Transcript Genetic Variation

Evolution
Genetic Variation,
Extinction
and Diversification
Overview
• Genetic Variation
•
Mendel’s Laws
Independent Assortment
Crossing Over
Mutations
Change in Chromosome Number
The Common Genetic Code
Extinction and Diversification
Reason for Mass Extinction
Diversification
Five Major Extinctions
The Sixth Extinction
Extinction Vs. Religion
• Readings and Questions
• Bibliography
Genetic Variation
• The inheritable traits of organisms lead to
genetic variation.
• The origins of genetic variation directly
relate to sexual reproduction, and include
mutations (changes in the base pairs of
DNA), segregation (random assortment of
one of each chromosome pair into an egg
or sperm cell), and recombination (an
event during meiosis in which specific
DNA is shuffled).
http://www.accessexcellence.org/AB/GG/
Mendel’s Laws
• Gregor Mendel is known as “the father of modern genetics” due to his
contributions to genetics made by studying heredity throughout various
generations of pea plants.
• His studies led to three laws:
1: The sex cell of a plant or animal contains only one
allele for different traits, but not both factors needed
to express the traits
2: Characteristics are inherited independently from
other characteristics (i.e. trait for hair colour isn’t
dependent on trait for eye colour)
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3. Each inherited characteristic is determined by two heredity genes,
one from each parent which determine whether a gene will be
dominant or recessive.
Independent Assortment
• Each somatic cell within a human contains two of each type of
chromosome.
• When gametes are produced, the chromosomes
separate so that each gamete only contains one
allele for each trait.
• Out of the 23 pairs of homologous
http://anthro.palomar.edu/biobasis/default.htm
chromosomes, it all comes down to
random chance as to which one of
the two chromosomes is inherited
by the offspring
• Because of random chance, two gametes
virtually never contain the same DNA
• This explains why everyone (except
identical twins) is unique because
http://anthro.palomar.edu/biobasis/default.htm
they are genetically different
Crossing Over
• Crossing over refers to the breaking during meiosis of one
maternal and one paternal chromosome, the exchange of
corresponding sections of DNA, and the rejoining of the
chromosomes.
• The result of crossing over is
a combinations of alleles not
present in either parent
• This process is also called
Crossing-over unlinks genes as homologous
chromosomes separate in the formation of sex cells
http://anthro.palomar.edu/biobasis/default.htm
recombination
• Crossing over leads to greater genetic variation amongst
populations than that resulting from independent
assortment alone.
Mutations
• Mutations are rare, random events which are very important for
evolution
• The are usually non-beneficial to organisms, however they are also
usually recessive, meaning unless two mutations are coupled together,
the mutation will not be expressed
• All mutations are alterations in a sequence of DNA and can occur from
chemicals, radiation, or through errors in DNA replication
• Genetic variation depends on mutations within germinal cells.
(Although mutations often occur in somatic cells – i.e. cancer – only
mutation in germinal cells have the chance to be inherited)
• Mutations or chromosomal abnormalities also occur, where species can
end up with too many or too few chromosomes.
• This can be fatal or lead to various disorders.
Point Mutations
•
Point mutations occur due to a mistake in the DNA replication, or
damage by a chemical mutagen, and they include:
1. Substitution:
2. Deletion:
AAA CCC GGC AAA
AAG CCC GGC AAA
AAG ACC GGC AAA
AAC CCG GCA AA
3. Addition/Insertion
AAG CCC GGC AAA
AAG ACC CGG CAA A
Nelson Biology 12
•
•
http://www.accessexcellence.org/AB/GG/
1 results in only one amino acid in the sequence being changed.
Both 2 and 3 are frameshift mutations which can result in either many
different amino acids being altered, or a stop codon being read (early or
later than usual).
