To Be or Not To Be....

Are mules a species?

They are a hybrid between a donkey and a horse and are sterile.

After you complete this lesson, you should be able to answer this question.

It's a Mechanism

To review, Evolution is a process in which a population changes over time.

HOW it happens is called a “mechanism”.

Different mechanisms include:

1. Natural Selection
2. Mutation
3. Migration
4. Genetic Drift

Natural Selection

We discussed this in a previous lesson. See the picture below for a review:

Mutation

A mutation is a change in the genetic code. In the case of evolution, the mutation would need to cause a change in phenotype or trait.

In the beetle example, a mutation in the color gene of one or both parents might cause green beetle parents to have a brown beetle offspring. The amount of brown alleles for this gene now increases slightly in this population .

Migration

Migration is the movement of one organism from one area (or population) to another.

If a brown beetle migrates to another population of beetles on the other side of the forest, it increases the amount of the brown gene (allele) in the green population.

Genetic Drift

Genetic drift occurs when, by random chance, a small number of individuals pass on a trait to more offspring in the next generation; this is not because the trait made the parents more "fit" (that would be the natural selection). The trait that ends up being more common does not have to be beneficial. It could be neutral or even detrimental.

Genetic drift is a common occurrence in populations and does not result in adaptations. Since the offspring are a result of the parental gene's random recombination, many variations are always present, even if not expressed.

In the beetle population above, a random event (the shoe in this case) has killed off most of the green beetles in the population. The next generation has more brown and fewer green beetles, but this is due to chance only .

Genetic Drift

Genetic drift can also occur through the following 2 ways:

Bottle Neck

Founder Effect

Genetic drift caused by a chance event reduces the entire population to a very small size; this is usually caused by an environmental event.

Example: American bison nearly went extinct from a combination of factors (over-hunting, railroads, drought, all happening at once).

Genetic drift that results when a small number of individuals start a colony.

Example: The Amish began with a small population of 200 and one members had an allele for Ellis-van Creveld Syndrome, a form of dwarfism. They have more individuals with this syndrome than any other population.

Speciation

Watch the video below to learn about specifications.

Example of Geographic Isolation

In the beetle example below, a stream is rerouted through the original population's habitat. It divides the original population into two groups that cannot cross to interbreed. Over time, enough mutations, natural selection or other mechanisms cause them to change enough that they later no longer can interbreed.

Reproductive Isolation

There are two types of reproductive isolation: those caused by pre-zygotic and post-zygotic barriers that can prevent mating.

1. Pre-Zygotic Barriers

The physical or behavioral barrier that keeps male and female sex cells from uniting to form a zygote.

Examples:

Flowers that bloom at different times of the year will not fertilize each other.
Animals whose mating cycles occur at different times will be unable to reproduce.
Organisms whose reproductive structures are incompatible will not be able to achieve fertilization.
If the sperm is unable to penetrate the egg no fertilization can occur.

2. Post-Zygotic Barriers

Events that occur after the egg is successfully fertilized and has developed into a zygote.

Examples:

Hybrid embryos do not develop correctly.
Hybrid adults are sterile or infertile.
Hybrid adults do not survive (aren't fit).
Inheriting a lethal allele from both parents would result in the eventual premature death of the offspring.
Weak or deformed offspring.

Patterns of Evolution

Modeling Changes in Populations

Changes in a population’s genes can lead to evolution.

Did Darwin and Mendel ever know each other?

Watch the video below to see what would happen if they had a conversation.

Mechanisms of Evolution Challenge and Before You Go

Practice Time! Use the knowledge you learned from the lesson to complete the practice activities below.

Before You Go - You Need To Know

The following key points are from the explore section of the lesson. You must know the following information before moving to the next lesson. This is just a summary of the key points.

How can evolution occur?
- Natural selection, mutation, genetic drift, or migration
What is speciation?
- It is the formation of new species.
True or False. Changes in a population’s genes can lead to evolution.
- True

IMAGES CREATED BY GAVS (FREEPIK)