5.4 Evolution

1. 5.4 Evolution Pp 134 - 142

2. 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

3. Evidence for Evolution • It 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.

4. 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

5. Selective Breeding • man 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

7. Homologous Structures • Homologous 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.

8. Populations tend to over-produce • Many 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.

9. 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.

10. Members of a species show variation • Populations of a species show variation i.e. differences in phenotypes • Variation maybe discontinuouse.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

11. 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

12. How natural selection leads to evolution • Theory 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 population • some varieties are better adapted than others • best 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

14. Examples of evolution in response to environmental change • examples of evolution in response to environmental change includes: • antibiotic resistance in bacteria • melanism in peppered moth & ladybugs • heavy metal tolerance in plants • beak size in Darwin’s finches

15. Antibiotic Resistance • 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 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

16. 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 finchespopulation 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

17. Revision Questions • Defineevolution [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]