Environmental Biology and Genetics (Intermediate 2)

Introduction

Environmental biology and genetics are of considerable economic and social importance. This unit focuses on the importance of biodiversity and illustrates this through a study of ecology which explores energy flow and the factors that affect the variety of species in an ecosystem. The contribution to biodiversity by variation within a species is illustrated through the study of fertilisation and genetics.

Energy flow

Components of an ecosystem:

An ecosystem is made up of:

Food chains and food webs.

In a food chain or web you should be able to identify:

• Producers and consumers
• Herbivores and carnivores
• Predators and prey
• Decomposers

• The arrows represent flow of energy between populations.

Pyramids

Energy in the environment can be represented as a pyramid of numbers, biomass or energy:

• Producing the pyramid of numbers is quick and relatively easy
  • however there are anomalies where very large or very small organisms occur.
• Pyramid of biomass represents the system better, easy to work out the numbers
  • however anomalies occur where there is a rapid turnover of biomass anywhere in the system.
• Pyramid of energy the best way of representing energy in the ecosystem,
  • however it is harder to work out.

Each level of the pyramid is smaller than the lower level because energy is lost from the food chain at each stage.

The importance of biodiversity at species level.

A species is defined as a group of organisms that can interbreed to produce fertile offspring.

• a group of horses is a species – they can breed together and produce fertile offspring
• a group of donkeys likewise
• when a horse and donkey are mated together a mule is produced
  • mules cannot breed together successfully
  • mules are not a species
• A similar situation exists with lions and tigers.

Biodiversity is the variation within and between species, it covers the whole variety of life.

• A biodiverse world is important to maintain because:
  • It is aesthetically pleasing to humans
  • It allows new genes to be brought into domestic populations from wild organisms.
  • There is the potential for new products, crops, medicines
  • Many species of plants and animals have never been investigated for their uses in these areas.
  • Retaining a wide genetic variety in a species to allow for changes in the environment and challenges by diseases.
  • Many wild plants and animals posses genes which potentially could be valuable in domesticated crops and cattle

Stability of ecosystems.

The more biodiverse an ecosystem is the more stable it is when environmental change occurs such as changes in use or changes in climate.

• There are a wide variety of species found in stable ecosystems.
• Each species occupies a single niche, and all niches are filled
• when conditions change within the ecosystem the wide variety of species are able to adapt.
• Consider the following fragments from a larger food web

• if one herbivore is removed from the first, complex food web there will be little effect on the other species in the web, the feeding relationships will shift slightly but there are plenty of individuals to make sure there is little overall effect.
• if one herbivore is removed from the second, less complex food web there will be a major effect on the other organisms a half – or in the case of carnivore 2 – all their food is lost.

Factors affecting Biodiversity

Adaptations to habitat and niche

The nature of species changes over time – this is referred to as evolution.

• It was first described by Charles Darwin during his epic voyages on the ship the "Beagle".
• He noticed that isolated species of finches changes slowly over time so that their beaks were better adapted to their food source.

Effects of Grazing

Grazing is an example of a factor that affects the variety of species in an ecosystem.

• Grazing can increase or decrease the biodiversity of an ecosystem depending on the circumstances
• Sheep grazing on grassland increase the biodiversity by keeping the size of large aggressive plants down so that the more delicate can survive
• Cattle overgrazing on marginal land reduces the biodiversity by tearing out the existing plants and causing erosion and desertification. The effects of human activity on habitat destruction and the biodiversity crisis.

Effects of Human Activity

Pollution is an example of a factor that affects the variety of species in an ecosystem.

• Pollution in any form usually reduces the biodiversity of an ecosystem:
• Humans are particularly aggressive on the vast biodiversity of tropical rain forest. Clearing it for:
  • Profit – timber is valuable
  • Space for expanding populations
  • Clearing forest for grazing and crops.

• There are changes in population structures brought about by disruption of food webs:
  • When a food web is disrupted in any way the numbers of organisms in it will change to adapt to the new circumstances.

Competition in Plants and Animals

Competition occurs usually:

• when two species are trying to occupy one niche:
• when a resources needed by a single species is in short supply:
  • e.g. planaria are small flatworms that graze on the mud of pond bottoms:

Plants compete mainly for:

• water
• light
• soil nutrients.

Animals compete for:

• food (e.g. predator prey interactions),
• water
• shelter.

Behavioural adaptations in animals

Animals respond to stimuli in the environment such as:

• Avoiding predators
• Finding food
• Avoiding abiotic conditions that might harm it
• Finding mates

Woodlice tend to move towards:

• darkness:
  • less chance of being seen by predators
• dampness:
  • less chance of drying out

Genetics

Fertilisation

Sex cells are called gametes:

• in mammals these are sperm and ova (singular – ovum)
• in plants these are pollen and ova

In mammals

• Sperm are produced in the testes (singular – testis) of the male
• Ova are produced in the ovaries of the female
• In the diagrams below move your mouse over the parts to see the name:

A process called meiosis produces gametes:

• The chromosome number is halved
• A single set of chromosomes is found in a gamete.
• When gametes fuse at fertilisation a double set of chromosomes results
• One set originates from the male parent, the other from the female parent.
• This is a source of variation

The fertilised ovum is referred to as a zygote.

