4B Diversity, Classification and Variation

What are gametes? What forms when they meet and fertilise?

sex cells- sperm and eggs.

A zygote forms

What is significant about chromosome numbers in gametes and normal cells?

Gametes contain a haploid number of chromosomes (n) compared to normal cells which have a diploid number of chromosomes (2n).

How are gametes formed?

Meiosis-

• before meiosis begins, the DNA replicates so that there are two copies of each chromosome, called chromatids.

• the DNA condenses to form double-armed chromosomes each made from two sister chromatids, and the sister chromatids are joined by the centromere.

• In Meiosis 1, the chromosomes arrange themselves into homologous pairs, which are then separated, halving the chromosome number.

• In Meiosis 2, the pairs of sister chromatids that make up each chromosome are separated as the centromere is divided.

• Four haploid gametes that are genetically different are produced.

What are the two main events in Meiosis that lead to genetic variation?

1- crossing over of chromatids- this leads to 4 daughter cells containing chromatids with different alleles.

2- independent segregation of chromosomes- each homologous pair is made up from one maternal and one paternal chromosome. In Meiosis 1, it is completely random which chromosome from each pair ends up in which daughter cell, so the gametes produced have different combinations of those chromosomes.

What are chromosome mutations?

errors in mitosis cause variations, leading to inherited conditions as the errors are present in the gametes.

What is non-disjunction?

A type of chromosome mutation where the chromosomes do not separate properly.

What are gene mutations?

Changes to the base sequence of DNA

What is a substitution gene mutation?

One base is substituted for another, eg. ATGCCT becomes ATTCCT. This will not always to lead changes in the amino acid sequence due as the genetic code is degenerate.

What is a deletion gene mutation?

A base is deleted, eg. ATGCCT becomes ATCCT. This will always lead to changes in the amino acid sequence as there will be a shift in all the base triplets after the deletion as the number of bases changes.

What is genetic diversity?

The number of different alleles of genes in a specific population.

How is genetic diversity in a population increased?

• mutations in DNA that form new alleles

• Different alleles being introduced into a population when individuals migrate into them- this is gene flow

What is a genetic bottleneck?

An event that causes a reduction in population, reducing the number of different alleles in the gene pool and reducing genetic diversity.

What is the founder effect?

A type of genetic bottleneck: when a few organisms from a population start a new colony and there are only a few different alleles in the initial gene pool. This leads to a higher incidence of genetic disease because the frequency of each allele in the new colony may be different to the frequency of the alleles in the original population.

Describe natural selection.

• there is differential reproductive success in a population as some individuals may have an allele that increases chance of survival, and they are therefore more likely to also reproduce and pass on genes including the beneficial allele.

• A greater proportion of the next generation inherits the beneficial allele.

• This group are also more likely to survive, reproduce and pass on their genes, so the frequency of the allele increases with each generation.

• This leads to evolution over generations as the advantageous alleles become more common in the population.

Give an example of a behavioural, physiological and anatomical adaptation.

Behavioural- possums playing dead

Physiological- processes inside the body eg. Hibernation

Anatomical- structure features eg. Blubber.

What types of natural selection affect allele frequency?

Directional and stabilising.

Describe directional selection.

Where individuals with alleles for characteristics of an extreme type are more likely to survive and reproduce, for example in response to an environmental change. Eg. Bacteria evolving antibiotic resistance.

Describe stabilising selection.

Where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce. This occurs when the environment isn’t changing and reduces the range of possible characteristics. Eg. Human birth weight.

What is phylogeny?

The study of the evolutionary history of groups of organisms.

What do phylogenetic trees show?

All organisms have evolved from shared common ancestors.

What is taxonomy?

The science of classification. The eight groups used to classify organisms are the taxa (taxon singular)

What are the eight taxa?

Domain

Kingdom

Phylum

Class

Order

Family

Genus

Species

What is a species?

A group of similar organisms able to reproduce to give fertile offspring.

What is the binomial naming system?

The nomenclature used for classification. The first part is the genus name and has a capital letter. The second part is the species name and does not have a capital letter. These are always in Latin and in italics.

How can courtship behaviour be used to classify species?

It is species specific as this allows members of the same species to recognise each other and prevent interbreeding, making reproduction more successful. The more closely related a species is, the more similar their courtship behaviour.

What technologies have been used to clarify evolutionary relationships and update classification?

• genome sequencing

• Comparing amino acid sequence

• Immunological comparisons

How has genome sequencing been used to clarify evolutionary relationships?

The entire base sequence of an organism’s DNA can now be determined, and compared to another organisms to see how closely related they are. Closely related species share a higher percentage of similarity in their DNA base order.

How have amino acid sequences been used to clarify evolutionary relationships?

The sequence of amino acids in a protein is coded for by the base sequence in DNA. Related organisms have similar DNA sequences and so similar amino acid sequences.

How have immunological comparisons been used to clarify evolutionary relationships?

Similar proteins bind to the same antibodies.

How can genetic diversity now be assessed by scientists?

By comparing the DNA base sequences of the same gene in different organisms in a population, allowing scientists to find out how many alleles of that gene there are in that population.

Different alleles also have slightly different mRNA base sequences, producing proteins with slightly different amino acid sequences which can also be compared.

How are the new technologies used to assess genetic diversity better?

They can give more accurate estimates of genetic diversity within a population or species and allow the genetic diversity of different species to be compared more easily.

What is variation?

The differences that exist between individuals, both between species and within species.

What is the cause of variation?

Genetic factors and environmental factors.

What is biodiversity?

The variety of living organisms in an area.

What is a habitat?

The place an organism lives.

What is community?

All the populations of different species in a habitat.

What is local biodiversity?

The variety of species in a small habitat

What is global biodiversity?

The variety of species on Earth.

How is biodiversity measured?

An index of diversity. Calculated using an equation that takes both the number of species in a community and the abundance of each species

What is species richness?

A measure of the number of different species in a community.

How can agricultural practices reduce biodiversity?

Woodland clearance to increase farming area reduces the number of trees and destroys habitats

Hedgerow removal

Pesticides kill pests and damage the population of their predators

Herbicides reduce plant diversity and food

Monoculture reduces biodiversity directly and means that fields support fewer organisms.

What are examples of conservation schemes?

Giving legal protection to endangered species.

Creating protected areas such as SSSIs

The environmental stewardship scheme.

What consequences could conservation schemes have on food production?

• reduce food output

• As biodiversity grows, pesticides could in part be replaced by natural predators.

• Increase the biodiversity on farms, helping predator and prey populations who eat pests and weeds.