Chapter 20: Selection and evolution

Genetic variation provides the raw material on which natural selection can act

Variation within a population means that some individuals heave features that given them an advantage over other members of that population

Genetic variation

Genetic variation is caused by: Independent assortment of chromosomes (and

therefore alleles) during meiosis Cross over between chromatids of homologous

chromosomes during meiosis Random mating between organisms within a

species Random fertilization of gametes Mutation

Genetic variation

These processes (minus mutation) reshuffle existing alleles in the population so that offspring have combination of alleles which differ from those of their parents This produces phenotypic variation

Genetic variation

Mutation can produce completely new alleles Ex: sickle cell allele of the gene that codes for

β- globin Such a change in a gene is called a gene

mutation The new allele is often recessive, so it

frequently does not show up in the population until some generations after the mutation originally ocurred

Genetic variation

Mutations that occur in body cells, or somatic cells, often have no effect at all on the organisms, and such mutations cannot be passed on to offspring by sexual reproduction

Genetic variation

Mutations in cells of ovaries or testes may be inherited by offspring if mutation divides to form gametes Gametes will have mutated gene

If mutated gamete is involved in fertilization the new organism will express the mutated gene in all somatic cells

Genetic variation

Genetic variation, whether caused by the reshuffling of alleles during meiosis and sexual reproduction or by mutation, can be passed on by parents to offspring

Variation in phenotype is also caused by the environment in which organisms live Ex: some organisms may be larger because they

has better access to food while they were growing. This type of variation is NOT passed on by parents to their offspring

Genetic variation

All organisms have the reproductive potential to increase their population numbers

As population increases, various environmental factors come into play to keep numbers in check

These factors may be: Biotic: caused by other living organisms

(predation, competition, pathogenic infection) Abiotic : caused by nonliving components (ex:

water supply or nutrient levels in the soil)

Natural Selection

If the pressure of the environmental factors is sufficiently great, then the population size will decreases

Over a period of time, population numbers will oscillate around a mean level

Natural Selection

The number of young produced is far greater than the number which will survive to reproduce

What determines if a newborn will survive to reproductive age? It may be just luck, but some individuals are born with a better chance of survival than others

Variation within a population means that some individuals will have features which give them an advantage in ‘the struggle for existence’

Natural Selection

Ex: coat color in rabbits Most rabbits have alleles that give them brown

color. However, a few will be homozygous for the recessive allele that gives a white color. These white rabbits will stand out to predators (foxes) more from other and are less likely to survive than brown rabbits. The chances of a white rabbit reproducing and passing of its alleles for white coat to its offspring are very small.

Natural Selection

Predation by foxes is an example of a selection pressure

Selection pressure increases the chances of some alleles being passed on to the next generation, and decrease the chances of others

Natural Selection

In the rabbit fur coat color example, the alleles for brown color have a selective advantage over the alleles for white

The alleles for brown coat will remain the more common alleles in the population

Natural selection

The effects of such selection pressures on the frequency of alleles in a population is called natural selection

Natural selection raises the frequency of alleles conferring an advantage, and reduces the frequency of alleles conferring a disadvantage

Natural Selection

Usually, natural selection keeps things they way they are. This is stabilizing selection Keeps allele numbers around a central mean

However, is a new environmental factor or new allele appears, the allele frequencies may also change This is called directional selection

Evolution

A new environmental factor Imagine we are plunged into a new Ice Age.

The climate becomes much colder and snow covers the ground almost all year. Now, white rabbits have a selective advantage in the new environment. Rabbits with white fur are more likely to survive and pass on their alleles.

The frequency of the white allele will increase

Evolution

Because they are random, most mutations produce features that are harmful

Other mutations may be neutral, meaning their neither benefit nor harm the organism

Mutations produce advantageous features only occasionally

Evolution

Imagine that mutation occurs in the coat color gene of a rabbit, producing a new allele which gives a better camouflaged coat than brown

Rabbits possessing this allele will have as selective advantage over brown rabbits, making them more likely to survive to reproduce

Over many generations, the new allele will become more common

Evolution

Such a change in allele frequency in a population are the basis of evolution Evolution occurs because natural selection

gives some alleles a better chance of survival than others.

Over many generations, populations may gradually change, becoming better adapted to their environments

Evolution