TYPES OF EVOLUTION
Evolution can happen through….. MICROEVEOLUTION and MACROEVOLUTION
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Microevolution is evolution on a small scale that results in a change in gene frequency in a
population.
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House sparrows have adapted to the climate of North America, mosquitoes have
evolved in response to global warming, and insects have evolved resistance to our
pesticides. These are all examples of microevolution — evolution on a small scale
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When the gene frequency changes we can detect microevolutionary change.
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Imagine that you go to the mountaintop this year, sample these beetles, and
determine that 80% of the genes in the population are for green coloration
and 20% of them are for brown coloration. You go back the next year, repeat
the procedure, and find a new ratio: 60% green genes to 40% brown genes.
Macroevolution is evolution on a grand scale — what we see when we look at the over-arching
history of life: stability, change, lineages arising, and extinction.
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A population is a group of organisms that share a common gene pool and live in a specified
area— like all the individuals of one beetle species living on a particular mountaintop
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Species: A group of organisms that can interbreed and produce viable offspring.
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Gene (allele) frequency is the amount of times a specific gene/allele occurs in relation to that of
other alleles/genes in a population.
MICROEVOLUTION CHANGES
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Consider the beetle population where we see an increase in the frequency of brown coloration
genes and a decrease in the frequency of green coloration genes in a beetle population. Any of the
following could be the cause;
o Mutation: Some "green genes" randomly mutated to "brown genes" (although since any
particular mutation is rare, this process alone cannot account for a big change in allele
frequency over one generation).
o
Gene flow or migration: Some beetles with brown genes immigrated from another
population, or some beetles carrying green genes emigrated.
o
Genetic drift: When the beetles reproduced, just by random luck more brown genes than
green genes ended up in the offspring. In the diagram at right, brown genes occur slightly
more frequently in the offspring (29%) than in the parent generation (25%).
o
Natural selection: Beetles with brown genes escaped predation and survived to reproduce
more frequently than beetles with green genes, so that more brown genes got into the next
generation.
MICROEVOLUTION
GENE FLOW
Gene flow — also called migration — is any movement of genes from one population to another.
Gene flow includes lots of different kinds of events, such as pollen being blown to a new destination
or people moving to new cities or countries. If genes are carried to a population where those genes
previously did not exist, gene flow can be a very important source of genetic variation. In the graphic
below, the gene for brown coloration moves from one population to another.
GENETIC DRIFT
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Genetic drift: Changes to allele frequency as a result of chance; such changes are much
more pronounced in small populations.
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In each generation, some individuals may, just by chance, leave behind a few more descendents
(and genes, of course!) than other individuals. The genes of the next generation will be the genes of
the "lucky" individuals, not necessarily the healthier or "better" individuals.
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Genetic drift affects the genetic makeup of the population but, unlike natural selection,
through an entirely random process. So although genetic drift is a mechanism of evolution,
it doesn't work to produce adaptations.
Genetic Bottleneck:
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Genetic bottleneck: a dramatic, often temporary reduction in population size resulting is
significant genetic drift
Results in loss of diversity. (E.g. when a population goes from 10,000 to only 50, then the new
population is likely to not have ALL of the alleles from the original population.
Founder Effect:
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The founder effect is genetic drift that results when a small number of individuals separate from
their original population and establish a new population.
The founding population, based on chance, would ultimately have a different mix of alleles than
the original population.
So depending on the mixture of alleles in the founding population, the most prevalent could end
up being the rarest in the original population.
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For example, if the presence of a large beak on the mainland occurred 1 in 1000 finches (0.1%)
and then after migrating to the islands the larger beak was found in 1 of 20 finches (5%), this
would indicate a 50 times increase of that allele.
ALTERS THE VARIETY OF THE GENE POOL
Gene pool- refers to the alleles present in a populations and their frequency
The Hardy-Weinberg Principle:
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Hardy-Weinberg principle: In large populations, in which only random chance is at work,
allele frequencies are expected to remain constant from generation to generation
If evolution occurs because of a selective pressure, then changes in an allele frequency
will be seen and the Hardy-Weinberg principle will be false.
For the Hardy-Weinberg to be true, the following CANNOT occur:
o Natural selection
o Genetic drift ; no bottleneck or founder affect (must be a large enough
population)
o Mutations that form new alleles
o Immigrations and emigrations (because it can introduce new alleles/remove
alleles)
o Horizontal gene transfer (alleles cannot be acquired from different species)
If any of the above is true then evolution is likely to occur.
