Natural selection and speciation
How do new species form?
Learning Objectives:
By the end of this session you should be able to:
- List the basic steps in allopatric speciation (and apply them to unfamiliar scenarios)
Species:
What is a species? How do we know two groups of animals are different species?
Reveal prior student responses
Reveal (evolutionary biologist's) definition of species
A category of organism (living thing) distinguishable from all other types of organisms by the fact that only if two individuals are members of the same species, do they have the potential to produce fertile, and viable offspring through sexual reproduction.
What is a population?
Organisms of a particular species, within a defined area.
So how do we form multiple species from one original specie?
The process of forming a new species is called speciation. Speciation is complete when sufficient genetic differences accumulate between populations of an original species, such that these populations are no longer capable of reproducing with each other produce fertile offspring even if they could physically meet.
How do two or more populations of the same species get to be genetically diverged (different)?
(One way)
- Isolation
- Natural selection + Mutation
- Lots of time / generations
… this process is called allopatrick speciation
Allopatric Speciation stages:
- Two populations are divided by geographic barrier with no gene flow/migration between them
- Each group experiences different selection pressures and different mutations accumulate, which allow differences in allele frequencies to develop
- Over time, the populations, if reintroduced, would be unable to produce viable fertile offspring together
Natural selection
Stages:
-
Heritable VARIATION:
There must be genetic variation within a population which causes phenotypic variation
-
Struggle to Survive:
Not all members of a population will survive long enough to reproduce.
-
HEREDITY and differential reproduction:
Individuals with phenotypes better equipped to suit their environment/selection pressures will be more likely to survive and reproduce. These genes will be inherited by their offspring.
-
Change to population (genotypic / phenotypic)
Over time the more successful phenotypic trait will become more common in the population (change in allele frequency)
Today’s TL;DR
- Genetic diversity helps populations deal with changing environmental pressures (often called selective agents)
- Genetic drift (the bottleneck effect, the founder effect), natural selection all typically reduce genetic diversity
- Mutations and gene flow typically increase genetic diversity
- Point mutations are single nucleotide changes to a DNA sequence, block mutations are larger changes in to a whole segment of DNA
- In allopatric speciation there are 3 big stages: geographic isolation, mutation and natural selection, and reintroduction