EVOLUTION BY NATURAL SELECTION

Evolution – Change over time. It the Process by which modern

organisms has descended from ancient species.

In 1859, On the Origin of Species, Darwin proposed a mechanism

for evolution called natural selection.

He describe Natural selection is the process by which genetic

mutation that lead to selective advantages and increased

fitness become, and remain, more common in successive

generation of a population. Natural selection operates on the

phenotypes of individuals produced by their particular

combination of allele.

Darwin’s process of natural selection has four components.

Variation.  Organisms (within populations) exhibit

individual variation in appearance and behavior.  These

variations may involve body size, hair color, facial

markings, voice properties, or number of offspring.  On the

other hand, some traits show little to no variation among

individuals—for example, number of eyes in vertebrates. 

Inheritance.  Some traits are consistently passed on from

parent to offspring.  Such traits are heritable, whereas

other traits are strongly influenced by environmental

conditions and show weak heritability.

High rate of population growth.  Most populations

have more offspring each year than local resources

can support leading to a struggle for resources. 

Each generation experiences substantial mortality.

Differential survival and reproduction.  Individuals

possessing traits well suited for the struggle for local

resources will contribute more offspring to the next

generation. 

Natural selection describes how populations, over time,

adapt to their environments. Adaptation is based on

the generational selection of certain beneficial alleles

that will, over time, increase in frequency (and

conversely, less advantageous alleles will decrease in

frequency). For example, let's say in a certain species

of bird there is an allele that leads to curly wing

feathers. While these curly feathers may be nice to

look at, they may not lead to an increase in fitness.

On the contrary, these curly-feathered birds are

likely poor flyers and they do not live sufficiently

long enough in the wild to reproduce. As a

consequence, the curly-wing allele is not very

prominent in the population because birds

having this allele don't live long enough to breed

and contribute the allele to the next generation.

There are main three types of natural selection:

Directional selection:

Directional selection is associated with gradually changing conditions

where the adaptive phenotype is shifted in one direction and one

aspect of a trait becomes emphasize. So, in Directional selection

Smaller individuals may have higher fitness (i.e. produce more

offspring) than larger individuals. It favors smaller individuals and will, if

the character is inherited, produce a decrease in average body size.

Directional selection could, of course, also produce an evolutionary

increase in body size if larger individuals had higher fitness.

An example of directional selection: pink salmon

Pink salmon (Onchorhynchus gorbuscha) in the Pacific Northwest

have been decreasing in size in recent years. In 1945,

fishermen started being paid by the pound, rather than per

individual, for the salmon they caught and they increased the

use of gill netting, which selectively takes larger fish. After gill

netting was introduced, smaller salmon had a higher chance of

survival; the selection favoring small size in the salmon

population was intense, because fishing is thorough: about 75 -

80% of the adult salmon swimming up the rivers under

investigation were caught in these years. The average weight of

salmon duly decreased, by about one-third, in the next 25 years.

Figure: the graph shows the decrease in size of pink salmon in two rivers in British Columbia. Two lines are drawn for each river: one for the salmon caught in odd-numbered years, the other for even years. Salmon caught in odd years are presumably heavier because of the two year life cycle of the salmon. From Ricker (1981).

Professor E. W. Bride wrote in "Nature":

Natural Selection' affords no explanation…of any…form of

evolution. It means nothing more than 'the survivors survive.'

Why do certain individuals survive? Because they are the fittest.

How do we know they are the fittest? Because they survive.

In his book he describes neither natural selection nor mutations

introduce any new genetic data into the organism's DNA.

Natural selection only selects out the disfigured, weak, or unfit

individuals of a population. It cannot produce new species, new

genetic information, or new organs and thus cannot make

anything evolve.

One commonly cited evidence for evolution is the development

of resistance in bacteria against antibiotics. Fifty years ago,

penicillin killed many types of disease-causing bacteria.

However, it is not that effective today. The development of

resistance against antibiotics is hypothetically direct

evidence for evolution.

