Evolutionary Mechanisms

Species A species is a group of individuals that

share many characteristics and are able to interbreed under natural conditions to produce fertile offspring.

Gene Pools Geneticists find it convenient to pool the

genotypes of all the individuals capable of reproducing and refer to this as the gene pool.

Thus the gene pool is the sum of all the alleles in a given population

Allele Frequency How often each allele of a gene occurs

in the gene pool for that population Knowing how often a characteristic

occurs in a population, the frequency of each of the alleles for a particular gene can be determined.

Allele Frequency Populations that differ in the

characteristics they posses are likely to be different in the frequencies of the various alleles of a gene in their respective gene pools

Thus, any two populations having differing characteristics are likely to have different gene pools

Allele Frequency Over time, the frequency of particular

alleles in a population may change or, in other words, the composition of the gene pool changes.

Changes may be due to chance events, e.g. a mutation that alters the expression of a gene, or by more natural means where changes in the environment may result in variations to the allele frequencies.

Changes to Allele Frequency Mutations Natural Selection Random Genetic Drift Migration

Mutations New variations in offspring, that show no

resemblance to either parent, may occur quite suddenly and purely by chance. Characteristics occurring like this are referred to as mutations.

Mutant- organism with a characteristic resulting from a mutation

There are two main types of mutations: Gene mutations and Chromosomal mutations

Mutations- Gene Mutations which are changes in the DNA

of a single gene so that the traits normally produced by that gene are changed or destroyed;

Mutations- Chromosomal All or part of a chromosome is affected.

Mutations When one considers the countless millions of times

that DNA is replicated and that cell division occurs, relatively few mutations occur in human populations.

Mutations can occur either in the body cells (Somatic Mutation) or in the reproductive cells of a person.

Only the characteristics of the individual with the somatic mutation are affected.

Reproductive cells are not affected so that when the individual dies, the mutation is lost. Somatic mutations are often the cause of many cancerous growths.

Mutations Mutations that affect the reproductive

cells can result in the gametes containing the changed genetic information that can then be passed on to the next and subsequent generations.

These are known as germline or germinal mutations.

Mutations- Germline Person in whom the mutation occurs is not

usually affected, but that individual produces gametes with changed DNA.

A germline mutation may, by chance, result in characteristics that improve the survival chances of the individual.

Such a characteristic is likely to be passed on to future generations resulting in a change to the allele frequencies of the gene pool.

Natural Selection proportion of alleles occurring in a large

population is very much the same from one generation to the next

This selection of favourable alleles is referred to as natural selection

Is not random

Random Genetic Drift In small populations, there is often a random,

non-directional variation in allele frequencies. This phenomenon is called random genetic

drift or, sometimes, the Sewall-Wright effect, after the man who first recognised its significance in causing changes to allele frequencies.

It is a purely chance occurrence, a change in allele frequencies that may occur where a population is very small.

Founder Affect occurs when a small group moves away from

its homeland to a totally new area and establishes a community, which later expands.

The migrant group, being such a small sample of the original population, is usually not genetically representative of them.

This new community, therefore, generally shows features that are not typical of the original homeland population.

Founder Affect Pitcairn Island, a small island in the

Pacific, populated in 1790 by the descendants of nine mutineers from the Bounty, together with six men and twelve women, Polynesians from Tahiti.

Due to isolation there have been few newcomers to the population and so few alleles have been introduced from outside.

Founder Affect In 1775, typhoon reduced population of Pingelap,

an island in Micronesia, to only 20. Among the survivors was a person heterozygous

for achromatopsia. After a number of generations, the incidence of

achromatopsia on Pingelap is 5% of the population. In other parts of the world it is 0.0033%. Furthermore 30% of the Pingelap population are

carriers; they are not colourblind but they do have the defective allele.

Founder Affect

Migration Geneticists describe migration as gene

flow from one population to another. If immigrants to a certain country bring

alleles that are not already in the population, the frequencies for the alleles of that gene will be altered.

Barriers to Gene Flow Populations kept apart by barriers that inhibit the amount of

interbreeding. As no two environments are exactly the same,

environmental pressures on one population will be different from the pressures on the other

Results in slightly different characteristics being favoured in one population compared to the other.

These changes, over many generations, result in the populations becoming less alike as they develop characteristics better suited to their respective environments.

Isolation results in the development of separate gene pools.

Barriers to Gene Flow Geographical barriers include oceans,

mountain ranges, large lake systems, deserts and expansive ice sheets.

For example, the original inhabitants of Australia were isolated for thousands of years by ocean barriers that formed as sea levels rose.

Barriers to Gene Flow sociocultural barriers are just as

effective in isolating groups as geographical ones.

Even today such things as economic status, educational background and social position are barriers to interbreeding.

Genetic Diseases Genetic diseases result in changes to allele

frequencies in a gene pool. An allele causing an inherited, fatal disease

would be expected to be gradually eliminated from a population because people with the allele would die and would not pass it on to the next generation.

