11.1 What is a Species?
A species is a group of individuals that can interbreed in nature and produce fertile offspring. All humans are the same species. A group of closely related species (such as lions, tigers and leopards) is called a genus. The scientific name is the genus name followed by the species name. For example, humans are Homo sapiens. Over time species will gradually evolve. Each gene comes in several different forms called alleles ( for example there are alleles for blood group A, group B, group O and group AB). All of the alleles in a population is called the gene pool.
Different species cannot produce fertile offspring, so they have reproductive isolation and each species remains seperate. This reproductive barrier may be as simple as different bird calls or displays (Fig 11.1), which is behavioral isolation. In other cases individuals never come into contact: spatial isolation, like tortoises on different islands.
Many plants bloom at different times of year (Fig 11.2), which is temporal isolation.
In many cases sperm from one species cannot fertilize the egg of another species, due to chemical differences between them: gamete incompatibility.
Even if the egg is fertilized, it may produce offspring that are sterile: for example when a horse and donkey ( two different species) mate the result is a sterile mule. Fig 11.3. In humans there are no reproductive barriers, so all humans are the same species.
Speciation means that one species splits over time to form two or more new species. There are usually three steps to this: - The gene pools of populations become isolated from each other (for example on different islands).
- One or both gene pools change due to evolution.
- Reproductive isolation evolves to keep the new species separate.
Once two populations are geographically isolated, new alleles can arise by mutation in one population that do not exist in the other population. For example the tortoises and birds on different islands of the Galapagos evolved into different species (see last chapter). Shrimp that were separated by the isthmus of Panama 3 million years ago have evolved into different species (Fig 11.7). Apple trees were first brought to America about 300 years ago, yet apple flies are already evolving into a different species from the original hawthorn flies (Fig 11.8).
This is an example of sympatric speciation, because the two flies are found in the same parts of the country. In contrast populations that are physically separated from each other (by an ocean or a mountain for example) are called allopatric.
In plants, new species can evolve very quickly because a hybrid plant formed from two species can become fertile due to polyploidy, where the number of chromosomes doubles. The crop canola (that canola oil comes from) is an example of this: it is a hybrid of kale and turnip. Fig 11.9.
11.2 Race
Races are populations of a single species that have diverged, or become slightly separate, from each other. Usually there is little gene flow between races, in other words different races do not usually mate with each other.
For example spotted owls in the west are divided into three races: the Northern race, California race and Mexican race (Fig 11.12). Each race is geographically separated from the others.
11.3 Humans and Race
In the past scientists have tried to identify human races based on skin color, hair texture and facial features. However they could not agree on the number of races, with estimates ranging from 5 to 26 races. The fossil record shows that Homo sapiens evolved in Africa about 200,000 years ago, and then spread throughout the world, replacing the older Homo erectus that was in Europe, Africa and Asia. Fig 11.15. This means that the "races" of humans have only been different for less than 200,000 years. Typically once two races are separated, they end up with different alleles (for example red butterflies would be a different race from blue: Fig 11.16). In fact human alleles do not follow this pattern: there is no allele that is only found in one "race". Fig 11.17. Similarly human "races" are often very variable: genes are not more similar within one race than between races. In other words you cannot divide humans into races based on their genes.
Evidence from genes, such as blood groups: Fig 11.19, shows that humans have migrated from one area to another for thousands of years, and married into other groups. So again there is no distinct gene pool for any particular part of the world.
11.4 Why human groups differ
Although humans cannot be divided into pure "races", people in different parts of the world look different. One reason for this is natural selection: genes that are useful in some environments are harmful in other environments.
For example sickle cell anemia is a genetic disease that is fatal if you inherit both alleles for it. However people with one normal allele and one sickle cell allele have a natural resitance to malaria. So in areas where malaria is common (most of Africa, parts of the Middle East and India) sickle cell anemia is also common. Fig 11.20.
Similarly UV light from the sun causes skin cancer and dark skin protects agains UV light. So in tropical environments people with dark skin have an advantage. Fig 11.23. In areas where the sun is not so strong (like northern Europe) people with light skin have an advantage, because they can make vitamin D more easily. Fig 11.24.
Apart from natural selection, some differences between groups is due to genetic drift. Genetic drift is a change in the gene pool of a small population due to chance. Two examples of this are:
- Founder effect: a small group starts a new population. For example the Amish in Pennsylvania are descended from about 200 people, and they have genetic diseases such as dwarfism at a much higher rate than other groups in Pennsylvania.
- Population bottleneck: a sudden decrease in the population removes some of the alleles. Fig 11.25. For example only 17 people on a South Pacific island survived a tsunami four hundred years ago, and the population of the island is now different from other islands.
One thing that may reinforce differences between populations is assortative mating. Individuals tend to mate with someone who is like themselves, for example tall women tend to marry tall men.
Last edited November 2006,
by David Byres, dbyres@fccj.edu