Unit 2: Section 8

Variation can occur:

• Within species - variation within a species is called intraspecific variation. 
• Between species - the variation between different species is called interspecific variation.

Continuous variation is when the individuals in a population vary within a range - there are no distinct categories:

• Animals - milk yield, mass
• Plants - number of leaves, mass
• Microorganisms - width, length

Discontinuous variation is when there are two or more distinct categories - each individual falls into only one of these categories, there are no intermediates:

• Animals - sex, blood group
• Plants - colour, seed shape
• Microorganisms - antibiotic resistance, pigment production

Causes of variation

Genetic factors

• Different species have different genes
• Individuals of the same species have the same genes, but different versions of them (alleles).
• The genes and alleles an organism has make up its genotype.
• The differences in genoty[e result in variation in phenotype - the characteristics displayed by an organism.
• Examples of variation caused only by genetic factors include blood group in humans and antibiotic resistance in bacteria.
• You inherit your genes from your parents. This means variation caused by genetic factors is inherited.

Environmental factors

• Variation can also be caused by differences in the environment, e.g. climate, food etc.
• Characteristics controlled by environmental factors can change over an organism's life.
• Examples of variation caused only by environmental factors include accents and whether people have pierced ears.

Both

• Genetic factors determine the characteristics an organism's born with, but environmental factors can influence how some characteristics develop. For example:
• Height - genes determine how tall an organism can grow, but diet or nutrient availability affect how tall an organism actually grows.
• Flagellum - genes determine if a microorganism can grow a flagellum, but some will only start to grow them in certain environments, e.g. if metal ions are present.

Adaptations make organisms well suited to their environment

• Being adapted to an environment means an organism has features that increase its chances of survival and reproduction, and also the chances of its offspring reproducing successfully.
• These features are called adaptations and can be behavioural, physiological and anatomical.
• Adaptations develop because of evolution by natural selection.
• In each generation, the best-adapted individuals are more likely to survive and reproduce - passing their adaptations on to their offspring. Individuals that are less well adapted are more likely to die before reproducing.

Behavioural adaptations - Ways an organism acts that invrease its chance of survival. For example:

• Possums sometimes 'play dead' - if they are being threatened by a predator they play dead to escape attack. This increases their chance of survival.
• Scorpians dance before mating - this makes sure they attract a mate of the same species, increasing the likelihood of successful mating.

Physiological adaptations - Processes in an organism's body that increase its chance of survival. For example:

• Brown bears hibernate - they lower their metabolism over winter. This conserves energy, so they don't need to look for food in the months when it's scarce - increasing their chance of survival.
• Some bacteria produce antibodies - these kill other species of bacteria in the area. This means there's less competition, so they're more likely to survive.

Anatomical (structural) adaptations - Structural features of an organism's body that increase its chance of survival. For example:

• Otters have a streamlined shape = making it easier to glide through the water. This makes it easier for them to catch prey and escape predators, increasing their chance of survival.
• Whales have a thick layer of blubber - this helps to keep them warm in the cold sea. This increases their chance of survival in places where their food is found.

Charles Darwin

Observations:

• Organisms produce more offspring than survive.
• There's variation in the characteristics of members of the same species.
• Some of these characteristics can be passed on from one generation to the next.
• Individuals that are best adapted to their environment are more likely to survive.

Theory:

• Individuals within a population show variation in their phenotypes.
• Predation, disease and competition create a struggle for survival.
• Individuals with better adaptations are more likely to survive, reproduce and pass on their advantageous adaptations to their offspring.
• Over time, the number of individual with the advantageous adaptations increases.
• Over generations this leads to evolution as the favourable adaptations become more common in the population.

Speciation

• Speciation is the formation of a new species.
• A species is defined as a group of similar organisms that can reproduc to produce fertile offspring.
• Species can exist as one or more populations.
• Speciation happens when populations of the same species evolve to become so different that they can't breed with one another to produce fertile offspring.

Darwin's finches

• Darwin observed 14 species of finch on the Galapagos Islands. Each species of finch was unique to a single island. Although the finches were similar, the size and shape of their beaks differed - they were adapted to the food sources found on their specific island:
• All the species of finch had a common ancestor
• Different populations became isolate on different islands.
• Each population evolved adaptations to their environment.
• The populations evolved to become so different that they could no longer breed together.
• They had evolved into separate species.

Evidence supporting evolution

Fossils

• Fossils are the remains of organisms preserved in rocks. By arranging fossils in chronological order, gradual changes in organisms can be observed that provide evidence of evolution.

DNA evidence

• The theory of evolution suggests that all organisms have evolved from shared common ancestors.
• Closely related species diverged more recently.
• Evolution is caused by gradual changes in the base sequence of organisms' DNA.
• So, organisms that diverged away from each other more recently should have more similar DNA, as less time has passed for changes in the DNA sequence to occur. This is exactly what scientists have found.

Molecular evidence

• In addition to DNA, the similarities in other molecules provide evidence.
• Scientists compare the sequence of amino acids in proteins and compare antibodies.
• Organisms that diverged away from each other more recently have more similar molecules, as less time has passed for changes in proteins and other molecules to occur.

Bacterial resistance to antibiotics

• Antibiotics are drugs that kill or inhibit the growth of bacteria.
• Scientists have observed the evolution of antibiotic resistance in many species of bacteria.
• For example, MRSA is a strain of bacteria that's resistant to methicillin.
• The evolution of antibiotic resistance can be explained by natural selection:
• There is variation in a population of bacteria. Genetic mutations make some bacteria naturally resistant to an antibiotic.
• If the population of bacteria is exposed to that antibiotic, only the individuals with resistance will survive to reproduce.
• The alleles which cause the antibiotic resistance will be passed on to the next generation, and so the population will evolve to become resistant to the drug.

Implications of antibiotic resistance for humans

• Infections caused by antibiotic-resistant bacteria are harder to treat - some species of bacteria are resistant to a lot of different antibiotics. It takes doctors a while to figure out which antibiotics will get rid of the infection, and in that time the patient could become very ill or die.
• There could come a point where a bacterium has developed resistance to all known antibiotics. To prevent this new antibiotics need to be developed. This takes time and a lot of money.

Insect resistance to pesticides

• Pesticides are chemicals that kill pests.
• Scientists have observed the evolution of pesticide resistance in many species of insect.
• For example, some populations of mosquito have evolved resistance to the pesticide DDT. Some populations of pollen beetles are sistant to pyrethroid pesticides.
• The evolution of pesticide resistance can be explained by natural selection:
• There is variation in a population of insects. Genetic mutations make some insects naturally resistant to a pesticide.
• If the population of insects is exposed to that pesticide, only the individuals with resistance will survive to reproduce.
• The alleles which cause the pesticide resistance will be passed on to the next generation, and so the population will evolve to become more resistant to the chemical.

Implications of pesticide resistance for humans

• Crop infestations with pesticide-resistant insects are harder to control - some insects are resistant to lots of different pesticides. It takes farmers a while to figure out which pesticide will kill the insect and in that time all the crop could be destroyed. If the insects are resistant to specific pesticides, farmers might have to use broader pesticides, which could kill beneficial insects.
• If disease-carrying insects become pesticide-resistant, the spread of disease could increase.
• A population of insects could evolve resistance to all pesticides in use. To prevent this new pesticides need to be produced. This takes time and costs money.