3. Speciation & Rates of Evolution

Success Criteria

Your learning has been successful if you can do the following:


Learn these so you can communicate this concept well.

Hei Mahi (Do Now)

Do Now in your OneNote/Notebook:

Think back to the previous lesson. Arrange these parts in the order that air travels through when breathing in:

bronchioles, nose, alveoli, traceha, bronchi

Exit Task

In your Learning Journal:

Re-write this interpreting question so it is asking about the Breathing:

What is the cause?

Then, write an answer for it.

Important Terminology to Describe Groups

Some important terminology to describe different groups of individuals, relating to speciation.


A population is a group of individuals of the same species that live in the same geographic area and have the potential to interbreed. 


A deme is a sub-population (a smaller group within the population) that can freely interbreed amongst itself. A population is usually made of several demes (groups) that are partially isolated from each other. Often a geographical barrier limits gene flow between demes, but not sufficiently to result in speciation. Enough gene flow occurs between demes ensuring that all of it's members remain the same species.

Highlighted in green is a deme, a group that is partially isolated form the rest of the population. Other groups in this image could also be considered demes. (Pathwayz)


A cline is the gradual change in the phenotypes and genotypes of a species across a geographical gradient. It is a biocommunity which shows a continuous gradient of variation. It shows a gradual phenotypic and genetic differences over a geographical area.

Ring Species

In a ring species, adjacent populations can interbreed and produce viable offspring, but as the distance between populations increases, genetic and phenotypic differences accumulate, eventually leading to reproductive isolation between distant populations.

The Species Concept

What is a species?

Species are members of the one species breed and produce fertile offspring. A species is a group of members in a population that can interbreed to produce fertile offspring. 

How are new species created?

Speciation is the process by which new species are formed from ancestral ones / existing species. 

For speciation to occur in the future, there needs to be something to stop gene flow so that reproductive isolating mechanisms (Concept 4) are built up in the different populations. 

Cladograms & Phylogenetic Trees

The term "CLADE" is used in evolutionary biology to describe a group of species that are thought to have evolved from a common ancestor. 

CLADOGRAMS and PHYLOGENETIC TREES illustrate the evolutionary relationships for organisms with a shared common ancestor.

Branches of a cladogram represent clades (groups of species with a common ancestor. However, cladograms do not show the time scale of evolutionary events. The branch lengths are the same and do not indicate the amount of evolutionary change that has taken place. And the branch points are meaningless in terms of time scales. 

Phylograms / phylogenetic trees are more detailed representations of evolutionary relationships that include information about the time scale of evolutionary events. Branch points differ according to the length of time since speciation, so they show the amount of evolutionary time separating two species. And branch lengths are proportional to the amount of evolutionary change, allowing for a more quantitative representation of evolutionary relationships.

To make phylograms, biologists must compare sequences that have a constant rate of mutation (‘evolutionary clocks’). Mitochondrial DNA is a useful source as it is maternally derived, has a known mutation rate and lacks recombination.

VIDEO: How to analyse a tree diagram (5:23, click the image)

Interactive Phylogenetic Tree (click the image). 

Hei Mahi (Do Now)

Do Now in your OneNote/Notebook:

What is the difference between demes and clines?

Exit Task

In your Learning Journal:

Re-write this interpreting question so it is asking about the Breathing:

What is the cause?

Then, write an answer for it.

Rates of Evolution

Phyletic gradualism is the process of small periodic changes in the gene pool over long periods of time. 

Populations slowly and continuously diverge by accumulating adaptive characteristics in response to different selective pressures and natural selection. Eventually, the accumulated changes results in speciation.

If species evolve by gradualism, there should be transitional forms seen in the fossil record.

It was thought that the only way new species could evolve was over a long time (millions of years), with a gradual transition from one form to another. 

Punctuated equilibrium is where there are long periods of stasis where no evolution is occurring, followed by rapid periods of change. Mutations are still occurring, but no great variant / allele is selected for due to similar environmental conditions. 

The fossil record shows evidence of punctuated equilibrium, when we see long periods of the same form being selected for (during stasis) followed by suddenly seeing new forms / species in the fossil record resulting from diverse selection pressures. 

Diverse selection pressures rapidly select for a change in form, so mutations that offered a different trait were selected for. 

(Instant speciation / polyploidy from Concept 5 is an example of punctuated equilibrium).

Tasks & Homework

Task: What is a Species? (Learn.Genetics Video)

B3.5 (3) What is a Species - Worksheet by Learn.Genetics.pdf

Task: Same or Different Species?

B3.5 (3) Same or Different Species by Learn.Genetics - Student.pdf

Worksheet: Cladograms & Phylogenetic Trees

B3.5 (1) Cladograms & Phylogenetic Trees.pdf

Reading: Speciation

B3.5 C3 Speciation and Rate of Evolution.pdf

sciPad Workbook

Education Perfect HOMEWORK

Work through the Education Perfect task called "B3.5 Concept 3: Speciation and Rates of Evolution"