Selection Selection PressurePressure

& Speciation& Speciation

Pick Me! Pick Me!Pick Me! Pick Me! We understand that direction of evolution is a We understand that direction of evolution is a

complex interaction of genetics, mutation and complex interaction of genetics, mutation and environmental pressures.environmental pressures.

Genetics acts as the source of variation but is Genetics acts as the source of variation but is the environment that determines if the allele will the environment that determines if the allele will become more prevalent in society.become more prevalent in society.

SelectionSelection is the pressure that exists against is the pressure that exists against those who are not favored by the environmental those who are not favored by the environmental conditions present.conditions present.

Stabilizing SelectionStabilizing Selection Stabilizing SelectionStabilizing Selection – A stable environment – A stable environment

where selection exists against individuals where selection exists against individuals exhibiting variations in a trait that deviate from exhibiting variations in a trait that deviate from the current population average. It is the most the current population average. It is the most common type of selection.common type of selection.

Hummingbird beak and tongue lengths are a Hummingbird beak and tongue lengths are a good example of stabilizing selection. Too big good example of stabilizing selection. Too big requires too much energy while too small can’t requires too much energy while too small can’t access food very well.access food very well.

Stabilizing SelectionStabilizing Selection

Directional SelectionDirectional Selection Directional SelectionDirectional Selection – The environment – The environment

selects for traits that represents an increase selects for traits that represents an increase or decrease in the value of a trait from the or decrease in the value of a trait from the current population average.current population average.

A change in the length of the flowers will see A change in the length of the flowers will see a change in the length of the beak of the a change in the length of the beak of the hummingbird.hummingbird.

The salmon populations shrunk with the The salmon populations shrunk with the introduction of gill nets.introduction of gill nets.

Directional SelectionDirectional Selection

Disruptive SelectionDisruptive Selection Disruptive SelectionDisruptive Selection – The environment – The environment

selects for variations of the trait that selects for variations of the trait that represent the extreme ends of the trait (they represent the extreme ends of the trait (they both differ from the current population both differ from the current population average).average).

Long or short flowers replace the medium Long or short flowers replace the medium length flowers so the population shifts length flowers so the population shifts towards longer or shorter beaks.towards longer or shorter beaks.

Disruptive selection is one of the Disruptive selection is one of the mechanisms that may lead to mechanisms that may lead to speciationspeciation..

Disruptive SelectionDisruptive Selection

Sexual SelectionSexual Selection Sexual SelectionSexual Selection – Traits that lead to mating – Traits that lead to mating

success are favored.success are favored. Sexual dimorphismSexual dimorphism may be apparent in which may be apparent in which

there are striking physical differences between there are striking physical differences between the male and female forms of the species.the male and female forms of the species.

Behaviors and abilities (fighting, singing) are Behaviors and abilities (fighting, singing) are also a source of sexual selection.also a source of sexual selection.

Sometimes the sexually selected trait comes into Sometimes the sexually selected trait comes into direct conflict with another selection pressure. A direct conflict with another selection pressure. A brightly coloured male lay look great to the brightly coloured male lay look great to the female form of his species but it also makes him female form of his species but it also makes him much more noticeable to predators as well.much more noticeable to predators as well.

Sexual SelectionSexual Selection

SpeciationSpeciation

SpeciationSpeciation The formation of a new species due to selective The formation of a new species due to selective

pressures is called pressures is called speciationspeciation.. Disruptive selection can result in two extreme Disruptive selection can result in two extreme

forms of a trait. These forms can be so different forms of a trait. These forms can be so different that the two populations no longer associate with that the two populations no longer associate with each other or recognize each other as members each other or recognize each other as members of the same species.of the same species.

Changes in gene frequency and phenotypic Changes in gene frequency and phenotypic traits within a population and/or species are traits within a population and/or species are termed termed microevolutionmicroevolution – it can lead to – it can lead to speciation.speciation.

