Cline

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.

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. 

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.

• All the members of the group of interest are more closely related to each other than they are to the outgroup.

• Variations in these sequences (either nucleotide or amino acid sequences) will occur due to mutations.

• The greater the differences between the sequences, the longer the time span since the two species had a common ancestor

• Each branch point (node) represents a speciation event by which distinct species are formed via divergent evolution.

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.