Units 4: How Does Life Change and Respond to Challenges ... · gradually lengthen their necks....

Units 4: How Does Life Change and Respond to Challenges Over Time?

Area of Study 1: How Are Species Related? Page 1

Area of Study 2: How Do Humans Impact on Biological Processes? Page 3

Area of Study 3: Practical Investigation Page 4

Conducting Controlled Experiments Page 8 Types of Errors Page 11 Accuracy, Precision and Validity Page 13 Presentation of Data Page 14

Background Genetics Page 19 Genes, Alleles and Genotypes Page 19 Environmental Influences and Phenotype Page 21

Genetic Engineering – Tools and Techniques Page 23 Isolating Cells Page 23 Obtaining the Cells’ DNA Page 23 Restriction Enzymes Page 24 Ligases Page 25 Plasmids Page 26 Recombinant DNA and Gene Cloning Page 27 Transformation of Bacteria Page 28 Identifying Recombinant Cells Page 29 Reverse Transcriptase and Copy DNA Page 30 Methods of Gene Transfer Page 31 Making DNA Visible for Gel Electrophoresis Page 33 Gel Electrophoresis Page 34 Southern Blotting Page 37 Fragments Used in Gel Electrophoresis Page 38 RFLPs Page 38 Intact Alleles (e.g. cystic fibrosis) Page 39 Minisatellites in DNA Fingerprinting Page 39 Microsatellites (STRs) in DNA Profiling Page 40 Mitochondrial DNA (Hypervariable region) Page 43 Polymerase Chain Reaction (PCR) Page 44 Features Distinguishing DNA Profiling from DNA Fingerprinting Page 45 DNA Sequencing Page 45

Changes in Allele Frequencies in Populations Page 46

Monogenic Traits Page 46 Polygenic Traits Page 47 Continuous and Discrete Variation Page 48 Gene Pool Allele Frequencies Page 49

Mutations Page 51

Point Mutations Page 52 Substitutions Page 52 Frameshift Mutations (Insertions and Deletions) Page 55 Block Mutations Page 57 Translocations Page 57 Inversions Page 59 Duplications Page 60 Deletions Page 61 Karyotypes Page 62 Chromosomal Abnormalities Page 63 Aneuploidy Page 63 Polyploidy Page 66

Variation, Natural Selection and Evolution Page 69

Species Page 69 Types of Variation Page 70 Causes of Genetic Variation Page 73 Sexual Reproduction Page 73 Mutation Page 73 Gene Flow Page 74 Genetic Drift Page 74 Environmental Selection Pressures Page 77 Heterozygote Advantage Page 78 Adaptation and Natural Selection Page 79 Development of Evolutionary Theory Page 81 Isolating Mechanisms Involved in Natural Selection Page 82 Prezygotic Isolating Mechanisms Page 83 Postzygotic Isolating Mechanisms Page 84 Forms of Natural Selection Page 85 Allopatric Speciation Page 87 Selective Breeding Page 88 Loss of Genetic Diversity Due to Selective Breeding Page 89 Increase in Genetic Abnormalities Due to Selective Breeding Page 90 Summary of Causes of Genetic Change in a Population Page 92 Biodiversity in Fragmented Populations Page 94 Extinction Page 95

Earth’s Geological History Page 98

Significant Changes in Biodiversity Over Time Page 99 Mass Extinction Events Page 101

Examination Questions – Book 1 Page 103

Solutions

© The School For Excellence 2019 Master Classes – Unit 4 Biology – Book 1 Page 80

Natural selection results in “survival of the fittest”. A fit organism is one that is better adapted to the environment.

1. Genetic variation, resulting in phenotypic

variation, exists within the population.

2. In a changing environment, organisms with the

favoured phenotype will have an improved

chance of surviving and reproducing over those

of less favoured phenotypes.

3. Hence more offspring will possess the favoured

characteristics.

