4.4 Variation and Evolution

Variation

The differences between organisms of the same species

Phenotype

The total appearance of an organism

Phenotypic variation between people include:

Discontinuous variation

Continuous variation

Discontinuous variation

Shows a limited number of phenotypes

It is normally controlled by one gene

The environment does not affect gene expression

Normally represented on bar charts

Heritable variation

Differences in phenotype due to genetic reasons

This type of variation can be inherited

Non-heritable variation

Differences in phenotype due to environmental reasons

This type of variation cannot be inherited

Continuous Variation

There is a gradation from one extreme to the other

Normally controlled by many genes

The phenotype is determined by the interaction of all the genes and the environment

A bell-shaped curve is obtained

Factors that affect populations of organisms (biotic)

predation

parasitism

disease

competition for resources

What is the major biotic factor affecting breeding success and survival

Competition

Therefore determining the density and growth rate of populations

Things that organisms may compete for

Resources such as sunlight, minerals and food

Nest sites or mates

Intra-specific competition

Competition between members of the same species

Some of the population may not survive or reproduce, so the growth of the population slows

If the resource is plentiful with no competition, then the population will increase

Inter-specific competition

Competition between members of different species

Contest competition

The winning competitor gains the resource

Scramble competition

Many individuals all compete for and gain some of the resource

Selection

The process by which organisms that are better adapted to their environment survive and breed, while those less well adapted fail to do so

Selective agencies

Environmental factors that can alter the frequency of alleles in a population, when they are limiting

Examples of selective agencies

Supply of food

Breeding sites

Climate

Human impact

Selection pressure

The effect of the selective agencies acting on the population through Natural Selection

Gene Pool

Total of all the alleles for all the genes in a population

A gene pool will remain stable under the following conditions:

The population is large

There is no selection pressure

Mating is random

No mutations occur

All genotypes are equally fertile

No emigration or immigration

Genetic drift

A change in the frequency of an allele by chance

The Hardy-Weinberg Principle

The frequency of alleles and genotypes within a population will remain constant from one generation to the next if certain conditions remain true

Hardy Weinberg conditions

The population is large

There is no selection for or against any phenotype

There is random mating throughout the population

There are no mutations

The population is isolated, i.e. there is no immigration or emigration

The Hardy Weinberg Equation

p = frequency of the dominant allele

q = frequency of the recessive allele

p + q = 1

p2 = frequency of homozygous dominant (AA)

2pq = frequency of heterozygote (Aa)

q2 = frequency of homozygous recessive (aa)

Charles Darwin and Alfred Russel Wallace's theory of evolution states that…

Existing species have arisen through the modification of ancestral species by natural selection

The theory of evolution is based on the following observations:

In any population, there is variation

Individuals within a population have the potential to produce large numbers of offspring, yet the number of adults tends to stay the same from one generation to the next

From these observations, two deductions can be made…

There is a struggle for survival (competition) with only the ‘fittest’ phenotypes surviving

The individuals who survive reproduce and pass on to their offspring alleles for the phenotypes that enable them to succeed, changing the allele frequency

Speciation

The process by which new species are formed

Speciation can occur due to:

Genetic drift in isolated populations

The founder effect of disproportionate allele frequencies in small populations

Natural Selection

Isolating mechanism for allopatric speciation

Geographical isolation

Isolating mechanism for sympatric speciation

Reproductive isolation

Allopatric speciation

Occurs as the result of two populations becoming geographically isolated

Deme

A group of individuals within a population who breed with one another

Sympatric Speciation

Occurs when organisms inhabiting the same area become reproductively isolated into two groups for reasons other than geographical barriers

The reasons for reproductive isolation include:

Behavioural isolation

Gametic isolation

Seasonal isolation