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 species is a group or category of organisms (living thing) which are distinguishable from all other types of organisms because only members of this species have the potential to produce fertile, and viable offspring through sexual reproduction with each other.
What is a population?
Organisms of a particular species, within a defined area.So how do we form multiple species from one original species?
The process of forming a new species is called speciation. Speciation is complete when sufficient genetic differences accumulate between populations the same original species. At this point, individuals are no longer capable of reproducing with an individual from another population and producing 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: there needs to be separation (no gene flow) between populations
- Natural selection + Mutation: there needs to be different selection pressures and genetic variation
- Lots of time / generations: there needs to be many generations to achieve sufficient genetic separation
… this process is called allopatric 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
- In allopatric speciation there are 3 big stages:
geographic isolation, mutation and natural selection, and reintroduction - In natural selection there are 4 big steps / requirements:
heritable variation, struggle to survive, herediable differential reproduction, change to population phenotypes and genotypes