Evidence for Evolution

Evolution occurs when heritable characteristics of a species change

How does Natural Selection Lead to Evolution

It is crucial that you remember Darwin’s steps of how natural selection leads to evolution. Be sure to memorize the following:

(1) overproduction of offspring

(2) variation within the population, as a result of meiosis, sexual reproduction, and mutations

(3) struggle for survival, because there are not enough resources for all members of the population

(4) differential survival, those individuals best fit for their environment tend to survive better

(5) reproduction, those who survive can pass on their genes to the next generation.

It is through these steps that populations evolve. Remember that, even though the changes can be observed in individuals from generation to generation, what is of importance is what happens at the level of populations rather than at the individual level.

The fossil record provides evidence by revealing the features of an ancestor for comparison against living descendants
Selective breeding provides evidence of evolution as targeted breeds can show significant variation in a (relatively) short period
Comparative anatomy of groups of organisms may show certain structural features that are similar, implying common ancestry
Vestigial Structures in some species show the presence of functionless and reduced remnants of organs that were once present in their ancestors
Biogeography provides evidence for evolution because it suggests that closely distributed species share a common lineage

Fossil Record

The fossil record provides evidence for evolution

A fossil is the preserved remains or traces of any organism from the remote past

Fossils can be dated by determining the age of the rock layer (strata) in which the fossil is found

Sedimentary rock layers develop in a chronological order, such that lower layers are older and newer strata form on top
Each strata represents a variable length of time that is classified according to a geological time scale (eons, eras, periods)

Different kinds of organisms are found in rocks of particular ages in a consistent order, indicating a sequence of development

Prokaryotes appear in the fossil record before eukaryotes
Ferns appear in the fossil record before flowering plants
Invertebrates appear in the fossil record before vertebrate species

This chronological sequence of complexity by which characteristics appear to develop is known as the law of fossil succession

This ordered succession of fossils suggests that newer species likely evolved as a result of changes to ancestral species

Transitional Fossils

While fossils may provide clues as to evolutionary relationships, it is important to realise that the fossil record is incomplete

Fossilisation requires an unusual set of specific circumstances in order to occur, meaning very few organisms become fossils
Only the hard parts of an organism are typically preserved, meaning usually only fragments of remains are discovered
With limited fossil data, it can be difficult to discern the evolutionary patterns that result from ancestral forms (‘missing links’)

Transitional fossils demonstrate the intermediary forms that occurred over the evolutionary pathway taken by a single genus

They establish the links between species by exhibiting traits common to both an ancestor and its predicted descendents
An example of a transitional fossil is archaeopteryx, which links the evolution of dinosaurs (jaws, claws) to birds (feathers)
As new fossils are discovered, new evolutionary patterns are emerging and old assumptions are challenged

While fossils may provide clues regarding evolutionary processes and ancestral relationships, it is important to realise that the fossil record is incomplete

Fossilization requires a unusual combination of specific circumstances to occur, meaning there are many gaps in the fossil record
Only the hard parts of an organism are preserved and often only fragments of fossilized remains are discovered

Archaeopteryx

Fossil Formation

Fossilisation is a rare process, the vast majority of deceased organisms disappear without leaving a trace

In order for fossilisation to occur, the following conditions are required:

Hard body parts (bones, teeth, shells) – soft body parts will not fossilise, but may leave behind trace evidence (e.g. imprints)
Preservation of remains (protection against scavenging, erosion and environmental damage)
High pressure to promote mineralisation of remains (i.e. turn hard body parts into fossilised rocks)
Anoxic (low oxygen) conditions to protect against oxygen damage and prevent decomposition by saprotrophs

The stages of fossilisation generally occur as follows:

1. Death and decay – Soft body parts are decomposed or scavenged, leaving only the hard body remains

2. Deposition – The hard remains are rapidly covered with silt and sand, and over time more layers continue to build

3. Permineralisation – Pressure from the covering layers of dirt/rock cause the hard organic material to be replaced by minerals

4. Erosion / exposure – Movement of earth plates may displace the fossil and return it to the surface for discovery

Summary Video

Selective Breeding/Artificial Selection

click for more info

Selective breeding is a form of artificial selection, whereby man intervenes in the breeding of species to produce desired traits in offspring

By breeding members of a species with a desired trait, the trait’s frequency becomes more common in successive generations
Selective breeding provides evidence of evolution as targeted breeds can show significant variation in a (relatively) short period

Selective breeding of plant crops has allowed for the generation of new types of foods from the same ancestral plant source

