Change in Populations. Evolutionary Change Evolution is the genetic change in a gene pool of a population over time. A gene pool is all the genes (alleles)

Change in Populations

Evolutionary Change

• Evolution is the genetic change in a gene pool of a population over time.

• A gene pool is all the genes (alleles) in a population or species.

• During the course of evolution, organisms respond to environmental changes, some survive and pass on genes, but others do not.

• The particular allele carried by the most successful individuals will increase in frequency over time. (A frequency is a percentage expressed as a decimal e.g. 100%=1).

• Shifting allele frequencies in a population are what drive evolutionary change.

Evolutionary Change

• Nearly all populations show variation between individuals for particular traits.

• Population variation in a trait is called polymorphism.

• Mutation and sexual reproduction both increase genotypic variation.

• Genotypic variation in turn gives rise to phenotypic variation.

• Evolutionary mechanisms act on this phenotypic variation resulting in changes in allelic frequency.

Modern Theory of EvolutionThe following points summarize the modern theory of evolution:• Reproduction – reproduction of organisms in a population produces descendant

populations.• Excess of potential offspring – parents have the potential to produce many more

offspring than actually survive.• Variation – members of a population vary. Variation that is genetically based is

passed onto offspring.• Selection – environmental resources, such as food and nest sites, are limited, so

there is competition between individuals. Individuals that can compete successfully will leave a greater proportion of offspring than less successful individuals. The limiting factor acts as a selection pressure.

• Adaptation over time – environments change over time. Heritable characteristics that suit a particular environment will be selected. Populations divide over time and become adapted to new conditions.

• Chance effects – in small populations, shifts in the frequency of certain characteristics can also occur by chance.

• Divergence and speciation – when populations are geographically isolated and thus cannot interbreed, divergence over time may result in them becoming different species.

Selection• Selection is the term used for survival of the fittest. • Selection occurs when some individuals, with particular favourable

features, have a greater chance than others of leaving fertile offspring.

• Survival of the fittest means that those phenotypes that are best suited to the environment are more likely to survive.

• Biological fitness is measured by the relative proportion of fertile offspring left by an individual leaves in the next generation.

• The inherited characteristic that allows the individual to survive and reproduce is called an adaptation.

• Fitness is a relative characteristic in that a phenotype that suits one type of environment may not suit another. Human example of this is predominance of sickle-cell anaemia gene in areas where malaria is present.

• Selection does not generate variation – it can only act on the variation already present in the population.

Selection in cattle tick populations• The cattle tick (Boophilus microplus) is a major pest for graziers in

northern Australia as it carries the protozoan parasite that causes tick fever in cattle.

• The first cattle dips to control fleas were arsenic-based, but 1936, the tick populations were showing resistance to this poison.

• DTT was introduced by 1946, but by early 1960s, however, the tick populations had become resistant to DDT insecticide.

• Organophosphate pesticides were next to be introduced and by the 1980s, resistance to many of these compounds had developed in tick populations.

Selection in cattle tick populations

Why did resistance occur?• Resistance to each chemical pesticide involves a different gene. • Before a particular pesticide was widely used, the frequency in the cattle tick

gene pool of the various alleles that confer resistance to these pesticides was low.

• Once a pesticide became widely used, however, it acted as a selecting agent that produced an increase in the survival and reproduction rates of resistant ticks relative to those of non-resistant ticks.

• As a result of this selection, the proportion of resistant ticks increased and the frequency of the resistant allele in the population increased.

• This re-occurred in turn with different alleles as each new pesticide was introduced.

• The pesticide does not produce the resistant allele, instead it selects for the allele by creating an environment in which resistant members of the tick population have a greater genetic fitness than the non-resistant variety.

Natural Selection

• Depends on selection by the environment.• Factors that can act as agents of natural

selection that can act on populations include: – physical agents, such as climate change or food

shortages– biological agents such as an infectious disease or

predation – chemical agents such as a pollutant in soil or water.

• Natural selection is not directed towards a particular goal.

