Diversity of organisms

variation

differences between organisms

causes of variation

a mix of environmental and genetic factors

genetic variation

• genes determine which proteins an organism is capable of producing, influencing their characteristics

• occurs when mutation happens and alleles are combined in different ways during sex

environmental variation

• determines whether the organism has the resources needed to produce a particular protein, may affect gene expression

• factors affecting environmental variation may be:

  • temperature

  • nutrient availability

variation between species

• can be used to classify organisms into different groups

variation within species

• different individuals have different combinations of alleles, despite similar genetic makeup

• sequences can be different due to mutations like SNPs

• genome size varies

• no 2 individuals are identical, even identical twins (environmental factors)

discontinuous variation

• characteristics fall into distinct categories

• e.g human blood type

continuous variation

• characteristics can be measured incrementally

• e.g height, weight

species classification using morphology

• putting organisms into groups is called classification

• science of classification is taxonomy

• problems with classifying by morphology

  • similarities in observable characteristics doesn’t mean the 2 organisms share a common ancestor

  • analogous structures

  • doesn’t consider DNA

morphological species concept

A species is a group of organisms that share observable characteristics

Linnaeus

• Linnaeus re-invented classification by giving organisms 2 part latin names instead of long ones

• he believed classification based on morphology led to mistakes

binomial nomenclature system

• two part Latin names

• recognised universally

• first part = Genus

• second part = species

• species in the same genus will have similar characteristics

• facilitates understanding and communication between scientists

biological species concept

a group of organisms that can interbreed to produce fertile offspring

limitations of the biological species concept

• asexual reproduction

  • organisms like bacteria can’t be classified

• fertile hybrids

  • rarely, animals of different species breed and make fertile offspring

  • e.g wholpin

• extinction

  • some species are extinct, so it’s impossible to breed them with members of an existing species to determine fertility of offspring

• horizontal gene transfer

  • the passing of genetic info from one cell to another within the same generation

  • many organisms contain genetic info from a species that are very distantly related

similarities and differences between morphological and biological species concept

• similarities

  • both state there can be variation within species

  • both seek to define a species

• differences

  • biological reflects evolutionary relationships, whereas morphological does not

  • morphological concept is applicable to both asexually and sexually reproducing organisms, biological is only applicable to sexually reproducing ones

other characteristics that can sometimes be used to determine species

• morphology

• DNA

• biochemistry

evolution

process of species change

speciation

• when one species gives rise to 2 or more species

• can occur when a population becomes isolated from other populations of the same species due to living in a different area

• isolation means that members of the separate populations cannot breed together and gene flow cannot take place between them

• if environmental conditions affecting each population are different, natural selection could act differently on each population

• once speciation has happened, the 2 species can no longer breed to produce fertile offspring; they are reproductively isolated and are said to be separate species

distinguishing between populations and species

• in most cases, the ability of 2 populations to interbreed successfully declines gradually, rather than a sudden cut off point

• difficult to pinpoint when the 2 populations become different species

• decision when to assign separate species status to 2 populations is subjective

diploid cell

a cell containing 2 complete sets of chromosomes

haploid cell

• a cell containing one complete set of chromosomes, meaning half the number of chromosomes in normal body cells, in most cases these are gametes

chromosome number and cross breeding

• for a zygote to be viable, both gametes have to contain the same number of chromosomes

• in the case of closely related species e.g horse and donkey:

  • the fusion of a horse and donkey gamete produces an uneven number of chromosomes

  • these chromosomes cannot pair up during meiosis, preventing the formation of gametes

  • mule is infertile

chromosome number

• number of chromosome varies between species, dependent on changes during species’ evolution

• each individual in the same species has the same number of chromosomes

• humans: 46

• chimpanzees: 48

types of chromosomes

• metrocentric: centromere is in the middle, arms of equal length

• acrocentric: centromere is near the end, arms unequal

• chromosomes stained with dye have a banded appearance

karyogram

• an image that shows all the chromosomes in a cell

• arranged by size, shape and banding pattern, placed with their homologous pair

• homologous pair: a pair of chromosomes, one maternal one paternal, with the same shape and size

karyotype

• the appearance of a complete set of an individuals chromosomes, including their number, shape, size and banding

chromosome evidence and human evolution

• humans share a recent common ancestor with chimps, this raises the question of how humans lost the 2 chromosomes

• possible answers:

  • a pair of chromosomes disappeared from genome

  • a pair of chromosomes fused with another pair to form a single pair

hypothesis about loss of chromosome pair

• chromosomes in pairs 12 and 13 in a common ancestor fused to form the chromosomes in human pair 2

telomere

region of repeating DNA base sequences located at ends of chromosomes

evidence supporting hypothesis

• chimp chromosomes 12 and 13 when placed end to end match length of human chromosome 2

• locations of centromeres match on chimp chromosome 12 and human chromosome 2

• the banding of long arms of acrocentric chimp 12 and 13 matches banding of metrocentric human 2

• human chromosome 2 contains telomeric DNA in middle of chromosome

evidence against hypothesis

• lengths are not a perfect match, there is a slight overlap

• location of centromere of chimp chromosome 13 doesnt match human chromosome 2

testable hypothesis

• needs to be access to evidence to support it

• hypothesis needs to be a statement that can be accepted or rejected

• shouldn’t contain vague statements such as “may” or “could”

• shouldn’t make predictions

genomes

all of the genetic information in an organism

alleles

alternative forms of a particular gene with different base sequences

single nucleotide polymorphisms

• differences between DNA sequences involving a single base change

• area of difference between individuals

comparing eukaryotic genomes

• can vary in size, determined by mass of DNA present in a nucleus

  • plants may have a gene that codes for enzymes involved in photosynthesis, humans don’t need these

• vary in base sequencing

• diversity between species is much larger than diversity within species

genome size and organism complexity

• genome size measured in mega-bases (Mb)

• no clear relationship between genome size and organism complexity

• some similar species may have very different genome sizes

• factors contributing to unexpected numbers (e.g wheat has a larger genome size than humans)

  • polyploidy in plants causing much larger genomes

  • a larger genome doesn’t necessarily mean an organism has more genes

DNA sequencing

• determining the entire base sequence of the genes within an organisms genome

• organisms within the same species share most of their genome

• sequencing methods continually advancing to become faster and cheaper, due to advances in tech

genome sequencing and evolutionary relationships

• phylogenetics is the classification of species based on evolutionary origins & relationships

• advances in sequencing allow scientists to understand true phylogeny of taxa

• the more similar the sequences, the more closely related the species are

genome sequencing and personalised medicine

• by combining info about the genome with other clinical information, patterns can be identified that can help determine an individuals risk of developing disease

• genomes can also be used to work out how patients will respond to specific treatments

dichotomous key

• used to identify unfamiliar species

• contains a series of paired statements

  • e.g the organism has 1 pair of wings, the organism has 2 pairs of wings

• statements must be yes or no

DNA barcodes

• a DNA sequence that can be used to quickly identify a species

• usually just a few hundred base pairs in length

eDNA sampling

• the process of extracting DNA from a habitat

  • eDNA sample is taken from habitat of interest containing unknown species

  • eDNA sample is amplified and sequenced

  • eDNA sample is compared with data base of known sequences

  • species is identified