Selection and Selection and EvolutionEvolution

Nature encourages no looseness, pardons no errors- Ralph Waldo Emerson

I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural

Selection. - Charles Darwin, The Origin of Species

Natural selectionNatural selection

Darwin's theory of evolution has four main Darwin's theory of evolution has four main parts:parts:Organisms have changed over time, Organisms have changed over time, All organisms are derived from common All organisms are derived from common ancestors by a process of branching. ancestors by a process of branching. Change is gradual and slow, taking place Change is gradual and slow, taking place over a long time. over a long time. The mechanism of evolutionary change The mechanism of evolutionary change was natural selection. was natural selection.

The Process of Natural SelectionThe Process of Natural SelectionNatural selection is a process that occurs over Natural selection is a process that occurs over successive generations. successive generations. If all the offspring that organisms can produce were to If all the offspring that organisms can produce were to survive and reproduce, they would soon overrun the survive and reproduce, they would soon overrun the earth. earth. This unbounded population growth resembles a simple This unbounded population growth resembles a simple geometric series (2-4-8-16-32-64..) and quickly reaches geometric series (2-4-8-16-32-64..) and quickly reaches infinity.infinity.There is a "struggle" to survive and reproduce, in which There is a "struggle" to survive and reproduce, in which only a few individuals succeedonly a few individuals succeedOrganisms show variation in characters that influence Organisms show variation in characters that influence their success in this struggle for existence. their success in this struggle for existence. The population in the next generation will consist of a The population in the next generation will consist of a higher proportion of individuals that possess whatever higher proportion of individuals that possess whatever adaptation enabled their parents to survive and adaptation enabled their parents to survive and reproduce.reproduce.

For natural selection to occur, two requirements For natural selection to occur, two requirements are essential:are essential:

There must be heritable variation for some trait. There must be heritable variation for some trait. Examples: beak size, color pattern, thickness of Examples: beak size, color pattern, thickness of skin, fleetness.skin, fleetness.

There must be differential survival and There must be differential survival and reproduction associated with the possession of reproduction associated with the possession of that trait.that trait.

Unless both these requirements are met, Unless both these requirements are met, adaptation by natural selection cannot occur. adaptation by natural selection cannot occur.

Some examples:Some examples:

If some plants grow taller than others and so are better If some plants grow taller than others and so are better able to avoid shading by others, they will produce more able to avoid shading by others, they will produce more offspring. However, if the reason they grow tall is offspring. However, if the reason they grow tall is because of the soil in which their seeds happened to because of the soil in which their seeds happened to land, and not because they have the genes to grow tall, land, and not because they have the genes to grow tall, than no evolution will occur.than no evolution will occur.If some individuals are fleeter than others because of If some individuals are fleeter than others because of differences in their genes, but the predator is so much differences in their genes, but the predator is so much faster that it does not matter, then no evolution will occur faster that it does not matter, then no evolution will occur (e.g. if cheetahs ate snails).(e.g. if cheetahs ate snails).In addition, natural selection can only choose among In addition, natural selection can only choose among existing varieties in a population.existing varieties in a population.

Evidence of natural selectionEvidence of natural selectionIndustrial melanism in the peppered moth, Industrial melanism in the peppered moth, Biston Biston betulariabetularia. . Prior to 1800, the typical moth of the species had a light Prior to 1800, the typical moth of the species had a light pattern.pattern.During the Industrial Revolution, soot and other industrial During the Industrial Revolution, soot and other industrial wastes darkened tree trunks and killed off lichens. The wastes darkened tree trunks and killed off lichens. The light-colored morph of the moth became rare and the light-colored morph of the moth became rare and the dark morph became abundant.dark morph became abundant.In 1819, the first melanic morph was seen; by 1886, it In 1819, the first melanic morph was seen; by 1886, it was far more common -- illustrating rapid evolutionary was far more common -- illustrating rapid evolutionary change. change. Eventually light morphs were common in only a few Eventually light morphs were common in only a few locales, far from industrial areas. The cause of this locales, far from industrial areas. The cause of this change was thought to be selective predation by birds, change was thought to be selective predation by birds, which favored camouflage coloration in the moth.which favored camouflage coloration in the moth.