4. And their frequency (and that of the favoured

alleles) within the population will increase

across generations.

In time, the evolution of a new species may occur.

QUESTION 5 The giraffe’s ancestor survived in tropical rainforests which once covered the entire African continent, feeding on the leaves of large shrubs and small rainforest trees. Most of the continent dried out over thousands of years, and the rainforests were replaced by open dry woodlands (savannah). During times of drought, today’s giraffes compete with smaller antelopes and other browsers for scarce vegetation growing high in the trees. Use the stages of natural selection depicted above to describe how today’s giraffe populations may have evolved longer necks from their shorter necked ancestors:

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es-art.ru

Possible giraffe ancestor,

more similar to today’s okapi


QUESTION 6 Under what circumstances would a small and isolated population, with a high level of inbreeding and limited variation, be best able to survive the forces of natural selection?

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Many theories (or beliefs) have been devised to explain the abundance of life on planet Earth:

Creation:

A belief (not a theory) that today’s organisms have not evolved from previous species. Instead, species are unchanging life forms, and some of these species (e.g. dinosaurs) have become extinct.

Lamarckism:

Jean Baptiste Lamarck theorised that individuals lose characteristics they do not require, or use, and develop, or acquire, characteristics that are useful. He also argued that the acquired traits were heritable. Examples of what is traditionally called "Lamarckism" would include the idea that when giraffes stretch their necks to reach leaves high in trees (especially Acacias), they strengthen and gradually lengthen their necks. These giraffes have offspring with slightly longer necks. Similarly, a weightlifter, through his work, strengthens the muscles in his arms, and thus his

sons will have similar muscular development when they mature.

Darwinian Evolution:

Charles Darwin and Alfred Wallace independently developed the theory of evolution via natural selection. Today, this is the most widely accepted theory for the origin of all life forms within the scientific community.

123RF


If sufficient time exists, the evolution of new species from a common ancestor can occur if gene flow ceases between different populations of the same species. The circumstances creating this break in gene flow can vary:

• Geographical barriers such as mountains, rivers, oceans, etc. can separate previously

continuous territories. When gene flow ceases in this way, allopatric speciation can occur.

If the two isolated populations (of the same species) are exposed to different environments, different phenotypes will be favoured in each area. The forces underlying natural selection will operate differently in each population. The populations may become reproductively isolated as interbreeding ceases between them. Over time a population may change so much that it becomes a new species via natural selection and mutation.

• e.g. Allopatric speciation in blind cave fish of the USA.

Any animal that lives in permanent darkness and doesn’t need vision to find food or avoid predators won’t really need their eyes or visual centres in the brain. Energy-saving eye loss, or the expensive tissue hypothesis, is a theory used to explain why sighted animals that took up life inside caves evolved to be blind, and compensated for the loss of vision with evolution of improved senses of touch (e.g. detecting vibrations), smell, hearing etc. Genetic studies show that about a dozen eye genes are mutated in Astyanax cavefish, and different genes are mutated in different cavefish populations, suggesting many selective pressures are in action.

e.g. Evolution of desert frog species in central Australia from ancestral non-desert frog species

in Gondwana. Similar desert frog species have evolved in southern Africa from the same ancestral frogs due to the same selective pressures.

Breviceps, an African desert frog - arkive.org

Cyclorana, an Australian desert frog


There are many different isolating mechanisms, called prezygotic isolating mechanisms, that prevent species from interbreeding with each other.

If different species do manage to produce offspring, postzygotic isolating mechanisms ensure the hybrid offspring are not selected for by natural selection.

Prezygotic Isolating Mechanism Examples

Geographical Isolation Populations are separated by a geographical or physical barrier e.g. deserts, mountain ranges, oceans, rivers, canyons

e.g. mainland emu and extinct King Island and Flinders Island emus

Ecological Isolation Populations occupy different niches within the same ecosystem or territory

e.g. blackbird (a woodland bird) and the closely-related ring ouzel which lives in moorlands of the same habitat in England

Temporal Isolation The breeding times of populations do not overlap in the same area

e.g. the American toad and the Fowler's toad are closely related species, but the American toad mates in the early part of summer, while the Fowler's toad mates later in the season.

