The Primary Category in Linnean Classification Is the Species

Chapter 12

Who Am I?

Species and Races

12.1 What is a Species

The primary category in Linnean classification is the species

Species are given binomial (two-part) names

First part consists of the genus

Second part is the specific epithet

Genus name is capitalized, both names are italicized or underlined when used

Same genus, different species

Panthera leo and Panthera pardus

12.1 What is a Species - The Biological Species Concept

The biological species concept states that species are reproductively isolated from one another.

In nature, members of the same species can potentially interbreed

Members of different species cannot interbreed

Sum total of alleles in a species is called gene pool

12.1 What is a Species - The Nature of Reproductive Isolation

Movement of alleles within a gene pool is called gene flow.

Gene flow does not occur between species, due to reproductive barriers.

Two general kinds of reproductive barriers:

Prefertilization – prevent fertilization from occuring

Postfertilization – fertilization occurs, but hybrid cannot reproduce

Five different prefertilization reproductive barriers

Spatial

Behavioral

Mechanical

Temporal

Gamete incompatibility

Spatial reproductive isolation

Species are separated by distance

Example: polar bear (Arctic) and spectacled bear (South America)

Behavioral reproductive isolation

Differences in mating behavior may interfere with reproduction

Example: many birds have mating songs or dances

Mechanical reproductive isolation

Sexual organs are incompatible

Example: many insects have “lock and key” genitals

Temporal reproductive isolation

Difference in timing of reproduction

Example: organisms might have different mating or flowering times

Gamete compatibility reproductive isolation

Eggs and sperm of different species unable to fuse

Common among organisms that release gametes into the environment

12.1 What is a Species - The Nature of Reproductive Isolation

Postfertilization barriers to reproduction:

Hybrid inviability

Hybrid sterility

Hybrid inviability

Zygote unable to develop because genetic instructions are incomplete

Example: sheep crossed with goat produces an embryo, but it dies early in development

Hybrid infertility

Product of interspecies cross is unable to reproduce

Example: mule

12.1 What is a Species - Speciation: an Overview

Three steps necessary for one species to give rise to a new species

1. Isolation of gene pools of populations

2. Evolutionary changes in gene pools of populations

3. Evolution of reproductive isolation between populations

Once reproductively isolated, how long does the process of evolution take?

Two general explanations

Gradualism – slow accumulation of small changes over long period of time

Punctuated equilibrium – rapid change followed by long periods of no change

Evidence that both processes are at work

Isolation and divergence of gene pools

Migration can lead to isolation of a population

Examples include oceanic islands

Because migrant populations are small, genetic changes can occur rapidly

More than 50 species of Hawaiian silversword are descended from a migrant population of California tarweed.

Geographic barriers can also intrude between populations

Isthmus of Panama connects North and South America, but divides an ocean gulf

6 pairs of snapping shrimp species exist. One species pair is on the Carribean side and the other is on the Pacific side

Genetic evidence indicates that each species pair is descended from ancestral species separated by rise of Panama

Species separated by barriers or distance are allopatric

Species occupying the same area are sympatric

Apple maggot flies appear to be speciating sympatrically.

Apples are not native to N. America, introduced by colonists

Apple maggot flies infest hawthorns and apples

Flies mate on fruit where they will lay their eggs

Hawthorns fruit 1 month after apples

Apple-preferring and hawthorn-preferring flies appear to have little gene flow

In plants, speciation can occur instantaneously, with no barriers between populations.

Hybrids between plant species are usually infertile.

Hybrids can occasionally become fertile through polyploidy.

Many plants produce male and female gametes and can self-pollinate.

Because of change in chromosome numbers, offspring are genetically isolated from their parent plants.

Canola developed as a result of polyploidy

Scientists suspect that this process is responsible for much of plant species diversity.

The evolution of reproductive isolation

No rule to tell with certainty when populations are truly isolated

The dragonflies in the picture below cannot interbreed

All dogs are capable of interbreeding

12.2 Races and Genealogical Species

Biologists do not agree on a definition of the term “race”, and some feel the concept is meaningless

Any biological definition of race would probably have the following concepts:

Races are populations of one species that have diverged

Little gene flow, so any evolutionary changes in one population do not occur in the others

Possible criteria for defining race

Genealogical species concept defines species as smallest group of reproductively compatible individuals descended from a single common ancestor

Spotted owl has 3 distinct populations that could theoretically interbreed, but are separated physically

Are human races like genealogical species?

