Natural Selection, Adaptations and Selection Pressure

1Overview

Why do polar bears have white fur, cacti have spines and deep-sea fish glow? The explanation common to all of them was proposed in the mid-1800s by two naturalists working independently, Charles Darwin and Alfred Russel Wallace. They presented their ideas together in 1858, and Darwin’s book On the Origin of Species followed in 1859. The idea is natural selection, and it ties together everything in this strand: variation arises by mutation, is inherited by the rules of genetics, and is sorted by the environment. This lesson covers natural selection, adaptations and selection pressure, including the effects of invasive species. The next lesson covers adaptive radiation and extinction.

2The logic of natural selection

Natural selection follows from a few observations about living things. Three principles underlie it: traits are inherited, more offspring are produced than can survive, and offspring vary in their inherited traits. Put in terms of conditions:

1. Variation. Individuals in a population differ in their traits.
2. Heredity. Some of that variation is genetic, so it is passed to offspring.
3. Differential reproduction. Because resources are limited, not all individuals survive and reproduce equally, and some traits give an advantage.

If all three conditions are met, the result is predictable: traits that improve survival and reproduction become more common in later generations. Darwin called this descent with modification.

The beetle example illustrates the process. A population includes green and brown beetles. Birds eat green beetles more often, so fewer green beetles survive to reproduce. Because colour is inherited, the next generation has a larger share of brown beetles, and over time the population may become entirely brown. Notice what did not happen: no beetle changed colour during its life, and the birds did not cause brown alleles to appear. The brown variation was already present, and selection changed how common it was.

3Adaptations and fitness

An adaptation is a heritable trait that aids survival and reproduction in the current environment. Adaptations can be physical (the shape of a beak), physiological (how a body handles a toxin) or behavioural (how an animal finds a mate or avoids predators). A trait is an adaptation only relative to a particular environment, and which traits are favourable can change when the environment changes.

Biologists measure success by fitness, which in this context means an organism’s relative contribution of offspring to the next generation compared with others in the population. Fitness is about reproduction, not strength: a small, unremarkable organism that leaves many surviving offspring is fitter than a powerful one that leaves none. Natural selection acts on whole organisms, so the combined effect of all of an organism’s traits matters.

The beaks of Galapagos finches provide a classic case. Different species have beak shapes suited to different foods. Peter and Rosemary Grant’s long-term study documented beak size shifting after a drought changed the availability of seeds: when large, hard seeds were what remained, birds with larger beaks survived better, and beak size in the population shifted.

4How selection can shape a population

Selection can act on a trait with a range of values in three main ways.

• Stabilizing selection favours the average and reduces variation. Examples include forest mice whose brown fur matches the ground, and robins that typically lay four eggs, because very large or small clutches do worse.
• Directional selection favours one extreme after an environmental change, shifting the population. The peppered moth in 18th- and 19th-century England is the traditional example: populations shifted toward darker forms as soot darkened trees, although some scientists have since questioned details of that evidence.
• Diversifying selection favours two or more extremes while intermediates do worse, increasing variation. Beach mice that are light (matching sand) or dark (matching grass) survive better than medium-coloured mice, and alpha males and sneaker males are favoured over medium-sized males in some species.

Selection is not always so simple. In frequency-dependent selection, the fitness of a trait depends on how common it is: side-blotched lizards with orange, blue and yellow throats cycle in frequency like a game of rock-paper-scissors. Selection also cannot produce a perfect organism. It acts only on existing variation, linked alleles can be kept or lost together, and a series of intermediate steps may be disadvantageous even if the end result would help.

5Selection pressure

A selection pressure is any factor in the environment that influences which individuals survive and reproduce. Common selection pressures include:

• predators, which favour camouflage, speed or defences;
• food supply, such as the seed sizes available to finches after a drought;
• climate and weather;
• disease, and in the case of bacteria, antibiotics;
• competition for resources or mates, including competition from introduced species.

Antibiotic resistance shows selection pressure at work and on a fast timescale. Antibiotics kill susceptible germs, while resistant germs survive, multiply and spread, so repeated exposure speeds up resistance. The drug does not create the resistance; it selects it. Resistant germs can also pass their resistance mechanisms to other germs. The article Antibiotics do not work on colds explains why using antibiotics wisely matters.