This has probably happened many times in the past but sometimes alleles that cause serious disease do persist in certain populations.

Genetic Diseases- Tay- Sachs Disease Hereditary disorder of lipid metabolism

that occurs most frequently in individuals of Jewish descent from Eastern Europe (the so-called Ashkenazi Jewish population).

Tay-Sachs caused by missing enzyme that results in the accumulation of a fatty substance in the nervous system.

Death usually occurs by the age of 4 or 5

Genetic Diseases- Tay Sachs Disease Frequency worldwide is very low, affecting approximately 1 in 500 000

births. For the Ashkenazi Jews the incidence around 1 in every 2500 births. Genetic drift suggested as reason frequency so high. Jewish populations tended to be small and isolated Individuals who are

heterozygous, appear to have increased resistance to tuberculosis (TB). Heterozygotes have advantage in situations where TB is prevalent:

homozygotes more susceptible to TB, and would possibly die, heterozygotes would die early in life.

Heterozygotes would have survival advantage and would be more likely to reproduce and pass their alleles on to the next generation.

Due to discrimination, the Ashkenazi Jews often found themselves isolated in overcrowded ghettos under conditions that would increase the threat of TB, situations, where the frequency of the allele for Tay-Sachs could be maintained in the population

Genetic Diseases- Sickle Cell Anemia

The Theory of Evolution Through Natural Selection The theory of evolution through natural

selection was put forward independently by Charles Darwin (Fig. 15.1) and Alfred Russel Wallace in 1858. However, it is Darwin’s name that is usually associated with this theory because of the massive amount of supporting evidence he collected

The Theory of Evolution Through Natural Selection Carolus Linnaeus- binomial system of

naming organisms using the generic (genus) and specific (species) names.

This system was important to Darwin as it enabled him to classify and organise the material he collected.

The Theory of Evolution Through Natural Selection Thomas Malthus’s An Essay on the Principle of

Population, pointed out that the human population was increasing at a rate far exceeding the rate of food production. He demonstrated that natural reproduction rates exceeded the available resources

Darwin realised that under these circumstances a struggle for existence would occur, with the favourable variations being preserved and the unfavourable ones being gradually lost from the population.

The Theory of Evolution Through Natural Selection In 1858, Darwin received copy Alfred Russel Wallace’s

essay, a naturalist On the Tendency of Varieties to Depart Indefinitely from the Original Type, covered the same ideas that Darwin had been working on.

Darwin had been painstakingly collecting evidence and refining his ideas for 20 years but Wallace’s essay was the stimulus for him to publish his views.

A joint essay was prepared by Darwin and Wallace and read before the Linnaean Society in 1858.

A year later Darwin published his first book, On the Origin of Species. Darwin’s theory of natural selection was based on three observations:

Darwin’s Observations- Variation Variation. Darwin noted that all

members of a species vary. He made no attempt to explain the source of this variation. However, he did point out that these variations were passed on from one generation to the next, characteristics displayed by the parents being passed on to their offspring.

Darwin’s Observations- Birth Rate Birth rate. Inspired by Malthus, Darwin

realised that all living organisms reproduce at a rate far greater than that at which their available food supply and other resources increase. This would normally result in overcrowding

Darwin’s Observations- Nature’s Balance Nature’s balance. Darwin observed

that, although the birth rate of organisms was very high, each species tended to maintain its numbers at a relatively constant level

Darwin’s Observations and Interpretations there must be a struggle for existence survival of the fittest: more organisms

with favourable characteristics survived, while many of those with unfavourable characteristics died before they had an opportunity to reproduce and pass on the unfavourable characteristics.

Survival of the fittest is possible because there is variation within any species.

The Theory of Evolution through Natural Selection An important point to note is that

individual organisms do not adapt. The species adapts to its environment

by natural selection and the process of adaptation takes many generations.

The Theory of Evolution Through Natural Selection If the environment tends to favour a

particular characteristic, more of the alleles for that trait will be passed on to the next generation.

This will result in a change in the frequency of that allele in the gene pool, resulting, over time, in that characteristic being more frequent in the population

Speciation Isolation is a barrier to gene flow. Reproductive isolation may lead to the development

of separate gene pools. No two environments are exactly the same, so

different alleles more favoured in one environment more than another. over time the allele frequencies of each gene pool will change, depending on which characteristics are favoured for survival.

Over many generations, the populations become less and less alike as they develop characteristics that better suit them to their respective environments

Speciation If two populations isolated for very long period of time,

and environmental influences on each are different enough, major changes in allele frequencies within each population could occur.

Members of those populations may become so different that, even if the barriers to reproduction were removed, interbreeding would no longer be possible.

If this occurred, two populations would be regarded as separate species.

The process of producing two species in this way is referred to as speciation

Modeling Natural Selection Activity