Patterns of EvolutionExternal 3 Credits

Do Now• Define the following terms;

• Population• Species• Gene pool• Natural Selection

Answers• Population; is a group of individuals of the same species in an

area.• Species; is a groups of individuals who normally interbreed to

produce fertile offspring and who belong to the same gene pool.

• Gene pool; all the alleles available to the population of a species.

• Natural Selection; the process were the organsims with the best suited phenotype in a particular environment is select for (has increased survival)

Lesson Objectives• Review Yr 12 Evolution

A new idea?The idea that life has

evolved is not new and goes back to the civilisations of the ancient Greeks.

Christianity has a different perspective and is detailed in various passages in Genesis in the bible.

EVOLUTION• Macro-Evolution

Large changes in a gene pool over a long period of time, as in the formation of a new species, extiction and adaptive radiation

• Micro-Evolution

Small changes in the frequency of alleles in a gene pool over successive generations

Who evolves?• Individuals do not evolve – only populations

evolve.• All the genes in a population are called a gene

pool - the ratio of alleles and genotypes in a population can change over time.

• As this changes, so evolution occurs.

Write this down!!!

Sources of Variation1. Meiosis (covered in Yr 12)2. Mutations

Processes of Evolution3. Genetic Drift4. Founder Effect5. Bottleneck Effect6. Gene migration 7. Natural Selection

Meiosis• Independent Assortment• Segregation• Crossing over

What is a gene pool?

Remember!! All the alleles present at

all gene loci in all members

of a population

GENE FLOW (results from Migration)

Individuals migrate between populations. Immigrating individuals introduce new

alleles.Emigrating individuals remove alleles.

• Gene flow can change a gene pool due to the movement of genes into or out of a population

► Mutation changes alleles► Natural selection leads to differential reproductive success

Genetic Drift• A change in the gene pool of a small population due to chance

Genetic Bottleneck Founder Effect

• Genetic drift is a change in a gene pool due to chance• Genetic drift can

cause the bottleneck effect

There are several potential causes of microevolution

Figure 13.11A

Originalpopulation

Bottleneckingevent

Survivingpopulation

• Population bottleneck – genetic drift due to high mortality in a population.• Unlikely that gene pool of

the remaining population is representative of original population.

Decreased genetic diversity among Cheetahs.

•

Cheetahs underwent 2 population bottlenecks:1st during last ice age 2nd during nineteenth century due to excessive hunting.Today, just two isolated populations live in South & East Africa, numbering only in a few thousand animals between them. The South African cheetahs are so genetically alike that even unrelated animals can accept skin grafts from each other.

• or the founder effect

Figure 13.11B, C

Founder effect

• Equivalent to genetic drift due to a few individuals leaving a large population to found a new group. Unlikely that gene pool of founding population is representative

of original population.

Mainland Population Island Population

Direction of movement

Founder effect

28 61%

12 26%

6 13%

4 44%

5 56%

Founder effect• Thus isolated populations of a species may have very different

genes from the parent population and would therefore have a different susceptibility to the effects of natural selection on them at these new localities.

The Laysan Finch Story• Small population on Laysan Island and

of conservation concern.• A group of 10 males and 10 females

were captured and transported to a similar island 500 nm away.

• They were individually marked and samples of their DNA were taken prior to release.

• The introduced population thrived and samples of their DNA taken 5 years later showed a greater variation than the initial population.

• How come?

Laysan Island

New Island

500 nm

5 genes go with the originalpopulation.

2 new genes appear, giving a new total of 7 genes in the ‘new’population.

Non-random Mating Non-random mating causes certain alleles to

become more common in future generations (some individuals leave more offspring than others).

Gene migration (Gene Flow)• Most populations

are not closed systems.

• Immigration from other populations brings in new gene combinations.

• Those individuals, who leave the population (emigrate), take their genetic combinations with them.

• I=in e=exit

Mutation• A change in the DNA - introduces ‘new’ alleles into the population. Mutations can be beneficial, have no effect (silent) or be harmful.

Increase or decrease of genetic diversity???• Mutations and immigration increase genetic diversity.• Natural selection, emigration, non-random mating and genetic

drift decrease genetic diversity.

Important Slide!!!!Write this down!!!!!

Natural Selection The differential survival and reproductive success of

organisms whose genetic traits- PHENOTYPES increase their chance to survive and reproduce in a particular environment.

It is considered to be the major driving force of evolution.

Write this Down!!

Natural Selection Summary• Over-production of young• Competition• Genetic Variation (ie different phenotypes)• Differential Fitness (Reproductive Success)• Fittest outcompete others to pass their “fit/favourable”

genes onto their offspring• These “favourable” genes will then increase in frequency

Speciation• Speciation is the formation of a new species• Remember: A species is a group of organisms that normally

interbreed in nature to produce fertile offspring & belong to the same gene pool

• There are 2 types of speciation: Allopatric Speciation Sympatric Speciation

Allopatric speciation• Species can be allopatric – living in geographically

different areas.

