5.4 Evolution

Define Evolution

• Evolution is the process of cumulative change in the heritable characteristics of a population

Charles Darwin at age 22 traveled on HMS Beagle in 1831

Evidence for Evolution

1. Fossil Record • Most fossil data gives us sporadic information

rather than continuous data• Overall, life which existed more than 500 mya was

very different from life today• Apart from organisms like sharks, cockroaches,

ferns most organisms are very different from fossilized ancestors

• Although most top predators are mammals, none of them existed during the Age of the Dinosaur

Evidence for Evolution

2. Selective Breeding of Domestic Animals• Provides good evidence for recent changes in

heritable characteristics• Idea is to breed best plant/animal specimens

together so offspring have higher likelihood of retaining those traits– Ex: milk and meat cows

Evidence for Evolution

3. Homologous Structures• All life is connected through evolutionary history

and therefore some organisms must share similar structures– Ex: five-fingered (pentadactyl) limb

a) Humerusb) Radiusc) Ulna

Natural Selection• Darwin noticed that plants and animals produce

more offspring than the carrying capacity of that environment can support– Ex: plants produce more seeds than propagate– Ex: mushrooms produce more spores than grow– Ex: fish lay more eggs than survive

• Not all offspring survive because of limited resources leads to struggle for survival

Consequences of Overproduction

1. Overpopulation and limited resources causes competition

2. Competition can lead to adaptive behaviors and allow organisms “more fit” for environment to have higher survival/reproduction rates and then these organisms’ genes are more likely to be passed on to the next generation

Variation

• Members of a species can show variation (variation refers to differences in phenotypes)

Continuous Variation Discontinuous Variation

Ex: blood groupsEx: height of trees in forest

Mutations and the Environment are a couple of

methods which increase variation

How does Sexual Reproduction Increase Variation?

1. Meiosis – due to random orientation and independent assortment, each gamete contains a unique set of parental chromosomes

How does Sexual Reproduction Increase Variation?

2. Fertilization – random fertilization (egg + sperm) allows new combination of genes in organism

3. Crossing over during Prophase I of Meiosis also increases variation

How Natural Selection leads to Evolution

• Natural selection is a process where the survival/thriving of organisms is dependent on the compatibility between their characteristics and the environment

• The Steps of Evolution by Natural Selection:– Overproduction of offspring with genetic variance• Better suited for enviro = higher likelihood to survive/repro• Less suited for enviro = lower likelihood to survive/repro

– Over many generations these effects can change the gene pool…EVOLUTION

Examples of Natural Selection in response to Environmental Change

1. Antibiotic Resistance:Antibiotics are medications given for bacterial infections, but some bacteria are resistant to the antibiotics. The resistant bacteria can transfer their plasmids to other bacteria, creating strains that are resistant

Ex: a new strain of Staph bacteria (MRSA) has evolved to be resistant to antibiotics

Examples of Natural Selection in response to Environmental Change

1. Peppered MothPrior to Britain’s Industrial Revolution (1780-1850), both moths lived in the area but by the mid-1800’s the environment in England had changed due to soot blanketing the trees so that the light colored moths were easier for birds to hunt therefore the darker moths had a natural advantage