Artificial Selection• Occurs when humans deliberately select particular plants or animals

to breed for a specific purpose.• When farmers, horticulturalists, horse breeders or geneticists

deliberately select particular organisms from a population to be the parents of the next generation, a process of artificial selection occurs.

• Under conditions of artificial selection, the parents that contribute to the next generation may not be the most fit in a genetic sense.

• For example, certain breeds of dog have been artificially selected to show phenotypes that in the wild would have low survival rates and fitness values. – The dwarf legs and long bodies of Dachshunds whose dwarf legs and

long bodies make them prone to dislocation of the spine, giving them a lower survival rate in the wild.

– hairless breeds of dogs would also be expected to have lower survival value in the wild, particularly in low temperatures

Not all evolutionary change is the result of selection• Gene flow is the migration of alleles from one population.

– Genetic variation may result from genetic exchange between populations when animals migrate between populations or when seed and pollen are dispersed.

– The Duffy blood group is an example of gene flow in the human population. The Fya allele has steadily increased in frequency in African-Americans in the northern states where marriages between black and white Americans occur more often compared to the southern states.

• Genetic drift is the change in frequency of alleles from generation to generation due to chance alone.

– It is a random process which in small, isolated populations can cause an evolutionary change referred to a as the founder effect.

– Perhaps the best example is the atoll of Pingelap in the Pacific Ocean. – Five percent of the population suffer from a form of colour-blindness called

achromatopsia. This figure is much higher than other populations as achromatopsia is a recessive autosomal disorder.

– The reason? In 1775 the atoll only had a population of 30 due to famine and a typhoon. Genealogical analysis shows that one of these people was a chief who had many children, and who also happened to be heterozygous for achromatopsia.

Patterns of Evolution

• Through the action of natural selection, gene flow or chance effects (in small populations), allele frequencies in populations may change over time.

• Evolution is defined as a change in the gene pool of a population over time.

• Patterns of evolution include:– divergent evolution– convergent evolution– parallel evolution.

Divergent Evolution• Is the evolution of different species (populations) from a common

ancestral species (population).• The homologous features shared by the species become different

over time due to natural selection or genetic drift.• Darwin’s finches are a classic example of divergent evolution and

natural selection.• Adaptive radiation is the rapid divergence of an evolutionary

lineage from a recent common ancestor.• Darwin’s finches are thought to have undergone this type of

evolution as the adapted to different habitats and different islands in the absence of competitors.

Divergent Evolution

Convergent Evolution• Is the independent development of

similar features in unrelated species or populations. The species become alike over time (converge).

• Natural selection may lead them to evolve one or more similar features which are analogous.

• The eyes of vertebrates and octopuses are a striking example of convergent evolution.

Parallel Evolution • Is similar to convergent evolution except that it occurs

when related species evolve similar features independently.

• The waxy coating evolved by a number of eucalypt species to protect themselves from frost damage is an example of parallel evolution.

• The backward-opening pouches in the Australian native marsupials, the marsupial mole (Notoryctes typhlops) and the bilby (Macrotislagotis), which reflect their burrowing behaviours

Co-evolution

• Refers to changes in two different species that have a close interaction, such as a parasite and its host, a predator and its prey or two species that have an interaction of mutualism.

• For example, bird-pollinated flowering plants and the birds that pollinate them have evolved features that favour their mutually beneficial interaction. – The flowers, for example, have evolved colours, shapes and

other features, such as nectar production, that attract and reward the birds.

– The birds, in turn, have evolved specialised features such as long bills that equip them to feed on the nectar while gathering pollen on their heads.

Speciation• Evolution by natural selection can result

in changes within members of one species and, if sufficient time elapses, can lead to the formation of new species.

• The process of formation of a new species is known as speciation.

• For living, sexually reproducing organisms, species are recognised as different when they are not able to interbreed.

• Two patterns of speciation are possible:– phyletic evolution in which one

population progressively changes over time to become a new species

– branching evolution in which a population of one species splits and one part of the population evolves separately to form a new species distinct from the original species

• A pattern of branching evolution is more common than phyletic evolution.