Plants of the genus Brassica have been bred to produce different foods by modifying plant sections through artificial selection
This includes broccoli (modified flower buds), cabbage (modified leaf buds) and kale (modified leaves)

Selective breeding of domesticated animals has also resulted in the generation of diverse breeds of offspring

Examples of selective breeding of domesticated animals can be seen in horse, cows and dogs

Compartive Anatomy

Comparative anatomy of groups of organisms may show certain structural features that are similar, implying common ancestry

Anatomical features that are similar in basic structure despite being used in different ways are called homologous structures
The more similar the homologous structures between two species are, the more closely related they are likely to be

Homologous structures illustrate adaptive radiation, whereby several new species rapidly diversify from an ancestral source, with each new species adapted to utilise a specific unoccupied niche

Homologous Structures

Adaptive Radiation

Adaptive radiation Adaptive radiation occurs when many similar but distinct species evolve relatively rapidly from a single species or from a small number of species. This happens as variations within a population allow certain members to exploit a slightly different niche in a more successful way. A niche is a position or role within a community of an ecosystem. By natural selection and the presence of some kind of barrier, a new species can evolve. A barrier separating populations might be a mountain range or a body of water.

An example of this are the primates found in Madagascar and the Comoro Islands off the southeast coast of Africa. Millions of years ago, without competition from monkeys or apes, lemurs on these islands were able to proliferate. Large numbers of offspring meant a greater chance for diversity.

Among the wide range of variation in lemur species, some are better adapted for living on the ground instead of in trees. Others are better adapted for living in lush rainforests, while some can survive in the desert. Most lemurs are active during the day (diurnal) but some are nocturnal. The reason why there are so many different species of lemur with different specialties is because of adaptive radiation.

Not a single species of living lemur has been found anywhere else in the world. And yet fossils of their ancestors have been found on the continents of Africa, Europe, and Asia. What happened? It is believed that lemurs were not successful in competing with apes and monkeys, because as soon as traces of the latter start to become more prevalent in the fossil record, the lemur-like organisms become rare.

The Pentadactyl Limb

A classical example of homologous structures is the pentadactyl limb in a variety of different animals

Mammals, birds, amphibians and reptiles all share a similar arrangement of bones in their appendages based on a five-digit limb

Despite possessing similar bone arrangements, animal limbs may be highly dissimilar according to the mode of locomotion:

Human hands are adapted for tool manipulation (power vs precision grip)
Bird and bat wings are adapted for flying
Horse hooves are adapted for galloping
Whale and dolphin fins are adapted for swimming

Analagous Structures

Structural traits are not commonly used to determine clades as such features may not necessarily indicate shared heritage

Traits that are similar because they are derived from common ancestry are termed homologous structures
Traits that are superficially similar but were derived through separate evolutionary pathways are termed analogous structures

Vestigial Structures

Some species show the presence of functionless and reduced remnants of organs that were once present in their ancestors

Changes to the environment have rendered these organs redundant and so over time they have lost their functionality

These structures are called vestigial organs and demonstrate the evolutionary divergence of a species from a past activity

An example of a vestigial organ is the pelvic bone in whales – this bone suggests that whale ancestors were terrestrial mammals

Comparitive Embryology

Some species show the presence of functionless and reduced remnants of organs that were once present in their ancestors

Changes to the environment have rendered these organs redundant and so over time they have lost their functionality

These structures are called vestigial organs and demonstrate the evolutionary divergence of a species from a past activity

An example of a vestigial organ is the pelvic bone in whales – this bone suggests that whale ancestors were terrestrial mammals

Biogeography

Biogeography describes the distribution of lifeforms over geographical areas, both in past and present times

Related species are usually found in close physical proximity (supporting the concept of speciation via gradual divergence)
Fossils found in a particular region tend to closely resemble the modern organisms of the region

Biogeography provides evidence for evolution because it suggests that closely distributed species share a common lineage

If speciation was random, the distribution of structurally similar species would be expected to be scattered

Examples of biogeographical distribution indicating shared ancestry can be observed by the fact that:

Most modern marsupials are found almost exclusively in Australia (~70% of extant species)
Australia has few placental mammals compared to South America, even though environmental conditions are similar

Exceptions to this correlation between biogeographical distribution and common ancestry can be explained by continental drift

Over 250 million years ago, there was a single continental landmass (Pangaea) which split into the 6 current land regions
Closely related species that were separated by the breaking landmass are localised to regions that were once connected
- For example, ratites (flightless birds) are distributed globally according to regions that were once part of Gondwanaland

Famous Examples

Darwin's Finches

Antibiotic Resistance

Google Sites

Report abuse