IB Bio: Topic 5 + 10.3 Evolution and Natural Selection

Evidence of evolution: Fossil records

• Fossils found in rock layers (strata) allow us to determine geological areas from which organisms belong and gives a timeline for life on earth

• Most found fossils do not exactly match current species: indicating life changed over time

Radiometric techniques to date fossils

• The ratio of isotopes present in fossils is determined

• C-14 is a parent atom to N-14, the daughter atom. The half-life of C-14 is known (5730 yrs), so the ratio between these two atoms can be used to communicate how old the fossil is

Fossil records direct evidence for evolution

• Direct evidence: Body fossils that provide direct evidence of ancestral forms and include bones, teeth, shells, leaves, etc.

• Can be preserved remains (soft tissue preserved) or petrified fossils, casts, molds (mineralised structures or preserved imprints)

Fossil records indirect evidence for evolution

• Trace fossils which provide information about ancient life

• Includes preserved footprints, burrows, coprolite (feces)

Limitations of fossil records as evidence for evolution

• The fossil record is incomplete

• Very specific conditions are required for the mineralization process for fossilization to take place

• Generally only hard parts of organisms preserved

• Fossils can be easily damaged, often it is only fragments recovered

Transitional fossils

• They establish the connection between species by exhibiting traits common to both an ancestor and its predicted descendants.

• eg. archaeopteryx connects dinos and birds

Law of fossil succession

• The age of the fossil is equal to the age of the strata it is found in

• The ordered succession of fossils suggests that newer specues likely evolved as a result of changes to ancestral species

Homologous structures

• anatomical features found in different species that are similar in structure but may have a different function

• Suggests a common ancestor; the structure may have evolved to be fit for the environmental conditions of different species

• eg. the pentadactyl limb is similar in structure (5 finger-like structures)a mong humans, birds/bats, horses, and whales/dolphins, but with different functions; tool manipulation, wings for flying, hooves for galloping, fins for swimming

Adaptive radiation

several new species diversify from an ancestral source, with each new species adapted to utilise a specific unoccupied niche

Divergent evolution

a species diverges significantly and can no longer reproduce with organisms of original species, then a new species is declared (speciation)

Vestigial structures

• Homologous features that no longer serve their original purpose — provide evidence for common ancestry

• May be unused or used for a different purpose

• eg. the human appendix

Analogous structures

• anatomical features that are different in structure but have a similar function

• May have confused earl evolutionary biologists as they may have though some species were more related than they are (no close common ancestor)

• Supports convergent evolution

Convergent evolution

species living in similar environments may evolve to have similar adaptations in order to survive/thrive

Evidence for evolution: Selective breeding

• when humans control the breeding of plants/animals, significant changes can occur in the gene pool over a short period of time

• Supports evidence that species would change when the environment favoured organisms that could survive and breed

• NOTE evolution is often slow, but some genes have a greater effect on change

Biochemical evidence of evolution

• Organisms with similar DNA/amino acid sequences are more closely related

• Cytochrome C (protein) is often used to compare because of its universality

Continuous variation

continuous variation in features across geographical ranges supports the concept of gradual divergence —species change slowly and constantly

Industrial melanism

peppered moths exist in a light form and dark form. The dark form has increased in population since the Industrial Revolution since pollution and soot turns lichen and trees black. Evolved to be better at camoflouge

Overproduction of offspring

• Species tend to produce more offspring than the environment can support to maximize chances of offspring surviving and therefore not all individuals can survive to maturity

• Limited nutrient supply, space, sunlight, etc.

• Leads to competition and struggle

Causes of variations in genotypes/phenotypes

• mutations (nonsense or missense)

• meiosis (crossing over/recombination and random assortment of homologous chromosomes)

• sexual reproduction (random fertilization by the egg and sperm will always generate different zygotes)

Beneficial alleles

if they lead to a trait that allows species to survive and reproduce in their environment, their frequence increases in a population over time

Mutations caused by…

• spontaneous replication errors or induced errors by mutagens (chemicals, viruses or radiation)

• they are not formed based on need!!

Requirements for a mutation to result in a new allele/variation

• mutations must be in the gamete because mutations in somatic cells are not heritable

• therefore must occur during meiosis or be passed down from parents

• must be in coding DNA

Why do harmful mutations often die out

• If it results in a selective disadvantage an organism may not be able to survive and reproduce

Genetic variety in asexually reproducing populations

relies on mutation or horizontal gene transfer (eg. plasmids exchanged between prokaryotes)

Adaptations

• changes in the traits of a population due to variations and selective pressures

• they are unconscious and made by populations not individuals

Steps of evolution by natural selection

• overproduction of offspring: genetic variation in this population leads individuals with useful variations to succeed and individuals with harmful variations find it more difficult to survive

