BIOL 2390 - Topic 5 (part 2)

Organization of species

1.) Species rarely consist of a single continuous interbreeding population

2.) Instead, they are organized into a metapopulation, which is divided into groups of subpopulations consisting of interbreeding individuals

Linkage of subpopulations

They are linked by the movement of individuals between one subpopulation to another (aka migration/gene flow)

Genetic variation

It can occur within and among subpopulations

Evolution within each subpopulation

Each subpopulation will likely have different selection pressures, therefore each subpopulation will have different evolutionary trajectories

Evolution requires…

Variation among individuals in some heritable trait, resulting in differential success of said trait

Natural selection acting on Daphne finch birds during a drought

1.) Beak size was the target of selection, with the selective agent being the food size and abundance

2.) When the drought occured, smaller seeds were unavailable, leaving behind the larger seeds, which is why the birds selected towards larger beaks rather than small ones

Types of natural selection

1.) Directional selection

2.) Stabilizing selection

3.) Disruptive selection

Directional selection

When selection (the mean value of a trait) prefers one extreme over the other

Directional selection example

Long lizard tails scare predators, therefore selection shifts towards long tails over short tails

Stabilizing selection

When selection favours the mean, at the expense of the extremes

Stabilizing selection example

Baby birth weights: too big and both the mother and offspring dies, too small and the offspring dies, therefore selection favours the middle

Disruptive selection

It is when selection favours both extremes, but not the mean, resulting in a bimodal distribution of the characteristic

When does disruptive selection occur

It typically occurs when a population is subjected to multiple selection pressures at the same time

Disruptive selection can lead to…

It can lead to the formation of a new species if the selection pressures continue to exist for a long time

Disruptive selection example

1.) Short squirrel tails help keep predators from catching you on the ground

2.) Long tails provide good balance in the trees

3.) Medium tails don’t help at all

Mutations

1.) A random event that causes a change in a gene or a chromosome

2.) It is the main source of genetic variation

Cause of mutations

They can be caused by mutagens, such as UV radiation, or due to errors that occur during replication

Inheritance of mutations

It can only be inherited when the mutation occurs in reproductive cells

Environment and mutations

1.) The environment determines whether a mutation is neutral, beneficial, or harmful

2.) Neutral and beneficial mutations stay

3.) Harmful mutations disappear