The magazine Scientific American has to say the following in

its March 1998 issue: "Many bacteria possessed resistance

genes even before commercial antibiotics came into use."

Before the development of penicillin, some bacteria species

were already resistant. Many were not. After penicillin was

used against the bacteria, the non-resistant bacteria were

killed. The resistant bacteria survived and reproduced to

produce more resistant bacteria. The population increase

of resistant bacteria is not evolution. A new species of

bacteria did not evolve. The bacteria are still the same

species as they were before. What happened was only a

weeding out of non-resistant bacteria.

Evolutionists frequently point to the development of antibiotic

resistance by bacteria as a demonstration of evolutionary

change. However, molecular analysis of the genetic events

that lead to antibiotic resistance do not support this common

assumption. Many bacteria become resistant by acquiring

genes from plasmids or transposons via horizontal gene

transfer. (Is Bacterial Resistance to Antibiotics an Appropriate Example

of Evolutionary Change? Kevin L. Anderson; Creation Research Society

Quarterly; volume 41, No. - 4; March 2005)

Disruptive selection

Diversifying selection (also referred to as disruptive

selection) favors individuals at both extremes of the

phenotypic range, usually during periods of environmental

change. Disruptive selection could favor both extremes

over the intermediate types. Disruptive selection favours

individuals with variation at opposite extremes of trait over

individual with intermediate variations. Sometimes

environmental conditions may favour more than one

phenotypes. It is associated with a fluctuating environment

and gives rise to balance polymorphism in the population.

Imagine a population in which the main food supply

has decreased. Because less preferred food exists,

the variants of both extremes may be able to utilize

different food supplies, whereas intermediates may

not be able to adjust. Selection against

intermediates would result, due to the decrease in

available preferred food.

AN EXAMPLE OF DISRUPTIVE SELECTION: BRISTLES ON FRUITFLIES.

In nature, sexual dimorphism is probably a common example

of disruptive selection; but here we use an experiment by

Thoday and Gibson on the Drosophila melangaster fruitfly

as an example. Thoday & Gibson (1962).

Thoday and Gibson bred from fruitflies with high, or low,

numbers of bristles on a certain region of the body;

individuals with intermediate numbers of bristles were

prevented from breeding. As the graph shows, after 12

generations of this disruptive selection, the population had

noticeably diverged.

Figure: experimental

disruptive selection on

sternopleural bristle

number in fruitflies.

Individuals with many or

few bristles were allowed

to breed, those with

intermediate numbers

were not; and the

population rapidly

diverged.

www.execulink.com/~ekimmel/mixed_flash.htm

Tutorial 23.1 Natural Selection bcs.whfreeman.com/thelifewire/content/chp23/2302001.html 

Three Modes of Natural Selection wps.pearsoncustom.com/wps/media/objects/3014/.../17_A02.swf

SEXUAL SELECTIONDefinition: Sexual selection involves any features physical or

behaviour that affect reproductive advantage over members of the same sex. It has led to sexual dimorphism which is the differences between male and females in the same species.

SEXUAL SELECTION

Two types of sexual selection(i) Intrasexual : where male compete with one

another for access to females or for resources required by females

(ii) Intersexual: when females decide on which mate to choose base on personal attributes such as song morphology or coloration.

SEXUAL SELECTIONIntrasexualWhere males compete with one another. Main forms of competition are:

- Contests- Scrambles- Endurance rivalries

SEXUAL SELECTIONIntersexual- Where female choose. - In 95% of species females spent a lot more time

raising offspring than males so they have a lot more vested in who they choose as a mate.

Tungara Frog

SEXUAL SELECTIONSEXUAL DIMORPHISMWhy are males and females so different in many

species? Darwin believed that sexual selection would account for differences in colouring in both sexes.

THE PEACOCK

SEXUAL SELECTIONNATURAL SELECTION

VS SEXUAL SELECTION

- Natural selection deals with forces between species while sexual selection deals with forces

within a species - Sexual selection focuses on reproduction rather

than on survival while natural selection relies on both.