Variation and evolution

Why organisms differ in their phenotypes

• They have a different genotype

• They have the same genotype but different epigenetic modifications

• They have different environments

Variation

The difference in phentypes between organisms of the same species

Heritable variation

If they result from different DNA nucleotides sequences or different epigenetic modifications, they can be inherited

in a asexually reproducing organisms, heritable variation could only be increased by mutation

Mechanisms that generate heritable variation from sexual reproduction

• Crossing over between homologus chromosomes during prophase l of meiosis

• the random distribution of chromosomes during metaphase lof meiosis

• random distribution of chromatids at metaphase ll of meiosis

• mixing of two different parental genotypes at fertilization

Non heritable variation

Variation imposed by the environment that is not a result of epigenetic change cannot be transmitted to offspring

Discontinuous variation

When there is only a certain number of possibilities for characteristics Eg a plant is either tall ordwarf

there are no intermediate types and the characteristics are discrete

each of these characteristics are controlled by single genes (monogenic)

the environment has no influence on the genes expression

the distribution of phenotype is shown best as a bar chart

Continuous variation

When there are many possible values of for characteristics eg. A baby born at 9 months tend to weigh between 2kg and 5kg

such characteristics are controlled by many genes (polygenic)

the environment has considerable influence on the genes expression and therefore has a role in determining phenotypic variation

Competition for breeding success and survival

When there is no environmental resistance organisms over produce: meaning to parents usually have more than two offspring

populations tend to get bigger from one generation to the next

Limits to population growth

• Some resources may be limited so individuals have to compete for them eg. soil, water, light etc

• animals compete for food and shelter

• as a result fewer offsprings are produced or offspring may die before maturity and can't reproduce

Inter-specific and intra-specific competition

• Interspecific: occurs between individuals of different species competing for the same resources

• Intraspecific: occurs between individuals of the same species

Selection pressure

when its limiting

An environmental factor that can alter the frequency of alleles in a population, when it is limiting

introduced by Charles Darwin

Natural selection

The increase the chance of survival and reproduction of organisms with phenotype suited to their environment, enhancing the transfer of favourable alleles from one generation to the next

Selective agencies

overcrowding and predators

• Overcrowding allows these diseases to spread. People with alleles that give them a more effective immune system are more likely to survive and pass those alleles on

• predation: when a population grows, predators have more food than their numbers increase. Some individuals in the prey population are more likely to survive and reproduce passing the alleles on that have made them successful. Eg:

• - some are better camouflaged

• - mimic species are harmless but have characteristics of other species that are toxic to the predator

Selective agencies

nesting sites

day length

temperature

human impact

• Availability of nesting sites: some animals raise young in various specific situations Eg. Must be a certain height above the ground and angle

• day length effects reproductive behaviour

• temperature effects survival eg. can only reproduce if cold enough or survive in hotter climates

• human impact: habitat lost has destroyed breeding grounds

The value of an allele depends on the environment

It's the phenotype that is suited or not to the particular environment

the phenotype is determined in part by the genotype

if the phenotype provides an advantage the alleles that produced it are transmitted to the next generation more successfully

the same allele may produce a phenotype that is suited to one environment but not to the other.

Gene pool

The total of all the alleles of all the genes in a population at a given time

Allele frequencies

The frequency of an allele in its proportion, fraction or percentage of all the alleles of that Gene in a gene pool

Proportions of alleles in the gene pool

Remains stable if the environment is stable

some phenotypes will be advantageous and will be selected for

so that alleles that produce them will be transmitted to the next generation

other phenotypes will be selected against and the alleles that produce them will not be passed on

the gene poo! is constantly changing and some alleles become more frequent and others

Genetic drift

Chance events\ variations in allele frequencies in a population

if a sexually reproducing population is in a stable environment and there is no mutation, immigration or emigration, the frequencies of all the alleles stay constant

genetic drift is most significant in small or isolated populations when a small number of alleles form a large proportion of the total

Hardy weinberg principle

States that in ideal conditions, alleles and genotype frequencies in a population are constant from generation to generation