Chromosomal Mutation
• Chromosomal Mutations lead to
an inactivation of the gene if the
translocation occurs within the
coding segment.
http://www.accessexcellence.org/AB/GG/
Translocation
Inversion
Chromosome 1: 5’ ATG GCA 3’
Normal
Chromosome 2: 5’ TAGC AAG 3’
Chromosome: 3’ TTA CCG ATA

5’ AAT GGC TAT 3’

Chromosome 1: 5’ TAGC GCA 3’
After 5’ AAT GCC TAT 3’
Chromosome 2: 5’ ATC AAG 3’
Inversion:
3’ TTA CGG ATA
Change in Chromosome Number
• Irregular number of chromosomes, as well as
structural modification in a chromosome can
have drastic effects on an individual
• Polyploidy results when an individual inherits
more than 46 chromosomes due to an improper
separation in meiosis.
•
Aneuploidy results when individuals inherit
less than 46 chromosomes.
• Down syndrome (trisomy 21) is the most
common disorder, causing mental retardation,
and distinctive physical traits
http://www.ndsu.nodak.edu/instruct/mcclean/plsc431/chromnumber/number2.htm
The Common Genetic Code
• Humans are 99.9% identical to chimanzees when
referring to their DNA.
• In 1987, scientists were amazed when British
researchers showed that a human gene could
be inserted into a lowly yeast cell and function
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perfectly well.
• The Human Genome Project suggests trends that genes performing
various functions in lower animals have been maintained through
evolution even in human DNA (though sometimes modified).
• The ‘thread’ of genetic similarity connects us to nearly 10 million other
species today, and more importantly, back to one common ancestor
over 3.5 billion years ago.
• Most importantly, amongst all living organisms, the instructions for
reproducing and operating are encoded in chemical language,
represented by A, C, T, and G, the initials of 4 chemicals.
Extinction and Diversification
• Extinction is the evolutionary termination
of a species caused by failure to reproduce
and death of all remaining members of the
species; the natural failure to adapt to
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environmental change.
• Diversification is the opposite; it refers to a species growth and
evolution into a greater variety of that species and potentially new
species
• They occur in cycles; a long period of diversification eventually halts
when an extinction occurs, and diversification must start over.
• Extinction always occurs, and can be caused by a species’ food
requirements, predation, or habitat.
• Extinction is the expected fate of a species, rather than a rarity.
Reasons For Mass Extinction
• Crater impact is one of the
major reasons that is believed
to cause mass extinction
• 65 million years ago, the
http://www.aros.net/~lambo/ele001/ele001.htm
Chixulub Crater smashed into the earth, releasing the equivalent
energy of 100 million megatons of TNT. At the same time, the
dinosaurs, along with many other species became extinct.
• As further support, rock samples from 95
locations worldwide show high levels of
iridium, a rare metal in the Earth’s crust,
abundant in meteorites.
• This event however is the only mass
extinction to be in certain correlation
with crater impact.
Terrestrial Impact Structures
http://www.aros.net/~lambo/ele001/ele001.htm
Abrupt Fall in Sea Level
• Another theory that is believed to lead
to mass extinction is abrupt falls in sea
level.
• Each of the three largest extinctions in
that last 250 million years corresponds
to a major sea level change.
• This lead to the conclusion that falls in
sea levels have detrimental effects on the
diversity of marine invertebrates.
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Giant Eruptions
• Giant eruptions coincide with mass extinctions more so than any other
factor
• The best known series of eruptions occurred throughout 1 million
years, between 66.5-64.5 million years ago, when over one million
cubic kilometers of basaltic lava was poured out from under the Earth’s
surface.
• Having no similar eruptions in our history
to compare these eruptions with, it is hard to
imagine the effects they would have on climate.
• The three largest extinctions during the past 250
million years occurred at times of both sea-level
fall and flood-basalt eruption.
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Diversification
• Diversification of new organisms rarely happens quickly, as life
seldom rebounds from extinction over a short time, possibly due to
extreme change in habitat.
