Continuity of Life on Earth

Continuity of life on EarthSarah Jones

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Life has existed on Earth for approximately 3.5 billion years …

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The processes that have transformed life on Earth from its earliest forms to the vast diversity that characterises it today…

Evolution

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Dawkins, 2011

… a change in the genes.

Dawkins, 2011

“Through the process of descent with modification, the common ancestor of life on Earth gave rise to the fantastic diversity that we see documented in the fossil record and around us today. Evolution means that we're all distant cousins: humans and oak trees, hummingbirds and whales.” (evolution.berkeley.edu, 2016)

Old Theories of Evolution• Jean-Baptiste Lamarck (early 1800’s)

proposed:“The inheritance of acquired

characteristics”• He proposed that by using or not using its

body parts, an individual tends to develop certain characteristics, which it passes on to its offspring.

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Lamarck believed that:A giraffe acquired its long neck because its ancestor stretched higher and higher into the trees to reach leaves, and that the animal’s increasingly lengthened neck was passed on to its offspring.

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H.M.S. Beagle (1831-1836)

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• Darwin set sail on the H.M.S. Beagle (1831-1836) to survey the south seas (mainly South America and the Galapagos Islands) to collect plants and animals.

• On the Galapagos Islands, Darwin observed species that lived no where else in the world.

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“As many more individuals of each species are born than can possible survive, and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving and thus be naturally selected. From the strong principle of inheritance any selected variety will tend to propagate its new and modified form.” Charles Darwin

• Written in 1859 “On the Origin of Species by Means of Natural Selection”

• Two main points:– Species were not created in their present

form, but evolved from ancestral species.– Proposed a mechanism for evolution -

Natural Selection.

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Darwin’s Five Points1. A population has variations.

2. Some variations are favourable.

3. More offspring are produced than survive.

4. Those that survive have favourable traits.

5. A population will change over time.

Natural SelectionIndividuals with favourable traits are more likely to leave more offspring better suited for their environment.

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Variation• Natural selection happens because there is

variation i.e. natural differences in populations.

• Variation is caused by differences in genes.

http://www.darwinwasright.org/genetics.html

Speciation• When one species splits into two or more

separate species.• Speciation is why we see biodiversity on Earth.• Steps of speciation

1. Variation2. Isolation3. Selection

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• Variation – within a populations is required before speciation can occur.

• Isolation – required for a new species to occur e.g. Grand Canyon ground squirrel.

• Selection – natural selection affects genotype. Changes prevent groups from breeding in the future.

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Sexual Selection

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Artificial Selection• The selective breeding of domesticated plants

and animals by man.• The ancestor of the domesticated dog is the

wolf.

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Evidence For Evolution

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Palaeontology• The study of fossils — any form of preserved

remains thought to be derived from a living organism.

• Fossils are an important source of evidence for evolution.

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Evidence From Fossils• The lowest rock layers are usually the oldest.

• The oldest rock layers contain the oldest fossils.

• The rock layers that formed later contain more complex kinds of organisms.

• The variety of fossils increases in the upper, more recent layers of rock.

• No fossil records exist of modern, living plants and animals.

http://www.detectingdesign.com/fossilrecord.html

Relative Age - Relative means that we can determine if something is younger than or older than something else. Relative time does not tell us how old something is, all we know is the sequence of events.

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Absolute Age - Absolute age means that we can more or less precisely assign a number (in years, minutes or seconds) to the amount of time that has passed. We can say how old something is.

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CorrelationEarly fossil examiners were able to correlate or match layers of sedimentary rock merely by looking at the fossils they contained (fossils in a particular rock were different from rocks above and below).

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Some plant and animals only lived a short time in geological history, yet had a wide spread distribution. Known as index fossils – used to correlate layers of rock and therefore determine the age of the layer.

Limitations of fossilsThere are many gaps in the fossil record. This is because:

– dead organisms decompose rapidly.

– dead organisms are eaten by scavengers.

– soft-bodied organisms do not fossilise easily.

– only a small fraction of organisms die in conditions favourable to fossilisation.

– only a fraction of the fossils have been unearthed.

Geophysical evidence suggests that geographical regions and climatic conditions have varied throughout the earth’s history, and these changes would have favoured a mechanism for evolutionary change.

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Ecological considerations also support this. Plants appeared on land before animals, and insects before insect-pollinated plants.

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Biogeography• Distribution gives clues to the evolutionary

history of organisms and of the Earth itself.

• A major cause of speciation in most groups is thought to be geographic fragmentation of ancestral species e.g. continental drift and consequent isolation of populations.

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Comparative Anatomy• Comparative anatomy is the study of similarities

and differences in the anatomy of organisms.

• Two major concepts of comparative anatomy are:

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Homologous and Analogous Structures

Homologous Structures• Those that are similar in structure and

development but which may have different functions.

