EVOLUTION
:is the accumulation of heritable changes through succeeding generations of organisms
that main result in the emergence of new species in the long term. The theory of evolution is
scientific. It looks at how different varieties of organisms came into being. It does NOT tell us how
the earth came into being and it does NOT tell us how the first life forms appeared.
NB TERMS:
Gene pool: All of the genes and their different alleles that are present in a population of a
particular species of organism.
Gene/Allele: The occurrence of an allele in a population in relation to all alleles of that gene at the
same locus. It is often expressed as a fraction.
Species: A group of organisms of common ancestry that closely resemble each other and that are
capable of interbreeding and producing fertile offspring.
Population: a group of organisms of the same species inhabiting a defined area close enough
that random interbreeding can take place.
MICROEVOLUTION:
:the process of changes occurring within a species through mutations, natural selection, genetic
drift etc.
MACROEVOLUTION:
:the process that results in the formation of new
species and as geological time has passed this
has resulted in more highly developed groups above
the species level being formed. It often
indicates that new genera occurred as a result of the
development of key innovations such as the
development of a tiny skeleton, 4 limbs, wings etc.
Factors that affect gene frequency in a species:
1. Mutation:
2. Migration:
3. Genetic Drift: The random change in the frequency of alleles in a population over successive
generations due to chance. In each generation, some individuals may, just by
chance, leave behind a few more descendants (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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4. Natural Selection: There is variation between individuals in a population. Some phenotypes
are more successful than others (fitter). Survive for longer to produce more
of the next generation.
Building Resistance:
Pesticide-resistance, herbicide-resistance, and antibiotic-resistance are all examples of
microevolution by natural selection. The enterococci bacteria, shown here, have evolved a
resistance to several kinds of antibiotics.
EVIDENCE OF EVOLUTION:
1. The Fossil Record
Fossils are animals or plants or other organisms that lived a very long time ago.
The remains of the organisms are preserved in rocks or amber. Thousands of years
go by, the mud piles up in layers and the weight of the mud presses down on the
bones. The mud turns to rock. As that happens, ground water seeps through the
changing layers of mud. Minerals are dissolved in the water..The water seeps into
all the tiny holes in the bones. The organism becomes permineralised and
becomes a fossil. Some fossils are actual parts of plants or animals that have
turned to stone. Sometimes a fossil is only an imprint of a plant or an animal.
Some are found in the frozen ground of the Arctic, like the bones of the ancient
mammoth.
Fossils are usually found in sedimentary rocks.
Sedimentary rocks are formed mostly from particles of older rocks that have been broken apart by
water, ice and wind. The particles of gravel, sand and mud, that are collectively called sediment,
settle into layers at the bottoms of rivers, lakes and oceans. With the passing of time, the layers
of sediments are compacted by the weight of overlying sediments and cemented together to
become sedimentary rock. The fossils are made when living organisms die and get buried by
sediments quickly before the hardest parts of the animal have a chance to decay. As sediments
accumulate, pressure causes the sediments to harden into rock.
STEPS:
1. The animal dies.
2. The flesh decays and the layers of sediments start covering it.
3. Sediments into bones.
4. Sediments replace the bones as they harden.
5. After 10 000 years it is a fossil.
The principle of superposi-on states that older beds are covered by younger beds so in a sedimentary sequence
the youngest unit is at the top. Or - in any sequence of undisturbed layered sedimentary rock, a given layer must
be older than any layer on top of it. By implica>on this means that the fossils found in the layers can be aged
rela>ve to each other: this is known as rela>ve da>ng. Rela>ve da>ng methods use geological principles to place
events in chronological order.
An index fossil is a fossil of a group that only during a par>cular period of geological >me and can therefore be
used to date the rock in which it is found.
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Beginning at the present and examining older and older layers of rock, eventually a level will be
reached where no fossils of humans are present.
If we continue backwards in time, we will successively come to levels where no fossils of
flowering plants are present, no birds, no mammals, no reptiles, no four-footed vertebrates,
no land plants, no fishes, no shells, and no animals.
Three concepts are important in the study and use of fossils:
1. Fossils represent the remains of once-living organisms.
2. Most fossils are the remains of extinct organisms; that is, they belong to species that are no
longer living anywhere on earth.
3. The kinds of fossils found in rocks of different ages differ because life on earth has changed
through time.
Finding Fossils: Natural processes such as erosion and mountain formation can expose deeper
rock layers bringing fossils to the surface.
Radiometric dating enables absolute age determined by measuring the amount of radioactive
decay that has taken place.
The fossil record indicates:
1. There is an increase in complexity of organisms. The simplest organisms appeared first.
2. There in an increase in diversity from the older fossils to modern species.
3. There are more extinct species further back in time.
4. Transitional fossils (intermediate forms between groups) exist.
The fossil record supports the theory that:
All species evolved from a simple common ancestor. Transformation of a simple common
ancestor into a large number of species as a result of accumulated genetic changes has been
termed Descent with Modification
EVOLUTION OF THE HORSE:
Horse ancestors lived in forest thickets but with the change of their environment to grassland, many
changes that enabled the survival of these animals occurred.
1. The original animals ate soft leaves and fruit. They probably lived in
thickets.
2. Grasses evolved and grasslands developed. Thickets receded.
Molars widened by natural selection to eat tougher material and
legs got longer for running through grasslands
3. Animal now running on middle toe and has strong cement on
teeth for chewing grasses. Has increased in size.
4. Ice age: as ice moved over North America so animals moved
south to the Pampas (grasslands of South America). The side
toes disappear and the cement covering thickens and flattens.
5. After the ice age. Horses became extinct in North America.
Reasons suggested: include climate change , a pandemic, or
hunting following the arrival of humans. Later horses migrated across
from Europe.