Ideal conditions for Hardy wienberg principle

organisms

• Organisms are diploid

• allele frequencies are equal in both sexes

• mating is random

• they reproduce sexually

• Generations don't overlap

Ideal conditions for Hardy Weinberg principles

population

• Population sizes very large

• no immigration or emigration

• no mutation and no selection

Hardy Weinberg equation

p + q = 1

p² + 2pq + q² = 1

Evolution

A change in the average phenotype of a population

Speciation

The formation of a new species

if the change in phenotype is profound enough, organisms with the altered phenotype are unable to reproduce successfully with the initial population, therefore speciation occurs

Founder effect

The loss of genetic variation in the new population, established by a very small number of individuals from a larger population

while the founder population remained small, it may undergo genetic drift and become even more different from a large parental population

Stabilising selection

a characteristic showing continuous variation

in certain environments the average phenotype may provide a greater advantage than either extreme

extreme values will be selected against

the normal curve displaying the range of phenotypes over successive generations will have a smaller standard deviation but higher peak. Average stays the same but more individuals have that value

Directional selection

in a changing environment, extreme phenotype may become advantageous then other values are selected against and overtime, the average phenotype changes

• the allele all for this trait increases

• other alleles decrease

Disruptive selection

in some situations the average phenotype doesn't provide an advantage and is selected against

over several generations, a lower and higher value is selected and so a curve displaying the proportions of the population with values of the characteristic is bimodal

Species

A group of phenotypically similar organisms that can interbreed to produce fertile offspring

Reasons why it may not be possible to tell whether organisms can interbreed

• Are few in numbers and widely separated

• are only known as fossils

• have a very long life cycle

• only reproduce asexually

if physical characteristics are very similarr, organisms are seen to be in the same species

New species may arise

abruptly

gradually

• Abruptly: by polyploidy when they chromosome number doubles by endomitosis, more common in plants

• gradually, by isolating groups of individuals

Reproductive isolation

The prevention of reproduction and, therefore gene flow between breeding groups within a species

subgroups within the population may breed more often with each other, than with the rest of the population- demes

if a deme becomes isolated, it can't breed with members of other demes and the gene flow in and out of this deme is prevented

If demes are isolated for many generations

They undergo changes in allele frequency and accumulate so many different mutations that they are no longer able to interbreed successfully with the members of the initial population

speciation has occurred and the separate species each have their own gene pool

Types of reproductive isolation

pre zygotic

post zygotic

• Pre zygotic: gametes are prevented from fusing and so a zygote is never formed

• post zygotic: gametes fuse and a zygote forms. Even if the organism develops and grows, it is sterile and so the genes of the parent species are kept separate and the species don't merge

Pre zygotic isolation

geographical isolation

This occurs when the population becomes split by a physical barrier into separate demes

• called allopatric speciation

Eg. Mountain ranges, roads, rivers, canyons

Allopatric speciation

The evolution of new species from demes isolated in different geographical locations

Charles Darwin

Charles Darwin (1809-1882) was a naturalist and became the ships naturalist on the ship HMS Beagle. 

Between 1831 and 1836 he sailed around the world making observations of the flora and fauna.

changes of evolution

Evolution is a property of a population describing a change in the average phenotype, produced by changes in allele frequency in the population

These changes are brought about by:

• mutation

• Gene flow

• genetic drift

• natural selection

Darwin's observations

• Members of a population show variation

• individuals within a population have the potential to produce large numbers of offspring, yet the numbers of adults remain constant from one generation to the next

• resources are Limited

What Darwin the deduced from the observations

• There is a “struggle for existence” with only the “fittest” surviving

• those that survive reproduce and their offspring have characteristics that enable them to succeed

• in time a group of individuals undergo many changes and become sufficiently distinct to belong to a new species

• if the environmental conditions change, the features needed to survive in it will change. So natural selection is a continuous process

Types of evolutionary product

• Adaptive traits/ adaptations are features used in an original form

• exaptations are structures that appear to have had their original use modified

Evolution and environment

Evolution can only modify what is there and it does so in response to environmental conditions

• evolution is not a direct process, it is entirely contingent upon the environment. Eg. Tapeworm

• some structures don't appear to be ideally suited to their current function, suggesting some features may still be in the process of evolutionary modification