5.1 ~ Evidence for Evolution

Evolution definition

The cumulative change in the heritable characteristics of a population

Three features shared by water and land mammals

1. Have placentas

2. Give live birth, feed milk to young

3. Warm blooded

4. No gills

Structure of bones in the pentadactyl limb

1 upper arm bone

2 lower arms bones

Several wrist bones

Five hands bones

Five finger bones

Evolution

• Evolution is the characteristics of a species changing over time

• Characteristics can be acquired

  • They are developed during the lifetime of an individual (e.g. scars, broken bones)

• Or heritable

  • Passed on from parent to offspring

• The mechanism of evolution is by natural selection

Evidence to support theory

• Fossils

  • Show different species existed in the past/species changer over time

• Selective breeding of domesticated animals/ crop plants

  • Shows that (artificial) selection can cause rapid change

• Homologous (anatomical) structures/ vestigial organs

  • Homologous structures/ pendactyl limbs/ other examples show common ancestry

How fossils are formed

• Sedimentary rock is formed by sediment (sand, mud, etc.) being deposited and compressed at high pressure over millions of years

• Organisms that die amongst this sediment may become trapped within the rock and form a fossil

• The strata (layers) of rock can be dated using radioisotope dating and linked to particular geological eras

• From this fossil record, we can look at the evolution of species over time

Fossils supporting theory of evolution

• Fossils can show the evolution of physical characteristics of organisms

• Fossils can be moles, imprints, casts or preserved parts of organisms (or even the whole organism)

• Trace fossils include footprints, faeces or burros, can give indicators of the behaviour of organisms

• The fossil record is incomplete as:

  • Only the hard parts of an organism (bones, shells, etc.) are preserved - soft tissue will decompose

  • Usually only parts of organism remain as rest may have been dispersed

  • Natural disasters may have destroyed fossil remans (may not have found them yet)

Selective breeding

• A form of artificial selection, breeding individual species based on desirable characteristics

• There is often huge variation between domestic livestock and closely related wild species

  • E.g. modern egg-laying hens and jungle fowl

  • In addition, there is huge variation between breeds of domesticated livestock, such as cattle, sheep and horses

• This variation is achieved by repeatedly selecting and breeding for particular characteristics

• Artificial selection also demonstrates how quickly considerable changes in characteristics can take place

Convergent evolution

Article organisms:

Clearly do not have the same ancestry but have evolved similar characteristics because of the environment they all lived in

Analogous structures

• Structures that have evolved independently in two different species to serve the same purpose

  • E.g. the tail fins of whales and fish

• However, close inspection of these structures often shows them to be very differnt

• This suggests that the species may have had different origins; however, evolved similar structures to perform the same function due to the environment they were living in

  • Convergent evolution

Homologous structures

• Structures that look superficially different and may perform a different function; however, have a “unity of type” - i.e. they appear to have the same origin

• Evolutionary explanation is that all of species had a common ancestor with a pentadctyl limb, but they evolved to become different because they performed different functions

  • Adaptive radiation

Evolution of homologous structures

• There are many examples of homologous structures, however, these don’t prove but suggests that organisms evolved or had a common ancestor

• Redimentary/vestigial organs are another example

  • Reduced structures that appear to have no function in present day organisms but are though to be ‘remnants’ of their ancestors

• Examples include the small pelvis and thigh bone found in the body wall of whales and snakes and the appendix in humans

• Evolution explains the presence of these structures: they once had a function but, due to changes in conditions and/or the environment, they no longer have a function

Structures which make up the pentadactyl limb

• Bone structure

  • Single bones in the proximal part

  • Two bones in the distal part

  • Group of wrist/ankle bones

  • Series of bones in each of five digits

• Forelimb

  • Humerus

  • Radius and Luna

  • Carpals

  • Metacarpals and phalanges

• Hindlimb

  • Femur

  • Tibia and fibula

  • Tarsals

  • Metarsals and phalanges

Speciation

1. Populations of a species can gradually diverge into separate species by evolution

2. Continuous variation across the geographical range of related populations matches the concept of gradual divergence

3. Development of melanistic insects in polluted areas

Populations of a species

• If populations of a species are separated so that they do not interbreed, they may evolve in different ways

  • Because the selections pressures on these populations may vary and so natural selection will act differently

• The characteristics of the population will gradually diverge until they are recognisably different

• Over time, these two populations would not interbreed even if they were to merge back together

  • They now indentifiably different species

  • Process is called speciation

Where does speciation occur

• When a population migrates; for example, to an island

• An endemic species is one that is found only in a certain geographical area

  • E.g. lava lizards on the Galápagos Islands

• This one species of lava lizards is found on the main islands of the Galapagos

  • On the smaller surrounding islands, there are closely-related but different species of the lizard

  • This speciation is thought to have occured when populations of the main species migrated to the smaller islands

Continuous variations and gradual divergence

• Galapagos finches show continuous variation in characteristics across their geographic range

  • E.g. their beaks change shape and size depending on the food available in their particular geographic location

  • However, there are not distinct ‘categories’ of beak size: there is continuous range in size across all species

• Demonstrates how each species gradually diverged from the original species

• The more similar the environment (habitat, diet) tend to be more genetically similar as the divergence is more subtle than organisms that exist in wildly different environments

Moths

• Dark varieties of typically light-coloured insects are called melanistic insects

• The peppered moth is probably the most famous example of a melanistic insect

• The peppered moth is an example of natural selection in industrialised areas

  • Adult moths tend to fly at night to mate and reproduce and roost during the day in the branches of threes

  • Births and other animals hunt they in daylight if they can find them

  • In unpolluted areas, tree branches are covered in pale lichens and the light moths are well camouflaged

  • In polluted areas, tree branches are covered in darker lichens and the light moths would be easily sported and eaten by predators

  • Over time, darker melanistic varieties of the moth in polluted areas as they were able to camouflage against the darker trees and survived to reproduce