17.2 Evolution as Genetic Change in Populations

Pages 487 - 497

Natural Selection in Action

Peppered Moths

White moths against a light colored background

White moth against a dark backgroundWhite

As a predator, which moth would be easier prey????

Industrial Revolution in England

• In the nineteenth century it was noticed that in towns and cities it was actually the black form of the moth that was more common than the pale peppered form. Industrialisation and domestic coal fires had caused sooty air pollution which had killed off lichens and blackened urban tree trunks and walls. So now it was the pale form of the moth that was more obvious to predators, while the melanic form was better camouflaged and more likely to survive and produce offspring. As a result, over successive generations, the black moths came to outnumber the pale forms in our towns and cities

Moth Populations in England

• In the mid-twentieth century, controls were introduced to reduce air pollution and as the air quality improved tree trunks became cleaner and lichen growth increased. Once again the normal pale Peppered Moths were camouflaged and the black forms were more noticeable. Now the situation in urban areas has again become the same as in the countryside, with normal pale Peppered Moths being far more common than the black forms. So natural selection has been seen to work in both directions, always favouring the moth that is best suited to the environmental conditions. The same thing has been observed throughout Europe and the USA.

Types of Natural Selection

In a normal population without selection pressure, individual traits, such as height, vary in the population. Most individuals are of an average height, while fewer are extremely short or extremely tall. The distribution of height falls into a bell curve

Stabilizing selection

eliminates extreme individuals. A plant that is too short may not be able to compete with other plants for sunlight. However, extremely tall plants may be more susceptible to wind damage. Combined, these two selection pressures act to favor plants of medium height.

Directional selection

selects against one extreme. In the familiar example of giraffe necks, there was a selection pressure against short necks, since individuals with short necks could not reach as many leaves on which to feed. As a result, the distribution of neck length shifted to favor individuals with long necks.

Disruptive selection

eliminates intermediate individuals. For example, imagine a plant of extremely variable height that is pollinated by three different pollinator insects: one that was attracted to short plants, another that preferred plants of medium height, and a third that visited only the tallest plants

• If the pollinator that preferred plants of medium height disappeared from an area, medium height plants would be selected against, and the population would tend toward both short and tall plants, but not plants of medium height.

Genetic Drift

• In small populations, individuals that carry a particular allele may leave more descendants than other individuals leave, just by chance.

• Over time, a series of chance occurrences can cause an allele to become more or less common in a population.

• This kind of random change in allele frequency is called genetic drift

Bottleneck Effect

• The bottleneck effect is a change in allele frequency following a dramatic reduction in the size of a population

• A sever bottleneck effect can sharply reduce a population’s genetic diversity

Founder Effect

• The founder effect occurs when a few individuals colonize a new habitat

• The gene pool changes as a result of the migration of a small subgroup of a population

The Genetic Basis for Evolution

• Darwin’s theory of natural selection and evolution rests on two crucial ideas:

• Variations exist in the individuals within a population.

• Those variations are passed down from one generation to the next.

gene pool

• the sum total of all the alleles within a particular population.

• Using genetics, one can create a new definition of evolution as the change in the allele frequencies in the gene pool of a population over time.

Hardy-Weinberg Equilibrium

• The Hardy-Weinberg principle states that a sexually reproducing population will have stable allelic frequencies and therefore will not undergo evolution, given the following five conditions:

• large population size • no immigration or emigration • random mating • random reproductive success • no mutation

• Hardy –Weinberg equations:• p + q = 1.0

•

• p2 + 2pq +q2 = 1.0

Hardy-Weinberg equilibrium

• Few natural populations ever experience Hardy-Weinberg equilibrium, though, since large populations are rarely found in isolation, all populations experience some level of mutation, and natural selection simply cannot be avoided.

Development of New Species

• reproductive isolation• Allopatric speciation occurs when populations

of a species become geographically isolated so that they cannot interbreed.

• adaptive radiation, which is the creation of several new species from a single parent species.

Convergent Evolution

• When different species inhabit similar environments, they face similar selection pressures, or use parts of their bodies to perform similar functions. These similarities can cause the species to evolve similar traits, in a process called convergent evolution.

• From the outside, the fin of a whale may look like the flipper of a penguin, but the bone structure of a whale fin is still more similar to the limbs of other mammals than it is to the structure of penguin flippers. More importantly, convergent evolution never results in two species gaining the ability to interbreed; convergent evolution can’t take two species and turn them into one.