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)

3. Each inherited characteristic is determined by two heredity genes, one from each parent which determine whether a gene will be dominant or recessive.

www.corbis.com

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 chromosomes, it all comes down to random chance as to which one of the two chromosomes is inheritedby the offspring

• Because of random chance, two gametes virtually never contain the same DNA

• This explains why everyone (except identical twins) is unique because they are genetically different

http://anthro.palomar.edu/biobasis/default.htm

http://anthro.palomar.edu/biobasis/default.htm

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 notpresent in either parent

• This process is also called recombination

• Crossing over leads to greater genetic variation amongst populations than that resulting from independent assortment alone.

Crossing-over unlinks genes as homologous

chromosomes separate in the formation of sex cellshttp://anthro.palomar.edu/biobasis/default.htm

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 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).

Nelson Biology 12

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

http://www.accessexcellence.org/AB/GG/

Chromosomal Mutation

• Chromosomal Mutations lead to

an inactivation of the gene if the

translocation occurs within the

coding segment.

Translocation Inversion

Chromosome 1: 5’ ATG GCA 3’ Normal 5’ AAT GGC TAT 3’

Chromosome 2: 5’ TAGC AAG 3’ Chromosome: 3’ TTA CCG ATA

Chromosome 1: 5’ TAGC GCA 3’ After 5’ AAT GCC TAT 3’

Chromosome 2: 5’ ATC AAG 3’ Inversion: 3’ TTA CGG ATA

http://www.accessexcellence.org/AB/GG/

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 traitshttp://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

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.

www.corbis.com

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

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.

www.corbis.com

Reasons For Mass Extinction

• Crater impact is one of the major reasons that is believedto cause mass extinction

• 65 million years ago, the 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 massextinction to be in certain correlation with crater impact.

http://www.aros.net/~lambo/ele001/ele001.htm

Terrestrial Impact Structures

http://www.aros.net/~lambo/ele001/ele001.htm

• 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. www.corbis.com

Abrupt Fall in Sea Level

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.www.corbis.com

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.

• 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.

www.corbis.com

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

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.

www.corbis.com

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

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.

www.corbis.com

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

• 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. www.corbis.com

www.corbis.com

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. 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).

2. Explain how mass extinctions are most likely the cause of multiple effects, rather than a single cause.

3. 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