Chromosome structure

Chromosomes:

Within the nucleus of cells chromosomes are:

• Thread like structures
• Made of the chemical DNA
• Consist of a sequence of bases (alkaline chemicals)
• The sequence of bases codes the sequence of amino acids in a protein
• This determines the structure and the function of the proteins made.

Meiosis

Meiosis occurs to produce gametes (sex cells)

• Body cells have two matching sets of chromosomes
• Gametes have one set of chromosomes.
  • The reduction in number of chromosomes to a single set occurs during meiosis
  • The full set of chromosomes is restored at fertilisation.
  • Matching chromosomes pair and separate during the production of gametes
  • The random assortment of chromosomes during gamete production leads to variety in offspring.

Sex Determination

In humans the X and Y chromosomes determine sex:

• In humans, each male gamete has an X or a Y chromosome in equal numbers
• Each female gamete has an X chromosome.
• At fertilisation the meeting of an X male gamete with a female X gamete results in a female offspring
• At fertilisation the meeting of a Y male gamete with a female X gamete results in a male offspring

The Monohybrid Cross

• A monohybrid cross a cross involving true breeding strains:
  • Chromosomes are divided into functional regions called genes.
  • Different forms of a gene are called alleles and each parent contributes one of the forms (with the exception of the X and Y chromosomes).
  • Each gamete carries one of the forms of the gene.

In a monohybrid cross:

• The experimenter begins with true breeding strains differing in one characteristic
  • This is the P (parental) generation

• Crossed together they produce the F1 (first filial) generation
  • All the organisms in the F1 generation show the dominant characteristic
  • The F1 generation is allowed to breed freely to produce the F2 (second filial) generation.

• The ratio of dominant to recessive characteristics in the F2 generation is 3:1
  • The true breeding organisms possessing the same alleles are homozygous
  • The non-true breeding organisms possessing different alleles are heterozygous
  • The type of alleles possessed by an organism is referred to as its genotype
  • The result of the genotype on the appearance or behaviour of the organism is its phenotype.

For example consider the cross of a white-eyed fruit fly (Drosophila) with a red-eyed fruit fly

We will call The red-eyed allele E

The white-eyed allele e

	P generation
	Phenotype	red eyed	x	white eyed
	Genotype	EE		ee
	gametes	all E		All e
		pair them up	Ee
	F1 generation
	Phenotype	red eyed		red eyed
	Genotype	Ee	x	Ee
	gametes	E and e		E and e
	F2 generation
	Genotypes	EE,  Ee,  eE,  ee
	Phenotypes	Red eyed flies and white eyed flies
	Ratios	3 : 1

Such inheritance is called Mendelian after Gregor Mendel who developed this process.

• It results when a single gene is involved
• Some inheritance is polygenic such as hair (four genes) and skin colour (three genes) involving several genes
• These give a more continuous form of inheritance.

Codominance

Codominance results when the alleles have the same weight – neither is dominant.

• In this case the phenotype is midway between the phenotypes of the two true breeding strains.
• For example consider the cross of a red shorthorn cow with a white shorthorn – the in between form is called roan.
• The white allele W The Red allele R

Environmental impact on phenotype

• The environment affects the final phenotype of the organism
• The environment and genotype interact to make the final appearance of an organism
• For example humans grow taller this century due to better feeding and medicine.
• These effects are not passed on to the next generation

Natural selection inheritance

The process by which Organisms that are better adapted to their environment survive and breed,

• While those less well adapted fail to do so.
• The better-adapted organisms are more likely to pass their characteristics to succeeding generations.
• Over many generations the species will change and become better adapted to its environment.
• The distribution of the peppered moth is a case in point.

The peppered form on the left is more common in unpolluted areas where it is well camouflaged on lichen covered tree bark.

The melanic (black) form on the right is better camouflaged on soot blackened tree bark in polluted areas.

Selective Breeding

Farmers and breeders have been breeding from selected stock for many generations:

By only breeding from organisms with a desired characteristic

over many generations this characteristic becomes part of the breed.

Genetic engineering.

Genes can be moved artificially between organisms and species.

• Bacteria have small circular chromosomes called plasmids
• Plasmids can be cut open using enzymes

Genes can be cut out of chromosomes using enzymes.

• An enzyme is used to insert the foreign gene into the plasmid and seal it back into a circle.
• The plasmid is placed back into the bacterium
• The bacterium will now produce the protein the foreign gene manufactures e.g.:
  • Insulin for diabetics
  • Factor VIII for blood clotting
  • Human Growth Hormone (HGH).
• Such a bacterium is referred to as transgenic

Advantages over traditional breeding methods are:

• Large-scale chemical production of human identical chemicals.
• Costs are much lower
• The time scale is much shorter

Genes can be moved between species and can add useful characteristics, e.g.

• disease resistant plants.

Disadvantages over traditional breeding methods are:

There is a danger of the release of genes or transgenic organisms into the environment where they could compete with normal species or cause damage to the environment.