Forces of Evolution Flashcards
The modern definition of evolution:
Change in allele frequency in a population from one generation to the next
Mutation
New changes in DNA sequence
Increase variation within a population
Increase differentiation between populations, increase variation
Natural selection
Differential reproductive success of individuals with/without successful traits
“Successful” alleles increase in frequency in a population
Usually decrease (sometimes maintain) variation within a population
Increase differentiation between populations
Types of natural selection:
Directional selection
Disruptive selection
Stabilizing selection
Gene flow
Migration of individuals between populations
Increases variation within a population
Decreases differentiation between populations
Genetic drift
Random fluctuations in allele frequency
Decreases variation within a population
Increases differentiation between populations
Speciation
Biological species concept
Cladogenesis
For two populations to become reproductively isolated, they MUST stop gene flow – remember how the four forces of evolution act to increase or decrease differentiation between populations
Barriers to gene flow
Ecological vs. geographic barriers
Allopatric speciation
Parapatric speciation
Sympatric speciation
There are four forces of evolution that have been proposed, tested extensively, and generally accepted by the scientific community that can lead to a change in the frequency of alleles in a population from one generation to the next:
1.Mutation
2.Natural Selection
3.Gene Flow
4. Genetic Drift
Mutations are changes in the DNA sequence induced by
radiation, chemical agents, the action of viruses, or by errors during replication. Changes in the DNA sequence introduce new alleles, thereby changing the allele frequencies in the population from generation to generation.
E.g.The most common mutation underlying blue eyes in European populations reduces the expression of the OCA2 gene, which is part of the melanin production pathway.
Gene flow: If there are two (or more) separated populations within a species whose members
do not commonly interbreed, then migration of individuals between these populations can lead to changes in allele frequencies within the populations. This is often called gene flow as alleles “flow” between gene pools
In genetic drift, allele frequencies change from generation to
generation through the action of random sampling. The allele frequencies “drift” from generation to generation.
The forces of evolution not only explain inherited changes from one generation to the next (microevolution), they also
form part of the explanation for long term changes in biological, such as speciation and diversification.
The process of speciation that leads to an increase in the number of species is called
cladogenesis. In this process, populations of an ancestral species become reproductively isolated and become two new species.
Before we go any farther, we need to rigorously describe what we mean by species. The definition used by most animal biologists is called the
biological species concept.
“Species are groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups” (Mayr, 1942)
In order to understand how cladogenic (splitting) speciationcan be explained by the forces of evolution, we must think about how the action of these processes affects
different populations of the same species.
Effect of Differentiation BETWEEN Populations of a Species: the increases of mutation, natural selection, and gene drift versus the decreases of gene flow
- For cladogenic (branching) speciation to occur, gene flow must eventually be
eliminated between populations for them to develop into full - reproductively isolated - species.
- Gene flow maintains species cohesion. If gene flow is terminated, mutation, natural selection, and genetic drift
increase differentiation between populations, ultimately leading to new species.
- Geographic and ecological differences between populations of an ancestral species are the
most important factors underlying the process of cladogenic speciation from different speciation pattern in the following order: allopatric; parapatric; and sympatric
- Allopatric speciation has the simplest mechanism for the elimination of gene flow: simple geographic separation. Allopatric speciation is the most commonly invoked method of cladogenesis for mammals, but there is growing evidence for parapatric speciation. Few good mammalian examples of sympatric speciation have been proposed.
cladogenesis for mammals, but there is growing evidence for parapatric speciation. Few good mammalian examples of sympatric speciation have been proposed.
- The previously discussed example of the hawthorn fly (apple maggot fly) is an example of
sympatric speciation.Gene flow between the hawthorn-preferring and the apple-preferring flies has been interrupted and they are now substantially differentiated in behavior and genetics.
