Selection Pressure & Speciation

Pick Me! Pick Me! We understand that direction of evolution is a complex interaction of genetics, mutation and environmental pressures. We understand that direction of evolution is a complex interaction of genetics, mutation and environmental pressures. Genetics acts as the source of variation but is the environment that determines if the allele will become more prevalent in society. Genetics acts as the source of variation but is the environment that determines if the allele will become more prevalent in society. Selection is the pressure that exists against those who are not favored by the environmental conditions present. Selection is the pressure that exists against those who are not favored by the environmental conditions present.

Stabilizing Selection Stabilizing Selection – A stable environment where selection exists against individuals exhibiting variations in a trait that deviate from the current population average. It is the most common type of selection. Stabilizing Selection – A stable environment where selection exists against individuals exhibiting variations in a trait that deviate from the current population average. It is the most common type of selection. Hummingbird beak and tongue lengths are a good example of stabilizing selection. Too big requires too much energy while too small can’t access food very well. Hummingbird beak and tongue lengths are a good example of stabilizing selection. Too big requires too much energy while too small can’t access food very well.

Stabilizing Selection

Directional Selection Directional Selection – The environment selects for traits that represents an increase or decrease in the value of a trait from the current population average. Directional Selection – The environment selects for traits that represents an increase or decrease in the value of a trait from the current population average. A change in the length of the flowers will see a change in the length of the beak of the hummingbird. A change in the length of the flowers will see a change in the length of the beak of the hummingbird. The salmon populations shrunk with the introduction of gill nets. The salmon populations shrunk with the introduction of gill nets.

Directional Selection

Disruptive Selection Disruptive Selection – The environment selects for variations of the trait that represent the extreme ends of the trait (they both differ from the current population average). Disruptive Selection – The environment selects for variations of the trait that represent the extreme ends of the trait (they both differ from the current population average). Long or short flowers replace the medium length flowers so the population shifts towards longer or shorter beaks. Long or short flowers replace the medium length flowers so the population shifts towards longer or shorter beaks. Disruptive selection is one of the mechanisms that may lead to speciation. Disruptive selection is one of the mechanisms that may lead to speciation.

Disruptive Selection

Sexual Selection Sexual Selection – Traits that lead to mating success are favored. Sexual Selection – Traits that lead to mating success are favored. Sexual dimorphism may be apparent in which there are striking physical differences between the male and female forms of the species. Sexual dimorphism may be apparent in which there are striking physical differences between the male and female forms of the species. Behaviors and abilities (fighting, singing) are also a source of sexual selection. Behaviors and abilities (fighting, singing) are also a source of sexual selection. Sometimes the sexually selected trait comes into direct conflict with another selection pressure. A brightly coloured male lay look great to the female form of his species but it also makes him much more noticeable to predators as well. Sometimes the sexually selected trait comes into direct conflict with another selection pressure. A brightly coloured male lay look great to the female form of his species but it also makes him much more noticeable to predators as well.

Sexual Selection

Speciation

Speciation The formation of a new species due to selective pressures is called speciation. The formation of a new species due to selective pressures is called speciation. Disruptive selection can result in two extreme forms of a trait. These forms can be so different that the two populations no longer associate with each other or recognize each other as members of the same species. Disruptive selection can result in two extreme forms of a trait. These forms can be so different that the two populations no longer associate with each other or recognize each other as members of the same species. Changes in gene frequency and phenotypic traits within a population and/or species are termed microevolution – it can lead to speciation. Changes in gene frequency and phenotypic traits within a population and/or species are termed microevolution – it can lead to speciation.

What is a Species? A species is a group or population of interbreeding individuals that are reproductively isolated from other groups and evolve independently. A species is a group or population of interbreeding individuals that are reproductively isolated from other groups and evolve independently. Species can be differentiated using their reproductive isolating mechanisms – any behavioural, structural or biochemical traits that prevent individuals of different species from reproducing successfully together. Species can be differentiated using their reproductive isolating mechanisms – any behavioural, structural or biochemical traits that prevent individuals of different species from reproducing successfully together.

Reproductive Isolating Mechanisms There are a variety of mechanisms at work that prevent the successful production of viable offspring between two separate species. There are a variety of mechanisms at work that prevent the successful production of viable offspring between two separate species. These mechanisms can be grouped into two major categories – prezygotic and postzygotic. These mechanisms can be grouped into two major categories – prezygotic and postzygotic. Basically, those that stops a zygote from forming and those that stop the being from developing and/or mating after the zygote has been formed. Basically, those that stops a zygote from forming and those that stop the being from developing and/or mating after the zygote has been formed.

Pre-zygotic Mechanisms Prezygotic mechanisms are those things that do not the actual mating and/or fertilization to ever take place between the two organisms. Ecological Isolation – Different habitats. Ecological Isolation – Different habitats. Temporal Isolation – Different reproductive timing. Temporal Isolation – Different reproductive timing. Behavioural Isolation – Different rituals or actions used to recognize a suitable mate. Behavioural Isolation – Different rituals or actions used to recognize a suitable mate. Mechanical Isolation – Reproductive parts of body that can only function or be used in presence of the same species. Mechanical Isolation – Reproductive parts of body that can only function or be used in presence of the same species. Gametic Isolation – Prevention of fertilization at the molecular level. Often seen in marine organisms. Gametic Isolation – Prevention of fertilization at the molecular level. Often seen in marine organisms.

Post-zygotic Mechanisms Post-zygotic mechanisms are isolating mechanisms that take effect after a zygote or offspring has been produced. Zygotic Mortality – No fertilized zygotes or embryos develop to maturity. Zygotic Mortality – No fertilized zygotes or embryos develop to maturity. Hybrid Inviability – Hybrid offspring do not live long after birth. Hybrid Inviability – Hybrid offspring do not live long after birth. Hybrid Infertility – Hybrid offspring are strong but sterile. Hybrid Infertility – Hybrid offspring are strong but sterile.

Modes of Speciation Anytime you have a series of events that leads to reproductive isolation, you may also have speciation taking place soon after. Things like environmental change and mutation are no longer shared between the two groups. Geographical isolation is the most common source of speciation. Allopatric speciation – The evolution of populations into separate species as a result of geographic isolation. The geography of the environment acts as a physical barrier that cuts off the two populations. Allopatric speciation – The evolution of populations into separate species as a result of geographic isolation. The geography of the environment acts as a physical barrier that cuts off the two populations. Sympatric speciation – The evolution of populations within the same geographic area into separate species. Perhaps the area is quite large and the two populations occupy their own areas within the larger habitat or a mutation may have occurred that instantaneously isolates the two populations. Sympatric speciation – The evolution of populations within the same geographic area into separate species. Perhaps the area is quite large and the two populations occupy their own areas within the larger habitat or a mutation may have occurred that instantaneously isolates the two populations.

Allopatric Speciation

Sympatric Speciation

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