Galapagos finches are the famous example from Galapagos finches are the famous example from Darwin's voyage. Darwin's voyage. Each island of the Galapagos that Darwin visited had its Each island of the Galapagos that Darwin visited had its own kind of finch (14 in all), found nowhere else in the own kind of finch (14 in all), found nowhere else in the world. world. Some had beaks adapted for eating large seeds, others Some had beaks adapted for eating large seeds, others for small seeds, some had parrot-like beaks for feeding for small seeds, some had parrot-like beaks for feeding on buds and fruits, and some had slender beaks for on buds and fruits, and some had slender beaks for feeding on small insects feeding on small insects Each was slightly modified from an original colonist, Each was slightly modified from an original colonist, probably the finch on the mainland of South America, probably the finch on the mainland of South America, some 600 miles to the east. some 600 miles to the east. It is probable that adaptive radiation led to the formation It is probable that adaptive radiation led to the formation of so many species because other birds were few or of so many species because other birds were few or absent, leaving empty niches to fill; and because the absent, leaving empty niches to fill; and because the numerous islands of the Galapagos provided ample numerous islands of the Galapagos provided ample opportunity for geographic isolation. opportunity for geographic isolation.

Environmental factorsEnvironmental factors

In a statistical way: Suppose that each In a statistical way: Suppose that each population can be portrayed as a frequency population can be portrayed as a frequency distribution for some trait -- beak size, for distribution for some trait -- beak size, for instance. instance. What will the frequency distribution look like in What will the frequency distribution look like in the next generation?the next generation?There are four possible answers:There are four possible answers:– Directional selectionDirectional selection– Disruptive selectionDisruptive selection– Stabilizing selectionStabilizing selection– Balancing selectionBalancing selection

Directional Selection

If there is a range of phenotypes in a population, one is likely to dominate. If conditions change to favour one of the others, then the balance will shift.

Disruptive Selection

Stabilizing SelectionWhen characteristics on either side of the bell shaped curve of distribution are selected against, the graph becomes narrower and taller.

Variation in the population is actually reduced.

Balancing SelectionBalancing Selection

Sickle cell anaemia and malariaSickle cell anaemia and malaria

Three groups of people in malaria areas:Three groups of people in malaria areas:Normal haemoglobin may contract malaria and die Normal haemoglobin may contract malaria and die (Hb(HbNN Hb HbNN homozygous) homozygous)Sickle cell will die from sickle cell anaemia (HbSickle cell will die from sickle cell anaemia (Hbnn HbHbnn homozygous) homozygous)BUT heterozygotes (HbBUT heterozygotes (HbNN Hb Hbnn) will survive as they ) will survive as they are less likely to suffer from a malarial attackare less likely to suffer from a malarial attack

The heterozygote is therefore fitter than the rest and The heterozygote is therefore fitter than the rest and will survive to pass on the sickle cell trait.will survive to pass on the sickle cell trait.

SpeciationSpeciation

Speciation refers to the evolutionary process by Speciation refers to the evolutionary process by which new biological species arise.which new biological species arise.

For speciation to occur the "parent" species For speciation to occur the "parent" species must somehow become separated into two must somehow become separated into two breeding groups that cannot interbreed. breeding groups that cannot interbreed.

There are two geographic modes of speciation in There are two geographic modes of speciation in nature: allopatric and sympatric.  nature: allopatric and sympatric.  

Allopatric SpeciationAllopatric Speciation

During allopatric speciation, a population splits During allopatric speciation, a population splits into two geographically isolated populations into two geographically isolated populations They undergo genotypic and/or phenotypic They undergo genotypic and/or phenotypic divergence as: divergence as: – (a) they become subjected to dissimilar selective (a) they become subjected to dissimilar selective

pressures; pressures; – (b) they independently undergo genetic drift; (b) they independently undergo genetic drift; – (c) different mutations arise in the two populations. (c) different mutations arise in the two populations.

When the populations come back into contact, When the populations come back into contact, they have evolved so they are reproductively they have evolved so they are reproductively isolated and are no longer capable of isolated and are no longer capable of exchanging genes.exchanging genes.

Sympatric SpeciationSympatric Speciation

In Sympatric Speciation the separation is In Sympatric Speciation the separation is non-geographical but more "behavioural" non-geographical but more "behavioural" or "social“ due to change in genetics.or "social“ due to change in genetics.Sympatric speciation is more common Sympatric speciation is more common in plants. For instance, parent plants in plants. For instance, parent plants produce offspring that are polyploid. produce offspring that are polyploid. Hence, the offspring live in the same Hence, the offspring live in the same environment as their parents but are environment as their parents but are reproductively isolated. reproductively isolated.

Artificial selectionArtificial selection

Particular individuals are chosen and Particular individuals are chosen and allowed to breed.allowed to breed.

The alleles favoured by humans are The alleles favoured by humans are therefore passed on.therefore passed on.

It is an example of directional selection.It is an example of directional selection.

Eg. wheatEg. wheat

Desirable characteristics:Desirable characteristics:– High yieldHigh yield– Short stem lengthShort stem length– Pest resistancePest resistance– High protein contentHigh protein content