Species A

Species B

Sympatric species• Species can by sympatric – living together in the

same geographical area.

Species A & B live in all areas in the same geographical area.

The mechanism of speciationAllopatric speciation

A single population occupying a uniform environment

This is how most species come about.

Migration into new environments on the edge of the distributionGives rise to subspecies as a result of different selection pressures.There is gene flow between all populations still.

Species undergoes an expansion of range

Further migration, environmental differences and thedevelopment of geographical barriers, gives rise to geographical isolation of some races and populations.This isolation halts gene flow between this andthe original population.

Vegetation change River course change

Selection the sameSelection the same

Some of the isolated populations develop genetic andchromosomal differences that no longer allow inter-breedingwith the parent population. The subspecies is geneticallyand geographically isolated from its ancestral population.

Selection the same Selection different

Different alleles beingselected for.

Further changes in the environment remove the geographicalbarrier and allow the groups to live side by side. There is nointerbreeding because some of the groups are nowreproductively isolated, due to the different selection pressuresthey have been exposed to.

Gene flow can occur between these populations still as they have been exposed to the same selection pressures.

There is NO gene flow betweenthese populations now,because of different selectionpressures resulting inGenetic changes.

These now become

Sympatric and Allopatric populations

Allopatric Speciation – an example

The mechanism of speciationSympatric speciation

There are very few authenticated reports of speciation by this route.If it does occur, it happens within one generation.

Starting with onepopulation

A very small portion of the population undergoes a random mutation, which gives them instantaneous reproductive isolation from the rest of the species. It has to occur in a male and female in the same generation and must confer an immediate evolutionary advantage and separation from the parent population.

Polyploidy• This is the abrupt and almost instantaneous formation of a

new species.• The main cause of this is a problem of separation of the

chromosomes at Meiosis into the gametes and we will deal with it later on.

• Rare in vertebrates but common in plants.

Aims for Today• To be able to explain the pre-zygotic and post-zygotic isolating

mechanisms that lead to speciation.

Formation of species• Prevention of gene flow between populations can result in the

formation of new species. • These are called isolating mechanisms.• There are pre and post-zygotic isolating mechanisms.

How does it happen?• Pre-zygotic isolating mechanisms prevent the fusion of

gametes to form a zygote.• Post-zygotic isolating mechanisms prevent the zygote from

developing further, if fertilization occurs.

Pre-zygotic mechanisms

Geographical barriers• Populations are

geographically isolated.

• Scale is important.

Pre-zygotic mechanisms

Ecological barriers• Live in different areas

with different temperatures, humidity, altitude tolerances etc.

e.g. Arctic Fox and Fennec Fox

Pre-zygotic mechanisms1. Habitat differences.2. Breeding season differences.3. Behavioural differences; territoriality & courtship and the context of

the displays.4. Mechanical differences.5. Mating takes place but no zygote formed (duck sperm does not

survive reproductive tract of a hen)

Post-mating mechanisms1. Hybrid inviability – zygote formed but it does

not develop.2. Hybrid sterility – hybrid forms but it is sterile –

mule.3. Hybrid breakdown – hybrid is fertile but

offspring cannot reproduce.

A lion x tiger hybridWeighs 450 kg and is 3 mFrom nose to tail.

We’ve talked about mulesWhat about a zeedonk?

Isolation by time

A species, which has gone extinct, can obviously not interbreed with a species existent today.

Isolating Mechanisms – an overview

Speciation• The formation of a new species - speciation and can

occur in the following ways:• Reduced selection pressure – a population moves into a

new area, where the selection pressures are different.• An increase in population results in the expression of

alleles, which were previously selected against.

More speciation

• Migration into new areas, which might have different selection pressures.

• Some isolated populations develop genetic and chromosomal differences that no longer allow interbreeding with the parent population.

Formation of species• Prevention of gene flow between populations can

result in the formation of new species.

• There are two main schools of thought as tohow this is achieved.

Amount of difference

Tim

ePhyletic gradualism – genetic change takes place gradually over a long time.

Amount of difference

Tim

ePunctuated equilibrium – rapid genetic change followed by long periods of stability.

Extinction

Patterns of Evolution• Sequential Evolution; species may accumulate genetic changes

that, over time, result in the emergence of what can be recongised as a different species.

• Coevolution; where two species reciprocally affect each other’s evolution.

• Convergent Evolution; Species from different evolutionary branches may come to resemble each other if they have similar ecological roles and natural selection has shaped similar adaptations.

• Divergent Evolution; the process where two species have diverged from one common ancestor. (most common form of evolution)

Convergent evolution • Species from different evolutionary branches may come to

resemble each other if they have similar ecological roles and natural selection has shaped similar adaptations.

Convergent evolution • Homologous-descended by inheritance from a common

ancestor e.g. the pentadactyl limbs shown in the diagram

Convergent Evolution• Analogous structures; similar function and often the same

superficial structure, but of different evolutionary origins

• E.g the wing of a bird and the wing of an insect.

Co-evolution

• Co-evolution is used to describe cases where two or more species reciprocally effect each other’s evolution.

