Course: BIS 105 Environmental Biology

Unit 3 Learning Module

Natural Selection

Darwin’s Theory of Evolution

Darwin lived in an increasingly scientific society which had begun to accept the idea that universal "laws" governed processes in nature - perhaps including life itself. Like Lamarck, Darwin understood that species change. With Lyell, he saw that the history of Earth and its life covered a vast amount of time. From his observations of animal breeding, he recognized that even within species, individuals showed variation in traits, and that the variations could be passed to offspring. Recalling Malthus, he knew that populations could produce far more offspring than the environment could support. He predicted that individuals with traits which suited the environment would survive and reproduce to pass their favorable traits to offspring, as shown below. Those whose traits were less suited to the environment would die. Just as humans select for breeding those cattle which produce more milk, he reasoned, nature (the limited environment) selects individuals which use resources most efficiently. Thus, he called his explanation of how species change natural selection.

Darwin defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved," and he later regretted that he had not named it “natural preservation.” Today it is often defined as the process by which a certain trait becomes more common within a population. Let’s look once more at the parts of this process, and then we will consider its consequences.

Natural selection involves heritable variation, overproduction of offspring, preferential survival of individuals having variations favorable for the environment, and reproduction by survivors. This diagram shows two selection events, with reproduction after each one.

By chance, heritable variations exist within a species.

Darwin did not know that genes made of DNA determine traits. Much later, scientists learned that mutations in DNA can change genes and produce variations in traits. However, his observations of animal breeding and his detailed studies of barnacles and orchids convinced him that small, heritable variations in traits were common among individuals within a species. Darwin probably recognized that sexual reproduction increased variety in offspring. He expressed considerable concern that his own health problems might be heritable, especially when his beloved daughter Annie grew ill and died. He believed that his marriage to his cousin may have contributed to his children's weaknesses.

Species produce more offspring than can survive.

Malthus argued that human populations grow exponentially if unchecked, but that disease, starvation, or war will limit population growth eventually. High birth rates and high death rates were characteristic of human history. Darwin himself had ten children; three died before maturity. Darwin reasoned that all species had the capacity to grow. However, his observations showed that most populations remained stable due to environmental limits. He concluded that many offspring must die. The phrases overproduction of offspring and struggle for existence summarize this idea.

Offspring with favorable variations are more likely to survive to reproduce.

Although heritable variations appeared to be random, death, Darwin reasoned, was not. Offspring which, by chance, had variations which “fit” or adapted them to their environment would have a greater chance to survive to maturity and a greater chance to reproduce. Offspring without such adaptations were more likely to die. Thus, well-adapted individuals produce more offspring. Differential survival and reproduction is a cornerstone of natural selection.

Gradually, individuals with favorable variations make up more of the population.

Can an individual organism evolve? No. The accommodation of an individual organism to its environment is not evolution. Though an individual organism can be better adapted to its environment, it still must mate with others of its species, so by definition, it is not a new species. It is just an individual with a better chance of survival in its environment. It is the gradual accumulation of many adaptations that, over many generations within one lineage of organisms, results in a new species. These adaptations occur through genetic change.

Through chance variation, overproduction of offspring, and differential survival and reproduction, the proportion of individuals with a favorable trait (or favorable phenotype) will increase. The result is a population of individuals adapted to their environment. It is the variation within a species that increases the likelihood that at least some members of a species will be adapted to their environment and survive under changed conditions.

It is important to note that natural selection is not directed or intentional. It depends on chance variations - due to genetic variations - and can work only with the “raw material” of existing species. Occasionally, variations which have no particular adaptive logic may survive. However, the limits set by resources and environment usually mean an increase in traits which help survival or reproduction, and the loss of traits which harm them. Gradually, species change. Eventually, changes accumulate and a new species is formed.

How Does Evolution Really Work? http://www.youtube.com/watch?v=xkwRTIKXaxg (6:14)

Let’s compare natural selection to inheritance of acquired characteristics (Lamarck’s idea mentioned previously). How would Darwin’s mechanism explain the long necks of giraffes?