• “If we substantially diminish biodiversity on Earth, we can’t expect the
biosphere to just bounce back. It doesn’t do that. The process of
diversification is too slow,” – James Kirchner, professor of earth and
planetary science, at UC Berkeley
• Scientists have developed methods through looking at fossils to
determine rates at which new organisms appear and disappear.
• From these studies, they have determined it takes nearly 10 million
years to recover from global extinction, thus proving evolution doesn’t
speed up in response to rapid bursts of extinction.
• Studies of diversification are fairly recent, and it is not yet clear on
what all the limiting factors of diversification are.
The 5 Major Extinctions
• The Ordovician Mass Extinction
(438 million years ago)
• Vertebrates, along with armored jawless
fish appeared; shell bearing marine
invertebrates dominated.
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• High levels of iridium are not associated
with this extinction, therefore ruling out crater impact.
• This extinction seems to be linked with a major climate change
• The extinction occurred in two waves; the first when an ice age began,
and the second when it ended.
• Although this is generally accepted as a major extinction, some
paleontologists feel that as more fossils are collected from all regions
of the world, this may in fact have been a relatively minor event.
The Late Devonian Mass Extinction
• This extinction occurred 360 million
years ago, when amphibians along with
trees and forests, insects, and bony fish
appear; land plants radiated.
• During this time period, there was a
worldwide extinction of coral reefs and
their related fauna, along with many other
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groups of plants and animals.
• Some iridium anomalies have been detected around that time period
from China and Western Europe, however they are hardly comparable
to the levels associated with the Chixulub Crator
• At the time there are indications of climatic changes, and major
changes in sea-level and ocean chemistry.
• Notably, carbon isotope shifts indicate a rapid period of diversification
before the extinction.
The Permo-Triassic Extinction
• 245 million years ago marked the largest extinction of all time.
• Douglas Erwin, a famous paleobiologist, marked it as the “Mother of
Mass Extinctions”.
• An estimated 57% of all families, and 97% of all marine animals
became extinct.
• This was a very rapid extinction, almost certainly taking place within 1
million years, and probably much faster than that.
• At this time, life on land had evolved enough so that a small coal bed
was created in Australia, however after the extinction, no more coal
was laid down anywhere for at least 6 million years.
• The levels of iridium from this time period are normal
• Most importantly, this extinction coincides with the largest volcanic
eruption known throughout the Earth’s history.
The End-Triassic Mass Extinction
• Occurring 208 million years ago, after reptiles, amphibians, and insects
all radiated, and coniferous trees appeared and modernized.
• In 1999, a team of geologists reported that a massive eruption took
place around this time period which marked the beginning of major
plate tectonic activity that began splitting the Atlantic Ocean
• However, many critics have pointed out that the eruption seemed to
have occurred after Triassic/Jurassic boundary, and hence after the
extinction.
• The extent of this extinction itself is partly at question, as scientists
have been unable to determine how big it actually was.
• Essentially more research is required to determine why this extinction
occurred, and to what extent it did.
The Cretacious/ Tertiary Extinction
• Also known as the extinction of the dinosaurs,
which marked the beginning of the current age
of the mammals.
• This extinction occurred 65 million years ago,
killing off all large reptiles, leaving mammals
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to radiate, and angiosperm plants to dominate.
• The Chixulub Crater smashed into earth around this time period, in
addition to drops in sea level, and large volcanic eruptions.
• In comparison to the Permio-Triassic Extinction, this remains
relatively small, as only 20% - 25% of all species were brought to
extinction.
• Although famous for the extinction of dinosaurs, many other species
became extinct at this time.
The Sixth Extinction
• What is the sixth extinction? “It’s the next annihilation of vast numbers
of species. It is happening now, and we, the human race, are its cause,”
according to Dr. Richard Leakey, the world’s most famous
paleoanthropologist.
• Each year, between 17 000 and 100 000 species are wiped out.
• According to Dr. Richard Leakey’s figures, 50% of all the Earth’s
species will have become extinct over the next 100 years, and humans
are using almost half of the energy available to sustain life on Earth.
