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Name:_______________________________
Date:________________________________
Lesson # 7: Evolution
(Processes and 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:
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1) Directional Selection
- Favours an ______________________ in the value of a trait from the current population
average
- Occurs when the environment favours one _______________trait
- Causes average to move in one direction, the average has moved to one extreme
- The result is a ___________ away from the average
- Often happens when environment changes in a ________________________ 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?
2) ___________________ Selection
- Selection against individuals exhibiting traits that ___________from the current
population average
- The most _____________trait is the favoured trait
- Most common form of selection
- Once a species adapts to its environment, selective pressures maintain the evolved
______________
Example
- Human birth weight
- Medium sized hummingbird bill length
What other examples can you think of?
3) Disruptive Selection
- Selection that favours _______ or more variations of a trait that differ from the current
population average
- Favours traits at both _________________
- Causes species to diverge
- Occurs when two different types of resources are in ________ 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))
4) Sexual Selection
- Favouring of any trait that specifically ___________ the mating success of an individuals
- 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 __________________________________
Name:_______________________________
Date:________________________________
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Some traits, while sexually _________________ 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
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Not all evolutionary changes are the result of natural selection
Sometimes there are changes in the genetic makeup 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
The _____________makeup of a population can change simply by chance
The _____________ shifting of the genetic makeup of the next generation
The smaller the number of individuals in a population, the greater the ______________ 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
2) Bottleneck Effect
A _____________ in genetic diversity following an extreme ____________________ 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 _______________the genetic makeup of future generations will
be _____________ to the traits carried by those 50 surviving individuals and any new mutations
3) __________ Effect
Occurs when a small number of __________________establish a new population
The new ______________________will begin with a different ________________ than the
original mainland population’s gene pool
4) The ___________________________________ Principle
In the large populations in which only random __________ is at work, allele frequencies are
expected to remain _____________ 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 ______________ size – increases the likelihood of genetic drift
Mutation – introduces new alleles to a population
_______________ or emigration - introduces or removes alleles in a population
Horizontal gene transfer – the gaining of new alleles from different species
Name:_______________________________
Date:________________________________
Patterns of Evolution
- Natural selection leads to predictable outcomes
1) Adaptive _______________
- Occurs when a __________ 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 _____________________ become available
(and are not being used by other species)
- Only real competition is with each other
2) Divergent _________________
- In any ecosystem, there are a number of ecological niches
- This is the ______________ evolution of a group into many different forms
- _________________________ of differences between groups which can lead to the formation of
new species (great______________ 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
3) ________________________Evolution
- Occurs when two ______________species, evolve to occupy similar ecological niches
- When two different species have evolved similar traits in the same ecological niche
Examples
- Cacti and euphorbia
4) Coevolution
- A process in which one species evolves in ____________ to the evolution of another species
Example
- Certain plants have evolved hard protective ___________to protect their seeds, while some seed
– eating mammals have evolved powerful jaws and teeth for ___________through hard shells
- Any seeds surrounded by a hard shell might be better ________________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 __________ more
food than an herbivore with a less powerful jaw.
- This is sometimes called an ____________________________________
 Struggle between competing sets of co-evolving genes, traits, or species, that develop
adaptations and counter-adaptations against each other, resembling an arms race