12.3 Humans and the Race Concept - The Morphological Species Concept

The morphological species concept emphasizes physical differences

A species is defined as a group of individuals with some reliable physical characteristics that distinguish them from all other species

Morphological differences are assumed to correlate with isolation of gene pools

12.3 Humans and the Race Concept - Modern Humans: A History

Immediate predecessor of Homo sapiens was Homo erectus

H. erectus first appears in fossil record ~1.8 MYA

H. sapiens first appears in fossil record ~250,000 years ago.

Debate about precise model of evolution of modern humans, but all ultimately have Africa as the place that humans came from

Most evidence suggests that moderns humans descended from African ancestors within the last 200,000 years.

Humans have less genetic diversity than any other great ape (indicates young species).

Among human populations, those in Africa have greatest genetic diversity.

Physical differences between humans must have arisen within about 10,000 generations (not very long).

Thus, all humans share a recent common ancestor.

12.3 Humans and the Race Concept - Genetic Evidence of Divergence

Evolution results in a change in allele frequency. If a race is isolated from other races, there are two expectations:

Some alleles unique to the race

Differences in allele frequency compared to other races

Hypothetical example of a race-specific allele and different allele frequencies between races.

12.3 Humans and the Race Concept - Using the Hardy-Weinberg Theorem to Calculate Allele Frequencies

The Hardy-Weinberg theorem states that allele frequencies will remain stable in populations that meet three conditions

Large size

Random mating

No migration

No natural selection

Also provides a means of making predictions of what will happen if assumptions are violated

HW Theorem is expressed as an equation

p2 + 2pq + q2 = 1

p and q are alleles of a gene

p2 and q2 are homozygous condition (i.e. AA or aa)

2pq is heterozygous condition (i.e. Aa)

12.3 Humans and the Race Concept - Human Races Are Not Biological Groups

No race-specific alleles have been identified

Although sickle cell anemia has long been thought of as a “black disease”, it is not found in all African populations and it is found in non-African populations

Populations classified in the same race do not have similar allele frequencies

The distribution of alleles within racial groups is about the same as between racial groups

Human races have never been truly isolated

B blood type first evolved in Asia, but is now widespread. There are no clear boundaries in the human gene pool.

12.4 Why Human Groups Differ - Natural Selection

Sickle-cell anemia is an adaptation to environments where malaria is common

Nose shape is correlated with climate factors. Populations in dry climates have narrower noses than those in moist climates.

12.4 Why Human Groups Differ - Convergent Evolution

Traits shared by unrelated populations due to similarities of environment are examples of convergent evolution

Human skin color appears to be result of convergent evolution

Strong correlation between skin color and exposure to UV light

12.4 Why Human Groups Differ - Genetic Drift

Change in allele frequency that occurs due to chance is genetic drift

Humans are highly mobile

Small groups colonizing new areas are prone to genetic drift

Often drift occurs in three different situations

Founder effect – genetic differences resulting from a small sample

Population bottleneck – genetic change resulting from a dramatic reduction of population numbers

Chance events – small populations are especially prone to loss of alleles though chance

12.4 Why Human Groups Differ - Sexual Selection

When a trait influences chance of mating it is sexually selected

Peacock tail sexually selected

Sexual selection often accounts for male/female differences in many animal species

There is some evidence that sexual selection accounts for differences in human male/female body size

12.4 Why Human Groups Differ - Assortative Mating

Tendency of organism to choose mate that resemble self is assortative mating

People tend to mate assertively by height (i.e., tall women marry tall men) and skin color

Positive assortive mating tends to exaggerate differences between groups

12.5 Race in Human Society

Scientific data indicate that racial categories are biologically meaningless

Racial categories are socially meaningful and are socially constructed

BUT, arbitrary groupings are not necessarily bad – we group ourselves into other categories (religious, sports fans, cat lovers, etc.)