Branching EvolutionBranching evolution typically involves:1. splitting of a smaller group from an original population so that the

small group becomes geographically isolated2. over many generations, the isolated population is subjected to

different selection pressures because of different environmental conditions and to other change factors such as genetic drift

3. the isolated population changes over time such that, even if the two populations were to come together again, their members are unable to reproduce successfully — they are now two distinct gene pools and two different species

Two models of speciation

• There are two general models of speciation: the gradual and rapid model.:

• Gradual Model– The original population of a species is split by a physical

barrier into two subpopulations of approximately equal size. – Each subpopulation gradually diverges to form a new species.

(This is Darwin’s model). – The two separated species diverge because their

environments differ and they are under different selection pressure.

• Rapid Model– A small population is isolated at the edge of the range of the

original species by a physical barrier. – Rapid divergence to form a new species is possible in the

small population.

Two models of speciation

(a) Gradual model of speciation (b) Rapid model of speciation

Microevolution vs Macroevolution

The two models of speciation reflect two different views of evolution:

• Macroevolution (gradual evolution)– Given enough time, small micro-evolutionary processes are sufficient to account

for large evolutionary changes – Over long periods of time new species will give rise to new genera, families,

orders and phyla

• Microevolution (punctuated equilibrium)– Natural selection is the force behind permanent gene pool changes and “fine

tunes” organisms to their environment however most morphological changes occur during abrupt, chance speciation events and once in existence, species then change very little.

• There is no doubt that evolution occurs, the debate is only about the relative importance of each of these mechanisms.

More on Speciation

• Speciation occurs when gene flow has ceased between populations where it previously existed.

• Speciation is brought about by the development of reproductive isolating mechanisms which maintain the integrity of the new gene pool.

• There are two main types of speciation thought to operate in populations:– Allopatric speciation– Sympatric speciation

Allopatric Speciation

• Occurs when one population becomes geographically separated from the rest of the species and involves in isolation

• Thought to be the most common form of speciation and has been the most important in the evolution of new animal species

Sympatric Speciation

• Occurs when a population forms a new species within the same area as the parent species.

• This gene pool divergence is most common in plants which are not mobile and can form ‘instant’ species through polyploidy.

• May also occur as different groups or members within a population prefer different microhabitats and begin to exploit these. Increasing adaptation to different micro niches results in the different groups diverging and eventually becoming reproductively isolated.

Races and Geographic Variation• A species that is geographically widespread usually

occurs in numerous local populations, each physically separated from one another.

• Natural selection and genetic drift can lead to divergence of populations, leading to the evolution of geographic races or subspecies within a species.

• Differences between races depend on how much gene flow occurs between them.

• Definition:– Races or subspecies are populations within a species that show

genetic differences across a geographic range.

Races and Geographic Variation• Australian eastern rosellas have three separate geographic races

between Tasmania and Cape York.• Each race has slightly different appearances, however in areas

where the races overlap, birds show intermediate colouration indicating the races interbreed.

Reproductive isolating mechanisms

• Prevent successful breeding between different species – they are barriers to gene flow

• A single barrier may not completely isolate a gene pool, but most species have more than one isolating mechanism operating to maintain a distinct gene pool.

• Geographical barriers prevent species interbreeding but are not considered to be RIMs because they are not operating through the organisms themselves.

• Reproductive isolating mechanism can be categorised according to when and how they operate:– Pre-zygotic (pre-fertilization) mechanisms

– Post-zygotic (post-fertilization) mechanisms

Pre-zygotic isolating mechanisms

• Ecological or habitat– Different species may occupy different habitats within the same region e.g.

aquatic and terrestrial species

• Temporal– Species may have different activity patterns – they may be nocturnal or diurnal,

or breed at different seasons

• Behavioural– Species may have specific calls, rituals, postures etc. which enable them to

recognise potential mates

• Structural– For successful mating, species must have compatible copulatory apparatus,

appearance and chemical make-up (odour, chemical attractants)