• Each of the species involved exerts selective pressures on the other and over time the species develop a relationship that involves mutual dependency.

• Co-evolution is likely to occur when species have close ecological interactions with one another.

Co-evolution

• When New Zealand split away from Gondwana its insects did not include types of bees or any other insects that are attracted to bright colours.

• So….. Insects and plant needed to evolve adaptations that enabled the insects/birds present to obtain food from the plant while at the same time carrying pollen from flower to flower.

New Zealand’s pollinators

• As a result insect pollinated flowers in New Zealand flowers become dull in colours with strong nectar scents. This attracted small beetles, butterflies, moths and small bats.

• Several of the birds of the forest developed adaptations such as long, feathers tongues for feeding on nectar. At the same time some forest trees adapted to attract birds by evolving bright colour and nectar production. E.g Kowhai and tui’s

Coevolution

• Adaptive radiation is the diversification (both structural and ecological) among descendants of a single ancestral group to occupy different niches.

• In adaptive radiation is a pattern of evolution that involves an ancestral species evolving into a variety of new species, each adapted to survive in a different niche.

Adaptive radiation

Adaptive RadiationEXAMPLE: The radiation of the mammals occurred after the extinction of the dinosaurs, which has made niches available for exploitation.

Megazostrodon an early mammal ancestor

Flying predator/ frugivore niche

Freshwater predator niche

Underground herbivore niche

Marine predator niche

Arboreal herbivore niche

Terrestrial predator niche

Browsing/ grazing niche

Divergence and Radiationof the Ratites

All other living birds

Moa 1: Anomalopteryx

Moa 2: Pachyornis

Moa 3: DinornisMoa 4: Megalapteryx

Little Spotted Kiwi

Great Spotted Kiwi

Brown Kiwi

Emu

Cassowary

Ostrich

Elephantbird

Rhea 1

Rhea 2

Tinamou (can fly)

Mesozoic Era Cenozoic Era

Fossil evidence suggests that ratite ancestors possessed a keeled breastbone and an archaic palate (roof of mouth)

Ratites diverge from the line to the rest of the birds about 100

million years ago

Birds evolved from a dinosaur ancestor about

150 million years ago

6 important events

1. Isolation2. Mountains3. Sea level changes 4. Climate change5. North and South extent

Isolation• Isolation is a key reason why the endemic species of New

Zealand are so different.• 300 million years ago New Zealand was part of

Gondwanaland. Gondwanaland was made up of Africa, South America, Antarctica, India and Australia

• Initially New Zealand was under the sea and was attached to the eastern side of Australia

• 70 to 80 million years ago, New Zealand was pushed away from the Australia

Isolation

Isolation• When New Zealand sperated from gondwanaland only the

plants and animals present at the time of sepration could become the ancestors of the present species

• As a result New Zealand only had a small range of kinds of plants and animals

• These species eveloved in isolation for many millions of years

Mountian Building• 24 million years ago New Zealand was sitting on tectonic plate

boundary.• Pressure within the earth caused uplift into large mountians• This created a new alpine habitat for things already living

there• It also caused isolation which seprated different parts of the

country and weather patterns such as wetland on the west coast of the south island and the dry plains in Canterbury and Otago

Climate Changes• Mountian building and ice ages contributed to dramatic

changes in New Zealand’s climate• 20,000 year ago most fo New Zealand was covered in snow

fields, this caused• Some plant species to move north e.g nikau tree• Some plants to become extinct e.g. eucalypts• Some plants to evolve to better suit the conditions.

Changing Sea levels• At times changes in the sea level isolated or joined different

parts of the country• As recently as 12,000 years ago the cook strait was dry land

and island such as great barrier were part of the main land of New Zealand

• As the last ice age finished the sea level rose and separated the north and south island

North to South Extent• New Zealand is long and narrow, with varying climates all the

way along it.• This enabled tropical species to evolve in the north and alpine

species to evolve in the south.

• There are more than 80 species of Hebe in New Zealand.• Most species are restricted by their adaptations to very

specific areas.• The original New Zealand Hebe was probably a shrub with

normal-sized leaves in an alternate pattern with pale flowers.• Today there are three main groups of hebes.

New Zealand Hebe

• Large hebes are most like the ancestrial hebe.• Leaves are untoothed and either broad or narrow but never

overlap.• Flowers are pale and larger than the leaves• Found in lowland shrub, on the coast, in forest margins• NOT FOUND IN EXTREME ENVIRONMENTS

Large –Leafed hebe

• Have features that would help the plant to withstand dry, arid conditions with wind and cold.

• Toothed, fleshy leaves• Leaves are flat and concave and are short and closely

set……………………• Flowers are spikes crowded together• Found in sub-alpine regions, mainly on rocks

Medium-leafed hebes

• Have adaptations to withstand cold and snow and are able to survive the harsh conditions of bare rock

• Plants are small and spreading………..• Leaves are very small, overlapping and tough…………..• Flowers have only a few spikes, crowed near the tips of the

branches.

Small-leafed hebes