Behavioural Isolation An isolating mechanism in which two species do not mate because of differences in courtship behaviour.

e.g. the Great Crested Grebe, blue-footed booby etc.

Structural Isolation Structural differences in reproductive organs prevent mating or pollen transfer

e.g. wolverine and duck

Gamete Mortality The sperm of one species may not be able to fertilise the egg of another species due to chemical barriers

e.g. goose sperm and duck eggs,

e.g. lily pollen and rose eggs


Isolating mechanisms that prevent a zygote of two different species from developing into a

fertile adult are postzygotic isolating mechanisms.

Hybrid Inviability

Hybrid inviability is an isolating mechanism which reduces a hybrid's capacity to mature into a healthy and genetically fit adult. The relatively low health of these hybrids relative to pure-breed individuals prevents gene flow between species. Hence, hybrid inviability limits hybridisation and allows for the differentiation of species. The barrier of hybrid inviability occurs after mating species overcome prezygotic barriers to produce a zygote. In general, the hybrid embryo dies before birth. However, sometimes, the offspring develops fully with mixed traits, forming a frail, often infertile adult. This hybrid displays reduced genetic fitness, marked by decreased rates of survival and reproduction relative to the parent species. The offspring fails to compete with purebred individuals, limiting gene flow between species.

Hybrid Sterility

Hybrid sterility is an isolating mechanism whereby the viable hybrid is unable to breed, i.e. the hybrid is sterile.

Problems typically occur during gamete formation. The parents (e.g. a horse and donkey) of the hybrid (e.g. a mule) contain different numbers of chromosomes, so their gametes do not possess matching chromosomes. When a male donkey (2n = 62) mates with a female horse (2n = 64) a mule results with 63 chromosomes each somatic cell. Without homologous pairs of chromosomes, the mule cannot reliably reproduce.

Tigers and lions do possess the same chromosome number (2n = 38), however, the chromosomes are not truly homologous, so their offspring, ligers or tions, are typically sterile.

© The School For Excellence 2019 Master Classes – Unit 4 Biology – Book 1 5

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• One extreme of the trait distribution experiences selection against it.

• The population's trait distribution shifts toward the other extreme.

• The mean of the population graph shifts.

Example:

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• When selective pressures select against the two extremes of a trait.

Example:

A plant that is too short may not be able to compete with other plants for sunlight. However, extremely tall plants may be more susceptible to wind damage. Combined, these two selection pressures select to maintain plants of medium height. The number of plants of medium height will increase while the numbers of short and tall plants will decrease.


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• Selection pressures act against individuals in the middle of the trait distribution.

• The result is a bimodal curve.

Example:

• Beak size in seed eating Galapagos finches.

• Big beaks are well suited for big seeds, and small beaks for small seeds.

• Medium-sized beaks cannot be used to retrieve small seeds, and are not tough enough to crack open the bigger seeds.

• A polymorphic population has resulted.

Polymorphic Populations

Polymorphism occurs when two or more clearly different phenotypes exist in the same population of a species. The variation can be discrete (discontinuous) or continuous.

kenpitts.net

Polymorphism in the jaguar


Speciation is the process of species formation.

Allopatric speciation can occur when members of a population are split into two separate groups, each of which is exposed to different selective forces. If populations become

geographically isolated (e.g. by a mountain range, a desert, the sea etc.) and gene flow ceases, allopatric speciation can result.

Example 1: Evolution of a new tiger beetle species.

Example 2: Evolution of the Eastern and Western Whipbirds in Australia.

Eastern whipbird

Western whipbird

Gene flow existed between populations of one ancestral whipbird species when prehistoric

Australia was forested from the eastern to western coasts. It ceased with the formation of

Australia’s dry interior, exposing different whipbird populations to different selective forces.

