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Evolution Review

v  In 1831 Charles Darwin left England on a ship called the HMS Beagle, a British survey ship used for scientific discovery.

v  The expedition had a primary mission to survey the coast of South America and allowed Darwin to study its natural history in various locations during the trip.

v  He was able to collect thousands of specimens from the Andes mountains to the Brazilian jungles. His most famous stop was in the Galapagos Islands where he was able to study finches that would eventually lead to his theory of evolution.

Important Discoveries 1.  Plants and animals observed in the temperate regions

of South America were more similar to plants and animals in the South American tropics than to plants and animals in other temperate regions in the world.

2.  Darwin found fossils of extinct animals (such as the glyptodon) that looked very similar to animals presently living in the same region (eg: the armadillo).

3.  Plants and animals living in the Galapagos Islands closely resembled plants and animals living on the nearest continental coast (the west coast of South America).

4. Species of animals (such as tortoises) that at first looked identical actually varied slightly from island to island in the Galapagos.

5. Finches collected in the Galapagos looked similar to finches from South America but were, in fact, different species. Finch species also varied from island to island.

6. After reading Lyellʼs work, Darwin understood that geological processes that are slow and subtle can result in substantial changes. As well, forces that affect change are the same now as in the past.

Summary of Darwin’s Ideas

v  The factors that identified natural selection:

1.  Organisms produce more offspring than can survive, and therefore organisms compete for limited resources.

2.  Individuals of a population vary extensively, and much of this variation is heritable.

3.  Those individuals that are better suited to local conditions survive to produce offspring.

4.  Processes for change are slow and gradual.

Adaptations v  An adaptation is a variation that allows an organism

to be better suited to its environment.

v  Organisms that are better adapted to their environment are more “fit”, thereby increasing their chance of survival and successful reproduction.

v  Adaptations can be classified as behavioural, physiological or structural.

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Behavioural adaptations v  Behavioural adaptations are associated with how

organisms respond to their environment.

v  Seasonal migration by monarch butterflies, birds and caribou, hibernation by bears and garter snakes, the bending of sunflowers toward the sun, and the shedding of leaves in the fall by deciduous trees are all examples of ways in which organisms have adapted their behaviour.

Physiological Adaptations v  Variations in organism’s metabolic processes across a

species are known as physiological adaptations.

v  Antibiotic-resistant bacteria and pesticide-resistant insects developed because some organisms developed adaptations that allowed them to survive in the presence of antibiotics and pesticides.

Structural adaptation v  Structural adaptations affect the shape or arrangement

of physical features of an organism.

v  For example, the blowholes of whales and dolphins are relocated nostrils, while the needles of a cactus are modified leaves that both protect the plant and reduce water loss.

v  Mimicry is a type of structural adaptation that allows one species to resemble another.

v  Camouflage is second type of structural adaptation to the appearance of an organism.

v  Adaptations such as these assist an organism’s chance of survival by allowing it to blend in with its environment.

v  The stripes on a tiger help it to blend in to the jungle.

Genetic Variation 1. Mutations - Mutations, although rare, do constantly occur. They provide the source of new alleles, or variation from which natural selection can take place.

2. Gene Flow As evolution occurs, natural selection will directly affect the variation within a population as the better adapted individuals survive and reproduce, passing on their genes to successive generations.

v  Gene flow (also known as gene migration) is the transfer of alleles of genes from one population to another. Immigration and emigration (movement in and out of an area) of individuals from a population, will affect allele frequencies with the addition and loss of some variation as animals move into new areas or leave old ones.

3. Genetic Drift: Genetic drift occurs when we see a change in the gene pool of a small population as a result of random chance.

v  The bottleneck effect is a form of genetic drift that results from the near extinction of a population. The surviving population is unlikely to represent the gene pool of the original population. Certain alleles are more frequent than other alleles. Therefore, the genetic variation in the population is reduced.

v  The founder effect is a form of genetic drift that results from a small number of individuals colonizing a new area. It is very likely that these “founders” do not contain all of the genes represented in the parent population.

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Achieving equilibrium v  5 conditions to achieve genetic equilibrium

They are: 1. No mutation can occur, therefore the alleles do not change. 2. Immigration and emigration do not occur (no Gene Flow). 3. The population must be large enough so that changes do not happen by chance alone (no Genetic Drift) 4. All reproduction must be totally random so that one form of the allele is not selected for over another. 5. All forms of the allele must reproduce equally well so that there is no natural selection.

Stablizling selection v  Favor's individuals with an “average” value for a trait

and selects against those with extreme values. An example is plant height.

v  Plants that are too short may not get enough sunlight to survive and tall plants may be damaged by wind. Therefore, the selection favours the medium height plants.

Directional selection v  Favors individuals possessing values for a trait at one

extreme of the distribution, and selects against the average and the other extreme. Giraffe neck length is an example.

v  Individuals with short necks were unable to reach the leaves on the trees, therefore, the distribution of neck length shifted to favor individuals with long necks.

Disruptive selction v  favours individuals at both ends of the distribution and

selects against the average. It is also known as diversifying selection.

v  Disruptive selection leads to the formation of distinct subpopulations of organisms. In time, the allele frequencies in the subpopulations may change to the extent that the two groups may no longer be able to interbreed.

Evidence for Natural Selection

v  Fossil: provide transitional links between groups of organisms

v  Biogeographical: Biogeography is the study of the distribution of plants and animals throughout the world. This evidence supports Continental Drift, the observation that continents are not fixed (eg: Pangea).

v  Anatomical: There are many anatomical similarities between organisms. Despite their different functions, all of these forelimbs contain the same sets of bones organized in similar ways

v  Biochemical: Almost all organisms use the same biochemical molecules including DNA, ATP and many enzymes. They also use the same triplet code of DNA and 20 amino acids in their proteins.

Punctuated Evolution v  The pattern of long stable period in which species

stayed much the same. These periods were interrupted (punctuated) by short periods in which the quick pace of evolution rapidly resulted in the formation of new species

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Gradualism v  the pattern of slow and gradual evolutionary change

over long periods of time. Populations slowly diverge from one another due to differing selective pressures.

Allopatric Speciation v  When a part of a population becomes geographically

isolated from the parent population, allopatric speciation can occur.

v  Geographic isolation can occur due to the formation of physical barriers such as mountains, canyons, rising sea levels, glaciers etc.

v  The physical barrier prevents gene flow between the two populations.

Sympatric Speciation v  This occurs when the gene flow between members of a

population is prevented because of ecological isolation (niche differentiation)

v  Some members of a population may be better adapted to a slightly different habitat in an ecosystem, and begin to specialize in that habitat.

v  Different selective pressures in the two habitats lead to genetic changes in the organisms. The two populations become reproductively isolated (can’t breed), and two distinct species result

Divergent Evolution v  Also known as adaptive radiation

v  is the process in which an ancestral species gives rise to a number of new species that are adapted to different environmental conditions.

v  This often occurs when a species colonizes a new environment in which there are unoccupied ecological niches.

v  For example: darwin’s finches

Convergent Evolution v  Convergent evolution is the process in which different

organisms that live in similar habitats become more alike in appearance and behaviour.

v  As they encounter similar environmental pressures, the organisms develop analogous structures

v  Different structures, same function, no common ancestor à similar environments

v  bat, butterfly and bird wings are analogous structures.

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v  Homologous structures provide evidence for evolution because they illustrate how organism can descend from a common ancestor, these are NOT a part of convergent evolution.

v  Changes due to common ancestor

v  Not always a different function

v  Seal & human forlimbs