D4.1 Natural selection

Evolution

Changes in the heritable characteristics of organisms over generations

Natural selection

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

Paradigm shift

Occurs when scientific research contradicts previous assumptions

Example of natural selection in rabbits

Rabbits have natural predators (eg. foxes), which act as a selection pressure. Brown rabbits are more likely to survive and reproduce due to having more effective camouflage. The frequency of brown fur alleles in the population will increase, and white fur will decrease

Mutation as a source of genetic variation

Mutation results in the generation of new alleles. They may be advantageous, disadvantageous, or have no effect. An advantageous allele is more likely to be passed on because it increases the chance of survival and reproduction. A disadvantageous allele will likely die out as it is less likely to survive and reproduce. Only mutations in germ cells are passed down

Meiosis as a source of genetic variation

Crossing over results in a new combination of alleles. Random orientation, due to independent assortment, creates a number of possible chromosome combinations in gametes

Random fertilisation as a source of genetic variation

The random fusion of gametes at fertilisation creates genetic variation between zygotes, as each will have a unique combination of alleles

Overproduction of offspring

It is more usual for organisms to produce multiple offspring. There are often more offspring produced than can be supported by the environment. This leads to competition for resources (density dependent factors), thus a large number of offspring fail to survive and reproduce, promoting natural selection

Intraspecific competition and natural selection

Survival of the fittest, individuals that are well adapted and survive into adulthood are more likely to reproduce, whereas individuals that are less well adapted do not survive long into adulthood and are less likely to reproduce.

Abiotic factor

A non-living factor within an ecosystem. They can act as selection pressures as they affect the survival of individuals in a population, causing fluctuations in population size. Examples include light intensity, temperature, oxygen levels, carbon dioxide levels

Sexual selection

A form of selection where individuals compete for mates and/or are chosen by mates based on specific traits. Differences may be viewed as indicators of overall fitness by females in the population

Selection pressures

External agents which affect an organism's ability to survive in a given environment

Effects of sexual selection

Reproductive isolation, occurs when changes in the alleles and phenotypes of some individuals prevent them from successfully breeding with individuals with unchanged alleles or phenotypes. Sexual dimorphism, which is the distinct difference in size or appearance between sexes of a species

John Endler's experiments with guppies

Conducted research to investigate the effects of natural and sexual selection on the evolution of guppies. Guppies were randomly assigned to go to ten ponds in the greenhouse, five with coarse gravel and five with fine gravel. The ponds were exposed to three different levels of predation, two with none, four with a dangerous predator, and four with a weak predator. Endler counted and measured the spots on all guppies. Endler found that in high predation the mean number of spots decreased, in low predation it increased. In coarse gravel they had larger spots, in fine gravel they had smaller spots (spot size mimicking gravel size). With no predation, in coarse gravel they had small spots, in fine gravel they had large spots (not matching their background makes males more visible, helping to attract females)

Gene pool

Consists of all the genes and their different alleles present in an interbreeding population

Stable gene pools

The population is large, each individual has an equal chance of mating, matings are random, there are no selection pressures. Means that the population is not evolving

Allele frequency

The relative frequency of an allele at a particular gene locus, they must add up to 1

Hardy-Weinberg formula

p+q=1, p is frequency of dominant, q is frequency of recessive

q = square root of recessive phenotype frequency

Directional selection

The change in a phenotype or genotype of a population in one direction away from the mean. Tends to happen when environmental conditions change

Stabilising selection

Favours the average individual within a population. Occurs when environmental conditions are stable. Favours the majority that is well adapted to the environment, thus decreasing variation

Disruptive selection

Selection pressures act against the average individuals in the population. Occurs when habitats or resources undergo a change (like directional). Can lead to the formation of a new species

Hardy-Weinberg principle

States that if certain conditions are met, the allele frequencies of a gene within a population will not change from one generation to the next

Hardy Weinberg main formula

p² + q² + 2pq = 1, used to estimate the allele and genotype frequencies in a population

Conditions for the Hardy-Weinberg principle

Organisms are diploid, organisms reproduce by sexual reproduction only, there is no overlap between generations, mating is random, the population is large, there is no migration, mutation, or selection, allele frequencies are equal in both sexes

Artificial selection (selective breeding)

The process by which humans choose organisms with desirable traits and selectively breed them to enhance the expression of these desirable traits over many generations

Natural vs artificial selection

Artificial selection involves a deliberate choice of traits by humans, which can sometimes result in animals being poorly suited to their environment