4. When the mountain

barrier is eroded

sufficiently, the beetle

populations can meet

again. However, their

respective features are

so different they are no

longer able to

interbreed and produce

fertile offspring

3. Gene flow ceases, and under

the influence of different

mutations and different selective

forces, each population adapts

to its particular environment.

Allopatric speciation occurs over

time.

2. A mountain range

emerges, dividing the

beetle population into

two isolated groups.

The climate differs from

one side of the

mountains to the other.

1. Genetic variation exists

within a population of tiger

beetles living in a moist habitat

with plenty of vegetation.


The process of selective breeding involves humans selecting and breeding organisms of their desired phenotype. Humans select the desirable features of the organism which will be bred, so this variety will become the fittest in the population.

E.g. sheep, cattle, dog and cat breeds, wheat varieties etc.

• To produce sheep with thick wool, humans continue to only breed the sheep that possess the desirable thick wool variation.

• The same principle is followed in developing other sheep varieties.

E.g. artificial selection of Brassica vegetables from wild cabbage (Brassica oleracea):

wild cabbage

(Brassica oleracea)

cabbage

broccoli

cauliflower

Brussel sprouts

kale

Collard greens

kohlrabi

http://www.bringyou.to/apologetics/p15.htm

http://www.bringyou.to/apologetics/p15.htm


Both traditional and current selective breeding programs have often resulted in a decline in genetic diversity of species, making the populations more vulnerable to environmental changes, such as disease, Issues have arisen with many crop species. In South America, for instance, well over 100 tomato varieties were grown by farmers prior to the restrictions placed on them by multinational food companies. The real risk of countries only growing a few varieties of a particular crop is that if a disease affects one crop, it is likely to ravage most others, with the potential for a food crisis. Multinational companies (e.g. Monsanto) are “encouraging” farmers to only grow their specific varieties of crops. Selective breeding leads to monocultures: entire farms of nearly genetically identical plants. Little to no genetic diversity makes crops extremely susceptible to widespread disease. Bacteria and viruses mutate and change constantly. When a disease causing bacterium mutates and attacks a specific genetic variation, it can easily wipe out vast quantities of the species. If the selectively bred genetic variation is prone to a disease, the entire crop will be wiped out. A tragic case of selective breeding which led to the deaths of millions of Irish in the 1800s was the potato blight, a fungal disease.

In order to preserve the potentially disease-resistant alleles of from the diversity of original crop varieties, massive underground caves have been constructed in the Arctic region where seeds from thousands of crop varieties are maintained.

