were to. While the offspring lives, or viable, it cannot reproduce, therefore it is known as a offspring. The main mechanism for how reproductive isolation leads to speciation is the interruption of . Without this, the organisms cannot travel to another population to breed and the environment is pressuring the organisms to adapt and eventually speciate. Speciation can occur at various rates, depending on the number of chromosomes that result from cell division. Plants have speciation rates compared to animals because the frequency in which they result in from cell division is much higher than animals. In summary, speciation and extinction are what give rise to evolution and within speciation there are different mechanisms that drive it: natural selection and reproductive isolation. Natural selection refers to the environmental pressures that are placed on organisms. Those that have favorable traits that can survive and reproduce will carry on the gene line. Reproductive isolation has three underlying processes: geographic isolation, prezygotic barriers, and postzygotic barriers. Geographic isolation refers to physical events that happen to the environment the organisms live in. Under geographic isolation, there are four types: allopatric, peripatric, parapatric, and sympatric. Allopatric refers to physical separation of the environment but eventually remerges. Peripatric refers to a niche isolation, essentially a smaller scale of allopatric speciation. Parapatric is when a portion of the population moves to an adjacent niche and diverges in their adaptation. Sympatric is when two populations share the same niche but a genetic change, such a polyploidy, causes one of the populations to change. Prezygotic barriers refer to the barriers before fertilization and that includes temporal, behavioral, and mechanical isolation. Temporal refers to the active mating seasons organisms have. Behavioral refers to the courtship behaviors males display to entice females. Mechanical isolation refers to the anatomy differences in different organisms that prevent reproduction. Postzygotic barriers refer to the barriers after fertilization and that includes zygote mortality and hybrid sterility. Zygote mortality refers to zygote death that prevents the offspring from reproducing. Hybrid sterility is the result of a hybrid cross that produces a sterile offspring that cannot reproduce.