1. Heritable variation: In the past, some giraffes had short necks, and some had long necks.

2. Overproduction of offspring: Giraffes produced more young than the trees in their environment could support.

3. Differential survival and reproduction: Because the long-necked giraffes could feed from taller trees, they were more likely to survive and produce more offspring. Short-necked giraffes were more likely to starve before they could reproduce.

4. Species change: The long-necked giraffes passed their long necks on to their calves, so that each generation, the population contained more long-necked giraffes.

Recall that Lamarck believed that giraffes could stretch their necks to reach tall trees, and pass their stretched necks on to offspring. If this were true, evolution would reward effort toward a goal. Darwin showed that evolution is not goal-directed. Instead, the environment reinforces variations which occur by chance.

Lyell studied the geology which surrounded him and saw that the environment had changed many times over a vast amount of time. Darwin studied the life across continents and saw, in addition to tremendous variation, that species had changed – in response to the changes in their environment – over that vast amount of time. Both proved, with careful observations and well-reasoned inferences, that the present arises from the past. Limited to our brief lifespans, we see today’s species as fixed. Darwin taught us how to see the relationships between them; to see that they developed from earlier, distinctly different species; to see that all of them - all of us - share common ancestors. The cartoons which showed Darwin as an ape did a great disservice to his theory of evolution. Far too many people limit their understanding of evolution to the simple phrase that “we came from apes.” We humans share common ancestors not only with the great apes, but with ALL of life – blue whales, gazelles, redwood trees, saguaros, fireflies, mosquitoes, puffballs, amebas, and bacteria. As Darwin said in closing the Origin, “There is grandeur in this view of life.”

Darwin delighted in the great diversity of life, but also saw unity within that diversity. He saw striking patterns in the similarities and differences. Seeking an explanation for those patterns, he developed the concept of natural selection. Natural selection explains how today’s organisms could be related – through “descent with modification” from common ancestors. Natural selection explains the story told by the fossil record – the long history of life on Earth. Natural selection is a scientific answer (if only partial) to the old questions: Who are we? How did we come to be?

In the light of natural selection, it is easy to see that variation – differences among individuals within a population – increases the chance that at least some individuals will survive if the environment changes. Here is a strong argument against cloning humans: if we were all genetically identical – if variation (or genetic variation) did not exist – a virus which previously could kill just some of us would either kill all of us, or none of us. Throughout the long history of life, variation has provided insurance that inevitable changes in the environmental will not mean the extinction of a species. Similarly, the diversity of species ensures that environmental change will not mean the extinction of life. Life has evolved (or, the Earth’s changing environment has selected) variation and diversity because they ensure survival. Causes of mutation may have pre-existed, but in a sense, life has embraced them. And sexual reproduction has evolved to add further to variation and diversity.

Adaptations are logical because the environment imposes limits on organisms, selecting against those who do not “fit.” Adaptations arise through gradual accumulation of chance variations, so they cannot be predicted, despite the fact that they appear to be goal-directed or intentional. Adaptations relate to every aspect of life: food, water, oxygen, nutrients, shelter, growth, response, reproduction, movement, behavior, ability to learn. Adaptations connect organisms to the resources in their environments. You are born with your adaptations; they are not changes you make to fit yourself into an environment. If the environment changes, the adaptive value of some of your inherited characteristics may also change. Our human appetites for salt and fat, for example, may remain from our past, when fat and salt were rare in our environment; now that they are easily available, we consume more than is good for us. Biologist E.O. Wilson believes adaptations reach every aspect of human life - that social, political, and even religious behaviors are rooted in our genes. Of course, we can learn – and learning allows us to adapt within our lifetimes to environmental change. The ability to learn is itself an adaptation – perhaps our greatest gift. But more and more, we are discovering that much of our behavior – including learning - is genetically programmed – a gift from our ancestors similar to vision and hearing, or breathing and digestion.

Darwin’s theory can be summarized in two statements:

- All living species share common ancestors, and

- Natural selection explains how species change.