Arctic global seed vault in Svalbard


Kennel Club Breeding of Deformed Pedigree Dogs

A recent expose by Panorama into the activities of the Kennel Club of UK revealed how serious genetic faults are perpetuated in pedigree pups. This is as a direct consequence of the club’s desire to produce dogs with characteristics deemed appealing to their owners, but highly deleterious to the health of the animal. Pups born without the favoured characteristic were also simply destroyed, along with those dogs born with extraordinary deformities. In the same way that inbreeding among human populations can increase the frequency of normally rare alleles that cause diseases, the selective breeding that created the hundreds of modern dog breeds has put purebred dogs at risk of a large number of health problems, affecting both body and behaviour. Some conditions are directly related to the features breeders have sought to perpetuate among their dogs. As they deliberately manipulated the appearance of dogs to create or accentuate physical characteristics that were considered aesthetically pleasing, like the flat face of a bulldog or low-slung eyelids of a bloodhound, breeders also created physical disabilities. The excessively wrinkled skin of the Chinese shar-pei causes frequent skin infection; bulldogs and other flat-faced (or brachycephalic) breeds such as the Pekingese have breathing problems because of their set-back noses and shortened air passages; bloodhounds suffer chronic eye irritation and infection. The unnaturally large and small sizes of other breeds encourage different problems. For example, toy and miniature breeds often suffer from dislocating kneecaps and heart problems are more common among small dogs. Giant dogs such as mastiffs, and great danes are nearly too big for their own good. Researchers have found a striking correlation between a dog’s large size and a frequency of orthopaedic problems like hip dysplasia. Large dogs are often prone to heat prostration because they can’t cool down their bodies (tiny dogs, by contrast, have a hard time staying warm), and because of the massive weight they must support, these breeds are prone to malignant bone tumours in their legs. Meanwhile, the huge head and narrow hips of the Bulldog can necessitate that their pups must be born by Caesarean section. Other health problems among purebreds are the product of both inbreeding and bad genetic luck. The alleles responsible for many genetic diseases are “recessive,” Individuals that carry only one copy of the disease allele don’t have the condition, and are carriers of the disease. Normally, because disease alleles are relatively rare, it is unlikely that both the mother and the father will be carriers, and even less likely that they’ll both give the disease allele to their offspring. But that’s not the case for purebred dog breeds, where genetically similar individuals are intentionally mated, increasing the concentration of disease alleles. This increases the chance of what might be allergies or a predisposition to cancer.


Example: Designer Brachycephalic (short-headed) Dogs

Many brachycephalic dogs, including pugs, English bulldogs, French bulldogs, Boston terriers, Cavalier king Charles spaniels and shih tzus, are put down every year by their traumatised owners after the dogs suffered agonising pain which could not be remedied despite expensive veterinary treatment. Many of these dogs cannot walk more than a few minutes at a time, suffer from sleep apnoea and shake with pain from cervical deformities.

There is a rising human

backlash against the breeding of these cute dogs. Experts say their flat faces, big eyes, little noses and ears – bred by design to shorten their muzzles to make them appear non-threatening – appeal to us because they look like human babies. To fight the trend, the Australian Veterinary Association and the RSPCA launched the Love is Blind campaign

warning buyers to avoid buying dogs with exaggerated

features, and is lobbying kennel clubs and dog show organisers to stop perpetuating

these genetic deformities within dog breeds.

Vets said nearly every short-faced dog needed surgery to address the snorts, wheezing, grunting and sleep apnoea that many people on social

media find “cute”. One French bulldog, bought from a reputable breeder for $3,000, had to be euthanised after agonising back problems that emerged within weeks of her adoption. The dog could not move, and would yell and cry as she was in so much pain. Another such dog was bought from an “accredited kennel” for $4,000. The owner was not prepared, however, for the endless $15,000 of surgeries which followed, including the widening of its nostrils so it could breathe.

Sunday Age, 2 July 2017


QUESTION 3 Complete the following diagram by adding the extra causes of genetic change in a population:


Long-distance dispersal of species has always resulted in new areas of land being colonised by organisms, including plants, invertebrates and vertebrates. Many insect and spider species, as well as seeds of certain plants, have been discovered circumnavigating the globe in a frozen state 2 km above the earth.


Fragmentation of populations is typically caused by habitat destruction. With a more limited gene pool within each isolated population, a high rate of inbreeding occurs within the population, which leads to a decrease in genetic variability in the species involved. Hence, biodiversity within the population is decreased. Inbreeding decreases the genetic fitness of the population for a variety of reasons: 1. It forces competition with relatives, which decreases the evolutionary fitness of the

species.

2. There is an increased possibility that a lethal homozygous recessive trait may be expressed. This decreases the average litter sizes, indirectly decreasing the population. When a population is small, the influence of genetic drift increases, which leads to less and/or random fixation of alleles. In turn, this leads to increased homozygosity, negatively affecting genetic fitness of individuals.

3. Less effective selection against the less genetically fit members of small populations causes an accumulation of deleterious mutations. Since individuals in small populations are more likely to be related, they are more likely to inbreed. A reduction in fitness may occur in small populations because of mutation accumulation, reduced genetic diversity, and increased inbreeding.

