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Lesson # 7: Evolution
(Processes + Patterns of Evolution)
Evolutionary Change via
Selection
- Types of selective pressures may result
from: i.e. climate conditions, food
availability, predators, and choice of
mate etc.
- There are four types of selection:
1) Directional Selection
2) Stabilizing Selection
3) Disruptive Selection
4) Sexual Selection
Types of Selection
1) Directional Selection
- Favours an increase or decrease in the value of a trait from the
current population average
- Occurs when the environment favours one EXTREME trait
- Causes average to move in one direction, the average has moved to
one extreme
- The result is a shift away from the average
- Often happens when environment changes in a consistent way
Examples
Fishing with nets
Hummingbirds using their bills to feed on nectar
- Very common in artificial selection… why?
- How would necks of giraffes or camouflage / mimicry help explain
directional selection?
DIRECTIONAL
SELECTION
Types of Selection
2) Stabilizing Selection
- Selection against individuals exhibiting traits that deviate from
the current population average
- The most common trait is the favoured trait
- Most common form of selection
- Once a species adapts to its environment, selective pressures
maintain the evolved feature
Example
- Human birth weight
- Medium sized hummingbird bill length
What other examples can you think of?
STABILIZING
SELECTION
Types of Selection
3) Disruptive Selection
- Selection that favours two or more variations of a trait that
differ from the current population average
- Favours traits at both EXTREMES
- Causes species to diverge
- Occurs when two different types of resources are in one area
- May lead to formation of new species
Examples
- (e.g., Darwin’s finches – small beaked ate small seeds and big
beaked ate large seed; yet, medium beaked could not eat or
compete for either seed size (unfavourable))
DISRUPTIVE
SELECTION
Types of Selection
4) Sexual Selection
- Favouring of any trait that specifically enhances the mating success
of an individual
- Often leads males and females of a species evolving appearances and
behaviours that are quite different from each other
- Most common behaviour is the female selects male, and the males
compete one against one another
- Most females choose mates based on physical traits, or behaviours
- Some traits, while sexually attractive are detrimental
Examples
- Bright coloured feathers of a male peacock is attractive for females
but easy for predators to see
- A very successful male elephant seal may mate with dozens of
females each year and hundreds of females in his lifetime, while a
weak male may live a longer life but produce no offspring. In this
case, the genes of the short lived but dominant male are destined to
become more common in succeeding generations
Evolutionary Change Without
Selection
- Not all evolutionary changes are the result of natural
selection
- Sometimes there are changes in the genetic make up of a
population that are not influenced by the traits of
individuals
- Each of these changes tends to reduce genetic diversity
within a population
Types of Evolutionary Change Without Selection
1) Genetic Drift
2) Bottleneck and the Founder Effect
3) The Hardy – Weinberg Principle
Evolutionary Change Without
Selection
1) Genetic Drift
- The genetic make up of a population can change
simply by chance
- The random shifting of the genetic makeup of the
next generation
- The smaller the number of individuals in a
population, the greater the influence of genetic drift
- In small populations, genetic drift can result in a
particular allele becoming either very common or
disappearing entirely over a number of generations
- Any lost alleles result in a reduction of genetic
diversity of the population
Evolutionary Change Without
Selection
2) Bottleneck Effect
- A loss in genetic diversity following an extreme reduction
in the size of a population
Example
- If an individual population of 10,000 individuals is reduced
to only 50 individuals, they are unlikely to contain all of the
traits found in the larger population
- Many traits, and in particular rare ones are likely to be
eliminated
- If the population is allowed to recover, the genetic make up
of future generations will be limited to the traits carried by
those 50 surviving individuals and any new mutations
Evolutionary Change Without
Selection
3) Founder Effect
- Occurs when a small number of individuals establish a new
population
- The new population will begin with a different gene pool
than the original mainland population’s gene pool
Evolutionary Change Without
Selection
4) The Hardy – Weinberg Principle
- In the large populations in which only random chance is at work,
allele frequencies are expected to remain constant from generation to
generation
- Based on the Hardy – Weinberg principle, biologists recognize that the
following conditions result in evolution:
Natural selection – favours the passing on os some alleles over others
Small population size – increases the likelihood of genetic drift
Mutation – introduces new alleles to a population
Immigration or emigration - introduces or removes alleles in a
population
Horizontal gene transfer – the gaining of new alleles from different
species
Patterns of Evolution
- Natural selection leads to predictable outcomes
1) Adaptive Radiation
- Occurs when a single species evolves into a number of
distinct but closely related species
- Each new species fills a different ecological niche
- This process usually occurs when a variety of new
resources become available (and are not being used by
other species)
- Only real competition is with each other
1. Species A migrates from the
mainland to the first island. 2.
Isolated from the mainland,
species A evolves to species B. 3.
Species B migrates to the second
island. 4. Species B evolves in
species C. 5. Species C recolonizes
the first islands, but is now unable
to reproduce with species B. 6.
Species C migrates to the third
island. 7. Species C evolves into
species D. 8. Species D migrates to
the first and second island. 9.
Species D evolves to species E. This
process might go on indefinitely
until a large diversity is reached.
Patterns of Evolution
2) Divergent Evolution
- In any ecosystem, there are a number of ecological niches
- This is the large scale evolution of a group into many different
forms
- Accumulation of differences between groups which can lead to
the formation of new species (great variety of species)
Example
- Ontario forests have over 20 species of closely related rodents
including deer mouse, flying squirrel, porcupine and beaver
- All which evolved from a single common ancestor
Patterns of Evolution
3) Convergent Evolution
- Occurs when two different
species, evolve to occupy similar
ecological niches
- When two different species have
evolved similar traits in the same
ecological niche
Examples
- Cacti and euphorbia
Patterns of Evolution
4) Coevolution
- A process in which one species evolves in response to the evolution of
another species
Example
- Certain plants have evolved hard protective shells to protect their
seeds, while some seed – eating mammals have evolved powerful
jaws and teeth for chewing through hard shells
- Any seeds surrounded by a hard shell might be better protected from
herbivores and better able to survive than seeds with thin shells
- Similarly, any herbivore with a slightly more powerful jaw might be
able to acquire more food than a herbivore with a less powerful jaw.
- This is sometimes called an “evolutionary arms race”
Struggle between competing sets of co-evolving genes, traits, or
species, that develop adaptations and counter-adaptations against each
other, resembling an arms race