Evolution Topic 5

Evolution = the cumulative change in the heritable characteristics of a population Measured by change in frequency of alleles in a population New species arise when members of a population evolve to a point where they can no longer mate and reproduce with members of the old population

If we accept not only that species can evolve, but also that new species arise by evolution from pre-existing ones, then the whole of life can be seen as unified by its common origins Variation within our own species is the result of different selection pressures operating in different parts of the world

Evidence for evolution Fossil record A fossil is the ancient preserved remains of an organism. The fossil can be dated from the age of the rock formation. Sequences of fossil can show the gradual change of an organism over geological time (the farther down a fossil is buried, the older it is) Continuous fossil records are rare with most containing large time gaps until subsequent discoveries are made.

Man has selectively breed animals and plants for thousands of years. Selective Breeding (aka artificial selection) Man has selectively breed animals and plants for thousands of years. If an animal posses a characteristic that is considered useful or valuable then this animal is selected for breeding. The hope then is that this characteristic will be present in the next generation and at a higher frequency than before. In subsequent generations it may even then be possible to select from an even more advantageous characteristic.

For example: In a population of cows (generation 1 = G1) it is noticed that some produce more milk during lactation than others. These cows are selected for breeding and the other low milk yield cows are rejected for breeding. The calves of these high milk yields cows (G2) are then produced and once mature they themselves will have calves.

These now mature cows (G2)will be producing on average, a higher yield of milk than G1 cows. The G2 population of cows will show variation in milk yield. The breeder will select higher yielding G2 cows for the next breeding population. The cycle is repeated until the cow population is producing very large yields of milk way beyond level seen in the G1 population.

Homologous Structures All of life is connected through evolutionary history and consequently those organisms more closely connected might reasonably be expected to share common structures or homologous. Groups of organisms that are closely related may share traits or common body structures – homologous structures - which have been inherited from the common ancestor.

Darwin and his travels H.M.S. Beagle - Galapagos Islands Varieties of animals Wrote “Origin of Species” Natural Selection The process in which Evolution occurs

Natural Selection Populations tend to produce more offspring than the environment can support The consequence of overproduction of offspring is a struggle for survival based on competition for resources Members of a species show variation of behaviors and physical traits

This variation is due to genetic differences (different alleles) created through new alleles being formed as a result of beneficial mutations (less common) New combinations of alleles created through meiosis and sexual reproduction (crossing over, random fertilization, independent assortment)

Those individuals whose variation helps them best compete for resources are most likely to survive and reproduce themselves Therefore the traits that are most advantageous to a particular environment will tend to increase in frequency within the population over time

The alleles that code for the advantageous trait will correspondingly increase in frequency over time It is this cumulative change in the frequencies of alleles that we call evolution Note that evolution does not always lead to speciation (formation of a new species)

There are three basic types of natural selection that can occur when there is a range of phenotypes for a particular trait (which can be seen on a bell curve)

Stabilizing Selection The most average phenotype for a particular trait is selected for Keeps the center of the curve at it’s current position, but narrows the shape of the curve. For example, babies with medium birth weight tend to survive best because underweight babies are less healthy and overweight babies have more trouble being born

Directional Selection One of the extreme versions of a phenotype is selected for Causes the entire curve to shift in one direction as the frequency of the trait changes An example might be Galapagos finches: During a period of starvation, those finches who had the longest beaks were best able to reach food and were therefore most likely to survive

Disruptive Selection Both extreme versions of a phenotype are selected for These phenotypes are shown at both ends of the bell curve Results in a bi-modal curve (two peaks) If the selection lasts long enough and is strong enough, the single curve can split in two and result in the creation of two completely different phenotypes

For example, in a population of birds that lives where medium seeds become increasing less common the long and short beaked birds will be favored by selection; the population may end up splitting into two subgroups

Commonly Discussed Examples of Evolution Antiobiotic resistance of staphylococcus aureus This bacteria is associated with skin and lung infections Treated with Methicillin (antibiotic) Methicillin is the environmental selective pressure Over time, usually because a methicillin treatment was not completed, certain members of an s. aureaus colony survived and developed resistance to the drug (through a mutation) These individuals multiplied to become resistance strains that can not be treated with Methicillin This is another example of disruptive selection

Industrial melanism in the peppered moth The peppered moth has two phenotypes for wing color: light grey and dark grey Prior to the mid 1800’s the light grey phenotype was most abundant due it allowing for the moth to blend in with bark of the trees it lived around (camoflauge) When the industrial revolution began (mid 1800’s) soot from factories quickly darkened the plant life in surrounding areas

As a result the light grey wing color was no longer advantageous and the dark grey wing color was selected for The life span of the peppered moth is only one year so after only a few years the change in frequency of wing color (and therefore alleles) was easy to see This is an example of Directional selection The term ‘industrial melanism’ refers to the fact that the soot produced by industrialism (the selective pressure) caused a change in wing color frequencies (wing color is determined by melanin pigment)

Adaptations Adaptations are genetic changes that increase the survival of a population

Types of Adaptations Mimicry – Organisms that copy the appearance of another species for protection