• The wing of a bird, the flipper of a whale and the leg of a dog all possess the structural plan of the pentadactyl (5-digit) limb although their functions differ.

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• Homology, therefore, implies common ancestral origins and suggests that differences in structure have evolved in response to different environmental conditions.

• This is called divergent evolution.

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Analogous Structures• Analogous structures have a similar function but

no structural relationship. The wing of an insect and the wing of a bird serve the same function — flight — but are structurally dissimilar.

• This suggests that these two groups have different ancestral origins.

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These structures are regarded as examples of convergent evolution, whereby structures with different origins have become adapted to a common function.

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voices.nationalgeographic.com onehdwallpaper.com

• Parallel evolution occurs when related species evolve similar features independently.

• For example, within eucalyptus, a number of species have evolved a white, waxy coating on their leaves, which protects them from frost damage at high altitudes or from water loss in dry conditions.

Vestigial OrgansAn organ that was once useful in an animal’s evolutionary past, but now has no apparent nor predictable function e.g. rudiments of pelvis and hind limbs in snakes, wings on many flightless birds.

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Comparative Embryology

Transitional FossilsArchaeopteryx

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Molecular BiologyDNA and proteins (amino acids) – sequencing of DNA and proteins indicates the degree of relatedness between organisms.

Hardy-Weinberg Principle

The concept that the shuffling of genes that occur during sexual reproduction, by itself, cannot change the overall genetic makeup of a population.

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The Hardy-Weinberg Principle will be maintained in nature if all five of the following conditions are met:

1. Large population2. Isolation = no migration3. No net mutations4. Random mating5. No natural selectionIf these conditions are met,

the population is at equilibrium.

Therefore, “No Change” or “No Evolution”.

Macroevolution

The origin of taxonomic groups higher than the species level.

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Microevolution• A change in a population’s gene pool over a

succession of generations.• Evolutionary changes in species over relatively

brief periods of geological time.

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Five Mechanisms of Microevolution

1. Genetic drift:Change in the gene pool of a small

population due to chance.

Two examples:a. Bottleneck effectb. Founder effect

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a. Bottleneck Effect

Genetic drift (reduction of alleles in a population) resulting from a disaster that drastically reduces population size.

Examples:1. Earthquakes2. Volcano’s

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b. Founder Effect• Genetic drift resulting from the colonization of

a new location by a small number of individuals.

• Results in random change of the gene pool.

Example:Islands (Darwin finch)

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2. Gene Flow:The gain or loss of alleles from a

population by the movement of individuals or gametes.

• Immigration or emigration.

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3. Mutation:Change in an organism’s DNA that creates a new allele.

4. Non-random mating:The selection of mates other than by chance.

5. Natural selection:Differential reproduction.

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SpeciationThe evolution of new species.

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Reproductive Barriers

Any mechanism that impedes two species from producing fertile and/or viable hybrid offspring.

Two barriers:1. Pre-zygotic barriers2. Post-zygotic barriers

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1. Pre-zygotic Barriers

a. Temporal isolation:Breeding occurs at different times for different species.

b. Habitat isolation:Species breed in different habitats.

c. Behavioral isolation:Little or no sexual attraction between species.

d. Mechanical isolation:Structural differences prevent gamete exchange.

e. Gametic isolation:Gametes die before uniting with gametes of other species, or gametes fail to unite.

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2. Post-zygotic Barriers

a. Hybrid inviability:Hybrid zygotes fail to develop or fail to reach

sexual maturity.

b. Hybrid sterility:Hybrid fails to produce functional gametes.

c. Hybrid breakdown:Offspring of hybrids are weak or infertile.

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Allopatric SpeciationInduced when the ancestral population becomes separated by a geographical barrier.

Example:Grand Canyon and ground squirrels

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Adaptive Radiation

Emergence of numerous species from a common ancestor introduced to new and diverse environments.

Example:Darwin’s Finches

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Sympatric Speciation

Result of a radical change in the genome that produces a reproductively isolated sub-population within the parent population (rare).

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Interpretations of SpeciationTwo theories:

1. Gradualist Model (Neo-Darwinian):Slow changes in species overtime.

2. Punctuated Equilibrium:Evolution occurs in spurts of

relatively rapid change.

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CoevolutionEvolutionary change, in which one species act as a selective force on a second species, inducing adaptations that in turn act as selective force on the first species.

Example:1. Acacia ants and acacia trees2. Humming birds and plants with flowers with long tubes

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ExtinctionPopulations with reduced genetic diversity face increased risk of extinction.

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Human Evolution

phys.org

www.nhm.ac.uk

There was no first human…

Dawkins, 2011

Dawkins, 2011

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ReferenceClarence Cockett, Slideplayer, http://slideplayer.com/user/2995729/, 2016

All other links on relevant slides and images.