• Gamete Mortality– If sperm and egg fail to unity, fertilization will be unsuccessful

Post-zygotic isolating mechanisms

• Zygote mortality– The fertilized egg may fail to develop properly

• Reduced reproductive success– Fewer young may be produced and they may have a low viability

(‘survivability’)

• Hybrid sterility– The hybrid of two species may be viable but sterile – unable to breed

e.g. the mule

• Hybrid breakdown– The first generation may be fertile but subsequent generations are

infertile or non-viable

Problems with the species definition

• A biological species is a group of interbreeding (or potentially interbreeding) individuals, reproductively isolated from other such groups.

• This does not apply in all situations:– The concept of a species being able to interbreed cannot apply to

extinct populations because this is unknown – extinct forms must be classified on purely morphological grounds (unless DNA sequence is available).

– Asexually reproducing organisms do not breed with each other and so are assigned to species on the basis of appearance or biochemistry

• Even for sexually reproducing organisms, a species may grade in phenotype over a geographic area. Such continuous gradual change is called a cline and often occurs along the length of a country or continent. All the populations are the same species as long as interbreeding populations link them.

Extinction

• Extinction is the evolutionary process where species and groups of species eventually die out.

• Most species that have ever lived are now extinct.

• Often the extinction of one group has allowed another to undergo extensive radiation into free niches – large scale species changes are probably opportunistic events.

• Radiations may follow extinctions but are rarely the cause of extinctions.

Background Extinction

• Is the average rate of natural loss of species due to environmental change and/or competition.

• The average life of a species varies depending on the type of organism, but is generally a few million years.

• Generally larger and more complex organism have higher rates of background extinction than simpler organisms.– Average species duration for moss and liverworts is 20+ million

years– Average species duration for snails is 10-13 million years– Average species duration for mammals is 1-2+ million years

Mass Extinction• Superimposed on average rates of background extinction are five events in

the Earth’s history when the rate of extinction rose markedly – called Mass Extinctions

• Mass extinction refers to an abrupt increase in extinction rates affecting huge numbers of species at the same time.

• Mass Extinctions are marked by rapid rate of extinction of groups of organisms in the fossil record.

• During these episodes the diversity of major groups declines.

• Examples include:– The coming together of all continents during the Permian period, resulting in a

harsher climate and loss of coastal seas.– An asteroid impact that occurred near the Yucatan Peninsula of Mexico is

thought to have caused the extinction of when more than 70 per cent of Earth’s marine and terrestrial species, including the dinosaurs, 65 million years ago. Traces of this impact are preserved in the 180-kilometre wide Chicxulub crater that was caused by an asteroid estimated to be 10 to 20 kilometres in diameter.

Humans as agents of extinction• Humans have caused an increase in the number

of extinctions. • Animals that have become extinct as a result of

human activity include the dodo, the passenger pigeon and the Tasmanian tiger.

• There have been about 500 animal and 600 plant extinctions recorded globally since 1600.

Genetic bottlenecksNot quite extinction!!!!

• Genetic bottlenecks occur when a species declines in number and therefore genetic variation decreases.

• Even if the species recovers, there is less genetic variation.

• Cheetahs are a good example – genetic studies suggest all cheetahs descend from a single pregnant female.

Forces Operating in EvolutionMolecular Level• Point mutations• Control of gene expression• Rate of protein synthesis

Chromosomal Level• Crossing over• Block mutations• Polyploidy• Aneuploidy• Independent assortment• Recombination

Cellular Level• Gamete viability• Fertilization• Genotype expression

Organism Level• Environmental modification of

phenotype• Reproductive success of phenotype• Selection pressures• ‘Fitness’ of phenotype

Population Level• Genetic drift and population size• Natural selection altering gene

frequencies • Mate selection• Intra-specific competition• Founder effect• Immigration/emigration

Species Level• Geographical barriers• Reproductive isolation (pre-zygotic and

post-zygotic)• Selection pressures• Inter-specific competition