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.)