• Individuals that are of better “fitness” to the environment reproduce and pass on successful genetic characteristics while, due to struggle, individuals not fit to the environment likely die off

• Over many generations, the accumulation of changes in heritable characteristics cause change in the gene pool ie. evolution occurs

Struggle

stresses (inter or intra species) may select for or against certain traits

Natural selection: Pesticide resistance in rats

• pesticides wipe out all but a few genetically distinct resistant rats

• those rats reproduce and thrive bc little competition

• now there is a largely pesticide resistant rat population

Natural selection: Antibiotic resistance in bacteria

• most bacteria are killed off by antibiotics so the patient improves, but one resistant bacterium remains bc of a modification to its genetic makeup

• it multiplies by binary fission, can make the patient sick again

• The same antibiotic will not affect the new bacteria population

Gene pool

• all the genetic information present in the reproducing members of a population at a given time

• large gene pools tend to have a greater variety of traits (opposite is true with small gene pools)

Inbreeding

practise of having closely related organisms mate, often results in small gene pools

Allele frequency

• measure of the proportion of a specific variation of a gene in a population

• not the same as the proportion of individuals with a trait

• can be changed by immigration and emigration (ie. gene flow)

Disappearing alleles

due to the death of organisms with the allele because of natural, artificial, or sexual selection

Genetic drift

• change in the composition of a gene pool as a result of chance or random events

• faster and more significant in smaller populations where chance events have a bigger impact on gene pool

• larger population less affected by random events and maintain more stable allele frequences with low genetic drift

Mechanisms and effects of reducing a large population to a smaller population

• Mechanisms are population bottlenecks and founder effect

• changes the allele frequencies of a population significantly

Population bottlenecks

• occurs when a naturally occurring or human-induced event reduces a population by more than 50%

• surviving population has less genetic variability so subject to a higher level of genetic drift

• the population descended from the surviving members will have a different gene pool to original population

Founder effect

• occurs when a small group breaks away from a larger population to colonize a new territory

• this population subset does not have the same degree of diversity as the original larger population, therefore it is more subject to genetic drift

• as this new colony increases, it will have a unique gene pool to original population

• differs from population bottleneck because the original population remains intact

Stabilising selection

• intermediate phenotype favoured over both extremes

• operates when environmental conditions are stable and competition is low

• eg human birth weights

Directional selection

• one phenotypic extreme selected at the cost of the other phenotypic extreme

• operates in response to gradual/sustained changes in environmental conditions

• typically followed by stabilising selection once an optimal phenotype has been normalised

• eg. antibiotic resistant bacteria populations

Disruptive selection

• both phenotypic extremes favoured at the cost of the intermediate

• results in bimodal spread

• occurs when fluctuating conditions favour the presence of two different phenotypes

• continued separation may lead to speciation

• eg black or white moths in regions of sharply contrasting colour extremes

Isolation barriers

• when a barrier prevents two populations from interbreeding and therefore keeps their gene pools separate

Prezygotic isolation

occurs before fertilisation can occur — no offspring are produced

Postzygotic isolation

occurs after fertilisation — offspring are either not viable or infertile

Temporal isolation

• two populations differ in their periods of activity or reproductive cycles

• eg. if species reaches sexual maturity at different times of the year

Behavioural isolation

• when the presence or absence of a specific behaviour prevents reproduction from taking place

• usually because two populations exhibit different specific courtship patterns

Geographical isolation

• leads to allopatric speciation (speciation caused by geographic barriers)

• two populations occupy different habitats or separate niches within a common region

• or a geographical barrier prevents reproduction

Effect of polyploidy on evolution

• quickly leads to speciation since the previous generation will be incompatible with the new generation

  • interbreeding can no longer occur since they have more than two full sets of chromosomes (triploidy=3n)

• Eg. many allium species have diversified via polyploidy

Phyletic gradualism

• a model of speciation that predicts that speciation occurs uniformly via steady and gradual transformation of whole lineages

• big changes result from cumulative small changes

• supported by gradual change in horse fossil records

Punctuated equilibrium

• a model of speciation that predicts that species remain stable for long periods before undergoing abrupt rapid changes

• seen as a periodic process

• changes may be in response to drastic ecological events

• supported by lack of transitional fossils for many species

Binomial nomenclature system

Genus species - underlined or italics

Point of binomial nomenclature

• allows for identification and comparison of organisms based on recognised characteristics

• shows how closely related organisms are, allowing for prediction of evolutionary links

• makes it easier to collect/sort/group information

Three domains

• Eukarya: eukaryotic organisms

  • protista - anamalia - plantae - fungi

• Archaea: prokaryotic cells lacking a nucleus, often extreomphiles

  • eg. methanogens, thermophites

• Eubacteria: prokaryotic cells lacking a nucleus and consisting of the common pathogenic forms

