4.4 Variation and Evolution

Define continuous variation

A character within a population which shows a gradation from one extreme to another

Define discontinuous variation

Characters that are clear cut and easy to tell apart because they are controlled by a single gene

Define variation

The differences between organisms of the same species

Word for a character controlled by a single gene

monogenic

Word for a character controlled by many genes

polygenic

is discontinuous variation heritable and non heritable

It is heritable

It is not non heritable

Is continuous variation heritable and non heritable

It is heritable and non heritable

Define selection pressure

An environmental factor that can alter the frequency of alleles in a population, when it is limiting

Define natural selection

The increased chance of survival and reproduction of organisms with phenotypes suited to their environment. enhancing the transfer of favourable alleles from one generation to the other

Examples of selection pressure

• Predation

• Overcrowding

• Availability of nesting sites

Define gene pool

The total of all the alleles of all the genes in a population at a given time

Define frequency

Proportion of an allele in the gene pool

State the Hardy-Weinberg equations

P² + 2pq + q²

p + q = 1

What are the ideal conditions for the Hardy Weinberg principle

1. Organisms are diploid

2. The allele frequencies are equal in both sexes

3. They reproduce sexually

4. Mating is random

5. Generations do not overlap

6. The population size is very large

7. There is no immigration or emigration

8. There is no mutation

9. There is no selection

What is genetic drift

When variations in gene frequencies in populations occur by chance

Define speciation

The formation of a new species

Define evolution

A change in the average phenotype of a population

Explain genetic drift and founder

1. A small number of individuals become isolated and start a new population.

2. The founder members are a small sample of the original population

3. By chance they have very different allele frequency from the original population

4. Founder population may undergo genetic drift and become even more different

5. In a small population chance variation in allele frequency from one generation to the next can represent a large change in phenotype for a larger proportion of the population

Name the three types of natural selection

Stabilising selection, directional selection and disruptive selection

What is the theory of natural selection based on

• 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 the observations above, what deductions were made on natural selection

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

• The individuals that survive and reproduce pass on to their offspring the characteristics that enable them to succeed (that is a selection advantage

• In time, a group of individuals that once belonged to the same species may give rise to two different groups that are sufficiently distinct to belong to two separate species

Two ways new species rise

Abruptly

Gradually

How do new species arise abruptly

by polyploidy when their chromosome number doubles by endomitosis

How do new species arise gradually

by isolating groups of individuals

What are demes

Sub groups within a population that breed with each other more often than the rest of the population

Describe reproduction isolation

If a deme becomes isolated, it cannot breed with members of other demes and so the gene flow in and out is prevented. The mechanism prevents gene flow and the groups are reproductively isolated

How does specification occur

If demes are isolated for many generation, they undergo changes in allele frequency and accumulate so many different mutations that they are no longer able to interbreed successfully with members of the initial population

Name the two different types of reproductive isolation

Pre-zygotic

Post zygotic

Pre-zygotic

gametes are prevented from fusing and so a zygote is never formed

Post zygotic

gametes fuse and a zygote forms. Even if the organism develops and grows, it is sterile and so the genes of the parent species are kept separate and the species do not merge

When does geographical isolation occur

When the population becomes split by a physical barrier into separate demes.

Name examples of geographical isolation

mountain, a river, desert

Allopatric speciation

The evolution of new species from demes isolate in different geographical locations

Sympatric isolation

The evolution of new species from demes sharing a geographical location

What is behavioural isolation

in animals with elaborate courtship behaviour, the steps in the display of one organism fails to attract the necessary response in a potential partner of another group.

What is morphological isolation

the genitalia of the two groups may be incompatible.

What is gametic isolation

in flowering plants pollination may be prevented because the pollen grain fails to germinate on the stigma whereas in animals, sperm may fail to survive in the oviduct of the female

What is seasonal isolation

If the breeding season of two groups (demes) does not coincide, they cannot interbreed.

Describe hybrid inviability

Fertilisation occurs but incompatibility between the genes of the parents prevents the development of an embryo

Can an embryo formed from the gametes of two species develop in hybrid sterility

yes

How is the hybrid sterile in hybrid sterility

1. The chromosomes are not sufficiently similar.

2. Prophase I does not occur

3. gametes cannot form