Patterns of Macroevolution

Divergent Evolution

a pattern of evolution in which species that were once similar to an ancestral species diverge, or become increasingly distinct

Ex. Elephants and mammoths originally have a common ancestor – similar structures but have evolved according to their environments!

Ex. Humans and apes

Convergent Evolution

a pattern of evolution in which similar traits arise because different species have independently adapted to similar environmental conditions

Ex. Sharks and Dolphins – very similarly adapted, but distantly related

Ex. A shark’s fin and a dolphin’s flipper are completely different structures!

Convergent Evolution - Analogous Structures

Because different animals have similar environmental needs, they have adapted similar structures – even from across the world

This is why we can see structures that are similar in un-related organisms, otherwise called analogous structures

This is evidence of convergent evolution

Divergent Evolution - Homologous Structures

On the other hand, some animals start out with a common ancestor, but their environments become quite different

We can now see homologous structures – which have different functions but similar anatomical structures

This suggests divergent evolution has occurred

Adaptive Radiation

Adaptive Radiation: a form of divergent evolution where a common ancestral species diversifies into a variety of differently adapted species

Islands are a great place to study this – “living laboratories”

Islands give animals the opportunity to change in response to new environmental conditions while being isolated from the parent population

Ex. The Hawaiian Islands: originally had no life at all, but over time, gradually populated by species travelling through the ocean or winds. Now one of the most biologically diverse places, with species not found anywhere else in the world!

Ecological Niche

the ecological role and physical distribution of a species in its environment

Darwin’s Finches

A population of finches in the Galapagos

In the past, members of the ancestral species reached one of the islands

There were many unoccupied ecological niches for this species to move into

Finches on different islands faced different selective pressures – divergent speciation took place!

By looking at DNA and individual characteristics (such as beak length), scientists have been able to organize a phylogenetic tree showing the evolution of 14 different finch species

Co-Evolution

Co-Evolution: The process by which two species evolve in response to changes in each other.

An Evolutionary change in one organism may be followed by a corresponding change in another organism.

Example: Insects have been feeding on plants since insects evolved.

• In response, many plants developed toxins or poisons to prevent insects from feeding on them.

• Natural selection favored any insect that could withstand the effects of the toxin.

• Those insects then survived and produced a population of offspring who could also withstand the effect of the toxin or poison.

Extinctions

Reasons of Extinction:

• Species have to complete for similar resources – often one species is better than the other. The competitor is driven to extinction.

• The Environment changes and some species have genes which allow them to adapt, while others perish.

• Gradual Extinctions occur due to natural selection.

Mass Extinction

Caused by catastrophic events in Earth’s history, which wipe out entire ecosystems.

Species become extinct due to the environmental collapse occurring around them rather than the inability to complete for resources.

Ex. A huge asteroid hit the Earth when the dinosaurs were alive, causing a large amount of dust and water to be released into the atmosphere. This caused rapid climate change, an environment which most species could not survive in.

5 recorded mass extinctions

Speech of Evolution

There are two models that attempt to explain evolutionary speed.

1. Gradualism

2. Punctuated Equilibrium

Gradualism

Views evolution as slow and steady until a divergence takes place. Big changes occur by the accumulation of many small changes.

Note: Fossil records do not always support this theory! New species appear and disappear suddenly

Punctuated Equilibrium

Dr. Niles Eldredge and Stephen Gould came up with a new model. They said evolutionary history consists of long periods of equilibrium, where nothing changes. This is interrupted, or punctuated, by periods of divergence.

Note: When species first diverge from the parent, they go through a fast period of change – then stay stagnant until they give rise to other species

Hardy-Weinberg Equilibrium

A mathematical model used to calculate allele frequencies of traits with dominant and recessive alleles. The model assumes that the population:

1. Is large

2. Has random mating

3. Experiencing no selection

4. Has no mutations

5. Has no immigration or emigration

Aka NO MICROEVOLUTION. If these assumptions are met, the allele frequencies in a population will remain stable over time (no evolution).

Human Activity and Speciation

Habitats can be split, and populations isolated, when humans:

• Convert large stretches of wilderness into croplands

• Develop wilderness areas for recreation or tourism

• Build roads and subdivisions

• Floor large areas of land to build dams for hydroelectric power generation

Human-made barriers prevent gene flow between populations, causing adaptive radiation if their environments are different.

Severely fragmented populations may die out if there is not enough genetic diversity for adaptation to harsh conditions.