2.5 - Natural Selection

Before we get into the details, here are some general facts you should know about natural selection:

Natural selection only occurs when the genotypes of individuals in a population differ in evolutionary fitness. As a result of NS, alleles associated with higher fitness will increase in a population over time. If the environment changes, natural selection favors whichever genotypes produce the highest reproductive success in the new environment.

Natural selection is the only thing that causes organisms to be adapted to their environment: "acting on phenotypes, NS indirectly adapts a population to its environment by increasing or maintaining favorable genotypes in the gene pool"

Natural selection acts on the whole organism; the unit of selection is the individual. NS cannot act on individual alleles (genes).

Natural selection acts on the phenotype, not the genotype. But it can cause changes in the genotype.

I. evolutionary fitness

The biggest headache in evolutionary biology is the word "fitness". That's because it has a much different meaning in everyday speech than it does in biology. And part of the headache is caused by biologists who don't want to have to say "evolutionary fitness" all the time, so they get lazy and just say "fitness". Then, people who aren't trained in biology think they mean "physical fitness", and that leads to all kinds of misunderstanding about evolution and biology. And to make things more confusing, "evolutionary fitness" is also called "Darwinian fitness". From now on, whenever we use the term "fitness" in this course, we mean "evolutionary fitness".

The term "evolutionary fitness" can be defined in a lot of ways. "The relative contribution an individual makes to the gene pool of the next generation" is one way. It means that when we are comparing members of the same population or species to each other, the ones with the most offspring have the highest evolutionary fitness.

So, an organism that is very "fit" in the physical sense may have a very low evolutionary fitness if it does not reproduce. It is true that individuals who are more physically fit may have a higher chance of surviving in a difficult environment, and that may also give them higher evolutionary fitness. But not necessarily!!!

II. modes of selection

This refers to how natural selection affects the frequency of certain traits in a population. There are three general modes in which natural selection acts: stabilizing, directional, or diversifying (diversifying selection is also called disruptive selection; you don't have to know the term "disruptive" selection in this course but you may run into it in other biology courses). For each mode of NS, you should be able to explain which individuals are selected against, and the result on the mean trait value. The graphs in Campbell are very important, and you should be able to draw and explain them. The chart below summarizes some of the things you have to know about these modes of selection.

Diversifying selection may also lead to speciation, if it continues to the point where the groups with the opposite extremes of the trait no longer reproduce with each other. In a less extreme situation, diversifying selection can create a stable polymorphism in which both extreme traits are maintained in the population.

A good example of this in humans is the coarse facial hair on most mature males. Facial hair growth in males is affected by genetics, so some males have a lot more facial hair than others. All humans have hair on their faces, but male beard and mustache hair is much longer and coarser than female facial hair in the same areas of the face. Female humans do not grow coarse facial hair unless they have abnormally high levels of certain hormones that may be caused by tumors in endocrine glands.

Another example is the difference in feather color in many bird species. Male cardinals have bright red feathers, but females have duller orange-brown feathers. Size may also be a sexually dimorphic trait. In humans, size differences between males and females is not always obvious--many males are shorter than some females, and many females are taller than some males. So size is not necessarily dimorphic in humans. But in some other species, all males or all females are larger than the opposite sex, and that would be a definite example of sexual dimorphism. In hawks the female is the larger sex, but in sea elephants it's the male who is larger.

A. There are two main mechanisms by which sexual selection occurs: direct and indirect.

1. direct sexual selection

In this mechanism, males compete with each other for mates. Competition may involve fighting between the males to determine which of them may mate with receptive females. Selection is for traits that are used in these direct male-to-male interactions. Body size, antlers, and tusks are examples of traits that one male can use to defeat other males.

In this situation, a receptive female has no "choice" of which male she mates with (a receptive female is usually physically or physiologically altered by hormones; males usually ignore non-receptive females).

Why does this lead to the formation of physical traits that are so different from those of females? If the male with the biggest tusks chases away the males with smaller tusks, only the alleles for big tusks will be passed on to the next generation because the male with the biggest tusks is the only one who will mate with the females.

2. indirect sexual selection, also called "female choice"

In this mechanism, females choose mates based on physical characteristics. Selection is for traits preferred by females. Males compete for the attention of females by displaying physical characteristics or behaviors. One good example of female choice sexual selection is the peacock's flashy tail. Peahens (the female pea fowl) prefer males with big, pretty tails. So if only males with big pretty tails get to mate, only the alleles for big, pretty tails are passed on to the next generation.

Other examples of traits that aren't so obvious as the peacock's tail are also due to female choice sexual selection. The size of a male's territory or his foraging ability (ability to gather food) fall in this category.

This situation exists because the female has one or more alleles that make her prefer the male with the favorable trait, and because the male has one or more alleles that confer that trait on him. This type of selection perpetuates both the female choice alleles and the male trait alleles in the population.

B. traits favored by sexual selection may be a survival liability

Evolution is often a compromise between natural selection and sexual selection. The trait that confers mating advantage to the male increases his evolutionary fitness because he will mate with more females. However, the same trait (the peacock's tail, for example) may be a serious liability when he has to escape from predators. It may seriously shorten his lifespan. So, the fitness advantage of the trait (mating with more females) must offset the survival disadvantage (shortened lifespan), or sexual selection would not continue.

IV. why NS does not result in perfectly adapted organisms

A. historical constraints

"natural selection cannot favor disadvantageous traits based on the knowledge that they will ultimately work out for the best" When upright posture evolved in humans, biomedical engineers didn't come in as consultants and tell natural selection that the plan would work better if the bones and muscles were seriously altered first. Our ancestors up to that point moved around with the benefit of four limbs, not two. So the existing structure was what natural selection had to work with. That physical structure was not optimal for upright posture or bipedal locomotion--it was a constraint on human evolution. We could not just go for optimal, we have had to settle for what was possible given the materials-bones and muscles--on hand at the time.

B. compromise - Examples covered in textbook are seals and humans.

C. nonadaptive evolution

In most cases, natural selection increases evolutionary fitness and results in adaptation of organisms to their environments. But sometimes it doesn't. For example, genetic drift may cause the loss of alleles from the population, and the remaining alleles may not be the best adapted to the current environment. Examples in textbook are insect founder effect and bottlenecks.

D. selection can only act on existing variation

Natural selection favors the most fit of the available choices.

read "Natural Selection as the mechanism..." on pages 457-461

1) Introduction to Evolutionary Biology, Version 2. 1996-1997. Chris Colby. http://www.talkorigins.org

23.11

1. What happens to natural selection when there is no difference in fitness among the individuals of a population?

2. What does natural selection do when the environment changes?

3. Explain the relationship between natural selection and adaptive evolution.

4. What is the unit on which natural selection acts?

5. Does natural selection act on the genotype or the phenotype.

6. What is the factor that determines an organism's evolutionary fitness?

7. Describe the three modes of natural selection including the effect of each mode on the average trait value and the frequencies of the average and extreme trait values. Draw and label graphs comparing the three modes.

8. What two outcomes could result from diversifying selection?

9. Explain the cause of sexual dimorphism and give examples.

10. Discuss the difference between direct and indirect sexual selection, and the consequences for male appearance.

11. What is the tradeoff value of sexual selection for the male?

12. Explain why natural selection does not produce perfectly adapted organisms.