Variation is the differences that exist between individuals.
Intraspecific variation occurs within a species e.g. different European robins show variation in mass, length, wingspan, colour, beak size etc.
Interspecific variation occurs between different species e.g. a bee hummingbird compared to an ostrich.
Continuous variation is when the individuals in a population vary within a range - no distinct categories (quantitative).
- Animals - height, mass, milk yield
- Plants - leaf count, mass
- Microorganisms - width, length
Discontinuous variation is when there are two or more distinct categories and no intermediates (qualitative).
- Animals - sex, blood group
- Plants - colour, seed shape
- Microorganisms - antibiotic resistance, pigment production
Causes of Variation
Genetic factors (inherited)
Different species have different genes. Individuals of the same species have the same genes but different versions - alleles. The alleles makes up the organisms' genotype. The differences in genotype result in variation in phenotype (characteristics displayed by an organism).
E.g. eye colour in humans, blood type in humans, antibiotic resistance in bacteria
Differences in environment can also cause variation e.g. climate, food, lifestyle etc. Characteristics controlled by environmental factors can change over an organism's life.
E.g. accents, piercings
Genetic and environmental factors
Genetic factors determine birth characteristics but environmental factors influence development.
E.g. height (genes determine natural height but diet influences development), flagellum (some only grow in certain environments)
Variation gives some organisms a selective advantage because they are better adapted to their environment. Its adaptations increases the chances of its survival and reproduction.
Adaptations develop because of evolution by natural selection. In each generation, the best adapted individuals are more likely to survive and reproduce, hence passing their adaptations on to their offspring. Less well-adapted individuals more likely to die before reproducing.
Behavioural adaptations - the way an organism acts to increase survival chances
E.g. possums play dead when threatened by a predator, scorpions dance before mating to make sure mate is of same species to increase likelihood of successful mating
Physiological adaptations - processes inside an organism's body to increase survival chances
E.g. brown bears hibernate to conserve energy by lowering metabolism over winter, when food is scarce, some bacteria produce antibiotics which kill other bacteria species to lower competition
Anatomical adaptations - structural features of an organism's body to increase survival chances
E.g. otters are streamlined to glide through water making it easier to catch prey and escape predators, whales have thick layer of blubber to keep warm in cold sea
Evolution is the slow and continual change of organisms from one generation to the next.
- Organisms produce more offspring than survive
- Variation exists in characteristics of members of same species
- Some characteristics can be passed from one generation to the next
- Individuals best adapted to environment more likely to survive
Theory of evolution by natural selection
- Individuals in population show variation in phenotypes due to differences in alleles
- Predation, disease and competition create struggle for survival
- Better adapted individuals have selective advantage and are more likely to survive and reproduce to pass on alleles to offspring
- Over time, number of individuals with advantageous adaptations increase
- Over generations the alleles become common in the population
E.g. peppered moths show colour variation, before 1800s there were more light moths than dark moths, during 1800s pollution blackened many trees that moths lived on so dark coloured moths better adapted and could be camouflaged from predators so dark moths increased.
Evidence for Evolution
Darwin's Theory of Evolution was first met in 1859 with a lot of criticism because it conflicted with some religious beliefs. He knew very little about genes and alleles so had no mechanism evidence to explain his theory. However over time the theory has become increasingly accepted as evidence has been found to support it and no evidence to disprove it.
Fossil record evidence
Fossils are the remains of organisms preserved in rocks. They can be arranged in chronological order so that gradual changes in organisms can be observed. E.g. horse fossil records show gradual increase in size, length of limbs and hoof development.
Molecular evidence - DNA
All organisms evolved from common ancestors and closely related species diverged more recently. Evolution is caused by gradual changes in the base sequence of DNA so recently diverged organisms should have more similar DNA, as less time has passed for changes to occur. E.g. humans/chimps - 94%, humans/mice - 85%.
Molecular evidence - proteins and other molecules
Sequence of amino acids in proteins, antibodies, more similarities, less time for changes.
Speciation is the formation of a new species. A species is defined as a group of similar organisms that can reproduce to produce fertile offspring.
Speciation happens when populations of the same species are isolated in some way and evolve to become so different they cannot breed with one another to produce fertile offspring.
- 14 species of finches on the Galapogas Islands in the Pacific Ocean
- Each species of finch was unique to a single island
- They were quite similar but had different sized and shaped beaks because they were adapted to the food sources on their island
- All finches had common ancestor
- Different populations became geographically isolated
- Each population evolved adaptations to their environment
- Populations evolved so different they cannot breed to produce fertile offspring
Evolution of Resistance
Antibiotics are drugs that kill or inhibit bacteria growth. Some bacteria e.g. MRSA have become resistant to antibiotics e.g. methicillin.
Pesticides are chemicals that kill pests. Some species of insects have evolved resistance e.g. mosquitos to DDT.
- Variation in a population of bacteria/insects, genetic mutations make some bacteria/insects naturally resistant to an antibiotic/pesticide
- If population of bacteria/insects exposed to antibiotic/pesticide, only individuals with resistance survive and reproduce
- Alleles for antibiotic/pesticide resistance passed on to next generation until population is resistant to drug/chemical
This makes the infections/infestations harder to treat as it takes time for doctors/farmers to find an effective antibiotic/pesticide, and in that time the person could become very ill or die/crop could be destroyed. New antibiotics/pesticides have to be developed which costs time and money. Broader pesticides may need to be used which can kill beneficial insects.