Taxa in order of increasing specificity

Kingdom - phylum - class- order- family - genus - species

Artificial classification

• Grouping organisms based on superficial characteristics, without considering their evolutionary relationships or genetic similarities

• Strength is they are easily recognisable

Flashcard: Natural Classification

• A system of organizing and grouping organisms based on their evolutionary relationships and shared characteristics, rather than on artificial criteria

• shows evolutionary relationships and taxonomic levels

• disadvantage: highly mutable, tends to change as new information if discovered

Phylogenetic classification

groups species based on shared DNA or amino acid sequences

List plant phylums

Bryophyta - Filicinophyta - Coniferophyta - Angiospermophyta

Bryophyta

• no true leaves, roots or stems

• no vascularization

• reproduces by spores

• anchored by rhizoids

• eg. mosses

Filicinophyta

• has leaves, roots, and stems

• has vascularization

• reproduces by spores

• leaves are pinnate

• eg. ferns

Coniferophyta

• has leaves, roots, and stems

• has vascularization

• reproduces by seeds (cones)

• has woody stems

• eg. conifers

Angiospermophyta

• has leaves, roots and stems

• has vascularization

• reproduces by seeds (fruit)

• has fruit and flowers

• eg. flowering trees

Invertibrates

do not have a backbone

Animalia subgroups

vertebrates and invertebrates

Invertebrates phylum

Porifera - Cnidaria - platyhelmintha - annelida - mollusca - arthropoda

Porifera

• No body symmetry (asymmetrical)

• No mouth or anus (have pores to facilitate the circulation of material)

• May have silica or calcium carbonate-based spicules for structural support

• No segmentation

• Examples include sea sponges

Cnideria

• Have radial symmetry

• Have a mouth but no anus (single entrance body cavity)

• May have tentacles with stinging cells for capturing and disabling prey

• Examples include jellyfish, sea anemones and coral

Platyhelmintha

• Have bilateral symmetry

• Have a mouth but no anus (single entrance body cavity)

• Have a flattened body shape to increase SA:Vol ratio and may be parasitic

• Examples include tapeworms and planaria

Annelida

• Have bilateral symmetry

• Have a separate mouth and anus

• Body composed of ringed segments with specialisation of segments

• Examples include earthworms and leeches

Mollusca

• Have bilaterial symmetry

• Have a separate mouth and anus

• Body composed of a visceral mass, a muscular foot and a mantle (may produce shell)

• Examples include snails, slugs, octopi, squid and bivalves (e.g. clams)

Arthropoda

• Have bilateral symmetry

• Have a separate mouth and anus

• Have jointed body sections / appendages and have a hard exoskeleton (chitin)

• Examples include insects, crustaceans, spiders, scorpions and centipedes

Chordata

• Have bilateral symmetry

• Have a separate mouth and anus

• Have a notochord and a hollow, dorsal nerve tube for at least some period of their life cycle

• Examples include mammals, birds, reptiles, amphibians and fish (also invertebrate sea squirts)

Vertebrates classes

Fish - amphibian - reptile - birds - mammals

Fish

• Covered in scales made out of bony plates in the skin

• Reproduce via external fertilisation (egg and sperm released into the environment)

• Breathe through gills that are covered with an operculum

• Does not maintain a constant internal body temperature (ectothermic)

Amphibians

• Moist skin, permeable to gases and water

• Reproduce via external fertilisation (usually spend larval state in water, adult state on land)

• Can breathe through skin but also possess simple lungs

• Do not maintain a constant internal body temperature (ectothermic)

Reptiles

• Covered in scales made out of keratin

• Reproduce via internal fertilisation and females lay eggs with soft shells

• Breathe through lungs that have extensive folding (increases SA:Vol ratio)

• Do not maintain a constant internal body temperature (ectothermic)

Birds

• Covered in feathers (made out of keratin)

• Reproduce via internal fertilisation and females lay eggs with hard shells

• Breathe through lungs with parabronchial tubes

• Maintain a constant internal body temperature (endothermic)

Mammals

• Skin has follicles which produce hair made out of keratin

• Reproduce via internal fertilisation and females feed young with milk from mammary glands

• Breathe through lungs with alveoli

• Maintain a constant internal body temperature (endothermic)

Full classification of humans

Animalia - chordata - mammalia - primate - homonidae - homo - sapiens

Full classification of buttercups

Plantae - angiospermophyta - eudicotidae - ranunculales - ranunculae - ranunculus - acris

Polyploidy in allium

• More common in plants - may lack separate sexes or be capable of self-pollination

• other organsism would need 2 polyploid parents to be a fertile polyploid - must have an even number of n

• allium can be triploidy, tetraploidy, → 8n

• speciation can only occur if the polyploid is viable and fertile but cannot interbreed with the original population

Reclassification in taxonomy

• species may be separated or grouped together in genera due to new evidence

• eg. humans and chimps were placed into homonidae