Climate Change and Evolution

evolution definition

Evolution can be defined as changes in the heritable characteristics of organisms over generations

• Heritable characteristics are those that can be inherited by, or passed on to, the next generation

  • Changes in characteristics that are not inherited, e.g. a plant having its leaves eaten, do not lead to evolution

what is process of natural selection

• Heritable characteristics are determined by the alleles of genes that are present in an individual

• Selection pressures drive natural selection and when the environment changes a new selection pressure is exerted on a population of organisms.

• Alleles may change as a result of random mutation, causing them to become more or less advantageous. Change in conditions mean some alleles become advantageous and others not

• Heritable characteristics that are advantageous are more likely to be passed on to offspring, leading to a gradual change in a species over time

  • This is the process of natural selection

natural selection definition

the process by which organisms that are better adapted to their environment survive, reproduce, and pass on their advantageous alleles, causing advantageous characteristics to increase in frequency within a population

stages in natural selection

• Variation exists between individuals in a population

• Natural selection can only take place if variation is present

• In any habitat there are environmental factors that affect survival chances

  • Environmental factors that influence survival chances are said to act as selection pressures

• In any population, due to the variation present, some individuals will have characteristics that make them better adapted for survival in the face of any selection pressures

  • This is sometimes described as 'survival of the fittest'

• Individuals that are well adapted and survive into adulthood are more likely to find a mate and reproduce, producing many offspring

• This means that they are more likely to pass on the alleles that code for these advantageous heritable characteristics to their offspring

• The characteristic increases in frequency as more individuals have it

• Eventually this favourable characteristic will become the most common of its kind in the population; the population can be said to have adapted to its environment by the process of natural selection

• Unfavourable characteristics decrease in frequency by same process

what causes variation

• Variation results from small differences in DNA base sequences between individual organisms within a population

• Sources of variation include

  • Mutation

  • Meiosis

  • Random fertilisation during sexual reproduction

what is speciation?

• Speciation can be defined as the development of new species from pre-existing species over time

what must happen for speciation to occur

• In order for speciation to occur two populations of the same species must be isolated from each other in some way

  • When this happens, there can no longer be an exchange of genes between the two populations

  • So reduced gene flow

what may cause isolation of populations

• Geographical isolation

  • This leads to a type of speciation known as allopatric speciation

• Random mutations that prevent them from interbreeding with each other

  • This leads to a type of speciation known as sympatric speciation

how speciation occurs

• Populations that are isolated from each other may face different selection pressures in their environment e.g. different predators or sources of food

• The different environmental conditions for the two populations might mean that different alleles are advantageous, so different alleles are more likely to be passed on and become more frequent in each population; this is the process of natural selection

  • The allele frequencies in the two populations change over time

  • Note that a process known as genetic drift can also affect allele frequencies

• Over time the two populations may begin to differ physiologically, behaviourally and morphologically to such an extent that they can no longer interbreed to produce fertile offspring; speciation has occurred

what is allopatric speciation?

• occurs as a result of geographical isolation

• Allopatric speciation occurs when populations of a species become separated from each other by geographical barriers

  • The barrier could be natural e.g. a body of water or a mountain range

  • It can also be man-made e.g. a motorway

• This creates two populations of the same species between which no gene flow is taking place

• Allele frequencies in the gene pools of the two populations may change in different ways due to 

  • Different selection pressures acting on them

  • The accumulation of random changes resulting from genetic drift

• Changing allele frequencies will lead to changes in the phenotypes of the two populations

• If enough allele frequency differences arise between the two populations, then they will eventually no longer be able to breed with each other and produce fertile offspring, and can be said to be separate species

allopatric speciation in trees

• A population of trees exists in a mountainous habitat

• A new mountain range forms that divides the species into two populations

• The geographical barrier prevents the two populations from interbreeding so there is no gene flow between them

• The two populations experience different environments, so different alleles become advantageous

• Different alleles are therefore more likely to be passed on in each population

• Different alleles become more frequent in each population

• Over thousands of years the divided populations form two distinct species that can no longer interbreed to produce fertile offspring

what is sympatric speciation

• takes place with no geographical barrier

• Isolation instead occurs when random changes in the alleles and therefore phenotypes of some individuals in a population prevent them from successfully breeding with other individuals in the population

what are the examples of phenotype changes that can lead to isolation

• seasonal changes

• mechanical changes

• behavioural changes

how seasonal changes lead to isolation

• Some individuals in a population may develop different mating or flowering seasons to the rest of the population i.e their reproductive timings no longer match up

Mechanical changes

• Some individuals in a population may develop changes in their genitalia that prevent them from mating successfully with individuals of the opposite sex i.e. their reproductive body parts no longer match up