Evolution and Speciation

Kingdom: AnimaliaPhylum: ChordataClass: Mammalia

Order: PrimateFamily: Hominidae

Genus: HomoSpecies: sapiens

Varieties and Breeds

Classification System

What is a species?

Its not as straightforward a question as most believe.

Evolution creates new species, but …

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

Defining Species

What is a Species?

There is only one extant (existing) 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 Be Difficult

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

What is a Species?

Lets start with the most widely accepted definition: A species is a group of individuals capable of interbreeding to produce fertile offspring.

This is the biological species concept. Like all attempts to define a species, it has a problem.

Hybrid Infertility

Horse X Donkey = Mule

Cannot Reproduce and produceFertile offspring.

So a better definition of a species might be:

A group of individuals capable of interbreeding to produce offspring that are

fertile and these offspring, in turn, can produce offspring that are fertile.

Another problem with the Biological Species Concept

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

How Many Species Are There?

We don’t know.

About 2 million species have been described.

Estimates of existing species number range from 4 million to 100 million (with 10-15 million being a more commonly considered upper estimate).

Two Patterns of Speciation

One species completelyreplacinganother

One speciesevolving fromanotherwith both thenexisting atsame time

How Do New Species Arise?

The key to speciation is reproductive isolation of populations

**It is reproduction that makes more organisms…so mess with reproductive success and you mess with population!

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What does it mean to have reproductive isolation?

It means that for whatever reason, one or more organisms are prevented from being able to reproduce with others in the population of that species.

1. Organism itself changes because of random mutation(s)

2.Environment changes, causing separation of population into two or more subgroups of the original population.

Intrinsic Reproductive Isolating Mechanisms

Something changes about the organisms in the population that interferes with reproductive success

but is not caused by geographic isolation!

Maybe something about mating ritualMaybe something about sperm or egg

Maybe something about time of year they mateEtc.

Extrinsic Reproductive Isolating Mechanisms

Something happens to the ENVIRONMENT of the organism which interferes with reproductive success.

Reproductive Isolation May Occur With or Without Geographic Isolation

Allopatric speciation occurs when geographic isolation creates a situation where reproduction can’t occur (an extrinsic mechanism).

Sympatric speciation occurs when reproduction is not inhibited by geographic isolation but something about the individual organisms of the population changes and interferes with reproductive capability. (anintrinsic mechanism)

Allopatric Speciation: A change in the environment

Two species of ground squirrel are believed 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…Even when allopatric speciation

has already occurred.

Intrinsic mechanisms involve changes to the 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

Intrinsic Isolating Mechanisms

(different habits within an overlapping range)

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

Intrinsic Isolating Mechanisms Continued:

Mating Ritual Blue-footed Booby

Mating Ritual Fiddler Crab Male Waving

Intrinsic Isolating Mechanisms Continued

Salamander Evolution

Subspecies evolving At different Locationsin California…ALL ARE SAME SPECIES

Speciation Occurs at Widely Differing Rates

A slow rate of speciation is seen with a living horseshoe crab which has diversified into 13 species within the last 300 million years.

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

What else, other than isolating Mechanisms, can contribute to theformation of new species?

Sometimes simply CHANCE EVENTScan contribute to speciation!

Genetic Drift any change in the allelic frequency in a population that is due chance events (floods, insects getting sprayed with insecticides, earthquakes, fires, etc)

Allelic frequency: a measure of how common an allele is in a population

Especially true in small populations because the chance of losing an allele completely becomes greater!

Type of Genetic Drift

Founder Effect: when a small populationcolonizes a new habitat (separate from the rest of that population) and interbreeds. Alleles thatwere once fairly uncommon in a population nowmay be more common because this subset of thepopulation is so small.

Amish population: Dwarfism, Polydactyly

Migration may contribute to speciation as well!

Transports genes to different gene pools if they mate while they are migrating.

b/c their genes are isolated from theiroriginal population and the alleles

enter a different gene pool.

Speciation RatesGeneralists (eat many different foods, well-adapted to many different habitats) like the horseshoe crab, tend to remain as stable species.

Specialists (eat more specific foods and only adapted to live in certain habitat), like the Galapagos finch, tend to be unstable as a species and More likely to change!

Speciation also becomes rapid when, new niches become available to the organisms, as seen with Galapagos islands.

Formation of New Species- Gradualism or Punctuated Equilibrium?

Punctuated Equilibrium

Gradualism

Does Evolution Create the Perfect Organism?

No, only organisms which arebetter suited to survive and reproduce in that environment at that time!

Scenario Groups:

Species Come and Go

Best estimates from the fossil record indicate that greater than 99% of species that have existed 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

65.5 mya

Asteroid Impact Theory

85% of allspecies died

Are We Now Causing a Mass Extinction?

Are humans causing the 6th mass extinction?

Humans and Extinction

Mass Extinction

Current Mass Extinction