• According to a United Nations report, almost 25% of the world’s
mammals face extinction in the next 30 years
• Extinction of species is mainly occurring due to industrialization
combined with rainforest and wetland destruction.
• The UN report also noted that factors leading to mammals’ extinction
continue with “ever increasing intensity”.
Extinction Vs. Religion
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• Over two centuries ago, the bones of a fossil
mammoth were collected in North America.
• At the National Institute of Sciences and Arts in 1796,
anatomist Baron Georges Cuvier argued that the bones
came from a unique species, which no longer lived,
and therefore was extinct.
• Cuvier’s deduction completely contradicted religious
beliefs of the time, as it was believed that a creator
would not allow any of his creations to disappear from
the earth.
• This stirred up numerous debates, culminating 60
years later, with the writings of Charles Darwin.
Summary
• Genetic variation relies on the inheritable traits of an organism.
• Mendel, the “father of modern genetics” gave insight to alleles,
independent assortment and dominant & recessive genes.
• Crossing over occurs between paternal and maternal chromosomes,
creating a chromosome different from those of either parent.
• Mutations, crucial for evolution, rarely occur, and are caused by erorrs in
DNA replication; including insertion, deletion, translocation, and inversion.
• We are related to all living organisms, getting down to A, C, T, and G.
• A combination of crater impact, change in sea level, and eruption is thought
to have caused the five major extinctions.
• Of the five extinctions, the Permo-Triassic Extinction is by far the largest.
• Both the UN and the world’s most famous paleoanthropologist agree that
humans are the cause of the sixth extinction.
Questions
1.
2.
3.
How do both crossing over and independent assortment increase
genetic variability? Include in your answer an explanation of why
two individuals will never be genetically the same (aside from
twins).
Explain how mass extinctions are most likely the cause of multiple
effects, rather than a single cause.
What is the Sixth Extinction? Why is it occurring, and what is it’s
main cause?
Readings:
Huge Genetic Variation Found;
http://www.wired.com/news/medtech/0%2C1286%2C45214%2C00.html
The Sixth Extinction; Leakey, Richard, & Lewin, Roger,
http://www.well.com/user/davidu/sixthextinction.html
Bibliography
•
Genetic Variation, Diversification and Mass Extinction; Nelson Biology 12
•
Biological Basis of Heredity; O’Niel, Dennis, http://anthro.palomar.edu/biobasis/default.htm
•
Independent Assortment of Chromosomes; No author,
http://www.biology-online.org/2/2_meiosis.htm
•
Biology Glossary; No author, http://www.pcsresearch.com/bt/Glossary.cfm?Term=E
•
Mutations, Mutagen, and DNA Repair; Montelone, Beth,
http://www-personal.ksu.edu/~bethmont/mutdes.html#types
•
Genetics; No author, http://mason.gmu.edu/~jlawrey/bio1471/genetic.html
•
Variation and Mendel’s Laws, Other Sources of Genetic Variation; No author,
http://www.anthro.mankato.msus.edu/biology/evolution/index.shtml
•
Biodiversity Glossary of Terms; No author,
http://www.wri.org/wri/biodiv/gbs-glos.html#CD
•
Extinction Level Event; No author, http://www.aros.net/~lambo/ele001/ele001.htm
Bibliography
•
The Sixth Extinction; Leakey, Richard, & Lewin, Roger,
http://www.well.com/user/davidu/sixthextinction.html
•
Extinction!; MacLeod, Norman, http://www.firstscience.com/site/articles/macleod.asp
•
The Common Genetic Code; No author,
http://www.pbs.org/wgbh/evolution/library/04/4/l_044_02.html
•
Quarter of Mammals ‘face extinction’; Podger, Corrine,
http://news.bbc.co.uk/1/hi/sci/tech/2000325.stm
•
Extinction, Cowen, Richard,
http://www-geology.ucdavis.edu/~GEL3/Cowenextinction.html