SPECIATION
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Speciation: the formation of a new species
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Speciation is a lineage-splitting event that produces two or more separate species.
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Each branching point, and every other branching point on the tree, is a speciation event. At that
point genetic changes resulted in two separate fruit fly lineages, where previously there had just
been one lineage.
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Species: All members of a population that can interbreed successfully under natural
conditions. (offspring must be able to interbreed successfully as well)
o When two organisms cannot interbreed, they are said to be reproductively isolated
o Populations of different species that do not exchange genetic information have
separate gene pools.
o Biologist today compare the genetic makeup of different populations to determine
relatedness and species type
Types of Evolution….
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Divergent Evolution: the large scale evolution of a group into many different forms.
o In order to fill different ecological niches species evolve into having a variety of different
forms and functions
o Example: deer mice, flying squirrels, porcupines and beavers all come from closely
related species but have evolved into different roles to fill ecological niches.
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Convergent Evolution: the evolution of similar traits in distantly related species.
o Due to similar environments organisms have evolved similar structures and appearances
o Example: Similar body shapes can be found in sharks and dolphins.
o Example: Cacti and euphorbia have evolved similar features in response to their hot and
dry environments
Coevolution: a process in which one species evolves in response to the evolution of another species.
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Species that exist in linked/ symbiotic relationships may evolve in response to changes in the
other
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The term coevolution is used to describe cases where two (or more) species reciprocally
affect each other's evolution.
Example: Brazil nut trees have evolved extremely hard protective shells and the agouti is
the only mammal that has evolved teeth and jaws strong enough to bite open the shell.
The Madagascar long spurred orchid has nectar at the end of a 30cm spur. It is
pollinated by the hawk moth, which has evolved to have a 30cm long tongue to reach
the nectar.
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Coevolution is likely to happen when different species have close ecological interactions with
one another. These ecological relationships include:
1. Predator/prey and parasite/host
2. Competitive species
3. Mutualistic species
Example: But we can see exclusive "matches" between plants and insects even when
pollination is not involved. Some Central American Acacia species have
hollow thorns and pores at the bases of their leaves that secrete nectar
(see image at right). These hollow thorns are the exclusive nest-site of
some species of ant that drink the nectar. But the ants are not just
taking advantage of the plant — they also defend their acacia plant
against herbivores.
This system is probably the product of coevolution: the plants would not
have evolved hollow thorns or nectar pores unless their evolution had
been affected by the ants, and the ants would not have evolved herbivore
defense behaviors unless their evolution had been affected by the plants.
MACROEVOLUTION
Macroevolution is evolution on a grand scale — what we see when
we look at the over-arching history of life: stability, change,
lineages arising, and extinction.
Consequences of Human Influence:
Humans interact with all other species, either directly or indirectly. We commercially harvest
many species from the wild; we alter habitats by clearing land for agriculture, urban expansion,
and mining; and we pollute the air, soil, and water. The greenhouse gases we emit are changing
Earth’s climate and the chemistry of the oceans. We also set aside large areas as parks and
intervene to protect endangered species. These interactions act as agents of natural selection
and have the potential to influence the evolution of species.
Using Table 2 (page 334) from the text, describes some consequences of human activities on
the evolution of species.
Human Selective pressure
Evolutionary change and
consequence
Example
Homework:
1.
2.
3.
4.
What is the bottleneck effect?
What is genetic drift?
What is the hardy-weinburg principle?
Biologists often describe evolution as a change in the frequency of alleles in
a population. How does this relate to natural selection?
5. Which type of selection (disruptive, sexual etc) led to the following
characteristics?
a. Hollow lightweight bones in birds
b. Hundreds of different but genetically very similar species of fruit flies
in the Hawaiin Island
c. Turtle species that have changed little over millions of years
d. Males of many frog species make calls every spring and females are
silent
6. What type of evolutionary force is at play in the following examples?
a. Ant colonies are destroyed in 30 backyards in Etobicoke due to
pesticide use leading to a slight change in that population’s allele
frequency
b. A volcanic eruption results in a population of fruit flies changing
allele frequency.
c. The cheetah population had a massive decrease in population
numbers and genetic diversity as a result of a natural disaster
d. Climate change on an island leads to a different pollination time for a
type of plant that is different than that of the same plant on the main
land.