5.2 Natural Selection

Who created the natural selection theory?

Charles Darwin

Conditions to which natural selection responds

* Inherited variation - there is genetic variation within a population which can be inherited
* Competition - There is a struggle for survival
* Selection - Environmental pressures lead to differential reproduction within a population
* Adaptations - Individuals with beneficial traits will be more likely to survive and pass these traits on to their offspring
* Evolution - Over time, there is a change in allele frequency within the population gene pool.

Three ways by which genetic variation between individuals in a species may occur:

* Mutations
* Meiosis
* Sexual reproduction

Mutations (ways for genetic variation)

Change in nucleotide sequence of a section of DNA which forms new alleles

Meiosis (ways for genetic variation)

* Crossing over → the exchange of segments of DNA between homologous chromosomes during prophase I


* The exchange of genetic material occurs between __non-sister chromatids__ at points called *chiasmata*


* Independent assortment → when homologous chromosomes line up in metaphase I, their orientation towards the opposing poles is __random__
* This causes different combinations of maternal and paternal chromosomes

Sexual reproduction (ways for genetic variation)

The fusion of two haploid gametes results in the formation of a diploid zygote

* This zygote can then divide by mitosis and differentiate to form a developing embryo

The Malthusian dilemma

* Populations multiply geometrically, while food resources only increase arithmetically.
* In other words, species tend to produce more offspring than the environment can sustainably support

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This causes competition

Types of adaptations

* Structural - differences in biological structure
* Behavioural - differences in patterns of activity
* Physiological - variation in detection and response by vital organs
* Biochemical - differences in molecular composition of cells and enzyme functions
* Developmental - variable changes that occur across the life span of an organism

Beneficial alleles and allele frequency

Beneficial alleles will lead to a higher allele frequency because it is passed on. Visa versa

Adaptive radiation

Describes the rapid evolutionary diversification of a single ancestral line

* occurs when members of a species occupy a variety of distinct niches with different environmental conditions
* consequently, members evolve adaptations in response to the different selection pressures.

example adaptive radiation

* Variety of beak types seen in the finches of the Galapagos Islands
* These finches have specialised beak shapes depending on their primary source of nutrition

Daphne Major

* A volcanic island that forms part of the archipelago (galapagos islands)
* Habitat of bird species; Darwin’s finches
* Darwin’s finches demonstrate adaptive radiation and show marked variation in beak size and shape according to diet
* In 1977, an extended drought changed the frequency of larger beak sizes within the population by natural selection
* Dry conditions result in plants producing larger seeds with tougher seed casings
* Between 1976 and 1978 there was a change in average beak depth within the finch population
* Finches with larger beaks were better equipped to feed on the seeds and thus produced more offspring with larger beaks

Example of antibiotic resistance

Staphylococcus auereus (Golden staph)

* Golden Staph can cause infection to skin
* Historically this was treated using antibiotic methicillin
* Bacterial strains developed that were resistant to this antibiotic (methicillin-resistant *Staphylococcus aureus* – or MRSA)
* These strains proliferated while susceptible strains died out (methicillin-sensitive *Staphylococcus aureus* – or MSSA)
* MRSA infections are now especially present in hospitals and nursing homes, where the use of methicillin was most common