4. Over time, the ability of a species to adapt to a changing environment, such as climate change, is decreased.

The endangered giant Brazilian otter.

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lloossss ooff vvaalluuaabbllee aalllleelleess))

LLiimmiitteedd ggeennee ppooooll && more likely genetic drift will be a driving force of evolution

rather than natural selection

HHiigghh rraattee ooff iinnbbrreeeeddiinngg

IInnccrreeaassee iinn eexxpprreessssiioonn ooff rreecceessssiivvee

ddeelleetteerriioouuss mmuuttaattiioonnss

HHiigghh cchhaannccee ooff eexxttiinnccttiioonn


When a particular species ceases to exist, or a particular variation no longer exists, (due to

an allele being eradicated), extinction is said to have occurred.

• Extinction can be caused by a change in environment that is too rapid to allow for the species to adapt, i.e. it is too rapid to allow for the course of evolution. The change is too dramatic for the species to survive.

• Small, inbreeding populations, with little variation, are more likely to suffer the adverse effects of sudden environmental change.

E.g. Introduced species may outcompete native fauna and flora by possessing features that make them selectively fitter than the native species.

• The action of humans has led to the extinction of many species of organisms in a very short period of time, particularly over the last 1,000 years. Today, species are being lost at the most rapid rate since the last worldwide extinctions during the reign of the dinosaurs. Humans are decimating species mainly via:

i. Habitat destruction (including land clearing for agriculture, pollution etc.).

ii. Introduction of feral species/noxious weeds.

iii. Exploitation (E.g. hunting tigers and rhinoceros for mythical medicinal properties).

iv. Global warming.


The average rate of natural loss of species is called the background extinction. The average life of a species varies according to the type of organism, e.g.

• certain marine animals: 5 – 10 my

• mammals: only ~ 1 my

• coelacanth: over 400 my

Plants: The first land plants, believed to have evolved from single celled aquatic organisms, were mosses and liverworts. Eventually plants with true veins evolved, giving rise to ferns and then seed plants. Ultimately, flowering plants evolved from a group of vascular seed plants, similar in appearance to today’s conifers. The rapid evolution of flowering plants occurred after the disappearance of dinosaurs, coinciding with the rapid diversification of another group of organisms. Can you suggest which group this was?

Animals: Unicellular aquatic organisms are thought to have evolved into multicellular aquatic animals.

Vertebrates first appeared in the form of primitive fish. Amphibians were the first vertebrates to move onto land. Reptiles evolved from certain amphibian species. Birds and mammals evolved at a later date from different reptile groups, i.e. mammals first evolved from a group of mammal-like reptiles, while birds later evolved from a branch of the dinosaurs.

rersource.wur.nl


The planet Earth appears to have solidified from a ball of molten rock around 4.6 billion years ago. Evidence of prehistoric life, known as fossils, clearly depicts a pattern of evolution of species since the first known bacteria evolved approximately 3.5 billion years ago. A geological time scale covers events that have occurred on Earth from its formation to the present.


Some key events which have occurred during the time of life on Earth are summarised in the following table:

Time (mya) Event

2.8 First humans (Homo sp.)

66 First primates

66 – 23 Small mammals benefit from dinosaur extinction, and grow larger species

66 Mass Extinction Event

130 First ‘modern’ bird

146 - 66 Flowering plants diversified along with pollinating flying insects

160 Earliest known placental mammal

200 - 146 First flowering plants

251 - 200 Mammals appear. First dinosaurs



359 - 299 Earliest known reptiles. Insects develop wings.

