5.4 Evolution

Pp 134 - 142

Define evolution. Evolution is the cumulative

change in the heritable characteristics of a population over time.

Not only does species evolve over time, but also new species can arise by evolution from pre-existing ones.

Variation within a species is as result of different selection pressures operating in different parts of the world, yet this variation is not so vast to justify a construct such as race having a biological or scientific basis

Evidence for EvolutionIt is difficult to prove

that all organisms on Earth are as result of evolution, but there is a lot of evidence that supports the theory of evolution, including;-fossil record, selective breeding of

domesticated animals &

homologous structures.

Fossil Records fossils show changes in organisms over time

fossilized organisms are different from those that are existing today, but they also share features with existing organisms (i.e. have homologous structures) suggest common ancestry;

fossil show intermediate stages in evolution of groups i.e. there are missing link fossils

Selective Breedingman has selectively breed

animals and plants for thousands of years.

the breeds of animal that are reared for human use are clearly related to wild species & in many cases domesticated animals can still interbreed with their wild relative

domesticated breeds have been developed by selecting desirable traits, and breeding from them

the striking differences in the heritable characteristics of domesticated breeds provides evidence that species can evolve rapidly

Homologous StructuresHomologous structures are

structures that have developed from a common ancestor , sharing same fundamental plan but performing different functions

The pentadactyl limb is an example of a homologous structure found in groups of organisms.

the forelimbs of all tetrapods (such as amphibians, reptiles, birds, mammals) have the same basic pattern of 5 metacarpals and 5 phalanges arising from the same embryological structures because development is determined by many shared genes from a common a ancestor

in each case the bones are modified and adapted to the function of the limb such as digging, swimming, flying, grasping etc.

Populations tend to over-produceMany species often

produce more offspring than can be supported by the environment in which they live i.e. more than the carrying capacity

For example, female frogs lay hundreds of eggs but only a handful survive to adulthood and plants often produce hundreds more seeds than necessary to propagate the species.

Consequence of the potential overproduction of offspring

The population produces more offspring than the carrying capacity of the environment can support

Overproduction results in struggle for existence i.e. offspring compete for limited resources (intraspecific competition)

Some individuals have characteristic (or combination ) that give them a competitive advantage.

These individuals are consequently 'fitter' in terms of freedom from disease, food availability etc.

These individuals are more likely to successfully reproduce (offspring survive)

Through inheritance of the genes for these advantageous characteristics the frequency of these characteristics become greater in the next generation.

Members of a species show variation

Populations of a species show variation i.e. differences in phenotypes

Variation maybe discontinuous e.g. blood groups or continuous e.g. skin colour

Discontinuous variation usually indicates the condition is controlled by one to two genes while Continuous variation normally indicates a polygenic condition or multiple alleles

How sexual reproduction promotes variation in a species.

segregation of alleles during meiosis

crossing over in prophase I of meiosis

random orientation i.e. assortment of homologues at metaphase I

fertilization by chance, one of many male gametes fertile the ovum

Large number of possible different gametes is 2n (223)

genes (alleles) combining from two parents

How natural selection leads to evolutionTheory of evolution by natural

selection originally advanced by Darwin & Wallace

based on observations that overproduction of offspring leads to struggle for survival

variation exists within the populationsome varieties are better adapted

than othersbest adapted individuals survive,

reproduce and pass on their characteristics (genes)to the next generation i.e. they are “selected for”

with time, advantageous variants become more frequent in the population

evolution is change in species (allele frequency) with time

evidence that species have evolved include observed evolution such as multiple antibiotic resistance

Examples of evolution in response to environmental change

examples of evolution in response to environmental change includes: antibiotic resistance

in bacteriamelanism in

peppered moth & ladybugs

heavy metal tolerance in plants

beak size in Darwin’s finches

Antibiotic Resistanceevolution is the process of

cumulative change over time the theory of evolution in

response to environmental change was proposed by Darwin & Wallace

populations tend to grow exponentially

more offspring than the environment can sustain are produced resulting in struggle for survival

but the populations still remain constant over time

individuals in populations show heritable variation due to mutations & sexual reproduction

individuals in a population may have traits that are better suit them to the environment e.g. resistance to antibiotics in bacteria

resistance to antibiotics in bacteria is heritable i.e. it can be passed from one generation to the next

individuals with antibiotic resistance will survive when antibiotic is applied

they will reproduce and leave more offspring with resistance to antibiotic

the population will tend to accumulate the adaptation of antibiotic resistance

therefore, the population will evolve with time

Evolution of beak size in Darwin’s finches evolution is the process of

cumulative change over time the theory of evolution in

response to environmental change was proposed by Darwin & Wallace

populations tend to grow exponentially

more offspring than the environment can sustain are produced resulting in struggle for survival

but the populations still remain constant over time

individuals in populations show heritable variation due to mutations & sexual reproduction

individuals in Darwin’s finches population show variation in beak size between weak long beaks & strong short beaks

change in environment; weather dried up, nuts become more common than fruits

individuals with short strong beaks got more food, survived & reproduced i.e. They had survival & reproductive advantage

they reproduced & passed the genes for strong beaks to their offspring

with time population will tend to more finches with short strong beaks

therefore, the population will evolve with time

Revision QuestionsDefine evolution

[1]Outline the

evidences for evolution [6]

Explain the consequence of the potential overproduction of offspring [3]

Explain how sexual reproduction promotes variation in a species [6]

Explain how natural selection leads to evolution [8]

Giving 2 examples, explain how evolution occurs in response to environmental change [9]