What is a species?

Its not as straightforward a question as most believe.

Macroevolution and SpeciationEvolution creates (and destroys) species, but …

These are members of different species - eastern (left) and western (right) meadowlark.

What is a Species?

There is only one human species.

What is a Species?

And these are all members of a single species.

Determining What Is and What Isn’t a Distinct Species Can Have Economic Consequences

Northern spotted owl (left) and barred owl (right).

Speciation - process by which a new species originates.

- Involves the creation of a population of organisms that are novel enough to be classified in their own group. 

- Two processes by which this can occur: - Anagenesis is the accumulation of heritable traits in a population, that transforms that population into a new species.

- Cladogenesis is branching evolution, in which a new species arises as a branch of from the evolutionary tree.  The original species still exists.  This process is the source of biological diversity.

What is a Species?

The definition we’ll use is this: A species is a group of individuals capable of interbreeding to produce fertile offspring.

One Problem in the Biological Species Concept

For asexually-reproducing organisms, like these bacteria, what constitutes a species?

How Do Species Arise?

Geographic isolation is the primary extrinsic reproductive isolating mechanism.

The key to speciation is reproductive isolation of populations.

There are extrinsic and intrinsic reproductive isolating mechanisms.

Reproductive Isolation May Occur With or Without Geographic Isolation

Allopatric speciation occurs when geographic isolation creates a reproductive barrier (an extrinsic mechanism).

Sympatric speciation occurs when a reproductive barrier is created by something other than geographic isolation (intrinsic mechanisms).

Allopatric Speciation

Two species of ground squirrel are postulated to have descended from a common ancestral population that was separated by formation of the Grand Canyon.

Harris’ antelope squirrel White-tailed antelope squirrel

Intrinsic Reproductive Isolating Mechanisms Are Always Required for Speciation

Intrinsic mechanisms involve changes to organisms that prevent interbreeding.

In allopatric speciation, intrinsic mechanisms come into play once populations are physically separated.

In sympatric speciation, intrinsic mechanisms are the only ones involved.

Harris’ antelope squirrel White-tailed antelope squirrel

Many Intrinsic Reproductive Isolating Mechanisms Drive Speciation

(different habits within an overlapping range)

Courtship rituals, like these, are critical for mating within a species, but ineffective for attracting members of other species.

Many Intrinsic Reproductive Isolating Mechanisms Drive Speciation

Behavioral Isolating Mechanisms

Many Intrinsic Reproductive Isolating Mechanisms Drive Speciation

Many Intrinsic Reproductive Isolating Mechanisms Drive Speciation

- Postzygotic barriers

a.  Reduced hybrid viability- Abort development of hybrid at some embryonic stage. b.  Reduced hybrid fertility- Meiosis doesn’t produce fertile gametes in vigorous hybrids. donkey + horse = mule (sterile hybrid)                        

c.  Hybrid breakdown- First-generation hybrids are fertile, but they cannot produce fertile offspring in the next generation (e.g. different species of cotton).

A summary of reproductive barriers between closely related species

Key question about allopatric species is whether they are indeed different enough that viable, fertile offspring would not be produced by mating. This can be tested sometimes as in…           

• Given time, any two isolated populations will diverge because of genetic drift

• Random divergence may affect traits responsible for reproductive isolation

-Speciation may occur

Divergence

Speciation Occurs at Widely Differing Rates

A slow rate of speciation evidenced by a living horseshoe crab (13 extant species) and a 300 million year-old fossil species

A rapid rate of speciation evidenced by Galapagos finches which have diversified into 13 species within the last 100,000 years.

Speciation Rates

Generalists, like the horseshoe crab, tend to remain as stable species.

Specialists, like the Galapagos finch, tend to be unstable as species.

Speciation also becomes rapid when, as occurred with Galapagos finches, new niches become available.

The Pace of Evolution

• Gradualism: the accumulation of small changes

• Punctuated equilibrium: long periods of stability followed by rapid change

The Pace of Evolution

Speciation Dynamics - Gradualism or Punctuated Equilibrium?

Punctuated equilibrium appears to be a more accurate view of speciation dynamics.

What would the fossil record look like if gradualism had occurred?

What would the fossil record look like if punctuated equilibrium had occurred?

Does Evolution Create the Perfect Organism?

No, only better organisms as evolution is constrained by history and buffeted by random events.

Essentially, every organism on earth is in significant part a sum of accidents.

Species Come and Go

Best estimates from the fossil record indicate that greater than 99% of species that have exited are now extinct.

A typical “lifetime” for a species is about 1 million years.

Mass Extinctions Are a Fact of Life

Gary Larson

The Cretaceous/Tertiary Mass Extinction

Adaptive Radiation

• Adaptive radiations: closely related species that have recently evolved from a common ancestor by adapting to different parts of the environment

• Adaptive Radiation occurs: – in an environment with few other species

and many resources– in many isolated island chains

1.Species A migrates from the mainland to the first island.

2. Isolated from the mainland, species A evolves to species B.

3. Species B migrates to the second island.

4. Species B evolves in species C.

5. Species C recolonizes the first islands, but is now unable to reproduce with species B.

6. Species C migrates to the third island.

7. Species C evolves into species D.

8. Species D migrates to the first and second island.

9. Species D evolves to species E.

Case 1: Hawaiian Drosophila

• More than 1000 species of Drosophila on Hawaiian Islands

• Diversity of morphological and behavioral traits

• Empty habitats resulted in fruit flies that are:-Predators -Parasites

-Herbivores -Detritivores

-Nectar eaters

Adaptive Radiation

Case 2: Darwin’s finches

• Ancestors were subjected to different selective pressures

• Geographic isolation on many islands

• Diverse population, some evolved into separate species

• Occupy many different habitats

Adaptive Radiation

• Ground finches– Feed on seeds: size of bill relates to

size of seed they eat• Tree finches

– All eat insects: one species uses a tool to get insects

• Vegetarian finch – Eats buds from branches

• Warbler finches – Eat insects from leaves and branches

Adaptive Radiation

Adaptive Radiation

The pattern of adaptive radiation represented by tridacnid clams