350 First land vertebrates (amphibians)

350 Oldest known land invertebrate (scorpion)



444 - 416 First algae on land – evolve into first terrestrial plants

440 Oldest known land-dwelling organism (fungus)

505 First known fish

543 - 488 First invertebrates in oceans

1,560 First multicellular life in oceans

3,500 Earliest known bacteria in oceans

Evolving from cone-bearing ancestor plants, fossil evidence indicates that flowering plants first appeared in the Lower Cretaceous, about 125 million years ago, and were rapidly

diversifying by the Middle Cretaceous, about 100 million years ago. Earlier traces of flowering plants are scarce.

bouquets

A 125 myo flowering plant from China


Marine life continues to evolve under the seas and oceans of planet Earth:

Diversity of Marine Animal Life over Geological Time

QUESTION 7 When compared with other living creatures, deep sea organisms have undergone an exceedingly slow rate of evolution. Explain a possible reason for this feature of deep sea creatures.

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Of the many mysterious and unexplained events that have happened on Earth over millions of years, mass extinctions are perhaps the most perplexing. Mass extinctions involve the monumental loss of plant and animal species over short time. These events leave Earth ready for evolutionary changes as new species develop to take the places of those lost. Scientists have discovered at least five different mass extinctions, over history when anywhere between 50% and 75% of life was lost. Countless questions remain unanswered about how and why these moments came to pass. Below is the information that experts have gathered so far.

Ordovician – Silurian Extinction (439 mya) 86% of life on Earth was wiped out. Scientists believe two major events resulted in this extinction: glaciation and falling sea levels. Some theories suggest that the Earth was covered in such a vast quantity of plants that they removed too much carbon dioxide from the air which drastically reduced the temperature. Falling sea levels were possibly a result of the Appalachian mountain range forming. The majority of the animal life lived in the ocean. Trilobites, brachiopods, and graptolites died off in large numbers but interestingly, this did not lead to any major species changes during the next era.

Late Devonian Extinction (364 mya) Estimates propose that around 75% of species were lost around 364 million years ago. Information is unclear as to whether the late Devonian extinction was one single major event or spread over hundreds of thousands of years. Trilobites, which survived the Ordovician-Silurian extinction due to their hard exoskeletons, were nearly exterminated during this extinction. Giant land plants are thought to be responsible as their deep roots released nutrients into the oceans. The nutrient rich waters resulted in mass amounts of algal blooms which depleted the seas of oxygen and therefore, animal life. Volcanic ash is thought to be responsible for cooling earth’s temperatures which killed off the spiders and scorpion-type creatures that had made it on land by this time. A distant amphibian cousin, elpistostegalians, had also ventured onto land but became extinct. Vertebrates did not appear on land again until 10 million years later, in the form of ichthyostegalians (extinct amphibians) from which we all evolved.

BBC


Permian – Triassic extinction (251 mya)

This mass extinction is considered the worst in all

history as approximately 96% of species were

lost. Ancient coral species were completely lost. “The Great Dying” was caused by an enormous volcanic eruption that filled the air with carbon dioxide which fed different kinds of bacteria that began emitting large amounts of methane. The Earth warmed, and the oceans became acidic. Life today descended from the 4% of surviving species. After this event, marine life developed a complexity not seen before and snails, urchins, and crabs emerged as new species.

Triassic – Jurassic extinction (199 - 214 mya) As in other mass extinctions, it is believed there were several phases of species loss. The blame has been placed on an asteroid impact, climate change, and flood basalt eruptions. During the beginning of this era, mammals outnumbered dinosaurs. By the end, dinosaurs’ ancestors (archosaurs) reigned the earth’s surface. This extinction laid the path that allowed for the evolution of dinosaurs which later existed for around 135 million years.

Cretaceous – Paleogene extinction (65 mya) Perhaps the most well-known mass extinction event, the end of the Cretaceous-Paleogene brought on the extinction of dinosaurs. A combination of volcanic activity, asteroid impact, and climate change effectively ended 76% of life on earth 65 million years ago. This extinction period allowed for the evolution of mammals on land and sharks in the sea.

Jewish Currents

behance

Paleocene mammals of the world

A small mammal of the Cretaceous

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