macroevolution

Macroevolution

is evolution over geologic time of groups larger than just an individual.

The process by which a new species evolves is called speciation.

Biological Species Concept

A biological species is a group of organisms that can reproduce with one another in nature and produce fertile offspring.

**Breeding behavior in nature can be different than in captivity (artifical enviornemtn)

How do new species arise?

some members of a species must become reproductively isolated from the rest of the species.

- Evolution within a species means a change in that population’s allele frequency.

When two populations are separated, their allele frequency changes separately from one another.

Since they no longer have migration between the populations, two separate species may develop.

**There are many ways to separate two populations.

Allopatric Speciation

Happens when a geographical barrier (mountains, rivers, oceans, distance) splits a population into two groups.

After being separated, each group develops differences that eventually prevent them from interbreeding.

“Allo” means other/different and “patric” means from → “from different places.”

Geographical isolation is the main cause that starts this type of speciation.

Over time, physical, behavioural, or genetic changes build up and keep the two groups reproductively isolated (unable to mate).

Example

A river forms and splits a population of squirrels.

Each group adapts to its side of the river.

Over many generations, their behaviours and traits change so much that even if they meet again, they no longer mate → they have become two different species.

Reproductive Isolation (formed by geographic isolation)

Any factor in nature that prevents interbreeding (breeding within a closed population) between individuals of the same or closely related species.

Extrinsic isolating mechanism – outside of the organisms in question

Geographic isolation is extrinsic

Intrinsic isolating mechanism – internal characteristics that prevent interbreeding

Differences in anatomy, physiology and behavior

Types of Intrinsic Reproductive Isolating Mechanisms

Ecological

Temporal

Behavioral

Mechanical

Gametic

Hybrid inviability

1. Ecological Isolation

A premating (prezygotic) isolating mechanism in which members of different species seldom, if ever, meet because each species prefers to live (is adapted to) different habitats.

(Two species don’t mate because they live in different places, so they never meet.)

When two species have different habitats, they will rarely have contact.

Lions prefer open grassland, tigers prefer forest

2. Temporal Isolation (time-based isolation)

Two or more species that share the same habitat but do not mate within the same time frame.

Two populations of the same species of plant release their pollen at different times of the year. If their reproductive periods did not overlap, they would have reduced gene flow.

Their reproductive periods don't overlap → they barely exchange genes → reduced gene flow.

3. Behavioral Isolation

Even if populations are in contact and breeding can occur, they must choose to mate.

Such a choice is based on specific courtship and mating displays.

4. Mechanical Isolation

Type of prezygotic isolation where reproductive organs differ in size or shape or another feature.

Different species of Alpine Butterfly look similar but have different reproductive organs.

5. Gametic Isolation

Even if mating occurs, offspring may not result if there are incompatibilities between sperm and egg, or between sperm and the female reproductive tract.

6. Hybrid Inviability or Infertility

Hybrids between closely related species are often inviable or, if they live, they are sterile.

This hybrid inviability and sterility, (hybrid incompatibility), can reduce the exchange of genetic variants between species.

Even if offspring develop, they are most often malformed or sterile

Sympatric Speciation

Sympatric speciation occurs when there are no physical barriers preventing any members of a species from mating with another, and all members are in close proximity to one another.

Even though they’re in the same place, a new species can still form if some individuals start using a different resource (like a new food) or develop a new trait that separates them reproductively.

In plants, this often happens through polyploidy, which is when the number of chromosome sets doubles or changes. Polyploid individuals can’t successfully mate with the original population, so they form a new species.s

Polyploidy

100,000 species of plants exist today because of polyploidy

A multiplication of the normal number of chromosomes in an organism.

Most often happens in plants that self pollinate

When does speciation occur?

Consider the horseshoe crab which has not changed much in 300 million years.

Consider Darwin’s finches, they developed into 13 separate species in 100,000 years.

Horseshoe crabs didn’t face strong pressures, so speciation was minimal.

Darwin’s finches faced different pressures on different islands, so speciation was fast.

Generalists' vs Specialists

The horseshoe crabs are generalists – their diet is diverse

The finches are specialists – their diet is very specific, especially when food is scarce.

When the food source changes, the finches must adapt, or they will not survive. The horseshoe crab just moves on and eats something else.

Adaptive Radiation

Rapid emergence of species from a single species that has been introduced into a new environment.

ex. When the finches arrived on the Galapagos there were no other birds of their kind living in the islands.

It was easy for them to specialize and fill the available niches. This is especially true on the Galapagos because there are 25 separate islands.

Water between the islands is a barrier = allopatric speciation

Two conditions that contribute to speciation

Specialization of food source and environment

Ability to migrate to a new environment, especially if there is no competition.

Convergent vs Divergent Evolution

Convergent evolution occurs when species have different ancestral origins but have developed similar features.

Divergent evolution occurs when two separate species evolve differently from a common ancestor