What is a Species?What is a Species? A A speciesspecies is a group or population of is a group or population of

interbreeding individuals that are reproductively interbreeding individuals that are reproductively isolated from other groups and evolve isolated from other groups and evolve independently.independently.

Species can be differentiated using their Species can be differentiated using their reproductive isolating mechanisms reproductive isolating mechanisms – any – any behavioural, structural or biochemical traits that behavioural, structural or biochemical traits that prevent individuals of different species from prevent individuals of different species from reproducing successfully together.reproducing successfully together.

Reproductive Isolating Reproductive Isolating MechanismsMechanisms

There are a variety of mechanisms at work that There are a variety of mechanisms at work that prevent the successful production of viable prevent the successful production of viable offspring between two separate species.offspring between two separate species.

These mechanisms can be grouped into two These mechanisms can be grouped into two major categories – major categories – prezygoticprezygotic and and postzygoticpostzygotic..

Basically, those that stops a zygote from forming Basically, those that stops a zygote from forming and those that stop the being from developing and those that stop the being from developing and/or mating after the zygote has been formed.and/or mating after the zygote has been formed.

Pre-zygotic MechanismsPre-zygotic MechanismsPrezygotic mechanismsPrezygotic mechanisms are those things that do not the are those things that do not theactual mating and/or fertilization to ever take place betweenactual mating and/or fertilization to ever take place betweenthe two organisms.the two organisms. Ecological IsolationEcological Isolation – Different habitats. – Different habitats. Temporal IsolationTemporal Isolation – Different reproductive timing. – Different reproductive timing. Behavioural IsolationBehavioural Isolation – Different rituals or actions used – Different rituals or actions used

to recognize a suitable mate.to recognize a suitable mate. Mechanical IsolationMechanical Isolation – Reproductive parts of body that – Reproductive parts of body that

can only function or be used in presence of the same can only function or be used in presence of the same species.species.

Gametic IsolationGametic Isolation – Prevention of fertilization at the – Prevention of fertilization at the molecular level. Often seen in marine organisms.molecular level. Often seen in marine organisms.

Post-zygotic MechanismsPost-zygotic MechanismsPost-zygotic mechanismsPost-zygotic mechanisms are isolating are isolatingmechanisms that take effect after a zygotemechanisms that take effect after a zygoteor offspring has been produced.or offspring has been produced. Zygotic MortalityZygotic Mortality – No fertilized zygotes – No fertilized zygotes

or embryos develop to maturity.or embryos develop to maturity. Hybrid InviabilityHybrid Inviability – Hybrid offspring do – Hybrid offspring do

not live long after birth.not live long after birth. Hybrid InfertilityHybrid Infertility – Hybrid offspring are – Hybrid offspring are

strong but sterile.strong but sterile.

Modes of SpeciationModes of SpeciationAnytime you have a series of events that leads toAnytime you have a series of events that leads toreproductive isolation, you may also have speciationreproductive isolation, you may also have speciationtaking place soon after. Things like environmental changetaking place soon after. Things like environmental changeand mutation are no longer shared between the two and mutation are no longer shared between the two groups. Geographical isolation is the most common source groups. Geographical isolation is the most common source of speciation.of speciation. Allopatric speciationAllopatric speciation – The evolution of populations into – The evolution of populations into

separate species as a result of geographic isolation. The separate species as a result of geographic isolation. The geography of the environment acts as a physical barrier that geography of the environment acts as a physical barrier that cuts off the two populations.cuts off the two populations.

Sympatric speciationSympatric speciation – The evolution of populations within – The evolution of populations within the same geographic area into separate species. Perhaps the the same geographic area into separate species. Perhaps the area is quite large and the two populations occupy their own area is quite large and the two populations occupy their own areas within the larger habitat or a mutation may have occurred areas within the larger habitat or a mutation may have occurred that instantaneously isolates the two populations.that instantaneously isolates the two populations.

Allopatric SpeciationAllopatric Speciation

Sympatric SpeciationSympatric Speciation

That’s All I Got…That’s All I Got…