Mechanisms of Evolution Flashcards
(13 cards)
Genetic Variation and Evolution
- Genetic variation makes
evolution possible
– Individuals inherit a new combination of alleles from their parents
– As a result, each new generation is genetically unique - New mutations randomly occurring in each generation also allows for the development of new traits
- Natural selection acts on genetic variation in that individuals with the fittest genetic makeup survive and reproduce
- The genetic traits present in a population make up the gene pool
– Successful traits are more common within a population
What changes allele frequencies
within a population?
- Mutations
- Gene flow (migration)
- Non-random mating
- Genetic drift
- Natural selection
Mutations
Mutation = a change occurring in the
DNA of an individual
→This can impact an entire gene pool
because a new trait is added
Example:
Warfarin-resistant Norway rats
Gene Flow
= the movement of alleles from one
population to another due to the
migration of individuals
→ This allows new alleles to be
introduced into a population
Example:
Lone wolf joining a new population
Non-random mating
= mating among individuals on the basis of mate selection for a particular phenotype or due to inbreeding
Preferred Phenotypes:
→ Choosing a mate based on
physical and behavioural traits
Sexual Selection
= natural selection for mating
based, in general, on
competition between males and
choice made by females
Sexual dimorphism: when
females and males in a species
exhibit very different traits
Genetic Drift
= the change in frequencies of alleles due to chance event in a breeding population (generally losing variation)
→Impact is greatest in small populations
Genetic Drift: The Founder Effect
Founder Effect = a change in gene pool
that occurs when a few individuals
start a new isolated population
* Because the founding group is so
small, only a fraction of the total
genetic diversity of the original
population is represented.
* The allele frequencies of the new
population will not be the same as
the original population
Genetic Drift: The Bottleneck Effect
Bottleneck Effect = changes in gene
distribution that result from a rapid
decrease in population size
* Caused by events such as
starvation, disease, natural
disasters, human activities
* Individuals who survive will only
have a fraction of the alleles present
before the population declined
→ gene pool loses diversity
Natural Selection
As we know, some individuals are
more likely to survive and
reproduce than others
→This is due to a selective
advantage caused by its genetic
makeup
There are 3 different types of
natural selection:
– Stabilizing selection
– Directional selection
– Disruptive selection
Stabilizing selection
- Favours individuals near the middle of the range.
- Acts against extreme variants of the
phenotype
The most common phenotype (the intermediate phenotype) is made more common in the population by removing the extreme forms.
This type of selection:
- Reduces variations
- Improves adaptation of the population to aspects of the environment that remain
constant
- Birth weight is a classic example of
a trait under strong stabilizing
selection, where high and low birth
weights are correlated with
reduced survival in both males and
females
Directional selection
- Favours the phenotypes at one end of the range
- Results in a shifted range of
phenotypes - Can occur as an adaptation to a
changing environment
Example: The changes in coloration of peppered moths
- An example of directional selection is the increase in darker forms of the peppered moth that occurred in industrial areas, where the moths with darker wing coloration are
better camouflaged than those with lighter wings against polluted tree trunks
Disruptive selection
- Favours both extremes of the range.
- Tends to eliminate the intermediate
phenotypes. - Can result in a split gene pool, and
may result in the formation of new
species.
Example:
Tail length in squirrels