6Invasive species as a selection pressure

The BC curriculum lists invasive species as an example of selection pressure. An invasive species is a nonnative organism (a disease, parasite, plant or animal) that spreads beyond where it was first introduced and can harm the environment, the economy or human health. Some are introduced unintentionally, such as zebra mussels, chestnut blight and the West Nile virus. Others are introduced on purpose, such as kudzu vine, house sparrows, starlings and nutria.

When an invasive species arrives, native species face a new selection pressure. The newcomer may outcompete natives for food and space, prey on them, spread disease, or alter the habitat. Invasive species often lack the predators and diseases that kept them in check at home. Harms include crop losses, clogged waterways, threats to fisheries, increased wildfire risk and the spread of disease to wildlife and humans. The effects are large: invasive species are one of the major threats to biodiversity, and increased global movement and trade have driven their spread.

What happens next depends on the variation in the native population. If some natives happen to have heritable traits that help them cope (for example, resistance to a new disease or avoidance of a new predator), those individuals leave more offspring and the population may adapt. If no such variation exists, or if the change is too rapid for selection to keep pace, the native population declines and may disappear locally. Extinction is covered in the next lesson.

7Evidence for evolution by natural selection

Natural selection explains how populations change. The wider theory of evolution is supported by several independent lines of evidence.

Two further ideas are worth distinguishing. Divergent evolution produces different species from a common ancestor. Convergent evolution produces similar traits in unrelated species facing similar pressures, such as the wings of bats and insects.

Natural selection links the genetics of the first lessons to the diversity of life: variation arises from mutation and from the shuffling of alleles in meiosis, and selection changes the frequencies of alleles in response to the environment. The next lesson shows what happens when this process continues over long periods and across many habitats. Related ideas about habitats and interactions appear in What is an ecosystem?.

8Practice problems with solutions

Problem 1: tracing a population over generations

A population of 200 moths is 50 percent light and 50 percent dark. Soot darkens the tree trunks, and birds eat light moths more often. Before breeding, 60 light moths and 20 dark moths are eaten. Survivors: 40 light and 80 dark, so dark moths now make up 80/120, about 67 percent. If colour is inherited, the next generation will have a higher share of dark moths. This is directional selection. The change is in the population, not in any single moth.

Problem 2: identifying the pressure

A lake is invaded by a nonnative fish that eats the eggs of a native fish. Which native fish are favoured? Those that guard eggs or lay eggs in places the invader cannot reach, if such variation exists and is inherited. If the native fish has no such variation, its numbers fall. The invader is the selection pressure.

Problem 3: separate the claims

“Giraffes stretched their necks and passed on longer necks.” Is this natural selection? No. Stretching during life does not change the alleles in eggs and sperm. Natural selection says that giraffes with heritable variation for longer necks left more offspring in an environment where reaching leaves mattered.

Summary

• Variation, heredity and differential reproduction together produce natural selection.
• Adaptations are heritable traits that help survival and reproduction in a particular environment.
• Selection pressures include predators, climate, food, disease and invasive species.
• Selection acts only on variation that exists, so it does not produce perfect organisms or plan ahead.

Questions for discussion

1. How would the beetle example change if the birds ate brown beetles more often?
2. Why might a trait that is an adaptation in one environment be a disadvantage in another?
3. What kinds of evidence would be needed to show that a trait in a wild population is an adaptation?

Connecting to genetics

At the DNA level, selection is a change in allele frequency. If the allele for brown colour is carried by half of the beetles, and brown beetles leave more offspring, the allele becomes more common in each generation. Mutation adds new alleles, meiosis and fertilization shuffle them into new combinations, and selection changes their frequency. This is the same chain of ideas that links Mendel to Darwin.

Reading a selection graph

When a graph shows a trait value on the horizontal axis and the number of individuals on the vertical axis, a curve that shifts to one side after selection indicates directional selection, a narrower curve indicates stabilizing selection, and a curve that splits into two peaks indicates diversifying selection. Comparing the graph before and after an environmental change is a common way to present evidence.

?Quick check

Try each question first, then reveal the answer.